Absorption of water mainly occurs in the stomach. Water absorption occurs in the blood. Absorption is the process of transporting food components from the cavity of the digestive tract into the internal environment - blood and lymph of the body. Suction in the mouth

1. Tell us about the structure of the stomach.

The stomach serves as a reservoir for storing and digesting food. Outwardly, it resembles a large pear, its capacity is up to 2-3 liters. The shape and size of the stomach depend on the amount of food eaten. The stomach has a body, a fundus and a pyloric section (the section bordering the duodenum), an inlet (cardia) and an outlet (pylorus). The wall of the stomach consists of three layers: mucous (the mucous membrane is collected in folds into which the excretory ducts of the glands that produce gastric juice open; the mucosa also contains endocrine cells that produce hormones, in particular gastrin), muscle (three layers of muscle cells: longitudinal, circular, oblique), serous.

2. What processes occur in the stomach?

Under the action of enzymes in the stomach, protein digestion begins. This process occurs gradually as the digestive juice permeates the food bolus, penetrating into its depths. This is facilitated by the constant mixing of food in the stomach, due to the alternate contraction of various muscle fibers. In the stomach, food lingers for up to 4–6 hours and, as it turns into a semi-liquid or liquid pulp and is digested, it passes in portions into the intestines.

3. How is the secretion of gastric juice regulated?

Regulation of juice secretion by the gastric glands occurs through reflex and humoral pathways. It begins with conditioned and unconditional secretion of juice at the sight or smell of food and when food enters the mouth immediately after the salivary glands of the oral cavity begin to work. Under the influence of the sympathetic nervous system, the secretion of digestive juices increases, while the parasympathetic nervous system decreases.

4. What does gastric juice contain?

Gastric juice is a clear liquid, 0.25% of its volume consists of hydrochloric acid (pH ≈ 2), mucins (protect the walls of the stomach) and inorganic salts and digestive enzymes themselves. Digestive enzymes are activated by hydrochloric acid. These are pepsin (breaks down proteins), gelatinase (breaks down gelatin), lipase (breaks down milk fats into glycerol and fatty acids), chymosin (curdles milk casein).

5. It is known that proteins are digested in the stomach. Why are the walls of the stomach itself not damaged?

The mucous membrane is protected from self-digestion by mucus (mucin), which abundantly covers the walls of the stomach.

6. What substances are digested in the duodenum?

In the duodenum, food is exposed to pancreatic juice, bile and intestinal juice. Their enzymes break down proteins into amino acids, fats into glycerol and fatty acids, and carbohydrates into glucose.

7. Using additional sources of information, as well as the drawing “Blood Movement in the Liver,” explain how the liver performs its barrier function.

The gates of the liver include the hepatic artery and the portal vein, which collects blood from all unpaired organs of the abdominal cavity. Blood passes through liver cells - hepatocytes, collected in hepatic acini, in which it is cleared of toxic substances, hemoglobin breakdown products, and some microorganisms. Next, the purified blood is collected in the hepatic vein, and the rest is mixed with the secretion of hepatocytes (together they make up bile) and moves along the bile ducts, which are collected at the gates of the liver into the common bile duct. Next, the bile either directly enters the duodenum, or is collected in the gallbladder and enters the intestine from the bladder as needed.

8. What role does bile play in the digestion process?

Bile increases the activity of enzymes in the intestinal juice and pancreas, and under its action, large drops of fat break up into small drops, which makes them easier to digest. Bile also activates absorption processes in the small intestine; has a detrimental effect on some microorganisms; creates an alkaline environment in the intestines; enhances motor activity (motility) of the intestine.

9. What stages can be distinguished in the process of digestion in the small intestine?

The digestion process in the small intestine consists of three stages: cavity digestion, parietal digestion and absorption.

10. What is parietal digestion? What is its significance?

Parietal digestion, the second stage of the digestion process, which occurs on the very surface of the intestinal mucosa. Food particles penetrating into the spaces between the villi are digested with the help of appropriate enzymes. Larger particles cannot get here. They remain in the intestinal cavity, where they are exposed to digestive juices and are broken down to smaller sizes. The process of parietal digestion provides the final stage of hydrolysis and the transition to the final stage of digestion - absorption.

11. What is the significance of the pendulum-like movements of the small intestine?

The small intestine is also capable of pendulum-like movements due to alternate lengthening and shortening of the intestine in a certain area. The contents of the intestine are mixed and moved in both directions.

12. What is the significance of the folding of the inner wall of the small intestine?

Due to folding, the surface area of ​​the intestinal mucosa sharply increases, so almost complete processing of food occurs here.

13. Where does the pancreatic duct flow into? What is the role of the enzymes it secretes?

The pancreatic duct as well as the common bile duct open in the large duodenal papilla on the lateral wall of the duodenum. The pancreas produces the following digestive enzymes: trypsin, chymotrypsin, elastase (break down proteins into peptides and amino acids); amylase (converts carbohydrates into glucose); lipase (breaks down fats into glycerol and fatty acids); nucleases (cleave nucleic acids into nucleotides).

14. What is the essence of suction? Where does the main absorption of nutrients occur? water?

Absorption is the process of moving nutrients from the intestines into the blood vessels; a complex physiological process based on the phenomena of filtration, diffusion and some others. Absorption occurs in the wall of the small and large intestines. The walls of the villi of the small intestine are covered with a single-layer epithelium, under which there are networks of blood and lymphatic capillaries and nerve fibers with nerve endings. Between the dissolved nutrient in the intestinal cavity and the blood there is only a thin barrier of two layers of cells - the intestinal walls and capillaries. Intestinal epithelial cells are active. They allow some substances to pass through (only in one direction), others not.

15. Name the end products of the breakdown of proteins, fats and carbohydrates. Which of them is absorbed into the blood and which into the lymph?

Proteins in our body are broken down into amino acids, carbohydrates into glucose, fats into glycerol and fatty acids. The breakdown products glucose, amino acids, and solutions of mineral salts are directly absorbed into the blood. In the cells of the body, these substances are converted into proteins and carbohydrates characteristic of humans. Fatty acids and glycerol are absorbed into the lymphatic capillaries.

Absorption occurs throughout the digestive tract, but with different intensity in its different departments. In the oral cavity, absorption is well expressed, but due to the short duration of food staying in it, it has no practical significance. Drugs can be absorbed, which is widely used in clinical practice. The stomach absorbs water and soluble mineral salts, alcohol, glucose and a small amount of amino acids. The main part of the digestive tract where absorption occurs is the small intestine. Already 1-2 minutes after nutrients enter the intestine, they appear in the blood. Partial absorption occurs in the colon. For the mechanism of absorption (transport of substances), see section 2.4. After eating, blood flow in the gastrointestinal tract increases by 30-130%, which accelerates absorption. Contraction of the villi of the small intestine also speeds up the absorption process. Each intestinal cell provides nutrients to approximately 100,000 other cells in the body. Let us note some features of the absorption of individual nutrients.

Water suction carried out according to the law of osmosis. Hydrostatic pressure in the gastrointestinal tract promotes water absorption. Water enters the digestive tract as part of food, liquids (2-2.5 l) and secretions of the digestive glands (6-8 l), and only 100-150 ml of water is excreted with feces, that is, almost all the liquid is absorbed. About 60% of water is absorbed in the duodenum and about 20% in the ileum.

Absorption of mineral salts can be carried out both through intestinal epithelial cells and through intercellular channels, primarily and secondarily actively (according to the laws of diffusion). For example, N+ ions enter the cytoplasm through the apical membrane of enterocytes according to the electrochemical gradient, and the transport of these ions from enterocytes to the interstitium occurs through the basolateral membranes of enterocytes

using a Na/K pump localized there. Ions N + , K + and SG also move through intercellular channels according to the laws of diffusion. Absorption of calcium ions and other divalent cations in the small intestine occurs much more slowly.

Absorption of monosaccharides occurs mainly in the small intestine; polysaccharides and disaccharides are practically not absorbed in the gastrointestinal tract. Glucose is absorbed at the fastest rate. The entry of monosaccharides from the cavity of the small intestine into the blood can occur in various ways, however, during the absorption of glucose and galactose, the sodium-dependent mechanism plays the main role. In the absence of Na+, glucose is absorbed 100 times slower (and only in the presence of a concentration gradient).

Products of hydrolytic breakdown of proteins absorbed in the form of free amino acids, di- and tripeptides. The main mechanism of absorption of amino acids in the small intestine is secondary active - sodium-dependent transport. Diffusion of amino acids according to an electrochemical gradient is also possible. Intact protein molecules in very small quantities can be absorbed in the small intestine by pinocytosis (endocytosis).

Absorption of fat breakdown products. Mixed micelles formed as a result of the interaction of monoglycerides, fatty acids with the participation of bile salts, phospholipids and cholesterol enter the membranes of enterocytes, where their lipid components dissolve in the plasma membrane and, according to a concentration gradient, enter the cytoplasm of enterocytes. In intestinal epithelial cells, resynthesis of triglycerides from monoglycerides and fatty acids occurs on microsomes of the endoplasmic reticulum. From newly formed triglycerides, cholesterol, phospholipids and glycoproteins, chylomicrons are formed - tiny fat particles enclosed in a thin protein shell. Chylomicrons accumulate in secretory vesicles, which merge with the lateral membrane of the enterocyte and, through the resulting hole, exit into the intercellular space, from where into the lymphatic system. Short- and medium-chain fatty acids are quite soluble in water and can diffuse to the surface of enterocytes without forming micelles. They penetrate through the intestinal epithelial cells directly into the portal blood.

Nutrient absorption is the final goal of the entire digestive chain that occurs in the human body. It occurs in almost all parts of the digestive system and plays a very important role in ensuring normal human life.

What is happening and where?

Nutrient absorption is a multifaceted process that occurs in each part of the digestive system. It often happens that its functioning is disrupted and in order to normalize the work, it is necessary to identify the department in which the failure occurred. And this can only be done by fully understanding all the stages of this complex physiological process.

In particular, the process is implemented:

1. In the oral cavity. Saliva includes special enzymes that allow you to break down any carbohydrates into glucose levels. However, the period of food being in the oral cavity is quite short - a maximum of 20 seconds. During this time, monosaccharides only begin the absorption process, which will end when the food enters the stomach. However, even there, saliva, which has soaked the food in the mouth, will actively participate in the process.
2. In the walls of the stomach. As a rule, it is in this section of the esophagus that the maximum percentage of water, already broken down mineral salts and amino acids are absorbed. Glucose and alcohol are partially absorbed. This is precisely what explains the pattern that people who drink on an empty stomach get drunk very quickly.
3. In the intestinal (small) area. The vast majority of nutrients from digested foods are absorbed in the small intestine. This can be explained by its specific structure, ideally adapted to perform this function. The internal cavity of the small intestine is strewn with villi, which significantly increase its area and only increase its absorption capacity. The maximum number of amino acids, monosaccharides and nutrients complete their breakdown and are absorbed into the blood here.
4. On the surface of the large intestine. This stage is the final one and allows you to complete the absorption of water, salt, numerous vitamins and even monomers not affected in the previous stages. After final absorption in the walls of the large intestine, food is considered completely processed and ready to be excreted from the body.

The process of breaking down food, on average, takes up to several hours and directly depends on the composition of the products that enter the body. Useful and healthy food is broken down as quickly as possible, while it takes much longer to process harmful and heavy foods.

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How exactly is absorption accomplished?

Despite its apparent simplicity, the absorption process occurs in strict accordance with various mechanisms through which its regulation is carried out.

In particular, salt and a complex of organic components enter the body following the laws of diffusion. Along with this, a number of other mineral substances enter there exclusively according to the laws of filtration, provoked by contractions of the muscular region of the intestine.

In addition, digestion in the intestines requires significant expenditure of energy resources, so after eating it is strongly recommended to minimize physical activity and just sit or lie down for at least an hour. During this time, glucose, any amino acids, vital sodium, and even substances such as fatty acids will be absorbed.

In the process of studying the issue of food absorption in the small intestine, scientists conducted special experiments, during which the absorption processes were disrupted by introducing certain poisons into the body. As a result of such an experiment, the functioning of the intestine did not stop. However, the process of assimilation of glucose and accompanying sodium ions completely stopped.

Moreover, the absorption of nutrients from food requires a significant increase in the intensity of cellular respiration and more active contraction of villi.

In fact, villi are a kind of pump that helps move food debris along the intestinal walls. Absorption occurs through them.

It is noteworthy that in just one day, about 10 liters of liquid are absorbed, approximately 8 liters of which are gastric juices. The main mechanism for carrying out such manipulations is the intestines.

People who have problems with the functioning of this element of the esophagus should seek help as quickly as possible! Indeed, in this case, not only does the condition worsen and discomfort is observed, but also a deficiency is formed for certain components that are no longer absorbed by this organ.

And, of course, having undergone the recommended treatment, you will need to monitor your own diet, completely eliminating harmful foods and substances that cause irritation and even inflammation of the intestinal walls. It is quite difficult to select such a list on your own, and it is better to consult a doctor on this issue.

Absorption is understood as a set of processes that ensure the transfer of various substances into the blood and lymph from the digestive tract.

A distinction is made between the transport of macro- and micromolecules. Transport of macromolecules and their aggregates is carried out using phagocytosis And pinocytosis and is called endocytosis. A certain amount of substances can be transported through intercellular spaces - by persorption. Due to these mechanisms, a small amount of proteins (antibodies, allergens, enzymes, etc.), some dyes and bacteria penetrate from the intestinal cavity into the internal environment.

Mainly micromolecules are transported from the gastrointestinal tract: nutrient monomers and ions. This transport is divided into:

Active transport;

Passive transport;

Facilitated diffusion.

Active transport substances is the transfer of substances across membranes against concentration, osmotic and electrochemical gradients with the expenditure of energy and with the participation of special transport systems: mobile carriers, conformational carriers and transport membrane channels.

Passive transport carried out without energy consumption along concentration, osmotic and electrochemical gradients and includes: diffusion, filtration, osmosis.

Driving force diffusion solute particles is their concentration gradient. A type of diffusion is osmosis, in which movement occurs in accordance with the concentration gradient of the solvent particles. Under filtering understand the process of transfer of solution through a porous membrane under the influence of hydrostatic pressure.

Facilitated diffusion, like simple diffusion, it occurs without energy consumption along a concentration gradient. However, facilitated diffusion is a faster process and is carried out with the participation of a carrier.

Absorption in various parts of the digestive tract. Absorption occurs throughout the digestive tract, but its intensity varies in different sections. In the oral cavity, absorption is practically absent due to the short-term presence of substances in it and the absence of monomeric hydrolysis products. However, the oral mucosa is permeable to sodium, potassium, some amino acids, alcohol, and some drugs.

In the stomach, the intensity of absorption is also low. Here water and mineral salts dissolved in it are absorbed; in addition, weak solutions of alcohol, glucose and small amounts of amino acids are absorbed in the stomach.

In the duodenum, the intensity of absorption is greater than in the stomach, but even here it is relatively small. The main process of absorption occurs in the jejunum and ileum, meaning in the absorption processes, since it not only promotes the hydrolysis of substances (due to the change in the parietal layer of chyme), but also the absorption of its products.

During absorption in the small intestine, contractions of the villi are of particular importance. Stimulators of villi contraction are products of hydrolysis of nutrients (peptides, amino acids, glucose, food extractives), as well as some components of the secretions of the digestive glands, for example, bile acids. Humoral factors also enhance the movements of the villi, for example, the hormone villikinin, which is formed in the mucous membrane of the duodenum and in the jejunum.

Absorption in the colon is negligible under normal conditions. Here, mainly the absorption of water and the formation of feces occurs. In small quantities, glucose, amino acids, and other easily absorbed substances can be absorbed in the colon. On this basis, nutritional enemas are used, i.e., the introduction of easily digestible nutrients into the rectum.

Absorption of protein hydrolysis products. Proteins, after hydrolysis to amino acids, are absorbed in the intestine. Absorption of different amino acids in different parts of the small intestine occurs at different rates. The absorption of amino acids from the intestinal cavity into its epithelial cells is carried out actively with the participation of a carrier and with the expenditure of ATP energy. From epithelial cells, amino acids are transported into the intercellular fluid through the mechanism of facilitated diffusion. Amino acids absorbed into the blood travel through the portal vein system to the liver, where they undergo various transformations. A significant portion of amino acids is used for protein synthesis. Amino acids in the liver are deaminated, and some undergo enzymatic transamination. Amino acids carried by the bloodstream throughout the body serve as the starting material for the construction of various tissue proteins, hormones, enzymes, hemoglobin and other protein substances. Some amino acids are used as a source of energy.

The intensity of amino acid absorption depends on age - it is more intense at a young age, on the level of protein metabolism in the body, on the content of free amino acids in the blood, on nervous and humoral influences.

Absorption of carbohydrates. Carbohydrates are absorbed mainly in the small intestine in the form of monosaccharides. Hexoses (glucose, galactose, etc.) are absorbed at the highest speed; pentoses are absorbed more slowly. The absorption of glucose and galactose is the result of their active transport through the apical membranes of intestinal epithelial cells. The transport of glucose and other monosaccharides is activated by the transport of sodium ions across the apical membranes. Glucose accumulates in intestinal epithelial cells. Further transport of glucose from them into the intercellular fluid and blood through the basal and lateral membranes occurs passively along a concentration gradient. Absorption of different monosaccharides in different parts of the small intestine occurs at different rates and depends on the hydrolysis of sugars, the concentration of the resulting monomers, and on the characteristics of the transport systems of intestinal epithelial cells.

Various factors, especially the endocrine glands, are involved in the regulation of carbohydrate absorption in the small intestine. Glucose absorption is enhanced by hormones of the adrenal glands, pituitary gland, thyroid and pancreas. Serotonin and acetylcholine enhance glucose absorption. Histamine somewhat slows down this process, and somatostatin significantly inhibits glucose absorption.

Monosaccharides absorbed in the intestine enter the liver through the portal vein system. Here, a significant part of them is retained and converted into glycogen. Some of the glucose enters the general bloodstream and is distributed throughout the body and used as a source of energy. Some of the glucose is converted into triglycerides and stored in fat stores. Mechanisms regulating the ratio of glucose absorption, glycogen synthesis in the liver, its breakdown with the release of glucose and its consumption by tissues ensure a relatively constant level of glucose in the circulating blood.

Absorption of fat hydrolysis products. Under the action of pancreatic lipase in the cavity of the small intestine, diglycerides are formed from triglycerides, and then monoglycerides and fatty acids. Intestinal lipase completes. lipid hydrolysis. Monoglycerides and fatty acids with the participation of bile salts pass into intestinal epithelial cells through the apical membranes using active transport. Resynthesis of triglycerides occurs in intestinal epithelial cells. From triglycerides, cholesterol, phospholipids and globulins are formed chylomicrons - tiny fatty particles enclosed in a lipoprotein membrane. Chylomicrons leave epithelial cells through the lateral and basal membranes, pass into the connective tissue spaces of the villi, from there they pass into the central lymphatic vessel with the help of contractions of the villi, thus, the main amount of fat is absorbed into the lymph. Under normal conditions, a small amount of fat enters the blood.

Parasympathetic influences increase, and sympathetic influences slow down the absorption of fats. Hormones of the adrenal cortex, thyroid gland and pituitary gland, as well as hormones of the duodenum - secretin and cholecystokinin-pancreozymin - enhance the absorption of fats.

Fats absorbed into the lymph and blood enter the general bloodstream. The main amount of lipids is deposited in fat depots, from which fats are used for energy and plastic purposes.

Absorption of water and mineral salts. The gastrointestinal tract takes an active part in the water-salt metabolism of the body. Water enters the gastrointestinal tract as part of food and liquids, and the secretions of the digestive glands. The main amount of water is absorbed into the blood, a small amount into the lymph. Absorption of water begins in the stomach, but it occurs most intensively in the small intestine. Some water is absorbed along the osmotic gradient, but it can also be absorbed in the absence of a difference in osmotic pressure. Actively absorbed solutes by epithelial cells “draw” water with them. The decisive role in the transfer of water belongs to sodium and chlorine ions. Therefore, all factors affecting the transport of these ions also affect the absorption of water. Water absorption is associated with the transport of sugars and amino acids. Many of the effects of slowing or accelerating water absorption result from changes in the transport of other substances from the small intestine.

Excluding bile from digestion slows down the absorption of water from the small intestine. CNS inhibition and vagotomy slow down water absorption. The process of water absorption is influenced by hormones:

ACTH enhances the absorption of water and chlorides, thyroxine increases the absorption of water, glucose and lipids. Gastrin, secretin, cholecystokinin-pancreozymin - weaken the absorption of water.

Sodium is intensively absorbed in the small intestine and ileum. Sodium ions are transferred from the cavity of the small intestine into the blood through intestinal epithelial cells and through intercellular channels. The entry of sodium ions into the epithelial cell occurs passively along an electrochemical gradient. From epithelial cells through their lateral and basal membranes, sodium ions are actively transported into the intercellular fluid, blood and lymph. Through intercellular channels, sodium ions are transported passively along a concentration gradient.

In the small intestine, the transfer of sodium and chlorine ions is coupled; in the large intestine, absorbed sodium ions are exchanged for potassium ions. With a decrease in sodium content in the body, its absorption in the intestine increases sharply. The absorption of sodium ions is enhanced by the hormones of the pituitary gland and adrenal glands, and inhibited by gastrin, secretin and cholecystokinin-pancreozymin.

Absorption of potassium ions occurs mainly in the small intestine using passive transport along an electrochemical gradient.

Absorption of chloride ions occurs in the stomach, and most actively in the ileum through the mechanism of active and passive transport. Passive transport of chlorine ions is coupled with the transport of sodium ions. Active transport of chlorine ions occurs through the apical membranes and is associated with the transport of sodium ions.

Of the divalent cations absorbed in the intestine, the most important are calcium, magnesium, zinc, copper and iron ions.

Calcium is absorbed along the entire length of the gastrointestinal tract, but its most intense absorption occurs in the duodenum and the initial part of the small intestine. In the same section of the intestine, magnesium, zinc and iron ions are absorbed. Copper absorption occurs primarily in the stomach.

The process of calcium absorption involves mechanisms of facilitated and simple diffusion. It is believed that the basement membrane of enterocytes contains a calcium pump, which pumps calcium out of the cell into the blood against an electrochemical gradient. Bile has a stimulating effect on calcium absorption. Absorption of magnesium and zinc ions, as well as the main amount of copper, occurs passively.

The absorption of iron ions is carried out both by the mechanism of passive transport - simple diffusion, and by the mechanism of active transport - with the participation of carriers. When iron ions enter the enterocyte, they combine with apoferritin, resulting in the formation of the metalloprotein ferritin, which is the main iron depot in the body.

Absorption of vitamins. Water-soluble vitamins can be absorbed by diffusion (vitamin C, riboflavin). Vitamin Bi2 is absorbed in the ileum. The absorption of fat-soluble vitamins (A, D, E, K) is closely related to the absorption of fats.

Digestion is the process of mechanical processing of food in the digestive canal, its enzymatic breakdown into simpler nutrients that can be absorbed into the blood. The main substances included in the products are proteins, fats, carbohydrates, vitamins, mineral salts and water. Functions of the digestive system:

• motor (mixing, chopping, moving food through the digestive tract);
• secretory (synthesis and secretion of digestive juices);
• absorption (ensuring the transition of nutrients from the intestines to the blood and lymph).

The digestive system consists of the alimentary canal and digestive glands. The alimentary canal includes the oral cavity, pharynx, esophagus, stomach, small and large intestines. The digestive glands include the salivary glands, pancreas and liver.

The oral cavity contains teeth, tongue, and salivary glands. The teeth are located in the sockets of the jaws. An adult has 32 of them; On each jaw there are 4 incisors, 2 canines, 4 small molars and 6 large molars. A tooth consists of a crown, neck and root. Inside the tooth there is a cavity - the pulp, which contains nerves and blood vessels. The hard substance of the tooth - dentin - is a modified bone tissue. The top of the tooth is covered with enamel.

In the oral cavity, the initial breakdown of carbohydrates is carried out by salivary enzymes that are active in a slightly alkaline environment. Saliva is secreted by three pairs of salivary glands: parotid, sublingual and submandibular. Food acts as an irritant on the nerve endings of the oral mucosa, from which the excitation is transmitted to the food center of the brain, and activates the functioning of the digestive organs.

A bolus of food soaked in saliva enters the stomach as a result of the reflex act of swallowing, in which the epiglottis descends and closes the entrance to the larynx, the soft palate rises and closes the nasopharynx, food is pushed into the esophagus, the walls of which contract in waves and push food into the stomach.

The stomach is a pouch-shaped extension of the digestive tract. It holds about 2-3 liters of food. Its walls contain glands, some of which secrete gastric juice. It contains the enzyme pepsin, which breaks down proteins into polypeptides. Other glands produce acid, which creates an acidic environment in the stomach and inhibits microorganisms that enter the stomach. Some cells in the stomach lining secrete mucus, which protects the stomach wall from

The duodenum is 25-30 cm long. The ducts of the pancreas and liver open into it. The pancreas produces the hormone insulin, which goes directly into the blood, and digestive enzymes involved in further breakdown. Under the influence of the enzyme trypsin, proteins are broken down into amino acids. Other enzymes are involved in the breakdown of nucleic acids, carbohydrates and fats.

The liver is the largest gland in our body. It is the “main chemical laboratory” of the body. The liver neutralizes toxic low-molecular substances that come with the blood. The liver produces bile, which is stored in the gallbladder and then passes into the duodenum.

The small intestine is 5-6 m long and forms loops in the abdominal cavity. The mucous membrane of the small intestine contains many glands that secrete intestinal juice. The mucous membrane forms outgrowths - villi. Inside them there are blood and lymphatic capillaries and nerves. Fatty acids and glycerol from the intestinal cavity pass into the epithelial cells of the villi, where they form fat molecules characteristic of the human body, which are then absorbed into the lymph and, having passed the barrier from the lymph nodes, enter the blood. Amino acids, glucose and other nutrients are absorbed into the blood, which collects in the portal vein and passes through the liver, where toxic substances are disinfected.

Water is absorbed in the colon and feces are formed. Here, fiber is digested with the help of bacteria that destroy the membranes of plant cells, and vitamins K and B are synthesized.

After food is absorbed into the blood, humoral regulation of digestion begins. Among the nutrients there are biologically active substances that, when absorbed into the blood, activate the functioning of the gastric glands. They begin to intensively secrete gastric juice, which ensures long-term secretion of juice.

Human stomach

Structure of the stomach

The stomach of most vertebrates and humans is a muscular, pouch-like extension of the intestine lying in the anterior part of the abdominal cavity. Anatomically, it is usually divided into a cardiac (fundal) section, consisting of the fundus, body and the cardiac region itself, and a pyloric (pyloric, or antral) section, which includes the antral region itself, the pylorus and the pyloric canal. The shape of the stomach changes depending on its functional state, the amount of gastric contents, diet and the condition of surrounding tissues.

The wall of the stomach has three main layers: the inner mucosa, the middle muscular layer, and the outer serous layer. Between the mucous and muscular layers there is an additional submucosal layer. The inner surface of the stomach, lined with epithelial cells, is strongly folded and dotted with mucous cells. In certain areas of the stomach there are glands deeply embedded in its walls that secrete digestive enzymes and mucus.

The stomach has powerful muscular walls, repeated local contractions of which crush and soften food, preparing it for processing in the intestine. Typically, muscle tissue is distributed more or less evenly in the wall of the stomach, but in omnivorous animals and granivorous birds it is concentrated in the distal (terminal) part of the stomach, called the muscular, or chewing, stomach. Mechanical and chemical processing of food occurs in this section, since along with food gastric juice also enters there from the proximal (located immediately behind the esophagus) section of the stomach, called the glandular, or digestive, stomach. The stomach of herbivorous mammals - rodents, sloths, ruminant artiodactyls - cows, sheep, deer is distinguished by the greatest features. In them, from the esophageal section of the stomach, 2 or 3 sections are formed that do not have glands and serve as a container for food. In ruminants, these are tripe, mesh and book. Here, with the help of symbiotic microflora, cellulose is fermented. Food processed in this way (“chewing gum”) is regurgitated for additional chewing, and then goes, bypassing the rumen and mesh, into the book for additional mechanical processing, and then into the abomasum, which can be considered a real stomach: it contains all types of glands and secretory cells characteristic of the fundic and pyloric sections of the human stomach.

Secretion of the human stomach

A feature of gastric juice, which is associated with its digestive functions, is the presence of acid proteases and hydrochloric acid. The leading acid protease that carries out protein hydrolysis, pepsin, is formed in the form of inactive pepsinogen and is activated in an acidic environment at pH 5 and below. Gastric juice, which is a colorless liquid with a pH of 1.5–1.8, is produced in humans in the amount of 2–3 liters per day by the glands and cells of the surface epithelium of the gastric mucosa. The glands of the fundus contain three types of cells: parietal, or parietal, producing hydrochloric acid; the main ones, producing a complex of proteolytic enzymes, additional (mucoid), secreting mucin (mucus), mucopolysaccharides and bicarbonates. The glands of the antrum consist mainly of mucoid cells. Parietal cells also secrete the so-called internal Castle factor, a glycoprotein necessary for the absorption of vitamin B 12 and normal bone marrow hematopoiesis.

Gastric juice in direct contact with the walls of the stomach (or duodenum) can have a significant damaging effect, primarily on the mucous membrane. Normal activity of the stomach and duodenum is possible only under conditions when the aggressive factors of gastric juice are resisted by natural protective mechanisms. First of all, this is the so-called mucus bicarbonate barrier - a glycoprotein gel with bicarbonate ions HCO 3 diffusing in it, secreted by the epithelial cells of the stomach, forming a thin continuous film 200-1500 microns thick on the surface of the mucous membrane. This gel, which is 95% water, forms a mixing zone in which bicarbonate ions interact with H+ ions from the stomach cavity. At the same time, a stable pH gradient is created on the surface of the mucous membrane: if in the stomach cavity the pH is less than 2, then on the surface of the epithelial cells it is more than 7. Thus, the mucus bicarbonate barrier prevents the penetration of pepsin and hydrochloric acid to the mucous membrane, maintaining a neutral or even alkaline environment near the epithelial cells. cells. The high adhesiveness and viscosity of this gel ensure its strong adhesion to epithelial cells. The normal functioning of the mucus bicarbonate barrier is ensured by adequate mucus formation and bicarbonate secretion. Although the mucous gel is easily passable for small ions, there is evidence that even H + ions diffuse in it 4 times slower than in water. For high-molecular compounds, including pepsin, the mucous gel is impenetrable. The mucus bicarbonate barrier is the first line of defense of the mucous membrane. The second line of defense is provided by mucosal epithelial cells, mainly by their lipoprotein membranes and the strong junction of their superolateral surfaces. These cells are distinguished by their high ability to fully regenerate. With minor damage, the mucous membrane is restored within 30 minutes, and complete renewal of all surface epithelial cells occurs within 2–6 days.

Violation of gastric secretion leads to various diseases - peptic ulcer, gastritis, pyloric stenosis, atony, achlorhydria, achylia.

Blood supply to the human stomach

The human stomach is supplied with blood from the celiac trunk, which arises from the abdominal aorta. Numerous branches of the first and second order depart from it, including the right and left gastric arteries. The branches of all these vessels form an arterial ring around the stomach, consisting of two arches located along its minor (left and right gastric arteries) and major (left and right gastroepiploic arteries) curvatures.

The gastric mucosa, which makes up half the weight of the stomach, as its most metabolically active part, receives three-quarters of the total volume of blood entering this organ. Normal blood flow protects the gastric mucosa by supplying it with oxygen, glucose and HCO 3 -, and carrying away metabolic products, toxic agents and H + ions. A feature of the microscopic structure of the gastric vasculature is the presence of numerous arteriovenous and capillary-capillary shunts between the vessels in both the submucosal and mucosal layers. This allows blood flow to be redistributed depending on local metabolic needs.

Motility of the human stomach

Gastric motility ensures mechanical processing of food: its storage, mixing, grinding and evacuation into the duodenum. Since the main purpose of the fundic section of the stomach is food storage, there are no rhythmic excitations or peristalsis in this section. The movement of solid contents in the stomach is carried out due to wave-like changes in muscle tone, which begin in the area of ​​the greater curvature and spread to the pyloric region. Strong circular peristaltic waves in the cardiac part of the stomach push its contents towards the pylorus, facilitating the evacuation of chyme into the duodenum. Gastric motility also plays an important role in ensuring a balance between aggressive and protective mechanisms in the stomach and duodenum. In healthy people, the relationship between the secretion of hydrochloric acid in the stomach and its motor-evacuation function is the opposite: the greater the secretion of acid, the lower the motor activity and vice versa. Hydrochloric acid stimulates the closure of the pylorus and its periodic activity. Acidification of the duodenal contents also slows down gastric emptying.

Regulation of the human stomach

The innervation of the stomach is carried out by the parasympathetic and sympathetic sections of the autonomic nervous system, the fibers of which pass as part of the vagus, splanchnic and phrenic nerves, and the enteric nervous system, located in the thickness of the walls of the gastrointestinal tract. The enteric system is represented by a number of plexuses, the most developed of which, the intermuscular one, is connected to the central nervous system through the vagus nerves. All functions of the stomach are regulated by both nervous and humoral mechanisms. The main physiological stimulus for the gastrointestinal tract is food. Events associated with the entry of food into the stomach, its distension, the chemical composition of food, etc. increase secretion, motility and blood flow in the stomach due to both central unconditioned and local, intraorgan reflexes, and due to humoral hormonal substances. Basal (interdigestive) secretion, accounting for 10% of the maximum, is due to gastrin. In the cerebral phase of secretion, nervous mechanisms predominate, and in the gastric and intestinal secretion, humoral mechanisms predominate. For example, gastrin and histamine enhance secretion, and somatostatin inhibits it. The vagus nerve increases gastric secretion. The participation of the sympathetic nervous system in the regulation of the secretory functions of the stomach has not yet been conclusively proven. The influences of the vagus and sympathetic nerves on the blood flow and motility of the stomach are opposite: the vagus nerves increase the blood flow of the stomach, the rhythm and strength of contractions of the stomach and its motor-evacuation functions, and the sympathetic nerves, respectively, decrease it. Humoral-hormonal substances released by stomach tissues also have different effects. Secretin and pancreozymin slow down motility and evacuation, and motilin enhances these functions.

G. E. Samonina

The liver is the largest human gland, occupying a central place in metabolism. More than 500 biochemical reactions of carbohydrate, fat and protein metabolism occur in it. In addition, the liver is the most important blood depot: at rest, a quarter of all blood in the body is retained in it. This is a multifunctional iron. It is involved in the processes of digestion, metabolism, blood circulation; controls the state of the internal environment of the body - homeostasis. The liver synthesizes and breaks down proteins, fats, carbohydrates, and vitamins (vitamin A is formed and accumulated here). The liver regulates blood sugar metabolism, removes toxins from the body, such as alcohol, and is involved in maintaining a constant body temperature. In addition, the liver has a unique property of regeneration - restoration of lost parts. The liver develops in the form of a hepatic outgrowth from the same part of the primary intestine as the duodenum. The mass of a cadaveric liver is 1.5 kg; in a living person, its mass, due to the presence of blood, is approximately 400 g more. The weight of the liver of an adult is about 1/36 of body weight. In a fetus, its relative mass is twice as large (about 1/18-1/20 of body weight), in a newborn it is 1/20 (about 135 g), and it occupies most of the abdominal cavity.

The liver is located in the abdominal cavity under the diaphragm on the right; only a small part of it extends to the left of the midline in an adult. The anterosuperior (diaphragmatic) surface of the liver is convex, corresponding to the concavity of the diaphragm to which it is adjacent, and a cardiac depression is visible on it. Its leading edge is sharp. The lower (visceral) surface has a number of depressions caused by the organs that are adjacent to it.

The falciform ligament, which is a duplicate of the peritoneum passing from the diaphragm to the liver, divides the diaphragmatic surface of the liver into 2 lobes - the large right one and the much smaller left one. Two sagittal and one transverse grooves are visible on the visceral surface. The latter is the portal of the liver, through which the portal vein, the hepatic artery proper and nerves enter it, and the common hepatic duct and lymphatic vessels exit. Sagittal grooves separate the ventrally located quadrate and dorsal caudate lobes. In the anterior part of the right sagittal groove between the quadrate and the right lobes of the liver there is a gallbladder, in its posterior part lies the inferior vena cava. The left sagittal groove in its anterior part contains the round ligament of the liver, which before birth was the umbilical vein. In the posterior part of this groove there is an overgrown venous duct, which connected the fetus's umbilical vein with the inferior cava.

These three grooves divide the lower surface of the liver into 4 lobes: the left one corresponds to the left lobe of the upper surface, the other three lobes correspond to the right lobe of the liver, which includes the right lobe itself, the quadrate and the caudate.

Currently, the accepted scheme for dividing the liver into 2 lobes, 5 sectors and 8 permanent segments. A sector is a section of the liver supplied by a branch of the portal vein of the second order and the same branch of the hepatic artery, from which the sectoral bile duct emerges.

A segment is a section of hepatic tissue supplied by a branch of the portal vein of the third order and the corresponding branch of the hepatic artery, from which the segmental bile duct emerges. The segment has a somewhat separate blood supply, innervation and bile outflow.

The surface of the liver is smooth and shiny, thanks to the serous membrane covering it on all sides, except for part of its posterior surface, where the peritoneum of the liver passes to the lower surface of the diaphragm. The peritoneum covering the liver forms double folds that are connected to form a ligament that holds the liver. Each lobe consists of thousands of tiny prismatic lobes formed by liver cells (hepatocytes). Inside the layers between the liver lobules there are branches of the hepatic artery, portal vein and bile duct - these formations form the so-called portal zone (hepatic triad).

Through the porta hepatis, two large blood vessels enter the liver: the hepatic artery and portal vein, and the hepatic vein and bile duct exit the liver. The hepatic artery, being a branch of the aorta, supplies the liver cells with arterial blood enriched with oxygen. The portal vein supplies the liver with venous blood from the abdominal organs. This blood contains products of the digestion of fats, proteins and carbohydrates from the stomach and intestines, as well as products of the breakdown of red blood cells from the spleen. After passing through the liver, this blood is collected by the hepatic veins and sent to the heart through the inferior vena cava. The liver plays a key role in carbohydrate metabolism. Glucose, which is absorbed in the small intestine during digestion, is converted into glycogen in liver cells - the main storage carbohydrate, often called animal starch. Glycogen is deposited in liver and muscle cells and serves as a source of glucose in case of its deficiency in the body. Simple sugars such as galactose and fructose are converted into glucose in the liver. In addition, in liver cells, glucose can be synthesized from other organic compounds (the so-called process of gluconeogenesis). Excess glucose is converted into fats and stored in fat cells in different parts of the body. The deposition of glycogen and its breakdown to form glucose is regulated by the pancreatic hormones insulin and glucagon. These processes play an important role in maintaining a constant blood glucose level.

The liver plays an important role in fat metabolism. Fatty acids coming from food are used in the liver to synthesize fats necessary for the body, including phospholipids - the most important components of cell membranes.

The participation of the liver in protein metabolism consists of the breakdown and transformation of amino acids, the synthesis of blood plasma proteins, as well as the neutralization of ammonia formed during the breakdown of proteins. Ammonia is converted to urea in the liver and excreted from the body in urine. The liver also neutralizes other substances toxic to the body.

Bile is secreted by hepatocytes and is a jelly-like substance with an alkaline reaction, reddish-yellow color and bitter taste with a specific odor. The color of bile is due to the content of hemoglobin breakdown products in it - bile pigments, and above all bilirubin. Bile also contains lecithin, cholesterol, bile salts and mucus. Bile acids play an important role in the digestion of fats: they contribute to their emulsification and absorption in the digestive tract.

About half of the bile produced by the liver goes to the gallbladder and is then used as needed. The gallbladder is adjacent to the lower surface of the right lobe of the liver. It is pear-shaped, its length is about 10 cm, and its volume is only 50-60 ml. The mucous membrane of the gallbladder has numerous folds, and underneath there are smooth muscle fibers. The liver produces 500-700 ml of bile per day. The gallbladder could not accommodate all this volume, so its mucous membrane is able to absorb water, and the bile thickens. Under the influence of the hormone cholecystokinin, produced by the duodenum, the gallbladder contracts and bile is released through the common bile duct into the duodenum.

Small intestine

Small intestine (lat. intestinum tenue), the longest part of the digestive tract. It starts from the pylorus of the stomach at the level of the border of the bodies of the XII thoracic and I lumbar vertebrae and is divided into the duodenum, jejunum and ileum. The last two are completely covered by the mesentery on all sides and therefore are allocated to the mesenteric part of the small intestine. The duodenum is covered by the mesentery on only one side. The length of the small intestine of an adult reaches 5-6 m, the shortest and widest is the duodenum, its length does not exceed 25-30 cm. About 2/5 of the length of the small intestine (2-2.5 m) is occupied by the jejunum and about 3/5 ( 2.5-3.5 m) ileum. The diameter of the small intestine does not exceed 3-5 cm. The thickness of the wall decreases along the course of the small intestine. The small intestine forms loops, which are covered in front by the greater omentum, and are bounded above and on the sides by the large intestine. The main processes of absorption occur in the small intestine. Here the chemical processing of food continues and the absorption of its breakdown products continues. The endocrine function of the small intestine is important: the production by enteroendocrine cells (intestinal endocrinocytes) of biologically active substances (secretin, serotonin, lutilin, enteroglucagon, gastrin, cholecystokinin, etc.).

Functions determine the structure of the small intestine. The intestinal mucosa forms numerous circular folds, due to which the absorption surface of the mucous membrane increases, the size and number of folds decrease towards the colon. On the surface of the mucous membrane there are intestinal villi and crypt recesses.

The duodenum (duodenum) is the initial section of the small intestine, begins immediately behind the stomach, covering the horseshoe head of the pancreas. The length of the duodenum in newborns is 7.5-10 cm, in an adult - 25-30 cm (about 12 finger diameters, hence the name). It is located for the most part retroperitoneally. The position of the intestine depends on the filling of the stomach. When the stomach is empty, it is located transversely; when the stomach is full, it rotates, approaching the sagittal plane. Only the initial (2-2.5 cm) and final sections are covered with peritoneum on almost all sides; the peritoneum is adjacent to the remaining sections of the intestine only in front. The shape of the intestine as it grows can be different: in adults, they are U-shaped (15% of cases), V-shaped, horseshoe-shaped (60% of cases), folded and ring-shaped (25% of cases).

The duodenum is divided into upper, descending, horizontal and ascending parts. When passing into the jejunum, the duodenum forms a sharp bend to the left of the body of the second lumbar vertebra.

The wall of the duodenum consists of 3 layers: the inner layer is the mucous membrane, the middle layer is the muscular layer, and the outer layer is the serous layer. The internal mucous membrane forms circular folds, densely covered with outgrowths - intestinal villi (22-40 of them per 1 mm2). The villi are wide and short. Their length is 0.2-0.5 mm. In addition to the circular ones, there is also a longitudinal fold running along the posteromedial wall of its descending part, which ends with a small elevation - the major duodenal papilla (Vaterov), at the top of which the common bile duct and the main pancreatic duct open. In the upper part of the intestine, in the submucosa, there are complex branched tubular duodenal glands, which in their structure and composition of the secreted juice are close to the glands of the pyloric part of the stomach. It opens into crypts. They produce secretions involved in the digestion of proteins, the breakdown of carbohydrates, mucus, and the hormone secretin. In the lower part, deep in the mucous membrane, there are tubular intestinal glands. Throughout the small intestine, the mucous membrane contains lymphatic follicles. The muscular layer consists of an inner circular and outer longitudinal layer. The serous membrane covers the duodenum only in front.

Acidic food gruel (chyle), which has passed from the stomach, continues to be digested in the duodenum under the influence of enzymes of pancreatic and intestinal juices, which have an alkaline reaction. Proteins are broken down into amino acids, carbohydrates into monosaccharides, fats into glycerol and fatty acids. Through the walls of the villi, the products of the breakdown of proteins and carbohydrates enter the blood, and the products of the breakdown of fats enter the lymph.

Jejunum and ileum

The mesenteric part of the small intestine consists of the jejunum and ileum, occupying about 4/5 of the entire length of the digestive tract. There is no clear anatomical boundary between them. This is the most mobile part of the intestine, since it is suspended on the mesentery and is enveloped by the peritoneum (located intraperitoneally). The loops of the jejunum are located vertically, occupying the umbilical and left iliac regions. The loops of the ileum are directed predominantly horizontally and occupy the right ileal region.

The length of the small intestine in a newborn is about 3 m, its intensive development continues until 3 years, after which growth slows down. In adults, the length of the small intestine is from 3 to 11 m; It is believed that the length of the intestine is determined by the diet. People who consume predominantly plant foods have longer intestines than people whose diets are dominated by animal products. The diameter of the mesenteric part of the small intestine in the initial section is about 45 mm, and then gradually decreases to 30 mm.

The digestive surface of the jejunum is larger than that of the ileum, this is due to its larger diameter and larger circular folds. The folds of the wall of the small intestine are formed by the mucous membrane and submucosa, their number in an adult reaches 600-650. The villi of the jejunum are longer and more numerous (22-40 per 1 mm2) than those of the ileum (18-31 per 1 mm2), and the number of crypts is also greater. The total number of villi reaches 4 million. The total surface area of ​​the small intestine, including microvilli, is 200 m 2 in adults.

Villi are outgrowths of the lamina propria of the mucous membrane, formed by loose fibrous connective tissue. The surface of the villi is covered with simple columnar (single-layer cylindrical) epithelium, in which there are three types of cells: intestinal epithelial cells with a striated border, cells that secrete mucus, goblet cells (enterocytes) and a small number of enteroendocrine cells (intestinal endocrinocyte) cells.

Most of the intestinal epithelial cells (columnar cells) have a striated border; on their apical surface there is a border formed by a huge number of microvilli (1500-3000 on the surface of each cell), which increase the absorption surface of these cells. The microvilli contain a large number of active enzymes involved in the breakdown (parietal digestion) and absorption of food products).

In the center of each villi runs a wide, blindly starting lymphatic capillary (central vessel). Fat processing products enter it from the intestine. From here, the lymph is directed to the lymphatic plexus of the mucous membrane and gives a milky color to the intestinal lymph flowing from the intestine. Each villus includes 1-2 arterioles of the submucosal plexus, which break up there into capillaries located near the epithelial cells. Simple sugars and protein processing products are absorbed into the blood. From the capillaries, blood collects into venules running along the villus axis.

Parietal digestion is very effective for the body. The fact is that a significant number of microbes are constantly in the intestines. If the main breakdown processes occurred in the intestinal lumen, a significant part of the digestion products would be used by microorganisms and a significantly smaller amount of nutrients would be absorbed into the blood. This does not happen because the microvilli do not allow microbes to reach the site of action of the enzymes, since the microbe is too large to penetrate the space between the microvilli. And nutrients located near the intestinal cell wall are easily absorbed.

Circular folds also help increase the suction surface. Their number in the entire intestine is 500-1200. They reach 8 mm in height and up to 5 cm in length. In the duodenum and upper parts of the jejunum they are higher, and in the ileum they are lower and shorter.

Absorption is also greatly facilitated by contraction of the villi. Each villi is covered with intestinal epithelium; Inside the villi there are blood and lymphatic vessels and nerves. In the walls of the villi there are smooth muscles, which, when contracting, squeeze the contents of the lymphatic vessel and blood capillary into larger vessels. Then the muscles relax, and small vessels again absorb the solution from the intestinal cavity. Thus, the villus acts as a kind of pump.

The mucous membrane of the small intestine contains up to 1000 glands per 1 mm 2 that produce digestive juice. It contains numerous enzymes that act on proteins, fats and carbohydrates and on the products of their incomplete breakdown formed in the stomach. Intestinal juice consists of the liquid part and exfoliated cells of the intestinal epithelium. These cells break down and release the enzymes they contain. Over 20 enzymes of intestinal juice have been discovered that can catalyze the breakdown of almost any food organic substances into easily digestible products.

The mouths of the intestinal crypts (Lieberkühn's crypts) open into the gap between the villi - depressions of the lamina propria of the mucous membrane in the form of tubes 0.25-0.5 mm long, up to 0.07 mm in diameter. The number of crypts reaches 80-100 per 1 mm 2. The crypts are lined with five types of epithelial cells: intestinal epithelial cells with a striated border (columnar cell), goblet enterocytes, enteroendocrine cells, borderless enterocytes and enterocytes with acidophilic granules (Paneth cells). Small cylindrical borderless enterocytes, located at the bottom of the crypts between Paneth cells, actively divide mitotically and are the source of restoration of the epithelium of the villi and crypts.

In the lamina propria of the mucous membrane of the small intestine there are many single lymphoid nodules with a diameter of 0.5-1.5 mm, as well as lymphoid (Peyer's patches) (clusters of lymphoid nodules). They are located mainly in the walls of the ileum, less often in the jejunum and duodenum.

The muscular layer consists of an outer longitudinal layer and a thicker inner circular layer. In both layers, the muscle bundles have a spiral direction, but in the circular one they form a very steep spiral (the length of one stroke is about 1 cm), and in the outer longitudinal one they form a very flat one (the length of the stroke is up to 50 cm).

The function of the muscularis mucosa is to mix food masses in the intestinal lumen and push them towards the colon. Mechanical irritation of the intestine with food causes contraction of the longitudinal and circular muscles of the intestinal wall. There are pendulum-like and peristaltic movements. Pendulum-like movements manifest themselves in variable shortening and lengthening of the intestine over a short area (from 15-20 to several tens of cm). In this case, the intestine is laced into small sections, and the folds play the role of filtering and retaining devices. Such movements are repeated 20-30 times per minute. At the same time, the contents of the intestine move in one direction and then in the opposite direction, which improves the contact of food with intestinal juices.

Peristaltic movements cover a wider area of ​​the intestine. In this case, above the portion of food, due to the contraction of circular muscle fibers, a narrowing is formed, and below, due to the contraction of the longitudinal muscles, an expansion of the intestinal cavity is formed. With such worm-like movements of the intestine, its contents move towards the large intestine. In addition, there is a constant tonic contraction of the muscles of the intestinal wall.

Large intestine (lat. intestinum crassum), the final section of the digestive system, the main role of which is to prepare undigested food debris for removal from the body. In the colon, the bulk of water and electrolytes are absorbed and some metabolic wastes and excess salts are released. It begins in the lower right part of the abdominal cavity (right groin area), rises to the lower surface of the liver, where it forms a bend to the left and runs horizontally across the abdominal cavity slightly above the navel. On the left side of the abdominal cavity it reaches the lower end of the spleen, where it bends down and goes down to the left groin area. Thus, the large intestine is divided into the cecum with the appendix, ascending, transverse, descending, sigmoid, colon and rectum. The length of the entire colon ranges from 1.5 m to 2 m with a diameter of about 6 cm. The width of the cecum reaches 7 cm, gradually decreasing to 4 cm in the descending colon. Undigested waste passes from the small intestine to the large intestine and is exposed to bacteria that inhabit the large intestine. In appearance, the large intestine differs from the small intestine in its large diameter, the presence of omental processes - processes of the peritoneum filled with fat, typical swellings (gaustra) and three longitudinal muscle bands formed by the outer longitudinal layer of the muscular lining of the intestinal wall, which does not create a continuous covering on the large intestine. The tapes run from the base of the appendix to the beginning of the rectum.

The mucous membrane of the colon is devoid of villi, but it contains many crescent-shaped folds formed by the mucous membrane and submucosa, which are located between the haustra. The colon has a larger number of crypts than the mucous membrane of the small intestine, they are larger (the length of each crypt reaches 0.4-0.7 mm), and wider. The mucous membrane is covered with a single-layer columnar epithelium, in which three types of cells are distinguished (intestinal epithelial cells with a striated border, goblet enterocytes and intestinal borderless enterocytes). The number of goblet cells is much greater than in the small intestine; enteroendocrine cells and enterocytes with acidophilic granules (Paneth cells) are very rare. Restoration of the epithelium occurs due to the mitotic division of small cylindrical borderless cells located in the area of ​​the bottom of the crypts.

At the point where the ileum flows into the colon there is a complex anatomical device - the ileocecal valve, equipped with a muscular sphincter and two lips. This valve closes the exit from the small intestine, periodically it opens, allowing the contents to pass in small portions into the large intestine. In addition, it prevents the contents of the large intestine from flowing back into the small intestine.

Following the ileocecal valve, a short section begins, located below the junction of the small intestine, the so-called cecum. A vermiform appendix, usually 7-9 cm long and 0.5-1 cm thick, extends from the cecum in humans. The appendix has a narrow cavity that opens into the cecum with a hole surrounded by a small fold of mucous membrane - the valve. The lumen of the appendix may partially or completely close with age. In addition to humans, apes and rodents have a vermiform appendage. There are several opinions about the role of the appendix in the human body. Some scientists consider it a vestige; others call it part of the immune system, since the mucous membrane of the appendix contains lymphoid tissue that neutralizes bacteria and toxins. Sometimes, for various reasons (damage to the mucous membrane, entry of a foreign body), the appendix becomes inflamed and appendicitis occurs.

The area of ​​the large intestine above the cecum is called the colon because of its location around the abdominal cavity. Its initial section is called the ascending colon, the following are called the transverse colon, descending and sigmoid colon. The entire colon is firmly attached to the posterior abdominal wall and covered by the peritoneum, riddled with blood vessels.

The ascending colon, 14-18 cm long, at the lower surface of the liver, bending approximately at a right angle (right, hepatic flexure), passes into the transverse colon, 30-80 cm long, which crosses the abdominal cavity from right to left. In the left part of the abdominal cavity at the lower end of the spleen, the colon bends again (left, splenic flexure), turns down and passes into the descending colon, its length is about 10 cm. In the left iliac fossa, the sigmoid colon forms a loop and descends into the pelvis, where goes down and passes at the level of the promontory of the sacrum into the rectum.

The external and internal muscles of the colon, contracting, promote the movement of food debris. The inner surface of the intestine is covered with a mucous membrane, which facilitates the passage of feces and protects the intestinal walls from mechanical damage and the harmful effects of digestive enzymes. Water is absorbed in the colon, due to which the stool becomes denser and its volume decreases by about 3 times. In addition to water absorption, bacterial synthesis of certain amino acids, B vitamins and vitamin K occurs in the colon, which are absorbed into the bloodstream. The final section of the large intestine is the rectum. The rectum forms two bends in the anteroposterior direction. The upper curve is called sacral, it corresponds to the concavity of the sacrum. At the coccyx, the rectum turns back and down, going around its apex, and forms a second bend, perineal, facing the concavity back. The upper section of the rectum, corresponding to the sacral flexure, is located in the pelvic cavity (pelvic). Downwards, the intestine expands, forming an ampoule, the diameter of which increases when filled. The final section, which goes back and down, is called the anal canal. It passes through the pelvic floor and ends at the anus (anus). The length of the upper part of the rectum is 12-15 cm, the anal canal (anal part) is 2.5-3.7 cm. In the front, the rectum with its wall, devoid of peritoneum, is adjacent in men to the seminal vesicles, vas deferens and the area of ​​the bottom lying between them bladder, even lower to the prostate gland. In women, the front borders with the posterior wall of the vagina along its entire length.

The mucous membrane of the rectum forms transverse folds in the upper section. In the lower section there are 8-10 longitudinal folds - anal columns, between which the anal sinuses are located. The epithelium of the pelvis and rectal ampulla is single-layered, cylindrical, the number of crypts is smaller than in the overlying parts of the colon. The mucous membrane of the anal canal is devoid of crypts. Here the single-layer epithelium of the mucous membrane of the upper rectum is replaced by multilayer cubic. In the anal canal there is a sharp transition from stratified cubic to stratified squamous non-keratinizing epithelium and finally gradually to keratinizing in the skin part. Longitudinal bundles of myocytes of the muscular membrane are located near the rectum not in the form of three ribbons, but in a continuous layer. The anus is surrounded by two powerful muscle rings that form the internal and external sphincters. The circular layer, thickening in the area of ​​the anal canal, forms the internal (involuntary) sphincter of the anus. The internal sphincter is formed by smooth muscles and is always in a tense state. The accumulation of feces in the rectum causes irritation of the nerve endings of the anus and involuntary relaxation of the internal muscles. Directly under the skin lies the external (voluntary) sphincter. It is formed by striated muscle fibers and is subject to conscious control. Bowel movement in young children occurs reflexively (involuntarily), but over time the child learns to control this process, and defecation occurs only when the external sphincter relaxes. The main signal for bowel movement is the urge that occurs in the rectum as a result of peristaltic movements in its walls. Feces contain from 65 to 80% water. The rest of the mass consists of bacteria, cellulose, dead cells of the mucous membrane, mucus, cholesterol and bile pigments, as well as (in trace quantities) inorganic substances. The color of excrement is determined mainly by the presence of bile pigments in it. Stool can remain in the colon for about 36 hours before reaching the rectum, where it is stored for a short time and then released. The daily amount of excrement can vary from almost 0.5 kg with a diet rich in vegetables and fruits, to 200 g with a protein diet and up to 30 g in case of fasting.

As a result of physical inactivity and “fast” eating, the number of people suffering from various diseases of the large intestine is rapidly growing. Residents of developed countries are susceptible to a whole bunch of such diseases. These include motor dysfunction, absorption problems, as well as inflammatory processes and neoplasms.

Impaired motor function is associated with increased or decreased peristalsis. When eating high-calorie foods with a small amount of dietary fiber (cellulose), the motor activity of the colon decreases, which leads to constipation. Constipation, in turn, leads to inflammatory diseases - colitis. Colitis can be acute or chronic. Chronic colitis can lead to the formation of ulcers, abscesses of the intestinal mucosa, or even cancer, which may appear as a tumor growing in the intestinal lumen or as an infiltrate narrowing the intestinal lumen. Most colon tumors appear in the final section of the colon, which greatly facilitates treatment. Advances in diagnosis and surgery have meant that colon cancer can be recognized and removed earlier. A modern endoscopic method - colonoscopy - allows you to directly see the inside of the colon. The endoscope tube is equipped with a light source and a miniature camera that transmits the image to a large color monitor. If polyps are detected, they can be immediately removed without resorting to major surgery.

14.8. SUCTION

14.8.1. GENERAL CHARACTERISTICS OF SUCTION

Suction- the physiological process of transfer of substances from the lumen of the digestive tract into the blood and lymph. It should be noted that the transport of substances through the mucous membrane of the digestive tract constantly occurs from the blood capillaries into the cavity of the digestive tract. If the transport of substances from the blood capillaries into the lumen of the digestive tract predominates, the resulting effect of two differently directed flows is secretion, and if the flow from the cavity of the digestive tract dominates, absorption.

Absorption occurs throughout the digestive tract, but with varying intensity in its different parts. In the oral cavity, absorption is insignificant due to the short stay of food in it. However, the absorption capacity of the oral mucosa is clearly manifested in relation to some substances, including drugs, which is widely used in clinical practice. The mucous membrane in the area of ​​the floor of the mouth and the lower surface of the tongue is thinned, has a rich blood supply, and absorbed substances enter directly into the systemic circulation. Water and

soluble mineral salts, alcohol, glucose and a small amount of amino acids. The main section of the digestive tract, where the absorption of water, minerals, vitamins, and hydrolysis products of nutrients occurs, is the small intestine. This section of the digestive tract has an extremely high rate of nutrient transfer. Within 1-2 minutes after the entry of food substrates into the intestine, nutrients appear in the blood flowing from the mucous membrane, and after 5-10 minutes their concentration in the blood reaches its maximum values. Part of the liquid (about 1.5 l) along with chyme enters the large intestine, where it is almost completely absorbed.

The structure of the small intestine is adapted to perform the absorption function. In humans, the surface of the mucous membrane of the small intestine increases 600 times due to circular folds, villi and microvilli and reaches 200 m2. Absorption of nutrients occurs mainly in the upper part of the intestinal villi. The peculiarities of the organization of microcirculation of the villi are essential for the transport of nutrients. The blood supply to the intestinal villi is based on a dense network of capillaries located directly under the basement membrane. Characteristic features of the microvasculature of the villi are a high degree of fenestration of the capillary endothelium and a large pore size, which allows fairly large molecules to penetrate through them. Fenestrae are located in the zone of the endothelium facing the basement membrane, which facilitates exchange between the vessels and the intercellular spaces of the epithelium. After eating, blood flow increases by 30-130%, and the increased blood flow is always directed to the part of the intestine where the bulk of the chyme is currently located.

Absorption in the small intestine is also facilitated by contraction of its villi. Thanks to the rhythmic contractions of the intestinal villi, the contact of their surface with the chyme improves, and lymph is squeezed out from the blind ends of the lymphatic capillaries, which creates a suction effect of the central lymphatic vessel.

In an adult, each intestinal cell provides nutrients to approximately 100,000 other cells in the body. This suggests high activity of enterocytes in hydrolysis and absorption of nutrients.

solid substances. Absorption of substances into the blood and lymph is carried out using all types of primary and secondary transport mechanisms.

14.8.2. ABSORPTION OF WATER, MINERAL SALTS AND CARBOHYDRATES

A. Water is absorbed according to the law of osmosis. Water enters the digestive tract as part of food and liquids (2-2.5 l), secretions of the digestive glands (6-8 l), and only 100-150 ml of water is excreted with feces. The entire remaining volume of water is absorbed from the digestive tract into the blood, a small amount into the lymph. Absorption of water begins in the stomach, but it occurs most intensively in the small and large intestines (about 9 liters per day). About 60% of water is absorbed in the duodenum and about 20% in the ileum. The mucous membrane of the upper small intestine is highly permeable to solutes. The effective pore size in these regions is about 0.8 nm, while in the ileum and colon it is 0.4 and 0.2 nm, respectively. Therefore, if the osmolarity of chyme in the duodenum differs from the osmolarity of the blood, then this parameter equalizes within a few minutes.

Water easily passes through cell membranes from the intestinal cavity into the blood and back into the chyme. Thanks to such movements of water, the contents of the intestine are isotonic with respect to blood plasma. When hypotonic chyme enters the duodenum due to the intake of water or liquid food, water enters the blood until the intestinal contents become isosmotic to the blood plasma. On the contrary, when hypertonic chyme enters the duodenum from the stomach, water passes from the blood into the intestinal lumen, due to which the contents also become isotonic to the blood plasma. As it moves further through the intestine, chyme remains isosmotic to the blood plasma. Water moves into the blood following osmotically active substances (ions, amino acids, glucose).

B. Absorption of mineral salts. The absorption of sodium ions in the intestine occurs very efficiently: from 200-300 mmol of Na + daily entering the intestine with food, and 200 mmol contained in digestive juices, it is excreted in feces

only 3-7 mmol. The main part of sodium ions is absorbed in the small intestine. The concentration of sodium ions in the contents of the duodenum and jejunum is close to their concentration in the blood plasma. Despite this, Na + is constantly absorbed in the small intestine.

The transfer of Na + from the intestinal cavity to the blood can occur both through intestinal epithelial cells and through intercellular channels. Na + enters the cytoplasm from the intestinal lumen through the apical membrane of enterocytes according to an electrochemical gradient (the electrical charge of the cytoplasm of enterocytes is 40 mV relative to the outer side of the apical membrane). The transfer of sodium ions from enterocytes to the interstitium and blood occurs through the basolateral membranes of enterocytes using the Na/K pump localized there. Na + , K + and SG ions also move through intercellular channels according to the laws of diffusion.

In the upper part of the small intestine, SG is absorbed very quickly, mainly according to the electrochemical gradient. In this regard, negatively charged chlorine ions move from the negative pole to the positive and enter the interstitial fluid after sodium ions.

HCOs contained in pancreatic juice and bile are absorbed indirectly. When Na+ is absorbed into the intestinal lumen, H+ is secreted in exchange for Na+. Hydrogen ions with HCO^ form H 2 CO 3, which, under the action of carbonic anhydrase, is converted into H 2 O and CO 2. Water remains in the intestines as part of the chyme, and carbon dioxide is absorbed into the blood and excreted through the lungs.

Absorption of calcium ions and other divalent cations in the small intestine occurs slowly. Ca 2+ is absorbed 50 times slower than Na +, but faster than other divalent ions: magnesium, zinc, copper and iron. Calcium salts supplied with food dissociate and dissolve in the acidic contents of the stomach. Only half of the calcium ions are absorbed, mainly in the upper part of the small intestine. At low concentrations, Ca 2+ is absorbed by primary transport. The specific Ca 2+ -binding protein of the brush border participates in the transfer of Ca 2+ across the apical membrane of the enterocyte, and transport through the basolateral membranes is carried out using a calcium pump localized there. At high concentration

Ca 2+ in chyme is transported by diffusion. Parathyroid hormone and vitamin D play an important role in regulating the absorption of calcium ions in the intestine. Bile acids stimulate the absorption of Ca 2+.

Absorption of magnesium, zinc and iron ions occurs in the same parts of the intestine as Ca 2+, and Cu 2+ - mainly in the stomach. Transport of Mg 2+ , Zn 2+ and Cu 2+ occurs by diffusion. Absorption of Fe 2+ occurs primarily and secondary actively with the participation of carriers. When Fe 2+ enters the enterocyte, they combine with apoferritin, resulting in the formation of ferritin, in the form of which iron is deposited in the body.

B. Absorption of carbohydrates. Polysaccharides and disaccharides are practically not absorbed in the gastrointestinal tract. Absorption of monosaccharides occurs mainly in the small intestine. Glucose is absorbed at the highest speed, and galactose is absorbed during the period of feeding with mother's milk.

The entry of monosaccharides from the cavity of the small intestine into the blood can be carried out in various ways, however, during the absorption of glucose and galactose, the sodium-dependent mechanism plays the main role. In the absence of Na +, glucose is transported through the apical membrane 100 times slower, and in the absence of a concentration gradient, its transport naturally stops completely. Glucose, galactose, fructose, pentose can be absorbed by simple and facilitated diffusion in the event of their high concentration in the intestinal lumen, which usually occurs when consuming carbohydrate-rich foods. Glucose is absorbed faster than other monosaccharides.

14.8.3. ABSORPTION OF PROTEIN AND FAT HYDROLYSIS PRODUCTS

Products of hydrolytic breakdown of proteins- free amino acids, di- and tri-peptides are absorbed mainly in the small intestine. The bulk of amino acids are absorbed in the duodenum and jejunum (up to 80-90%). Only 10% of amino acids reach the colon, where they are broken down by bacteria.

The main mechanism of absorption of amino acids in the small intestine is secondary active - sodium-dependent transport. At the same time, diffusion of amino acids according to an electrochemical gradient is also possible. The presence of two transport mechanisms

amino acids is explained by the fact that D-amino acids are absorbed in the small intestine faster than L-isomers that enter the cell by diffusion. There are complex relationships between the absorption of various amino acids, as a result of which the transport of some amino acids is accelerated and others are slowed down.

Intact protein molecules in very small quantities can be absorbed in the small intestine by pinocytosis (endocytosis). Endocytosis, apparently, is not essential for the absorption of proteins, but can play an important role in the transfer of immunoglobulins, vitamins, and enzymes from the intestinal cavity to the blood. In newborns, breast milk proteins are absorbed through pinocytosis. In this way, antibodies enter the newborn's body through mother's milk, providing immunity to infections.

Absorption of fat breakdown products. The digestibility of fats is very high. Over 95% of triglycerides and 20-50% of cholesterol are absorbed into the blood. A person with a normal diet excretes up to 5-7 g of fat per day in feces. The bulk of fat hydrolysis products are absorbed in the duodenum and jejunum.

Mixed micelles formed as a result of the interaction of monoglycerides, fatty acids with the participation of bile salts, phospholipids and cholesterol enter the membranes of enterocytes. Micelles do not penetrate cells, but their lipid components dissolve in the plasma membrane and, according to the concentration gradient, enter the cytoplasm of enterocytes. Bile acid micelles remaining in the intestinal cavity are transported to the ileum, where they are absorbed through the primary transport mechanism.

In intestinal epithelial cells, resynthesis of triglycerides from monoglycerides and fatty acids occurs on microsomes of the endoplasmic reticulum. From newly formed triglycerides, cholesterol, phospholipids and glycoproteins, chylomicrons are formed - tiny fat particles enclosed in a thin protein shell. The diameter of chylomicrons is 60-75 nm. Chylomicrons accumulate in secretory vesicles, which merge with the lateral membrane of the enterocyte, and through the resulting hole they exit into the intercellular space, from where they enter the blood through the central lymphatic and thoracic ducts. Main amount of fat

absorbed into the lymph. Therefore, 3-4 hours after eating, the lymphatic vessels are filled with a large amount of lymph, reminiscent of milk (lacty juice).

Short- and medium-chain fatty acids are quite soluble in water and can diffuse to the surface of enterocytes without forming micelles. They penetrate through the intestinal epithelial cells directly into the portal blood, bypassing the lymphatic vessels.

The absorption of fat-soluble vitamins (A, D, E, K) is closely related to the transport of fats in the intestines. If fat absorption is impaired, the absorption and assimilation of these vitamins is inhibited.

Absorption is the process of transport of digested nutrients from the cavity of the gastrointestinal tract into the blood, lymph and intercellular space.

It occurs throughout the entire digestive tract, but each section has its own characteristics.

In the oral cavity, absorption is insignificant, since food is not retained there, but some substances, for example, potassium cyanide, as well as medications (essential oils, validol, nitroglycerin, etc.) are absorbed in the oral cavity and very quickly enter the circulatory system, bypassing intestines and liver. This finds use as a method of administering medicinal substances.

The stomach absorbs some amino acids, some glucose, water with mineral salts dissolved in it, and quite significantly the absorption of alcohol.

The main absorption of hydrolysis products of proteins, fats and carbohydrates occurs in the small intestine. Proteins are absorbed in the form of amino acids, carbohydrates - in the form of monosaccharides, fats - in the form of glycerol and fatty acids. The absorption of water-insoluble fatty acids is aided by water-soluble bile salts.

The absorption of nutrients in the large intestine is insignificant, a lot of water is absorbed there, which is necessary for the formation of feces, in small amounts glucose, amino acids, chlorides, mineral salts, fatty acids and fat-soluble vitamins A, D, E, K. Substances from the rectum are absorbed like this the same as from the oral cavity, i.e. directly into the blood, bypassing the portal circulatory system. The effect of so-called nutritional enemas is based on this.

Mechanisms of the suction process

How does the absorption process occur? Different substances are absorbed through different mechanisms.

Laws of diffusion. Salts, small molecules of organic substances, and a certain amount of water enter the blood according to the laws of diffusion.

Filtration laws. Contraction of intestinal smooth muscles increases pressure, which triggers the penetration of certain substances into the blood according to the laws of filtration.

Osmosis. An increase in blood osmotic pressure accelerates water absorption.

Large energy costs. Some nutrients require significant energy expenditure for the absorption process, including glucose, a number of amino acids, fatty acids, and sodium ions. During the experiments, with the help of special poisons, energy metabolism in the mucous membrane of the small intestine was disrupted or stopped, as a result, the process of absorption of sodium and glucose ions stopped.

Absorption of nutrients requires increased cellular respiration of the small intestinal mucosa. This indicates the need for normal functioning of intestinal epithelial cells.

Contractions of the villi also aid in absorption. The outside of each villi is covered by intestinal epithelium; inside it there are nerves, lymphatic and blood vessels. Smooth muscles located in the walls of the villi, contracting, push the contents of the capillary and lymph vessels of the villi into larger arteries. During the period of muscle relaxation, small vessels of the villi take the solution from the cavity of the small intestine. Thus, the villus functions as a kind of pump.

During the day, approximately 10 liters of liquid are absorbed, of which approximately 8 liters are digestive juices. Absorption of nutrients is carried out mainly by intestinal epithelial cells.

Barrier role of the liver

Nutrients absorbed through the intestinal walls through the bloodstream first of all enter the liver. In liver cells, substances harmful to health that accidentally or intentionally enter the intestines are destroyed. At the same time, the blood passing through the capillaries of the liver contains almost no chemical compounds toxic to humans. This function of the liver is called the barrier function.

For example, liver cells are capable of destroying poisons such as strychnine and nicotine, as well as alcohol. However, many substances harm the liver, causing its cells to die. The liver is one of the few human organs capable of self-healing (regeneration), so for some time it can tolerate tobacco and alcohol abuse, but up to a certain limit, followed by the destruction of its cells, cirrhosis of the liver and death.

The liver is also a storehouse of glucose, the most important source of energy for the entire body, and especially the brain. In the liver, part of the glucose is converted into a complex carbohydrate - glycogen. Glucose is stored in the form of glycogen until its level in the blood plasma decreases. If this happens, glycogen is converted back into glucose and enters the bloodstream for delivery to all tissues, and most importantly, to the brain.

Fats absorbed into the lymph and blood enter the general bloodstream. The main amount of lipids is deposited in fat depots, from which fats are used for energy purposes.

The gastrointestinal tract takes an active part in the water-salt metabolism of the body. Water enters the gastrointestinal tract as part of food and liquids, and the secretions of the digestive glands. The main amount of water is absorbed into the blood, a small amount into the lymph. Absorption of water begins in the stomach, but it occurs most intensively in the small intestine. Actively absorbed solutes by epithelial cells “draw” water with them. The decisive role in the transfer of water belongs to sodium and chlorine ions. Therefore, all factors affecting the transport of these ions also affect the absorption of water. Water absorption is associated with the transport of sugars and amino acids. Excluding bile from digestion slows down the absorption of water from the small intestine. Inhibition of the central nervous system (for example, during sleep) slows down water absorption.

Sodium is intensively absorbed in the small intestine.

Sodium ions are transferred from the cavity of the small intestine into the blood through intestinal epithelial cells and through intercellular channels. The entry of sodium ions into the epithelial cell occurs passively (without energy consumption) due to the difference in concentrations. From epithelial cells through membranes, sodium ions are actively transported into the intercellular fluid, blood and lymph.

In the small intestine, the transfer of sodium and chlorine ions occurs simultaneously and according to the same principles; in the large intestine, absorbed sodium ions are exchanged for potassium ions. With a decrease in sodium content in the body, its absorption in the intestine increases sharply. The absorption of sodium ions is enhanced by the hormones of the pituitary gland and adrenal glands, and inhibited by gastrin, secretin and cholecystokinin-pancreozymin.

Absorption of potassium ions occurs mainly in the small intestine. Absorption of chlorine ions occurs in the stomach, and is most active in the ileum.

Of the divalent cations absorbed in the intestine, the most important are calcium, magnesium, zinc, copper and iron ions. Calcium is absorbed along the entire length of the gastrointestinal tract, but its most intense absorption occurs in the duodenum and the initial part of the small intestine. In the same section of the intestine, magnesium, zinc and iron ions are absorbed. Copper absorption occurs primarily in the stomach. Bile has a stimulating effect on calcium absorption.

Water-soluble vitamins can be absorbed by diffusion (vitamin C, riboflavin). Vitamin B 2 is absorbed in the ileum. The absorption of fat-soluble vitamins (A, D, E, K) is closely related to the absorption of fats.

Absorption is the process of transporting digested nutrients from the gastrointestinal tract into the blood, lymph and intercellular space.

It occurs throughout the entire digestive tract, but each section has its own characteristics.
In the oral cavity, absorption is insignificant, since food is not retained there, but some substances, for example, potassium cyanide, as well as medications (essential oils, validol, nitroglycerin, etc.) are absorbed in the oral cavity and very quickly enter the circulatory system, bypassing intestines and liver. This finds use as a method of administering medicinal substances.

The stomach absorbs some amino acids, some glucose, water with mineral salts dissolved in it, and quite significantly the absorption of alcohol.
The main absorption of hydrolysis products of proteins, fats and carbohydrates occurs in the small intestine. Proteins are absorbed in the form of amino acids, carbohydrates in the form of monosaccharides, fats in the form of glycerol and fatty acids. The absorption of water-insoluble fatty acids is aided by water-soluble bile salts.
The absorption of nutrients in the large intestine is insignificant, a lot of water is absorbed there, which is necessary for the formation of feces, in small amounts glucose, amino acids, chlorides, mineral salts, fatty acids and fat-soluble vitamins A, D, E, K. Substances from the rectum are absorbed like this the same as from the oral cavity, i.e. directly into the blood, bypassing the portal circulatory system. The effect of so-called nutritional enemas is based on this.

As for other parts of the gastrointestinal tract (stomach, small and large intestines), the substances absorbed into them first enter the portal veins into the liver, and then into the general bloodstream. Lymphatic drainage from the intestines occurs through the intestinal lymphatic vessels into the lacteal cistern. The presence of valves in the lymphatic vessels prevents the return of lymph into the vessels, which flows through the thoracic duct into the superior vena cava.
Suction depends on the size of the suction surface. It is especially large in the small intestine and is created by folds, villi and microvilli. Thus, for 1 mm2 of the intestinal mucosa there are 30 x 40 villi, and for each enterocyte there are 1700 x 4000 microvilli. Each villus is a microorgan containing muscle contractile elements, blood and lymphatic microvessels and a nerve ending.

The microvilli are covered with a layer of glycocolyx, consisting of mucopolysaccharide threads interconnected by calcium bridges, forming a layer 0.1 μm thick. This is a molecular sieve or network, which, due to its negative charge and hydrophilicity, allows low molecular weight substances to pass through the microvilli membrane and prevents high molecular weight substances and xenobiotics from passing through it. The glycocalyx, together with the mucus covering the intestinal epithelium, adsorbs from the intestinal cavity hydrolytic enzymes necessary for the cavity hydrolysis of nutrients, which are then transported to the microvilli membrane.
A major role in absorption is played by contractions of the villi, which contract weakly on an empty stomach, and in the presence of chyme in the intestine, up to 6 contractions per minute. The intramural nervous system (submucosal, Meissner's plexus) takes part in the regulation of villous contraction.
Extractive substances from food, glucose, peptides, and some amino acids enhance contraction of the villi. The acidic contents of the stomach promote the formation of a special hormone, villikinin, in the small intestine, which stimulates contraction of the villi through the bloodstream.

Suction mechanisms
Several types of transport mechanisms are used to absorb micromolecules, hydrolysis products of nutrients, electrolytes, and drugs.
1. Passive transport, including diffusion, filtration and osmosis.
2. Facilitated diffusion.
3. Active transport.

Diffusion is based on the concentration gradient of substances in the intestinal cavity, in the blood or lymph. By diffusion, water, ascorbic acid, pyridoxine, riboflavin and many drugs are transferred through the intestinal mucosa.
Filtration is based on a hydrostatic pressure gradient. Thus, an increase in intraintestinal pressure to 810 mm Hg. increases the rate of absorption of table salt solution from the small intestine by 2 times. Increased intestinal motility promotes absorption.

The passage of substances through the semi-permeable membrane of enterocytes is aided by osmotic forces. If a hypertonic solution of any salt (table salt, Epsom salt, etc.) is introduced into the gastrointestinal tract, then according to the laws of osmosis, the liquid from the blood and surrounding tissues, i.e. from an isotonic environment, will be absorbed towards the hypertonic solution, i.e. into the intestines and have a cleansing effect. This is the basis of the action of saline laxatives. Water and electrolytes are absorbed along the osmotic gradient.
Facilitated diffusion also occurs along a concentration gradient of substances, but with the help of special membrane carriers, without energy consumption and faster than simple diffusion. Thus, fructose is transported through facilitated diffusion.

Active transport occurs against an electrochemical gradient, even at low concentrations of this substance in the intestinal lumen, with the participation of a carrier and requires energy expenditure. Na+ is most often used as a transporter, through which substances such as glucose, galactose, free amino acids, bile salts, bilirubin, and some di- and tripeptides are absorbed.
Vitamin B12 and calcium ions are also absorbed through active transport. Active transport is extremely specific and can be inhibited by substances that are chemically similar to the substrate.
Active transport is inhibited at low temperatures and lack of oxygen. The absorption process is affected by the pH of the environment. Optimal pH for absorption is neutral.

Many substances can be absorbed through both active and passive transport. It all depends on the concentration of the substance. At low concentrations, active transport predominates, and at high concentrations, passive transport predominates.
Some high molecular weight substances are transported by endocytosis (pinocytosis and phagocytosis). This mechanism is that the enterocyte membrane surrounds the absorbed substance to form a vesicle, which is immersed in the cytoplasm and then passes to the basal surface of the cell, where the substance enclosed in the vesicle is released from the enterocyte. This type of transport is important when transferring proteins, immunoglobulins, vitamins, and enzymes from breast milk to a newborn.

Some substances, for example, water, electrolytes, antibodies, allergens, can pass through the intercellular spaces. This type of transport is called persorption.

14.8. SUCTION

14.8.1. GENERAL CHARACTERISTICS OF SUCTION

Suction- the physiological process of transfer of substances from the lumen of the digestive tract into the blood and lymph. It should be noted that the transport of substances through the mucous membrane of the digestive tract constantly occurs from the blood capillaries into the cavity of the digestive tract. If the transport of substances from the blood capillaries into the lumen of the digestive tract predominates, the resulting effect of two differently directed flows is secretion, and if the flow from the cavity of the digestive tract dominates, absorption.

Absorption occurs throughout the digestive tract, but with varying intensity in its different parts. In the oral cavity, absorption is insignificant due to the short stay of food in it. However, the absorption capacity of the oral mucosa is clearly manifested in relation to some substances, including drugs, which is widely used in clinical practice. The mucous membrane in the area of ​​the floor of the mouth and the lower surface of the tongue is thinned, has a rich blood supply, and absorbed substances enter directly into the systemic circulation. Water and

soluble mineral salts, alcohol, glucose and a small amount of amino acids. The main section of the digestive tract, where the absorption of water, minerals, vitamins, and hydrolysis products of nutrients occurs, is the small intestine. This section of the digestive tract has an extremely high rate of nutrient transfer. Within 1-2 minutes after the entry of food substrates into the intestine, nutrients appear in the blood flowing from the mucous membrane, and after 5-10 minutes their concentration in the blood reaches its maximum values. Part of the liquid (about 1.5 l) along with chyme enters the large intestine, where it is almost completely absorbed.

The structure of the small intestine is adapted to perform the absorption function. In humans, the surface of the mucous membrane of the small intestine increases 600 times due to circular folds, villi and microvilli and reaches 200 m2. Absorption of nutrients occurs mainly in the upper part of the intestinal villi. The peculiarities of the organization of microcirculation of the villi are essential for the transport of nutrients. The blood supply to the intestinal villi is based on a dense network of capillaries located directly under the basement membrane. Characteristic features of the microvasculature of the villi are a high degree of fenestration of the capillary endothelium and a large pore size, which allows fairly large molecules to penetrate through them. Fenestrae are located in the zone of the endothelium facing the basement membrane, which facilitates exchange between the vessels and the intercellular spaces of the epithelium. After eating, blood flow increases by 30-130%, and the increased blood flow is always directed to the part of the intestine where the bulk of the chyme is currently located.

Absorption in the small intestine is also facilitated by contraction of its villi. Thanks to the rhythmic contractions of the intestinal villi, the contact of their surface with the chyme improves, and lymph is squeezed out from the blind ends of the lymphatic capillaries, which creates a suction effect of the central lymphatic vessel.

In an adult, each intestinal cell provides nutrients to approximately 100,000 other cells in the body. This suggests high activity of enterocytes in hydrolysis and absorption of nutrients.

solid substances. Absorption of substances into the blood and lymph is carried out using all types of primary and secondary transport mechanisms.

14.8.2. ABSORPTION OF WATER, MINERAL SALTS AND CARBOHYDRATES

A. Water is absorbed according to the law of osmosis. Water enters the digestive tract as part of food and liquids (2-2.5 l), secretions of the digestive glands (6-8 l), and only 100-150 ml of water is excreted with feces. The entire remaining volume of water is absorbed from the digestive tract into the blood, a small amount into the lymph. Absorption of water begins in the stomach, but it occurs most intensively in the small and large intestines (about 9 liters per day). About 60% of water is absorbed in the duodenum and about 20% in the ileum. The mucous membrane of the upper small intestine is highly permeable to solutes. The effective pore size in these regions is about 0.8 nm, while in the ileum and colon it is 0.4 and 0.2 nm, respectively. Therefore, if the osmolarity of chyme in the duodenum differs from the osmolarity of the blood, then this parameter equalizes within a few minutes.

Water easily passes through cell membranes from the intestinal cavity into the blood and back into the chyme. Thanks to such movements of water, the contents of the intestine are isotonic with respect to blood plasma. When hypotonic chyme enters the duodenum due to the intake of water or liquid food, water enters the blood until the intestinal contents become isosmotic to the blood plasma. On the contrary, when hypertonic chyme enters the duodenum from the stomach, water passes from the blood into the intestinal lumen, due to which the contents also become isotonic to the blood plasma. As it moves further through the intestine, chyme remains isosmotic to the blood plasma. Water moves into the blood following osmotically active substances (ions, amino acids, glucose).

B. Absorption of mineral salts. The absorption of sodium ions in the intestine occurs very efficiently: from 200-300 mmol of Na + daily entering the intestine with food, and 200 mmol contained in digestive juices, it is excreted in feces

only 3-7 mmol. The main part of sodium ions is absorbed in the small intestine. The concentration of sodium ions in the contents of the duodenum and jejunum is close to their concentration in the blood plasma. Despite this, Na + is constantly absorbed in the small intestine.

The transfer of Na + from the intestinal cavity to the blood can occur both through intestinal epithelial cells and through intercellular channels. Na + enters the cytoplasm from the intestinal lumen through the apical membrane of enterocytes according to an electrochemical gradient (the electrical charge of the cytoplasm of enterocytes is 40 mV relative to the outer side of the apical membrane). The transfer of sodium ions from enterocytes to the interstitium and blood occurs through the basolateral membranes of enterocytes using the Na/K pump localized there. Na + , K + and SG ions also move through intercellular channels according to the laws of diffusion.

In the upper part of the small intestine, SG is absorbed very quickly, mainly according to the electrochemical gradient. In this regard, negatively charged chlorine ions move from the negative pole to the positive and enter the interstitial fluid after sodium ions.

HCOs contained in pancreatic juice and bile are absorbed indirectly. When Na+ is absorbed into the intestinal lumen, H+ is secreted in exchange for Na+. Hydrogen ions with HCO^ form H 2 CO 3, which, under the action of carbonic anhydrase, is converted into H 2 O and CO 2. Water remains in the intestines as part of the chyme, and carbon dioxide is absorbed into the blood and excreted through the lungs.

Absorption of calcium ions and other divalent cations in the small intestine occurs slowly. Ca 2+ is absorbed 50 times slower than Na +, but faster than other divalent ions: magnesium, zinc, copper and iron. Calcium salts supplied with food dissociate and dissolve in the acidic contents of the stomach. Only half of the calcium ions are absorbed, mainly in the upper part of the small intestine. At low concentrations, Ca 2+ is absorbed by primary transport. The specific Ca 2+ -binding protein of the brush border participates in the transfer of Ca 2+ across the apical membrane of the enterocyte, and transport through the basolateral membranes is carried out using a calcium pump localized there. At high concentration

Ca 2+ in chyme is transported by diffusion. Parathyroid hormone and vitamin D play an important role in regulating the absorption of calcium ions in the intestine. Bile acids stimulate the absorption of Ca 2+.

Absorption of magnesium, zinc and iron ions occurs in the same parts of the intestine as Ca 2+, and Cu 2+ - mainly in the stomach. Transport of Mg 2+ , Zn 2+ and Cu 2+ occurs by diffusion. Absorption of Fe 2+ occurs primarily and secondary actively with the participation of carriers. When Fe 2+ enters the enterocyte, they combine with apoferritin, resulting in the formation of ferritin, in the form of which iron is deposited in the body.

B. Absorption of carbohydrates. Polysaccharides and disaccharides are practically not absorbed in the gastrointestinal tract. Absorption of monosaccharides occurs mainly in the small intestine. Glucose is absorbed at the highest speed, and galactose is absorbed during the period of feeding with mother's milk.

The entry of monosaccharides from the cavity of the small intestine into the blood can be carried out in various ways, however, during the absorption of glucose and galactose, the sodium-dependent mechanism plays the main role. In the absence of Na +, glucose is transported through the apical membrane 100 times slower, and in the absence of a concentration gradient, its transport naturally stops completely. Glucose, galactose, fructose, pentose can be absorbed by simple and facilitated diffusion in the event of their high concentration in the intestinal lumen, which usually occurs when consuming carbohydrate-rich foods. Glucose is absorbed faster than other monosaccharides.

14.8.3. ABSORPTION OF PROTEIN AND FAT HYDROLYSIS PRODUCTS

Products of hydrolytic breakdown of proteins- free amino acids, di- and tri-peptides are absorbed mainly in the small intestine. The bulk of amino acids are absorbed in the duodenum and jejunum (up to 80-90%). Only 10% of amino acids reach the colon, where they are broken down by bacteria.

The main mechanism of absorption of amino acids in the small intestine is secondary active - sodium-dependent transport. At the same time, diffusion of amino acids according to an electrochemical gradient is also possible. The presence of two transport mechanisms

amino acids is explained by the fact that D-amino acids are absorbed in the small intestine faster than L-isomers that enter the cell by diffusion. There are complex relationships between the absorption of various amino acids, as a result of which the transport of some amino acids is accelerated and others are slowed down.

Intact protein molecules in very small quantities can be absorbed in the small intestine by pinocytosis (endocytosis). Endocytosis, apparently, is not essential for the absorption of proteins, but can play an important role in the transfer of immunoglobulins, vitamins, and enzymes from the intestinal cavity to the blood. In newborns, breast milk proteins are absorbed through pinocytosis. In this way, antibodies enter the newborn's body through mother's milk, providing immunity to infections.

Absorption of fat breakdown products. The digestibility of fats is very high. Over 95% of triglycerides and 20-50% of cholesterol are absorbed into the blood. A person with a normal diet excretes up to 5-7 g of fat per day in feces. The bulk of fat hydrolysis products are absorbed in the duodenum and jejunum.

Mixed micelles formed as a result of the interaction of monoglycerides, fatty acids with the participation of bile salts, phospholipids and cholesterol enter the membranes of enterocytes. Micelles do not penetrate cells, but their lipid components dissolve in the plasma membrane and, according to the concentration gradient, enter the cytoplasm of enterocytes. Bile acid micelles remaining in the intestinal cavity are transported to the ileum, where they are absorbed through the primary transport mechanism.

In intestinal epithelial cells, resynthesis of triglycerides from monoglycerides and fatty acids occurs on microsomes of the endoplasmic reticulum. From newly formed triglycerides, cholesterol, phospholipids and glycoproteins, chylomicrons are formed - tiny fat particles enclosed in a thin protein shell. The diameter of chylomicrons is 60-75 nm. Chylomicrons accumulate in secretory vesicles, which merge with the lateral membrane of the enterocyte, and through the resulting hole they exit into the intercellular space, from where they enter the blood through the central lymphatic and thoracic ducts. Main amount of fat

absorbed into the lymph. Therefore, 3-4 hours after eating, the lymphatic vessels are filled with a large amount of lymph, reminiscent of milk (lacty juice).

Short- and medium-chain fatty acids are quite soluble in water and can diffuse to the surface of enterocytes without forming micelles. They penetrate through the intestinal epithelial cells directly into the portal blood, bypassing the lymphatic vessels.

The absorption of fat-soluble vitamins (A, D, E, K) is closely related to the transport of fats in the intestines. If fat absorption is impaired, the absorption and assimilation of these vitamins is inhibited.