Transport of gases by blood. Physiology of respiration Transport function of proteins

If an animal has a circulatory system, there is an oxygen carrier in the blood. In a dissolved state, a person has only 2% oxygen in his arterial blood.

All oxygen carrier pigments are organometallic compounds, most contain Fe, some contain Cu.

Hemoglobins are iron porphyrins (heme) bound to globin (protein). Hemoglobin in humans and mammals is always found in specialized blood cells, erythrocytes. More than 90 types of hemoglobins, differing in protein components, have been identified. The hemoglobin molecule consists of several monomers, each of which contains one heme connected to globin. In humans, hemoglobin contains 4 such monomers. Myoglobin contains only 1 heme.

Chemically, heme is a protoporphyrin consisting of 4 pyrrole rings with an iron atom in the center.

Oxygenation of hemoglobin is the reversible addition of oxygen to ferrous (ferrous) iron in quantities depending on the oxygen tension in the surrounding space.

Oxygen is added to each of the iron atoms according to the equilibrium equation

Formally, in this reaction there is no change in the valence of iron. Nevertheless, oxygenation is accompanied by a partial transfer of an electron from ferrous iron to oxygen, and oxygen is partially reduced.

Heme iron may have a different valence value during the formation of methemoglobin, when Fe changes valency and becomes trivalent. In this case, with true oxidation of iron, hemoglobin loses its ability to carry oxygen.

The heme in the hemoglobin molecule is capable of attaching other molecules. If it adds carbon dioxide, it is called carbohemoglobin. When carbon monoxide is added to heme, carboxyhemoglobin is formed. The affinity of hemoglobin for CO is 300 times higher than for O 2. Therefore, carbon monoxide poisoning is very dangerous. If the inhaled air contains 1% CO, mammals and birds can die.

Arterial blood is saturated with oxygen by 96-97%. This process occurs very quickly, in just a quarter of a second in the alveolar capillaries.

In the literature, it is customary to evaluate the oxygen content in the blood according to the indicator blood oxygen capacity.

The oxygen capacity of blood is the maximum amount of oxygen that 100 ml of blood can absorb.

Since 96% of oxygen is combined with hemoglobin, the oxygen capacity of the blood is determined by this pigment. It is known that the oxygen-binding capacity of 1 g of hemoglobin is determined by the value of 1.34 - 1.36 ml of O 2, at normal atmospheric pressure. This means that with a blood content of 15 g% Hb (and this is close to the average), the oxygen capacity is 1.34´15@20 volume percent, that is, for every 20 ml of O 2 for every 100 ml of blood, or 200 ml About 2 per liter of blood. 5 liters of blood (the total oxygen capacity of an individual who has 5 liters of blood in the circulatory system) contains 1 liter of oxygen.

The hemoglobin oxygenation reaction is reversible

HHb 4 +4O 2 = HHb 4 (O 2) 4

Or simpler Hb+O 2 = HbO 2

It turned out that in practice it is more convenient to analyze this process if you plot the dependence of the HbO 2 concentration in the sample on the partial pressure/tension of oxygen. The more oxygen in the environment, the more the equilibrium in the reaction shifts towards oxygenation, and vice versa.

Each PO 2 value corresponds to a certain percentage of HbO 2. At PO 2 values characteristic of arterial blood, almost all hemoglobin is oxidized. In peripheral tissues, at low values of oxygen tension, the rate of its dissociation to oxygen and hemoglobin increases.

The hemoglobin dissociation curve is available in every textbook.

Analysis of the oxyhemoglobin dissociation curve shows that when the oxygen tension in the environment is 60-100 mm Hg. (conditions of the plain and the rise of a person to a height of up to 2 kilometers) the saturation of blood oxygen occurs completely. In tissues, oxygen release also proceeds satisfactorily, at oxygen tensions of about 20 mm Hg.

In other words, the nature of the curve provides information about the properties of the transport system.

The dissociation of oxyhemoglobin depends not only on the partial pressure of oxygen in the tissues, but also on some other conditions. When carbon dioxide enters the blood from tissues, the affinity of hemoglobin for oxygen decreases and the dissociation curve shifts to the right. This is a direct Verigo-Bohr effect. The Verigo-Bohr effect improves the dissociation of oxyhemoglobin in tissues. The opposite effect is observed in the lungs, where the release of carbon dioxide leads to a more complete saturation of hemoglobin with oxygen. The effect is not due to CO 2 itself, but to the acidification of the environment during the formation of carbonic acid (or the accumulation of lactic acid in actively working muscles).

Most of the oxygen in the body of mammals is carried in the blood in the form of a chemical compound with hemoglobin. Free dissolved oxygen in the blood is only 0.3%. The oxygenation reaction, the conversion of deoxyhemoglobin into oxyhemoglobin, occurring in the red blood cells of the capillaries of the lungs can be written as follows:

HB + 4O 2 ⇄ Hb(O 2 ) 4

This reaction occurs very quickly - the half-saturation time of hemoglobin with oxygen is about 3 milliseconds. Hemoglobin has two amazing properties that allow it to be an ideal oxygen carrier. The first is the ability to attach oxygen, and the second is to give it away. Turns out The ability of hemoglobin to attach and release oxygen depends on the oxygen tension in the blood. Let's try to graphically depict the dependence of the amount of oxygenated hemoglobin on the oxygen tension in the blood, and then we will be able to find out: in which cases hemoglobin adds oxygen, and in which it releases it. Hemoglobin and oxyhemoglobin absorb light rays differently, so their concentration can be determined by spectrometric methods.

The graph reflecting the ability of hemoglobin to attach and release oxygen is called the “Oxyhemoglobin dissociation curve.” The abscissa axis in this graph shows the amount of oxyhemoglobin as a percentage of the total hemoglobin in the blood, and the ordinate axis shows the oxygen tension in the blood in mmHg. Art.

Figure 9A. Normal oxyhemoglobin dissociation curve

Let's consider the graph in accordance with the stages of oxygen transport: the highest point corresponds to the oxygen tension that is observed in the blood of the pulmonary capillaries - 100 mm Hg. (the same amount as in the alveolar air). The graph shows that at this voltage, all hemoglobin turns into the form of oxyhemoglobin - it is completely saturated with oxygen. Let's try to calculate how much oxygen binds hemoglobin. One mole of hemoglobin can bind 4 moles ABOUT 2 , and 1 gram of Hb binds 1.39 ml of O 2 ideally, but in practice 1.34 ml. With a hemoglobin concentration in the blood, for example, 140 g/liter, the amount of bound oxygen will be 140 × 1.34 = 189.6 ml/liter of blood. The amount of oxygen that hemoglobin can bind if it is completely saturated is called the blood oxygen capacity (BOC). In our case, KEK = 189.6 ml.

Let us pay attention to an important feature of hemoglobin - when the oxygen tension in the blood decreases to 60 mm Hg, the saturation remains virtually unchanged - almost all hemoglobin is present in the form of oxyhemoglobin. This feature allows you to bind the maximum possible amount of oxygen when its content in the environment decreases (for example, at an altitude of up to 3000 meters).

The dissociation curve has an s-shaped character, which is associated with the peculiarities of the interaction of oxygen with hemoglobin. The hemoglobin molecule binds 4 oxygen molecules in stages. Binding of the first molecule dramatically increases the binding capacity, and the second and third molecules do the same. This effect is called cooperative action of oxygen

Arterial blood enters the systemic circulation and is delivered to the tissues. Oxygen tension in tissues, as can be seen from Table 2, ranges from 0 to 20 mm Hg. Art., a small amount of physically dissolved oxygen diffuses into the tissues, its tension in the blood decreases. A decrease in oxygen tension is accompanied by the dissociation of oxyhemoglobin and the release of oxygen. The oxygen released from the compound becomes physically dissolved and can diffuse into the tissue along a voltage gradient. At the venous end of the capillary, the oxygen tension is 40 mm Hg, which corresponds to approximately 73% hemoglobin saturation. The steep part of the dissociation curve corresponds to the normal oxygen tension for body tissues – 35 mmHg and below.

Thus, the hemoglobin dissociation curve reflects the ability of hemoglobin to accept oxygen if the oxygen tension in the blood is high, and to release it when the oxygen tension decreases.

The transition of oxygen into tissues occurs by diffusion, and is described by Fick’s law, and therefore depends on the gradient of oxygen tension.

You can find out how much oxygen is extracted by the tissue. To do this, you need to determine the amount of oxygen in the arterial blood and in the venous blood flowing from a certain area. Arterial blood, as we were able to calculate (KEK), contains 180-200 ml. oxygen. Venous blood at rest contains about 120 ml. oxygen. Let's try to calculate the oxygen utilization rate: 180 ml. - 120 ml. = 60 ml. is the amount of oxygen extracted by tissues, 60 ml./180  100 = 33%. Consequently, the oxygen utilization rate is 33% (normally from 25 to 40%). As can be seen from these data, not all oxygen is utilized by tissues. Normally, about 1000 ml is delivered to the tissues within one minute. oxygen. When the recovery rate is taken into account, it is clear that between 250 and 400 ml of tissue are recovered. oxygen per minute, the rest of the oxygen returns to the heart as part of the venous blood. With heavy muscular work, the utilization rate increases to 50–60%.

However, the amount of oxygen that tissues receive depends not only on the utilization rate. When conditions change in the internal environment and those tissues where oxygen diffusion occurs, the properties of hemoglobin may change. A change in the properties of hemoglobin is reflected in the graph and is called a “curve shift.” Let us note an important point on the curve - the half-saturation point of hemoglobin with oxygen is observed at an oxygen tension of 27 mm Hg. Art., at this voltage, 50% of hemoglobin is in the form of oxyhemoglobin, 50% in the form of deoxyhemoglobin, therefore 50% of bound oxygen is free (approximately 100 ml/l). If the concentration of carbon dioxide, hydrogen ions, and temperature in the tissue increases, then the curve shifts to the right. In this case, the half-saturation point will move to higher values of oxygen tension - already at a voltage of 40 mm Hg. Art. 50% of oxygen will be released (Figure 9B). Hemoglobin in intensively working tissue will release oxygen more easily. Changes in the properties of hemoglobin are due to the following reasons: acidification environment as a result of an increase in the concentration of carbon dioxide acts in two ways: 1) an increase in the concentration of hydrogen ions promotes the release of oxygen by oxyhemoglobin because hydrogen ions bind more easily to deoxyhemoglobin, 2) direct binding of carbon dioxide to the protein part of the hemoglobin molecule reduces its affinity for oxygen; increasing the concentration of 2,3-diphosphoglycerate, which appears during the process of anaerobic glycolysis and is also integrated into the protein part of the hemoglobin molecule and reduces its affinity for oxygen.

A shift of the curve to the left is observed, for example, in the fetus, when a large amount of fetal hemoglobin is detected in the blood.

Figure 9 B. Impact of changes in internal environmental parameters

O2 transport occurs in physically dissolved and chemically bound form. Physical processes, i.e., gas dissolution, cannot meet the body’s demand for O2. It is estimated that physically dissolved O2 can support normal O2 consumption in the body (250 ml*min-1) if the minute volume of blood circulation is approximately 83 l*min-1 at rest. The most optimal mechanism is the transport of O2 in a chemically bound form.

According to Fick's law, O2 gas exchange between alveolar air and blood occurs due to the presence of an O2 concentration gradient between these media. In the alveoli of the lungs, the partial pressure of O2 is 13.3 kPa, or 100 mmHg, and in the venous blood flowing to the lungs, the partial tension of O2 is approximately 5.3 kPa, or 40 mmHg. The pressure of gases in water or in body tissues is designated by the term “gas tension” and is designated by the symbols Po2, Pco2. The O2 gradient on the alveolar-capillary membrane, equal to an average of 60 mm Hg, is one of the most important, but not the only, according to Fick's law, factors in the initial stage of diffusion of this gas from the alveoli into the blood.

O2 transport begins in the capillaries of the lungs after its chemical binding to hemoglobin.

Hemoglobin (Hb) is capable of selectively binding O2 and forming oxyhemoglobin (HbO2) in an area of high O2 concentration in the lungs and releasing molecular O2 in an area of low O2 content in tissues. In this case, the properties of hemoglobin do not change and it can perform its function for a long time.

Hemoglobin carries O2 from the lungs to the tissues. This function depends on two properties of hemoglobin: 1) the ability to change from a reduced form, which is called deoxyhemoglobin, to an oxidized one (Hb + O2 à HbO2) at a high rate (half-life 0.01 s or less) at normal horn in the alveolar air; 2) the ability to release O2 in tissues (HbO2 à Hb + O2) depending on the metabolic needs of the body’s cells.

The dependence of the degree of oxygenation of hemoglobin on the partial pressure of O2 in the alveolar air is graphically presented in the form of an oxyhemoglobin dissociation curve, or saturation curve (Fig. 8.7). The plateau of the dissociation curve is characteristic of O2-saturated (saturated) arterial blood, and the steep descending part of the curve is characteristic of venous, or desaturated, tissue blood.

The affinity of oxygen for hemoglobin is influenced by various metabolic factors, which is expressed as a shift of the dissociation curve to the left or right. The affinity of hemoglobin for oxygen is regulated by the most important factors of tissue metabolism: Po2 pH, temperature and intracellular concentration of 2,3-diphosphoglycerate. The pH value and CO2 content in any part of the body naturally change the affinity of hemoglobin for O2: a decrease in blood pH causes a shift of the dissociation curve accordingly to the right (the affinity of hemoglobin for O2 decreases), and an increase in blood pH causes a shift of the dissociation curve to the left (the affinity of hemoglobin for O2 increases) ( see Fig. 8.7, A). For example, the pH in red blood cells is 0.2 units lower than in blood plasma. In tissues, due to the increased CO2 content, the pH is also lower than in blood plasma. The effect of pH on the oxyhemoglobin dissociation curve is called the “Bohr effect”.

An increase in temperature reduces the affinity of hemoglobin for O2. In working muscles, an increase in temperature promotes the release of O2. A decrease in tissue temperature or 2,3-diphosphoglycerate content causes a shift to the left in the oxyhemoglobin dissociation curve (see Fig. 8.7, B).

Metabolic factors are the main regulators of O2 binding to hemoglobin in the pulmonary capillaries, when the level of O2, pH and CO2 in the blood increases the affinity of hemoglobin for O2 along the pulmonary capillaries. In the conditions of body tissues, these same metabolic factors reduce the affinity of hemoglobin for O2 and promote the transition of oxyhemoglobin to its reduced form - deoxyhemoglobin. As a result, O2 flows along a concentration gradient from the blood of tissue capillaries to the body tissues.

Carbon monoxide (II) - CO, is able to combine with the iron atom of hemoglobin, changing its properties and reaction with O2. The very high affinity of CO for Hb (200 times higher than that of O2) blocks one or more iron atoms in the heme molecule, changing the affinity of Hb for O2.

The oxygen capacity of the blood is understood as the amount of O2 that is bound by the blood until hemoglobin is completely saturated. With a hemoglobin content in the blood of 8.7 mmol*l-1, the oxygen capacity of the blood is 0.19 ml of O2 in 1 ml of blood (temperature 0oC and barometric pressure 760 mm Hg, or 101.3 kPa). The oxygen capacity of the blood is determined by the amount of hemoglobin, 1 g of which binds 1.36-1.34 ml of O2. Human blood contains about 700-800 g of hemoglobin and can thus bind almost 1 liter of O2. There is very little O2 physically dissolved in 1 ml of blood plasma (about 0.003 ml), which cannot provide the oxygen demand of the tissues. The solubility of O2 in blood plasma is 0.225 ml*l-1*kPa-1

The exchange of O2 between capillary blood and tissue cells is also carried out by diffusion. The O2 concentration gradient between arterial blood (100 mm Hg, or 13.3 kPa) and tissues (about 40 mm Hg, or 5.3 kPa) is on average 60 mm Hg. (8.0 kPa). The change in the gradient can be caused by both the O2 content in arterial blood and the O2 utilization coefficient, which averages 30-40% for the body. The oxygen utilization coefficient is the amount of O2 given up when blood passes through tissue capillaries, related to the oxygen capacity of the blood.

Ticket 11

1. The membrane is a double lipid layer in which integral proteins are immersed, functioning as ion pumps and channels. Using the energy of ATP, the pumps pump K, Na, Ca ions against the concentration gradient. Peripheral proteins form the cytoskeleton of the cell, which gives strength and at the same time elasticity to the cell. Membranes are composed of three classes of lipids: phospholipids, glycolipids and cholesterol. Phospholipids and glycolipids (lipids with carbohydrates attached) consist of two long hydrophobic hydrocarbon tails that are connected to a charged hydrophilic head. Cholesterol gives the membrane rigidity by occupying the free space between the hydrophobic tails of lipids and preventing them from bending. Therefore, membranes with a low cholesterol content are more flexible, and those with a high cholesterol content are more rigid and fragile. Cholesterol also serves as a “stopper” that prevents the movement of polar molecules from the cell and into the cell. An important part of the membrane consists of proteins that penetrate it and are responsible for the various properties of membranes. Their composition and orientation differ in different membranes. The cell membrane is the separation barrier between the cytoplasm and the extracellular environment. Transport of substances through the cell membrane into or out of the cell, carried out using various mechanisms - simple diffusion, facilitated diffusion and active transport. The most important property of a biological membrane is its ability to pass various substances into and out of the cell. This is of great importance for self-regulation and maintaining a constant cell composition. This function of the cell membrane is performed thanks to selective permeability, that is, the ability to let some substances through and not others.

There are 4 main types of transport in the cell: 1) Diffusion, 2) Osmosis, 3) Active transport, 4) endo and exocytosis. 1) Diffusion is the movement of substances along a diffuse gradient, i.e. from an area of high concentration to an area of low concentration. Ions, glucose, amino acids, lipids, etc. diffuse slowly. Fat-soluble molecules diffuse quickly. Facilitated diffusion is a modification of diffusion. It is observed when a specific molecule helps a certain substance pass through the membrane, i.e. this molecule has its own channel through which it easily passes (glucose enters red blood cells). 2) Osmosis is the diffusion of water through semi-permeable membranes. 3) Active is the transport of molecules or ions across a membrane, against a concentration gradient and an electrochemical gradient. Carrier proteins (sometimes called pump proteins) transport substances across the membrane using energy, which is usually supplied by the hydrolysis of ATP. In a cell, a potential difference is maintained between the two sides of the plasma membrane - the membrane potential. The external environment is a positive charge, and the internal one is negative. Therefore, Na and K cations will tend to enter the cell, and chlorine anions will be repelled. An example of active transport found in most cells is the sodium-potassium pump. 4) Endo and exocytosis. The plasma membrane takes part in the removal of substances from the cell; this occurs through the process of exocytosis. This is how hormones, polysaccharides, proteins, fat droplets and other cell products are removed. They are enclosed in bubbles bounded by a membrane and approach the plasma membrane. Both membranes merge and the contents of the vesicle are expelled. Phagoctosis is the capture and absorption of large particles by a cell. Pinocytosis is the process of capturing and absorbing liquid droplets.

Potassium/sodium pump. Initially, this transporter attaches three ions to the inner side of the membrane. These ions change the conformation of the active site of ATPase. After such activation, the ATPase is able to hydrolyze one ATP molecule, and the phosphate ion is fixed on the surface of the carrier on the inside of the membrane. The released energy is spent on changing the conformation of the ATPase, after which three ions and an ion (phosphate) appear on the outside of the membrane. Here the ions are split off and replaced by two ions. Then the conformation of the carrier changes to its original one, and the ions end up on the inner side of the membrane. Here the ions are split off, and the carrier is ready for work again.

Oxygen in the blood is dissolved and combined with hemoglobin. A very small amount of oxygen is dissolved in plasma; every 100 ml of blood plasma at an oxygen tension (100 mm Hg) can carry only 0.3 ml of oxygen in a dissolved state. This is clearly not enough for the life of the body. With such an oxygen content in the blood and the condition of its complete consumption by the tissues, the minute volume of blood at rest would have to be more than 150 l/min. Another mechanism of oxygen transfer is important by combining it with hemoglobin.

Each gram of hemoglobin is capable of binding 1.34 ml of oxygen. The maximum amount of oxygen that can be bound by 100 ml of blood is the oxygen capacity of the blood (18.76 ml or 19 vol%). The oxygen capacity of hemoglobin is a value that reflects the amount of oxygen that can contact hemoglobin when it is completely saturated. Another indicator of the respiratory function of the blood is the oxygen content in the blood, which reflects the true amount of oxygen, both bound to hemoglobin and physically dissolved in the plasma.

100 ml of arterial blood normally contains 19-20 ml of oxygen, the same volume of venous blood contains 13-15 ml of oxygen, while the arteriovenous difference is 5-6 ml.

An indicator of the degree of oxygen saturation of hemoglobin is the ratio of the amount of oxygen associated with hemoglobin to the oxygen capacity of the latter. The oxygen saturation of hemoglobin in arterial blood in healthy individuals is 96%.

The formation of oxyhemoglobin in the lungs and its restoration in tissues depends on the partial oxygen tension of the blood: when it increases, the saturation of hemoglobin with oxygen increases, and when it decreases, it decreases. This relationship is nonlinear and is expressed by an S-shaped oxyhemoglobin dissociation curve.

Decrease in O2 voltage from 100 to 15-20 mm Hg. Art. has virtually no effect on hemoglobin oxygen saturation (HbO; it decreases by 2-3%). At lower O2 voltage values, oxyhemoglobin dissociates much more easily (the zone of steep decline in the curve). So, when the voltage 0 2 decreases from 60 to 40 mm Hg. Art. hemoglobin oxygen saturation decreases by approximately 15%.

The position of the oxyhemoglobin dissociation curve is usually expressed quantitatively by the partial oxygen tension at which hemoglobin saturation is 50%. The normal P50 value at a temperature of 37°C and pH 7.40 is about 26.5 mm Hg. Art..

Under certain conditions, the oxyhemoglobin dissociation curve can shift in one direction or another, maintaining an S-shape, under the influence of changes:

3. body temperature,

In working muscles, as a result of intense metabolism, the formation of CO 2 and lactic acid increases, and heat production also increases. All these factors reduce the affinity of hemoglobin for oxygen. In this case, the dissociation curve shifts to the right, which leads to easier release of oxygen from oxyhemoglobin, and the ability of tissues to consume oxygen increases.

With a decrease in temperature, 2,3-DPG, a decrease in CO 2 tension and an increase in pH, the dissociation curve shifts to the left, the affinity of hemoglobin for oxygen increases, as a result of which the delivery of oxygen to tissues decreases.

6. Transport of carbon dioxide in the blood. Carbon dioxide is transported to the lungs in the form of bicarbonates and in a state of chemical bonding with hemoglobin (carbohemoglobin).

Carbon dioxide is a metabolic product of tissue cells and is therefore transported by the blood from the tissues to the lungs. Carbon dioxide plays a vital role in maintaining the pH level in the internal environments of the body by mechanisms of acid-base balance. Therefore, the transport of carbon dioxide in the blood is closely related to these mechanisms.

In blood plasma, a small amount of carbon dioxide is dissolved; at PC0 2 = 40 mm Hg. Art. 2.5 ml/100 ml of blood carbon dioxide is tolerated, or 5%. The amount of carbon dioxide dissolved in plasma increases linearly with the PC0 2 level. In blood plasma, carbon dioxide reacts with water to form H + and HCO 3 . An increase in carbon dioxide tension in the blood plasma causes a decrease in its pH value. The carbon dioxide tension in the blood plasma can be changed by the function of external respiration, and the amount of hydrogen ions or pH can be changed by the buffer systems of the blood and HCO 3, for example, by their excretion through the kidneys in the urine. The pH value of blood plasma depends on the ratio of the concentration of carbon dioxide dissolved in it and bicarbonate ions. In the form of bicarbonate, the blood plasma, i.e. in a chemically bound state, transports the main amount of carbon dioxide - about 45 ml/100 ml of blood, or up to 90%. Erythrocytes transport approximately 2.5 ml/100 ml of carbon dioxide, or 5%, in the form of a carbamine compound with hemoglobin proteins. The transport of carbon dioxide in the blood from tissues to the lungs in the indicated forms is not associated with the phenomenon of saturation, as with the transport of oxygen, i.e., the more carbon dioxide is formed, the greater its amount is transported from the tissues to the lungs. However, there is a curvilinear relationship between the partial pressure of carbon dioxide in the blood and the amount of carbon dioxide carried by the blood: the carbon dioxide dissociation curve.

The role of red blood cells in the transport of carbon dioxide. Holden effect.

In the blood of the capillaries of the body's tissues, the carbon dioxide tension is 5.3 kPa (40 mm Hg), and in the tissues themselves - 8.0-10.7 kPa (60-80 mm Hg). As a result, CO 2 diffuses from tissues into the blood plasma, and from it into erythrocytes along the partial pressure gradient of CO 2. In erythrocytes, CO2 forms carbonic acid with water, which dissociates into H+ and HCO3. (C0 2 + H 2 0 = H 2 CO 3 = H + + HCO 3). This reaction proceeds quickly, since C0 2 + H 2 0 = H 2 CO 3 is catalyzed by the enzyme carbonic anhydrase of the erythrocyte membrane, which is contained in them in high concentration.

In red blood cells, the dissociation of carbon dioxide continues continuously as the products of this reaction are formed, since hemoglobin molecules act as a buffer compound, binding positively charged hydrogen ions. In red blood cells, as oxygen is released from hemoglobin, its molecules will bind with hydrogen ions (C0 2 + H 2 0 = H 2 C0 3 = H + + HCO 3), forming a compound (Hb-H +). In general, this is called the Holden effect, which leads to a shift of the oxyhemoglobin dissociation curve to the right along the x-axis, which reduces the affinity of hemoglobin for oxygen and promotes more intense release of it from red blood cells into tissues. In this case, as part of the Hb-H + compound, approximately 200 ml of CO 2 is transported in one liter of blood from the tissues to the lungs. The dissociation of carbon dioxide in erythrocytes can be limited only by the buffer capacity of hemoglobin molecules. HCO3 ions formed inside erythrocytes as a result of the dissociation of CO2 are removed from the erythrocytes into the plasma with the help of a special carrier protein of the erythrocyte membrane, and in their place Cl - ions are pumped from the blood plasma (the “chlorine” shift phenomenon). The main role of the CO 2 reaction inside erythrocytes is the exchange of Cl - and HCO3 ions between the plasma and the internal environment of erythrocytes. As a result of this exchange, the dissociation products of carbon dioxide H + and HCO3 will be transported inside erythrocytes in the form of a compound (Hb-H +), and in the blood plasma - in the form of bicarbonates.

Red blood cells are involved in the transport of carbon dioxide from tissues to the lungs, since C0 2 forms a direct combination with - NH 2 - groups of hemoglobin protein subunits: C0 2 + Hb -> HbC0 2 or carbamine compound. Transport of CO2 in the blood in the form of a carbamine compound and hydrogen ions by hemoglobin depends on the properties of the molecules of the latter; both reactions are determined by the magnitude of the partial pressure of oxygen in the blood plasma based on the Holden effect.

Quantitatively, the transport of carbon dioxide in dissolved form and in the form of a carbamine compound is insignificant compared to its transport of CO 2 in the blood in the form of bicarbonates. However, during the gas exchange of CO 2 in the lungs between the blood and alveolar air, these two forms become of primary importance.

When venous blood returns from the tissues to the lungs, CO 2 diffuses from the blood into the alveoli and PC0 2 in the blood decreases from 46 mm Hg. Art. (venous blood) up to 40 mm Hg. (arterial blood). At the same time, in the total amount of CO 2 (6 ml/100 ml of blood) diffusing from the blood into the alveoli, the proportion of the dissolved form of CO 2 and carbamic compounds becomes more significant relative to bicarbonate. Thus, the proportion of the dissolved form is 0.6 ml/100 ml of blood, or 10%, carbamic compounds - 1.8 ml/100 ml of blood, or 30%, and bicarbonates - 3.6 ml/100 ml of blood, or 60% .

In the red blood cells of the capillaries of the lungs, as the hemoglobin molecules become saturated with oxygen, hydrogen ions begin to be released, carbamine compounds dissociate and HCO3 is again converted into CO 2 (H+ + HCO3 = = H 2 CO 3 = CO 2 + H 2 0), which is excreted through diffusion through lungs along the gradient of its partial pressures between venous blood and alveolar space. Thus, hemoglobin in erythrocytes plays a major role in the transport of oxygen from the lungs to the tissues, and carbon dioxide in the opposite direction, since it is able to bind with 0 2 and H +.

At rest, approximately 300 ml of CO2 is removed from the human body through the lungs per minute: 6 ml/100 ml of blood x 5000 ml/min minute volume of blood circulation.

7. Regulation of breathing. Respiratory center and its divisions. Automation of the respiratory center.

It is well known that external respiration constantly changes under various conditions of the body’s vital activity.

Respiratory need. The activity of the functional respiratory system is always subordinated to satisfying the respiratory need of the body, which is largely determined by tissue metabolism.

Thus, during muscular work, compared to rest, the need for oxygen and the removal of carbon dioxide increases. To compensate for the increased respiratory demand, the intensity of pulmonary ventilation increases, which is expressed in an increase in the frequency and depth of breathing. The role of carbon dioxide. Animal experiments have shown that excess carbon dioxide in the air and blood (hypercapnia) stimulates pulmonary ventilation by increasing and deepening breathing, creating conditions for the removal of excess carbon dioxide from the body. On the contrary, a decrease in the partial pressure of carbon dioxide in the blood (hypocapnia) causes a decrease in pulmonary ventilation up to a complete stop of breathing (apnea). This phenomenon is observed after voluntary or artificial hyperventilation, during which carbon dioxide is removed from the body in excess. As a result, immediately after intense hyperventilation, breathing stops - posthyperventilation apnea.

The role of oxygen. A lack of oxygen in the atmosphere and a decrease in its partial pressure when breathing at high altitudes in a rarefied atmosphere (hypoxia) also stimulate breathing, causing an increase in the depth and especially frequency of breathing. As a result of hyperventilation, the lack of oxygen is partially compensated.

Excess oxygen in the atmosphere (hyperoxia), on the contrary, reduces the volume of pulmonary ventilation.

In all cases, ventilation changes in a direction that helps restore the altered gas state of the body. A process called respiratory regulation is the stabilization of a person's respiratory parameters.

Under the main respiratory center understand the set of neurons of the specific respiratory nuclei of the medulla oblongata.

The respiratory center controls two main functions; motor, which manifests itself in the form of contraction of the respiratory muscles, and homeostatic, associated with maintaining the constancy of the internal environment of the body during shifts in the content of 0 2 and CO 2. The motor, or motor, function of the respiratory center is to generate the respiratory rhythm and its pattern. Thanks to this function, breathing is integrated with other functions. By breathing pattern one should mean the duration of inhalation and exhalation, the tidal volume, and the minute volume of breathing. The homeostatic function of the respiratory center maintains stable values of respiratory gases in the blood and extracellular fluid of the brain, adapts the respiratory function to the conditions of a changed gas environment and other environmental factors.

Oxygen in the blood is dissolved and combined with hemoglobin. There is very little oxygen dissolved in plasma. Since the solubility of oxygen at 37 °C is 0.225 ml * l -1 * kPa -1 (0.03 ml-l -1 mm Hg -1), then every 100 ml of blood plasma at an oxygen tension of 13.3 kPa (100 mm rg. Art.) can carry only 0.3 ml of oxygen in a dissolved state. This is clearly not enough for the life of the body. With such an oxygen content in the blood and the condition of its complete consumption by the tissues, the minute volume of blood at rest would have to be more than 150 l/min. This makes clear the importance of another mechanism of oxygen transfer through its connections with hemoglobin.

Each gram of hemoglobin is capable of binding 1.39 ml of oxygen and, therefore, with a hemoglobin content of 150 g/l, every 100 ml of blood can carry 20.8 ml of oxygen.

Indicators of respiratory function of blood

1. Oxygen capacity of hemoglobin. The value reflecting the amount of oxygen that can contact hemoglobin when it is completely saturated is called oxygen capacity of hemoglobinA .

2. Oxygen content in the blood. Another indicator of the respiratory function of the blood is blood oxygen content, which reflects the true amount of oxygen, both bound to hemoglobin and physically dissolved in the plasma.

3. Degree of hemoglobin saturation with oxygen . 100 ml of arterial blood normally contains 19-20 ml of oxygen, the same volume of venous blood contains 13-15 ml of oxygen, while the arteriovenous difference is 5-6 ml. The ratio of the amount of oxygen associated with hemoglobin to the oxygen capacity of the latter is an indicator of the degree of oxygen saturation of hemoglobin. The oxygen saturation of hemoglobin in arterial blood in healthy individuals is 96%.

Educationoxyhemoglobin in the lungs and its restoration in tissues depends on the partial oxygen tension of the blood: when it increases. The saturation of hemoglobin with oxygen increases, and when it decreases, it decreases. This relationship is nonlinear and is expressed by an S-shaped oxyhemoglobin dissociation curve.

Oxygenated arterial blood corresponds to a plateau of the dissociation curve, and desaturated blood in tissues corresponds to a steeply decreasing part of it. The gentle rise of the curve in its upper section (zone of high O 2 tension) indicates that a sufficiently complete saturation of hemoglobin in arterial blood with oxygen is ensured even when the O 2 voltage is reduced to 9.3 kPa (70 mm Hg). A decrease in O tension, from 13.3 kPa to 2.0-2.7 kPa (from 100 to 15-20 mm Hg) has practically no effect on the saturation of hemoglobin with oxygen (HbO 2 decreases by 2-3%). At lower O2 voltage values, oxyhemoglobin dissociates much more easily (the zone of steep decline in the curve). Thus, when the O 2 tension decreases from 8.0 to 5.3 kPa (from 60 to 40 mm Hg), the saturation of hemoglobin with oxygen decreases by approximately 15%.

The position of the oxyhemoglobin dissociation curve is usually expressed quantitatively by the partial oxygen tension at which hemoglobin saturation is 50% (P 50). The normal P50 value at a temperature of 37°C and pH 7.40 is about 3.53 kPa (26.5 mm Hg).

The dissociation curve of oxyhemoglobin under certain conditions can shift in one direction or another, maintaining an S-shape, under the influence of changes in pH, CO 2 voltage, body temperature, and the content of 2,3-diaphosphoglycerate (2,3-DPG) in erythrocytes, on which it depends the ability of hemoglobin to bind oxygen. In working muscles, as a result of intense metabolism, the formation of CO 2 and lactic acid increases, and heat production also increases. All these factors reduce the affinity of hemoglobin for oxygen. In this case, the dissociation curve shifts to the right (Fig. 8.7), which leads to easier release of oxygen from oxyhemoglobin, and the possibility of oxygen consumption by tissues increases. With a decrease in temperature, 2,3-DPG, a decrease in CO tension, and an increase in pH, the dissociation curve shifts to the left, the affinity of hemoglobin for oxygen increases, resulting in a decrease in oxygen delivery to tissues.