Regulation of breathing. Functional system of oxygen supply to the body
To maintain the gas composition of the alveoli (removing carbon dioxide and the supply of air containing a sufficient amount of oxygen), ventilation of the alveolar air is necessary. It is achieved through breathing movements: alternating inhalation and exhalation. The lungs themselves cannot pump or expel air from the alveoli. They only passively follow the change in the volume of the chest cavity due to negative pressure in the pleural cavity. The diagram of respiratory movements is shown in Fig. 5.9.
Rice. 5.9.
At inhale the diaphragm moves down, pushing the organs away abdominal cavity, and the intercostal muscles lift the chest up, forward and to the sides. The volume of the chest cavity increases, and the lungs follow this increase, as the gases contained in the lungs press them against the parietal pleura. As a result, the pressure inside the pulmonary alveoli drops and outside air enters the alveoli.
Exhalation begins with the intercostal muscles relaxing. Under the influence of gravity chest wall goes down and the diaphragm goes up as the abdominal wall presses on internal organs abdominal cavity, and with their volume they raise the diaphragm. The volume of the chest cavity decreases, the lungs are compressed, the air pressure in the alveoli becomes higher than atmospheric pressure, and some of it comes out. All this happens when calm breathing. When you inhale and exhale deeply, additional muscles are activated.
Nervous regulation of breathing
The respiratory center is located in the medulla oblongata. It consists of inhalation and exhalation centers that regulate the functioning of the respiratory muscles. The collapse of the pulmonary alveoli, which occurs during exhalation, reflexively activates the inhalation center, and the expansion of the alveoli reflexively activates the exhalation center - thus respiratory center functions continuously and rhythmically. The automaticity of the respiratory center is due to the peculiarities of metabolism in its neurons. The impulses arising in the respiratory center along the centrifugal nerves reach the respiratory muscles, causing them to contract and, accordingly, providing inhalation.
Of particular importance in the regulation of breathing are impulses coming from the receptors of the respiratory muscles and from the receptors of the lungs themselves. The depth of inhalation and exhalation largely depends on their character. Physiological mechanism breathing regulation is built on the principle feedback: when inhaling, the lungs stretch and excitation occurs in the receptors located in the walls of the lungs, which travels along the centripetal fibers vagus nerve reaches the respiratory center and inhibits the activity of neurons in the inhalation center, while excitation occurs in the exhalation center through the mechanism of reverse induction. As a result, the respiratory muscles relax, the chest shrinks and exhalation occurs. By the same mechanism, exhalation stimulates inhalation.
When you hold your breath, the muscles of inhalation and exhalation contract simultaneously, as a result of which the chest and diaphragm are held in one position. The work of the respiratory centers is also influenced by other centers, including those located in the cerebral cortex. Thanks to their influence, you can consciously change the rhythm of your breathing, hold it, and control your breathing when talking or singing.
For irritation of abdominal organs, blood vessel receptors, skin, receptors respiratory tract breathing changes reflexively. Thus, when inhaling ammonia, the receptors of the mucous membrane of the nasopharynx are irritated, which causes activation of the act of breathing, and when high concentration vapors – reflexive holding of breath. This same group of reflexes includes sneezing and coughing - protective reflexes that serve to remove foreign particles that have entered the respiratory tract.
Humoral regulation of respiration
During muscle work, oxidation processes intensify, which leads to an increase in carbon dioxide levels in the blood. Excess carbon dioxide increases the activity of the respiratory center, breathing becomes deeper and more frequent. As a result of intense breathing, the lack of oxygen is replenished, and excess carbon dioxide is removed. If the concentration of carbon dioxide in the blood decreases, the work of the respiratory center is inhibited and involuntary holding of breath occurs. Thanks to the nervous and humoral regulation the concentration of carbon dioxide and oxygen in the blood is maintained at a certain level under any conditions.
Like all systems in the body, breathing is regulated by two main mechanisms - nervous and humoral.
The basis of nervous regulation is the implementation of the Hering-Breer reflex, which essentially consists of a series of successively alternating reflexes during the breathing process, similar to those described in various physiology textbooks. Here we note that all reflexes can be combined as one, the essence of which is as follows: inhalation, exhalation stimulates inhalation.
The change in respiratory phases is facilitated by signals coming from mechanoreceptors lungs along the afferent fibers of the vagus nerves. Impulses coming from the receptors of the lungs ensure the change of inhalation to exhalation and the change of exhalation to inhalation (Fig. 7)
Fig.7. A diagram reflecting the basic processes of self-regulation of inhalation and exhalation.
I – a set of inspiratory neurons that provide inspiration; And p – inspiratory late neurons that interrupt inhalation: light – excitatory, dark – inhibitory.
In the epithelial and subepithelial layers of all airways, as well as in the region of the roots of the lungs, there are so-called irritant receptors, which simultaneously possess the properties of mechano- and chemoreceptors. They become irritated when there are strong changes in lung volume. Irritant receptors are also excited under the influence of dust particles, vapors of caustic substances and some biological active substances, for example, histamine. However, for regulating the change between inhalation and exhalation, receptors sensitive to lung stretching (mechanical irritation) are of greater importance.
In the medulla oblongata, the neurons responsible for the rhythmic alternation of the acts of inhalation and exhalation form several nuclei of the dorsal and ventral groups, among which the latter is of greater importance in the implementation of the Hering-Breer reflex. Conventionally, all nuclei of the ventral and dorsal groups can be united under the common name of the respiratory center (RC). Neurons of the respiratory center medulla oblongata seem to be divided into two groups. One group of neurons gives fibers to the muscles that provide inspiration; this group of neurons is called inspiratory neurons(inspiratory center, IC), i.e. inhalation center. Another group of neurons that send fibers to the internal intercostal and intercartilaginous muscles is called expiratory neurons(expiratory center, EC), i.e. the center of exhalation. Neurons of the expiratory and inspiratory sections of the respiratory center of the medulla oblongata have different excitability and lability. The excitability of the inspiratory region is higher. In addition, the IC has pronounced automation.
Inhalation begins with the excitation of the IC, which is largely provided by automatic processes in it. Descending impulses travel through motor neurons spinal cord, axons that make up the phrenic, external intercostal and intercartilaginous nerves that innervate the main muscles of inspiration. The contraction of these muscles increases the size of the chest, air enters the alveoli, stretching them. The deeper the inhalation occurs, the more the lung receptors are activated. The frequency of afferentation from them increases, heading to the EC, which is excited. Excitation of the EC induces inhibition on the IC, motor impulses from it to the inspiratory muscles stop, which relax. Passive exhalation occurs under the influence of gravity. That. inhalation stimulates exhalation.
When you exhale, the lung parenchyma collapses, the activation of its mechanoreceptors stops, which means that afferentation to the EC disappears. The excitation of the EC stops and it ceases to exert inhibition on the IC. In the latter, automatic processes increase and he becomes excited. A new act of inhalation begins, i.e. exhalation stimulates inhalation.
Of course, the formation of the breathing pattern by stem structures, described here by Lumsden (1920), is given here in a simplified form. In fact, the respiratory neurons of the medulla oblongata form several ventral and dorsal groups responsible for the generation of various types of motor impulses at different moments (beginning - end), both inhalation and exhalation. It seems inappropriate to present in detail in this edition modern ideas about the mechanisms of respiratory rhythmogenesis. Let us only emphasize that the two main properties of the respiratory center that ensure the implementation of the Hering-Breer reflex are automaticity and reciprocity. The ability to self-excite is present not only in inspiratory neurons, as described above, but also in the EC. In addition, both the EC is capable of inducing inhibition on the IC, and vice versa. There is an antagonistic (reciprocal) relationship between these two groups of respiratory neurons.
In addition, let us note that the respiratory center, located in the medulla oblongata, is capable of forming a rhythm external respiration through a nervous (reflex) mechanism. However, it is known that the intensity of breathing largely depends on humoral factors, for example, blood acidity, and can also be changed arbitrarily.
A significant contribution to the study of these mechanisms was made by the domestic physiologist N.A. Mislavsky, on the basis of whose work the concept of a central mechanism of breathing regulation can be introduced (Fig. 8)
Fig.8. Respiratory center (its components) and efferent nerves.
K – bark; GT – hypothalamus; PM – medulla oblongata; cm – spinal cord; Th 1 - Th 6 – thoracic spinal cord; C 3 – C 5 – cervical region spinal cord.
The central mechanism of respiratory regulation (CMRM) is the entire set of brain nuclei involved in the formation of the rhythm and depth of respiratory movements. The main elements of the CMRD are the DC of the medulla oblongata, the pneumotoxic center (PTC) of the midbrain, and the cerebral cortex (CHC).
The respiratory center of the medulla oblongata is influenced by the overlying parts of the central nervous system. For example, in the anterior part of the pons there is a PTC, which promotes the periodic activity of the respiratory center, it increases the rate of development of inspiratory activity, increases the excitability of the mechanisms for switching off inhalation, and accelerates the onset of the next inspiration. In other words, the PTC intensively exchanges excitatory and inhibitory impulses with inspiratory and expiratory neurons of the medulla oblongata. PTC increases or decreases the excitability of the DC, thereby changing external respiration
By modern ideas excitation of the cells of the inspiratory part of the medulla oblongata activates activity apnoestic and pneumotaxic centers. The apneic center inhibits the activity of expiratory neurons, while the pneumotaxic center excites. As the excitation of inspiratory neurons increases under the influence of impulses from mechano- and chemoreceptors, the activity of the pneumotaxic center increases. By the end of the inhalation phase, the excitatory influences on the expiratory neurons from this center become dominant over the inhibitory influences coming from the apnoestic center. This leads to excitation of expiratory neurons, which have an inhibitory effect on inspiratory cells. Inhalation slows down, exhalation begins.
There is an independent mechanism for inhalation inhibition at the level of the medulla oblongata. This mechanism includes special neurons (I beta), excited by impulses from lung stretch mechanoreceptors, and inspiratory inhibitory neurons, excited by the activity of I beta neurons. Thus, with increasing impulses from the mechanoreceptors of the lungs, the activity of I beta neurons increases, which at a certain point in time (towards the end of the inhalation phase) causes excitation of inspiratory inhibitory neurons. Their activity inhibits the work of inspiratory neurons. Inhalation is replaced by exhalation.
The activity of PTC depends on many factors:
Firstly, the PTC receives afferentation from various organs and systems of the body: receptors of the lung parenchyma, vascular reflexogenic zones, and other receptive fields.
Secondly, PTC has its own central chemoreceptors, sensitive to changes in the acidity and gas composition of the cerebrospinal fluid. Thus, the humoral regulation of external respiration is carried out largely due to PTC.
Thirdly, the PTC is in close interaction with the CGM and is under its control, which ensures voluntary regulation of breathing.
If you cut the paths connecting the PTC with the CGM, then external respiration will practically not change. The animal will fully retain the ability to adapt the intensity of breathing to changing conditions of existence, which will be carried out unconditionally reflex type with the participation of PTC and DC. However, voluntary regulation will be impossible; for example, breathing will be held when the head is immersed in water.
If the brainstem is then transected below the mesencephalic region ( midbrain), thereby turning off the PTC, external respiration will remain the same, but will change significantly (Fig. 9)
Fig. 9 Effect of transections at different levels of the brain and spinal cord on breathing.
A – nature of respiratory movements, B – levels of transections.
It will consist of alternating phases of deep inhalation and exhalation, i.e. will be realized only in accordance with the Hering–Breer reflex. In this case, humoral regulation will be practically impossible; for example, blood acidification will not lead to an increase in the depth of respiration.
Finally, complete severing of the brain from the spinal cord results in respiratory arrest.
In the regulation of breathing great importance have hypothalamic centers. Under the influence of the centers of the hypothalamus, breathing increases, for example, during painful stimulation, during emotional arousal, and during physical exertion.
Speaking about the humoral regulation of external respiration, it should be noted that the activity of the respiratory center largely depends on the tension of gases in the blood and the concentration of hydrogen ions in it. Leading value in determining the value pulmonary ventilation has a tension of carbon dioxide in the arterial blood, it seems to create a request for the required amount of ventilation of the alveoli.
The content of oxygen and especially carbon dioxide is maintained at a relatively constant level. The normal level of oxygen in the body is called normoxia, lack of oxygen in the body and tissues is hypoxia, and lack of oxygen in the blood is hypoxemia. An increase in oxygen tension in the blood is called hyperoxia. The normal level of carbon dioxide in the blood is called normocapnia, increase in carbon dioxide content - hypercapnia, and a decrease in its content - hypocapnia.
Normal breathing at rest is called eipnea. Hypercapnia, as well as a decrease in blood pH (acidosis) are accompanied by an increase in pulmonary ventilation - hyperpnea, which leads to the release of excess carbon dioxide from the body, an increase in ventilation of the lungs occurs due to an increase in the depth and frequency of breathing.
Hyperoxia, hypocapnia and increased blood pH levels lead to a decrease in ventilation of the lungs, and then to respiratory arrest - apnea.
The activity of the DC depends on the composition of the blood entering the brain through the common carotid arteries. In 1901, this was shown by L. Frederick in experiments with cross-circulation. In two anesthetized dogs, the carotid arteries were cut and cross-connected and jugular veins. In this case, the head of the first dog was supplied with blood from the second dog, and vice versa. If in one of the dogs, for example, the first, the trachea was blocked and in this way asphyxia was caused, then hyperpnea developed in the second dog. In the first dog, despite an increase in CO 2 tension in the arterial blood and a decrease in O 2, apnea developed, since in its carotid artery blood was received from a second dog, in which, as a result of hyperventilation, the CO 2 tension in the arterial blood decreased (Fig. 10)
Fig. 10. Experience with cross circulation (according to L. Frederick)
Carbon dioxide, hydrogen ions and mild hypoxia cause increased respiration. These factors enhance the activity of the respiratory center, influencing peripheral and central chemoreceptors that regulate breathing.
The role of reflexogenic zones in the regulation of breathing.
Chemoreceptors, sensitive to an increase in carbon dioxide tension and a decrease in oxygen tension are located in the carotid sinuses and the aortic arch. Of greater importance for the regulation of breathing are carotid chemoreceptors. With normal oxygen content in arterial blood in afferent nerve fibers, extending from the carotid bodies, impulses are recorded. When oxygen tension decreases, the pulse frequency increases especially significantly, because hypoxia has a stimulating effect on arterial chemoreceptors. In addition, afferent influences from the carotid bodies increase with an increase in the carbon dioxide tension and the concentration of hydrogen ions in the arterial blood. Chemoreceptors of the carotid bodies inform the respiratory center about the tension of O 2 and CO 2 in the blood, which is sent to the brain.
Breathing depends on reflex influences from vascular reflexogenic zones, and in particular from the baroreceptors of the zone vertebral arteries(PAD). In particular, PAD causes combined changes in respiration and systemic blood pressure.
Central chemoreceptors are located in the medulla oblongata and are constantly stimulated by hydrogen ions found in the cerebrospinal fluid. Perfusion of this area of the brain with a solution with a reduced pH sharply increases breathing, and at a high pH, breathing weakens, up to apnea. The same thing happens when this surface of the medulla oblongata is cooled or treated with anesthetics. Central chemoreceptors, providing strong influence on the activity of the respiratory center, significantly change the ventilation of the lungs.
Central chemoreceptors respond to changes in CO 2 tension in arterial blood later than peripheral chemoreceptors, since it takes more time for CO 2 to diffuse from the blood into the cerebrospinal fluid and further into the brain tissue. Hypercapnia and acidosis stimulate, and hypocapnia and alkalosis inhibit central chemoreceptors.
Pulses coming from central and peripheral chemoreceptors are a necessary condition periodic activity of neurons of the respiratory center and compliance of ventilation of the lungs with the gas composition of the blood.
The uniqueness of the external respiration function is that it is both automatically and voluntarily controlled.
Regulation of breathing movements
Nervous regulation
The respiratory center (the center of inhalation and exhalation) is located in the medulla oblongata of the brain. The work of the Respiratory Center depends on pain and temperature, as well as blood pressure, medications and other factors.
The cerebral cortex allows you to voluntarily delay, change the rhythm and depth of breathing.
Humoral regulation
With an increase in the concentration of carbon dioxide (CO g) in the blood, the excitability of the respiratory center increases - breathing becomes more frequent. As the CO2 concentration decreases, the excitability of the respiratory center decreases.
External respiration is one of the most important functions of the body. Stopping breathing leads to certain death within 3-5 minutes. The amount of oxygen in the body is insignificant, so it is important that it constantly enters through the external respiration system. This explains the formation in the process of evolution of a regulatory mechanism that would ensure high reliability of breathing. The regulation of breathing is based on maintaining a constant level of body indicators such as Pco8, Po? and pH. The main principle of regulation is self-regulation, in which the deviation of these parameters from the normal level immediately causes a number of processes aimed at their restoration. In the respiratory regulation system, internal and external links of self-regulation can be distinguished. Internal links are associated with the state of the blood ( buffer properties, hemoglobin content) and the cardiovascular system, external - with the mechanisms of external respiration. The variable parameters of the external respiration regulation system are the depth and frequency of respiratory movements. The main regulated object is the respiratory muscles, which belong to skeletal muscles. In addition to them, the smooth muscles of the pharynx, trachea and bronchi, which affect the condition of the respiratory tract, should be included in the object of breathing regulation. The transport of gases in the blood and gas exchange in tissues is carried out by the cardiovascular system, the regulation of whose functions will be discussed in the relevant sections. Breathing is regulated mainly by the reflex pathway, which includes 3 elements: 1) receptors that perceive information and afferent pathways that transmit it to the nerve centers, 2) nerve centers, 3) effectors - paths for transmitting commands from the centers and the actual executive elements.
Involuntary regulation of breathing is carried out by the respiratory center located in the medulla oblongata (one of the parts of the hindbrain). The ventral (lower) part of the respiratory center is responsible for stimulating inhalation; it is called the inhalation center (inspiratory center). Stimulation of this center increases the frequency and depth of inspiration. The dorsal (upper) part and both lateral (lateral) parts inhibit inhalation and stimulate exhalation; they are collectively called the exhalation center (expiratory center). The respiratory center is connected to the intercostal muscles by the intercostal nerves, and to the diaphragm by the phrenic nerves. The bronchial tree (a collection of bronchi and bronchioles) is innervated by the vagus nerve. Rhythmically repeating nerve impulses directed to the diaphragm and intercostal muscles ensure the implementation of ventilation movements. The expansion of the lungs during inhalation stimulates those in bronchial tree stretch receptors (proprioceptors) and they send more and more impulses through the vagus nerve to the expiratory center. This temporarily suppresses the inspiratory center and inspiration. The external intercostal muscles now relax, the stretched lung tissue contracts elastically - exhalation occurs. After exhalation, the stretch receptors in the bronchial tree are no longer stimulated. Therefore, the expiratory center is switched off and inhalation can begin again. This entire cycle is repeated continuously and rhythmically throughout the life of the organism. Forced breathing is carried out with the participation of the internal intercostal muscles. The basic rhythm of breathing is maintained by the respiratory center of the medulla oblongata, even if all the nerves entering it are cut. However, under normal conditions, this basic rhythm is subject to various influences. The main factor regulating the respiratory rate is not the concentration of oxygen in the blood, but the concentration of CO2. When the level of CO2 increases (for example, during physical activity), the chemoreceptors of the carotid and aortic bodies present in the circulatory system send nerve impulses to the inspiratory center. The medulla oblongata itself also contains chemoreceptors. From the inspiratory center, through the phrenic and intercostal nerves, impulses enter the diaphragm and external intercostal muscles, which leads to their more frequent contraction and, consequently, to an increase in the respiratory rate. The CO2 that accumulates in the body can cause great harm to the body. When CO2 combines with water, an acid is formed that can cause denaturation of enzymes and other proteins. Therefore, in the process of evolution, organisms have developed a very rapid reaction to any increase in CO2 concentration. If the concentration of CO2 in the air increases by 0.25%, then pulmonary ventilation doubles. To produce the same result, the oxygen concentration in the air must decrease from 20% to 5%. Oxygen concentration also affects breathing, but under normal conditions there is always enough oxygen, and therefore its effect is relatively small. Chemoreceptors that respond to oxygen concentration are located in the medulla oblongata, carotid and aortic bodies, as well as CO2 receptors. Within certain limits, the frequency and depth of breathing can be regulated arbitrarily, as evidenced, for example, by our ability to “hold our breath.” We resort to voluntary regulation of breathing during forced breathing, when talking, singing, sneezing and coughing.
Physiological role pulmonary respiration consists of ensuring the optimal gas composition of arterial blood. For the normal intensity of tissue respiration processes, it is necessary that the blood entering the tissue capillaries is always saturated with oxygen and does not contain CO in quantities that prevent its release from the tissues. Since, when blood passes through the capillaries of the lungs, almost complete gas equilibrium is established between plasma and alveolar air, the optimal content of gases in arterial blood determines the corresponding composition of alveolar air. The optimal content of gases in the alveolar air is achieved by changing the volume of pulmonary ventilation depending on the conditions currently existing in the body.
However, inspiratory and expiratory neurons are considered as two functionally different populations, within which neurons are interconnected by a network of axons and synapses. Studies of the activity of single neurons of the reticular formation of the medulla oblongata led to the conclusion that the area of the respiratory center cannot be delineated strictly and unambiguously. The so-called respiratory neurons are found almost throughout the entire length of the medulla oblongata. However, in each half of the medulla oblongata there are areas of the reticular formation where respiratory neurons are grouped at a higher density.
In the rostral parts of the pons, in the medial parabrachial nucleus, in areas of brain tissue ventral to it, as well as in structures related to the control of accessory respiratory muscles, i.e. found in the place identified as the pneumotaxic center greatest number respiratory neurons of the pons. Unlike the neurons of the medulla oblongata, which stably retain the nature of volley activity, in the pons the same respiratory neuron can change the nature of its activity. The respiratory neurons of the pons are organized into groups consisting of 10-12 neurons of different types. Among them there are many so-called transitional (phase-spanning) neurons that exhibit a maximum frequency when changing phases of the respiratory cycle.
These neurons are attributed the function of binding different phases respiratory cycle, preparing conditions for stopping the inhalation phase and transitioning to exhalation. The pneumotaxic center of the pons is connected to the respiratory center of the medulla oblongata by ascending and descending pathways. Axons of neurons of the solitary fasciculus and retroambigual nucleus arrive from the medulla oblongata to the medial parabronchial nucleus and the Kölliker-Fuse nucleus. These axons are the main entrance to the pneumotaxic center. Distinctive feature The activity of the respiratory neurons of the pons is that if the connection with the medulla oblongata is disrupted, they lose the volley nature of impulses and modulation of the frequency of impulses in the breathing rhythm.
It is believed that the pneumotaxic center receives impulses from the inspiratory part of the respiratory center of the medulla oblongata and sends impulses back to the respiratory center in the medulla oblongata, where they excite expiratory and inhibit inspiratory neurons. The respiratory neurons of the pons are the first to receive information about the need to adapt breathing to changing conditions and accordingly change the activity of the neurons of the respiratory center, and the transition neurons ensure a smooth change from inhalation to exhalation. Thus, thanks to joint work with the pneumotaxic complex, the respiratory center of the medulla oblongata can carry out a rhythmic change of phases of the respiratory cycle with an optimal ratio of the duration of inhalation, exhalation and respiratory pause. However, for normal life activity and maintaining breathing adequate to the body’s needs, the participation of not only the pons, but also the overlying parts of the brain is necessary.
Breathing functions
The role of lung mechanoreceptors in the regulation of breathing
The source of information from the respiratory center about the condition of the lungs and extrapulmonary bronchi and trachea are sensitive nerve endings, located in smooth muscles, in the submucosal layer and in the epithelium of the airways.
Depending on the location, type of perceived irritation and the nature of reflex responses to irritation, three types of receptors are distinguished:
1) lung stretch receptors;
2) irritant receptors;
3) J-receptors (“juxtacapillary” receptors of the lungs).
Lung stretch receptors are located mainly in the smooth muscles of the airways - in the trachea and bronchi of all calibers. There are about 1000 such receptors in each lung and they are connected to the respiratory center by large myelinated afferent fibers of the vagus nerve with high speed conducting excitation (about 40 m/s). The direct stimulus of this type of mechanoreceptors is the internal tension in the tissues of the walls of the airways, which is determined by the pressure difference on both sides of the walls and a change in their viscoelastic properties depending, for example, on the intensity of bronchomotor tone.
With moderate stretching of the lungs during inspiration, the frequency of impulses from these receptors depends linearly on lung volume. The stimulation thresholds of individual mechanoreceptors vary significantly. Some of them have a high threshold and generate impulses only during inspiration, when the volume of the lungs increases beyond the functional residual capacity. Others (low-threshold) remain active during passive exhalation. The frequency of impulses in afferent fibers from stretch receptors especially increases during the development of the inhalation process. If the achieved lung volume is maintained at a constant level for a long time, then the activity of the stretch receptors changes little, therefore, they have slow adaptation.
Inflation of the lungs causes a reflex inhibition of inhalation and a transition to exhalation, and sharp decrease lung volume (by, for example, artificial suction of air through the intubated bronchus of one lung) leads to activation of inhalation. When the vagus nerves are cut, these reactions disappear, and breathing becomes sharply slower and deeper. These reactions, called Hering-Breuer reflexes, formed the basis of the idea of reflex self-regulation of breathing. Its essence lies in the fact that the duration of the phases of the respiratory cycle and the breathing frequency are determined by the impulses coming to the respiratory center from the mechanoreceptors of the lungs along the afferent fibers of the vagus nerve.
Stretch receptors provide feedback between the lungs and the respiratory center, signaling lung volume and the rate of change. When the lungs reach a certain critical volume, under the influence of impulses from the mechanoreceptors of the lungs, the expiratory neurons of the respiratory center are excited, the activity of the inspiratory neurons is inhibited, so inhalation is replaced by exhalation. It is believed that reflexes from lung stretch receptors play a major role in the regulation of pulmonary ventilation; the depth and frequency of breathing depend on them. However, it has been shown that in an adult, the Hering-Breuer reflexes are activated when the tidal volume exceeds 1 liter (as, for example, during physical exertion). It is possible that these reflexes may be of great importance in newborns.
Throughout the trachea and bronchi in the epithelium and subepithelial layer there are so-called irritant receptors(other names: rapidly adapting mechanoreceptors of the airways, receptors of the mucous membrane of the trachea and bronchi). They respond to sudden changes in lung volume, as well as when the mucous membrane of the trachea and bronchi is exposed to mechanical or chemical irritants: dust particles, mucus accumulating in the airways, vapors of caustic substances (ammonia, ether, tobacco smoke).
Excessive collapse (pneumothorax, collapse, atelectasis) or stretching of the lungs leads to changes in the tension of the walls of the intrapulmonary airways and excitation of irritant receptors. Unlike pulmonary stretch receptors, irritant receptors have quick adaptation. When tiny foreign bodies (dust, smoke particles) enter, activation of irritant receptors causes a cough reflex in a person, as well as unpleasant sensations in the chest such as soreness and burning. Excitation of the irritant receptors of the bronchi causes increased breathing, primarily due to shortening of exhalations, breathing becomes frequent and superficial. Activation of these receptors also causes reflex bronchoconstriction.
In the interstitium of the alveoli and respiratory bronchi, near the capillaries, there are J receptors(“juxtacapillary” receptors of the lungs). The stimulus for these receptors is an increase in pressure in the pulmonary circulation, as well as an increase in the volume of interstitial fluid in the lungs. Strong and time-stable excitation of J-receptors occurs with stagnation of blood in the pulmonary circulation, pulmonary edema, embolism of small vessels of the lungs and other damage to the lung tissue, which occurs, for example, with pneumonia. J-receptors are sensitive to a number of biologically active substances (nicotine, prostaglandins, histamine) that penetrate into the interstitium of the lungs either from the airways or with the blood of the pulmonary circulation.
Impulses from these receptors are sent to the respiratory center along slow, unmyelinated fibers of the vagus nerve, causing rapid shallow breathing. With the development of left ventricular circulatory failure and interstitial pulmonary edema, stimulation of J-receptors in a person causes a feeling of shortness of breath, i.e. feeling of difficulty breathing. In response to irritation of these receptors, in addition to rapid breathing (tachypnea), reflex bronchoconstriction also occurs. Excitation of J-receptors, caused by an increase in blood supply to the lungs during excessively heavy muscular work, can lead to reflex inhibition of skeletal muscle activity.
Reflexes with pro-prioreceptors of the respiratory muscles. The intercostal and abdominal muscles have specialized stretch receptors (muscle spindles and Golgi tendon receptors). In the diaphragm, such receptors are contained in small quantity. The proprioceptors of the respiratory muscles are excited with an increase in the length and degree of tension of the muscle fibers. The impulse from these receptors spreads mainly to the spinal centers of the respiratory muscles, as well as to the centers of the brain that control the condition of the skeletal muscles. Intercostal and abdominal muscles have stretch reflexes, which are under the control of suprabulbar structures of the brain.
Meaning segmental proprioceptive reflexes of the respiratory muscles consists of automatically regulating the strength of contractions depending on the initial length of the muscles and the resistance they encounter during contraction. Thanks to these features of the intercostal muscles, compliance of the mechanical parameters of breathing with the resistance of the respiratory system is achieved, which increases, for example, with a decrease in the compliance of the lungs, narrowing of the bronchi and glottis, and swelling of the nasal mucosa. In all cases, segmental stretch reflexes enhance the contraction of the intercostal muscles and muscles of the anterior abdominal wall. In humans, impulses from the proprioceptors of the respiratory muscles are involved in the formation of sensations that occur when breathing is impaired.
The role of chemoreceptors in the regulation of respiration
The main purpose of the regulation of external respiration is to maintain optimal gas composition of arterial blood - O 2 voltage, CO 2 voltage and, thus, to a large extent, the concentration of hydrogen ions. In humans, the relative constancy of the O 2 and CO 2 tension in arterial blood is maintained even during physical work, when the consumption of O 2 and the formation of CO 2 increases several times. This is possible because during work, ventilation of the lungs increases in proportion to the intensity of metabolic processes. An excess of CO 2 and a lack of O 2 in the inhaled air also causes an increase in the volumetric rate of respiration, due to which the partial pressure of O 2 and CO 2 in the alveoli and in the arterial blood remains almost unchanged.
A special place in humoral regulation the activity of the respiratory center has a change in CO 2 tension in the blood. When inhaling a gas mixture containing 5-7% CO 2, an increase in the partial pressure of CO 2 in the alveolar air delays the removal of CO 2 from venous blood. The associated increase in CO 2 tension in arterial blood leads to an increase in pulmonary ventilation by 6-8 times. Due to such a significant increase in breathing volume, the concentration of CO 2 in the alveolar air increases by no more than 1%. An increase in CO 2 content in the alveoli by 0.2% causes an increase in pulmonary ventilation by 100%.
The role of CO 2 as the main regulator of respiration is also revealed in the fact that the lack of CO 2 in the blood reduces the activity of the respiratory center and leads to a decrease in breathing volume and even to a complete cessation of respiratory movements (apnea). This happens, for example, with artificial hyperventilation: an arbitrary increase in the depth and frequency of breathing leads to hypocapnia- reducing the partial pressure of CO 2 in the alveolar air and arterial blood. Therefore, after the cessation of hyperventilation, the appearance of the next breath is delayed, and the depth and frequency of subsequent breaths initially decreases.
These changes in the gas composition of the internal environment of the body affect the respiratory center indirectly, through special chemosensitive receptors, located directly in the structures of the medulla oblongata ( "central chemoreceptors") and in vascular reflexogenic zones (“peripheral chemoreceptors”).
Central (medullary) chemoreceptors, Constantly involved in the regulation of breathing are called neuronal structures in the medulla oblongata, sensitive to CO 2 tension and the acid-base state of the intercellular brain fluid washing them. Chemosensitive zones are present on the anterolateral surface of the medulla oblongata near the exits of the hypoglossal and vagus nerves in a thin layer of the medulla at a depth of 0.2-0.4 mm. Medullary chemoreceptors are constantly stimulated by hydrogen ions in the intercellular fluid of the brain stem, the concentration of which depends on the CO 2 tension in the arterial blood. The cerebrospinal fluid is separated from the blood by the blood-brain barrier, which is relatively impermeable to H + and HCO 3 ions, but freely allows molecular CO 2 to pass through. When the CO 2 voltage in the blood increases, it diffuses from the blood vessels of the brain into the cerebrospinal fluid, as a result of which H + ions accumulate in it, which stimulate medullary chemoreceptors. With an increase in CO 2 voltage and the concentration of hydrogen ions in the fluid washing the medullary chemoreceptors, the activity of inspiratory neurons increases and the activity of expiratory neurons of the respiratory center of the medulla oblongata decreases. As a result of this, breathing becomes deeper and ventilation of the lungs increases, mainly due to an increase in the volume of each breath. On the contrary, a decrease in CO 2 tension and alkalization of the intercellular fluid leads to the complete or partial disappearance of the reaction of increasing the volume of respiration to excess CO 2 (hypercapnia) and acidosis, as well as to a sharp inhibition of inspiratory activity of the respiratory center, up to respiratory arrest.
Peripheral chemoreceptors sensing the gas composition of arterial blood, located in two areas: the aortic arch and the fission site (bifurcation) common carotid artery (carotid sinus), those. in the same areas as baroreceptors that respond to changes blood pressure. However, chemoreceptors are independent formations contained in special bodies - glomeruli or glomus, which are located outside the vessel. Afferent fibers from the chemoreceptors come from the aortic arch in the aortic branch of the vagus nerve, and from the carotid sinus in the carotid branch of the glossopharyngeal nerve, the so-called Hering nerve. Primary afferents of the sinus and aortic nerves pass through the ipsilateral nucleus of the solitary tract. From here, chemoreceptive impulses travel to the dorsal group of respiratory neurons of the medulla oblongata.
Arterial chemoreceptors cause a reflex increase in pulmonary ventilation in response to a decrease in oxygen tension in the blood (hypoxemia). Even in ordinary (normoxic) conditions, these receptors are in a state of constant excitation, which disappears only when inhaled by a person pure oxygen. A decrease in oxygen tension in arterial blood below the normal level causes increased afferentation from the aortic and sinocarotid chemoreceptors. Inhalation of a hypoxic mixture leads to increased frequency and regularity of impulses sent by the chemoreceptors of the carotid body.
An increase in arterial blood CO2 tension and a corresponding increase in ventilation is also accompanied by an increase in impulse activity directed to the respiratory center from chemoreceptors of the carotid sinus. The peculiarity of the role played by arterial chemoreceptors in the control of carbon dioxide tension is that they are responsible for the initial, fast phase of the ventilatory response to hypercapnia. When they are denervated, this reaction occurs later and turns out to be more sluggish, since it develops under these conditions only after the CO 2 tension in the region of chemosensitive brain structures increases.
Hypercapnic stimulation arterial chemoreceptors, like hypoxic, is permanent. This stimulation begins at a threshold CO 2 voltage of 20-30 mm Hg and, therefore, takes place already under conditions of normal CO 2 tension in arterial blood (about 40 mm Hg).
An important point for regulating breathing is interaction of humoral stimuli of respiration. It manifests itself, for example, in the fact that against the background of increased arterial CO 2 tension or increased concentration of hydrogen ions, the ventilatory response to hypoxemia becomes more intense. Therefore, a decrease in the partial pressure of oxygen and a simultaneous increase in the partial pressure of carbon dioxide in the alveolar air cause an increase in pulmonary ventilation that exceeds the arithmetic sum of the responses that these factors cause, acting separately. Physiological significance This phenomenon lies in the fact that the specified combination of respiratory stimulants occurs during muscular activity, which is associated with a maximum increase in gas exchange and requires an adequate increase in the functioning of the respiratory apparatus.
It has been established that hypoxemia lowers the threshold and increases the intensity of the ventilatory response to CO 2 . However, in a person with a lack of oxygen in the inhaled air, an increase in ventilation occurs only when the arterial CO 2 tension is at least 30 mm Hg. When the partial pressure of O 2 in the inhaled air decreases (for example, when breathing gas mixtures with a low content of O 2, at low atmospheric pressure in a pressure chamber or in the mountains), hyperventilation occurs, aimed at preventing a significant decrease in partial pressure O 2 in the alveoli and its tension in arterial blood.
In this case, due to hyperventilation, a decrease in the partial pressure of CO 2 in the alveolar air occurs and hypocapnia develops, leading to a decrease in the excitability of the respiratory center. Therefore, during hypoxic hypoxia, when the partial pressure of CO 2 in the inhaled air decreases to 12 kPa (90 mm Hg) and below, the respiratory regulation system can only partially ensure that the tension of O 2 and CO 2 is maintained at the proper level. Under these conditions, despite hyperventilation, O2 tension still decreases, and moderate hypoxemia occurs.
In the regulation of respiration, the functions of central and peripheral receptors constantly complement each other and, in general, exhibit synergy. Thus, stimulation of the chemoreceptors of the carotid body enhances the effect of stimulation of medullary chemosensitive structures. The interaction of central and peripheral chemoreceptors is vital important for the body, for example, under conditions of O 2 deficiency. During hypoxia, due to a decrease in oxidative metabolism in the brain, the sensitivity of medullary chemoreceptors weakens or disappears, as a result of which the activity of respiratory neurons decreases. The respiratory center under these conditions receives intense stimulation from arterial chemoreceptors, for which hypoxemia is an adequate stimulus. Thus, arterial chemoreceptors serve as an “emergency” mechanism for the respiratory response to changes in the gas composition of the blood, and, above all, to a deficiency of oxygen supply to the brain.
The relationship between the regulation of external respiration and other body functions
Exchange of gases in the lungs and tissues and its adaptation to the demands of tissue respiration during various states The body is provided by changing not only pulmonary ventilation, but also blood flow both in the lungs themselves and in other organs. Therefore, the mechanisms of neurohumoral regulation of respiration and blood circulation are carried out in close interaction. Reflex influences emanating from receptive fields of cardio-vascular system(for example, the gynocarotid zone), change the activity of both the respiratory and vasomotor centers. Neurons of the respiratory center are subject to reflex influences from the baroreceptor zones of blood vessels - the aortic arch, carotid sinus. Vascular-motor reflexes are inextricably linked with changes in breathing function.
Increased vascular tone and increased cardiac activity, respectively, are accompanied by increased respiratory function. For example, during physical or emotional stress, a person usually experiences a coordinated increase in minute blood volume in the systemic and pulmonary circulation, blood pressure, and pulmonary ventilation. However, a sharp increase in blood pressure causes excitation of the sinocarotid and aortic baroreceptors, which leads to reflex inhibition of breathing. A decrease in blood pressure, for example, during blood loss, leads to an increase in pulmonary ventilation, which is caused, on the one hand, by a decrease in the activity of vascular baro-receptors, and on the other, by excitation of arterial chemoreceptors as a result of local hypoxia caused by a decrease in blood flow in them. Increased breathing occurs when blood pressure increases in the pulmonary circulation and when the left atrium is stretched.
The work of the respiratory center is influenced by afferentation from peripheral and central thermoreceptors, especially with sharp and sudden temperature effects on skin receptors. Immersion of a person in cold water, for example, inhibits exhalation, resulting in a prolonged inhalation. In animals that do not have sweat glands (for example, a dog), with an increase in external temperature and a deterioration in heat transfer, ventilation of the lungs increases due to increased breathing (temperature polyp) and the evaporation of water through the respiratory system increases.
The reflex influences on the respiratory center are very extensive, and almost all receptor zones, when irritated, change breathing. This feature of reflex regulation of breathing reflects general principle neural organization of the reticular formation of the brain stem, which includes the respiratory center. Neurons of the reticular formation, including respiratory neurons, have abundant collaterals from almost all afferent systems of the body, which provides, in particular, versatile reflex effects on the respiratory center. The activity of neurons in the respiratory center is affected by a large number of different nonspecific reflex influences. Thus, painful stimulation is accompanied by an immediate change in respiratory rhythm. The breathing function is closely related to emotional processes: almost all emotional manifestations of a person are accompanied by changes in the breathing function; Laughter and crying are altered breathing movements.
The respiratory center of the medulla oblongata directly receives impulses from the receptors of the lungs and the receptors of large vessels, i.e. receptive zones, the irritation of which is especially significant for the regulation of external respiration. However, in order to adequately adapt the respiratory function to the changing conditions of the body’s existence, the regulatory system must have complete information about what is happening in the body and in the environment. Therefore, all afferent signals from various receptive fields of the body are important for the regulation of breathing. However, all this signaling does not come directly to the respiratory center of the medulla oblongata, but to various levels of the brain (Fig. 8.10), and from them can be directly transmitted to both the respiratory and other functional systems. Various centers of the brain form with the respiratory center functionally mobile associations, providing complete regulation of respiratory function.
Fig.8.10. Organization diagram central office regulation of breathing.
The arrows indicate the paths of transmission of regulatory influences to the respiratory center of the medulla oblongata.
As can be seen in Fig. 8.10, included in the central mechanism regulating breathing different levels CNS. The significance of the structures of the brainstem, including the pons and midbrain, for the regulation of respiration is that these parts of the central nervous system receive and switch to the respiratory center proprioceptive And interoceptive signaling, A diencephalon — signaling about metabolism. The cerebral cortex, as the central station of analytical systems, absorbs and processes signals from all organs and systems, making it possible to adequately adapt various functional systems, including breathing, to the subtlest changes in the vital functions of the body.
The uniqueness of the function of external respiration lies in the fact that it is to the same extent automatic, And arbitrarily controlled. A person breathes well in sleep and under anesthesia; In animals, breathing remains practically normal character even after removing the entire forebrain. At the same time, any person can arbitrarily, albeit briefly, stop breathing or change its depth and frequency. Voluntary control of breathing is based on the presence of respiratory muscles in the cerebral cortex and the presence of corticomedullary descending activating and inhibitory influences on the efferent part of the respiratory center. The ability to voluntarily control breathing is limited to certain limits of changes in oxygen and carbon dioxide tension, as well as blood pH. In case of excessive voluntary breath-holding or a sharp deviation of the actual minute volume of ventilation from the physiologically justified one, a stimulus arises that returns breathing under the control of the respiratory center, overcoming the cortical influence.
The role of the cerebral cortex in the regulation of breathing was shown in experiments on animals with electrical stimulation of various areas of the cerebral hemispheres, as well as with their removal. It turned out that as soon as a crustless animal takes a few steps for 1-2 minutes, it begins to experience pronounced and prolonged shortness of breath, i.e. significant increase in breathing rate and intensification. Consequently, if adaptation of breathing to environmental conditions is required, for example during muscular activity, participation is necessary higher departments central nervous system. Cortless animals retain uniform breathing only in a state of complete rest and lose the ability to adapt breathing to changes in the external environment during muscular work.
The influence of the cerebral cortex on breathing in humans is manifested, for example, in increased breathing even in the starting conditions before performing muscular efforts, immediately after the command “get ready.” Breathing intensifies in a person immediately after the start of movements, when the humoral substances formed during muscular work have not yet reached the respiratory center. Consequently, increased breathing at the very beginning of muscle work is caused by reflex effects that increase the excitability of the respiratory center.
Cortical influences on breathing are clearly manifested during training to perform the same work: in this case, there is a gradual development and improvement of functional relationships between muscle work and breathing that are adequate for this work. This is indicated by the dynamics of changes in external respiration during, for example, training to work on a bicycle ergometer with variable intensity. If the pace of work is constant, and its intensity periodically changes according to a predetermined schedule, then as you train with such a program, the average level of pulmonary ventilation decreases, but the change in ventilation when switching to new level intensity comes faster.
Consequently, as a result of training for work of variable intensity, the ability to switch activities more quickly develops. breathing apparatus to a new level of functional activity adequate to new working conditions. Better consistency in time of the processes of coordination of the function of external respiration during the transition from one working conditions to another is associated with the functional restructuring of the higher parts of the central nervous system. As a result of this, as you train for muscle work, fluctuations in breathing volume become smaller and breathing becomes more even. The dynamic stereotype thus developed is manifested in the fact that during the transition to work with constant intensity, lung ventilation has a pronounced wavy character.
The role of the higher parts of the central nervous system in the regulation of breathing in humans is manifested not only in his ability to voluntarily change the tempo, rhythm and amplitude of respiratory movements, but also in his ability to “consciously” perceive his hypoxic, or hypercapnic state.
A person cannot directly perceive the content of oxygen and carbon dioxide in the inhaled air due to the lack of adequate receptors in the respiratory tract and lungs. However, using the method active choice preferred breathing mixtures (the so-called gas preference) It has been shown that people avoid breathing gas mixtures that cause hypoxic or hypercapnic changes in the body. For example, a person was asked to choose one of two alternately inhaled mixtures of gases with different, unknown oxygen contents. Under such conditions, people did not yet distinguish mixtures containing 15% O 2 or more from ordinary air; a 12% oxygen content already caused a negative reaction in some people, and a mixture with 9% oxygen was rejected by almost all subjects. Similarly, a person avoided breathing mixtures enriched with carbon dioxide.
Studies on athletes have revealed their ability to evaluate hypoxic and hypercapnic changes in their body not only when inhaling the corresponding gases, but also during intense muscle activity. In particular, after sports training, the subjects could almost accurately determine the degree of oxygenation of their own arterial blood by their sensations.
When breathing gas mixtures that have a physiologically inadequate composition, a person, regardless of the intensity of developing hyperventilation, sometimes declares that it is “difficult to breathe,” i.e. complains of shortness of breath. The feeling of shortness of breath is a reflection of the mismatch between chemoreceptive signaling and other parts of the reflex regulation of breathing, including reverse afferentation emanating from the working respiratory muscles. These kinds of sensations underlie self-monitoring of backup performance when a person performs significant muscle load.
Breathing regulation
The body's need for oxygen during rest and during work is not the same; therefore, the frequency and depth of breathing must automatically change to adapt to changing conditions. During muscular work, oxygen consumption by muscles and other tissues can increase 4-5 times.
Breathing requires coordinated contraction of many individual muscles; this coordination is carried out by the respiratory center - a special group of cells located in one of the parts of the brain called the medulla oblongata. From this center, volleys of impulses are rhythmically sent to the diaphragm and intercostal muscles, causing regular and coordinated contraction of the corresponding muscles every 4-5 seconds. Under normal conditions, breathing movements occur automatically, without control from our will. But when the nerves going to the diaphragm (phrenic nerves) and intercostal muscles are cut or damaged (for example, in infantile paralysis), breathing movements immediately stop. Of course, a person can arbitrarily change the frequency and depth of breathing; he may even not breathe at all for some time, but he is not able to hold his breath for such long time so that it causes any significant harm: the automatic mechanism comes into action and causes inhalation.
The question naturally arises: why does the respiratory center periodically send volleys of impulses? Through a series of experiments, it was found that if the connections of the respiratory center with all other parts of the brain are interrupted, that is, if the sensory nerves and pathways coming from the higher brain centers are cut, then the respiratory center sends a continuous stream of impulses and the muscles involved in breathing , having contracted, remain in a contracted state. Thus, the respiratory center, left to its own devices, causes complete contraction of the muscles involved in breathing. If, however, either the sensory nerves or the pathways coming from the higher brain centers remain intact, then the respiratory movements continue to occur normally. This means that normal breathing requires periodic inhibition of the respiratory center so that it stops sending impulses that cause muscle contraction. Further experiments showed that the pneumaxic center, located in the midbrain (Fig.: 268), together with the respiratory center, form a “reverberating circular path”, which serves as the basis for regulating the respiratory rate. In addition, stretching the walls of the alveoli during inhalation stimulates the pressure-sensitive muscles located in these walls. nerve cells, and these cells send impulses to the brain that inhibit the respiratory center, which leads to exhalation.
The respiratory center is also stimulated or inhibited by impulses coming to it along many other nerve pathways. Strong pain in any part of the body causes a reflex increase in breathing. In addition, in the mucous membrane of the larynx and pharynx there are receptors that, when irritated, send impulses to the respiratory center that inhibit breathing. These are important protective devices. When any irritating gas, such as ammonia or strong acid vapors, enters the respiratory tract, it stimulates the receptors in the larynx, which send inhibitory impulses to the respiratory center, and we involuntarily “take our breath away”; thereby harmful substance does not penetrate the lungs. In the same way, when food accidentally enters the larynx, it irritates the receptors in the mucous membrane of this organ, causing them to send inhibitory impulses to the respiratory center. Breathing instantly stops, and food does not enter the lungs, where it could damage the delicate epithelium.
During muscular work, the frequency and depth of breathing must increase to satisfy the body's increased need for oxygen and prevent the accumulation of carbon dioxide. The concentration of carbon dioxide in the blood is the main factor regulating respiration. An increased content of carbon dioxide in the blood flowing to the brain increases the excitability of both the respiratory and pneumotaxic centers. An increase in the activity of the first of them leads to increased contraction of the respiratory muscles, and the second leads to increased breathing. When the carbon dioxide concentration returns to normal, stimulation of these centers stops and the frequency and depth of breathing return to normal levels.
This mechanism also works in the opposite direction. If a person voluntarily takes a series of deep breaths and exhalations, the carbon dioxide content in the alveolar air and in the blood will decrease so much that after he stops breathing deeply, respiratory movements will stop altogether until the level of carbon dioxide in the blood reaches normal again. The first breath of a newborn baby is caused mainly by the action of this mechanism. Immediately after the birth of a child and his separation from the placenta, the carbon dioxide content in his blood begins to increase and causes the respiratory center to send impulses to the diaphragm and intercostal muscles, which contract and produce the first breath. Sometimes, when a newborn baby's first breath is delayed, air containing 10% carbon dioxide is blown into his lungs to activate this mechanism.
Experiments have shown that the main factor stimulating the respiratory center is not so much a decrease in the amount of oxygen as an increase in the amount of carbon dioxide in the blood. If a person is placed in a small hermetically sealed chamber, so that he has to breathe the same air all the time, the oxygen content in the air will gradually decrease. If, in addition, a chemical substance is placed in the chamber that can quickly absorb the released carbon dioxide so that the amount in the lungs and blood does not increase, then the respiratory rate will increase only slightly, even if the experiment is continued until the oxygen content decreases very much. If you do not remove carbon dioxide, but allow it to accumulate, then breathing will sharply increase and the person will experience discomfort and a feeling of suffocation. When a person is allowed to breathe air with a normal amount of oxygen, but with increased content carbon dioxide, again there is an increase in breathing. Obviously, the respiratory center is stimulated not by a lack of oxygen, but mainly by the accumulation of carbon dioxide.
To ensure greater reliability of the proper response to changes in the concentration of carbon dioxide and oxygen in the blood, another regulatory mechanism has been developed. At the base of each of the internal carotid arteries (arteria carotid) there is slight swelling, called the carotid sinus, which contains receptors that are sensitive to changes in blood chemistry. When carbon dioxide levels increase or oxygen levels decrease, these receptors send nerve impulses to the respiratory center in the medulla oblongata and increase its activity.
Effect of training. Exercises and practice in sports training increase the body's ability to perform a particular task. First, muscles increase in size and strength during training (due to the growth of individual muscle fibers, and not an increase in their number). Secondly, when performing a particular action repeatedly, a person learns to coordinate the work of muscles and contract each of them with exactly the force with which it is necessary to achieve the desired result, which leads to energy savings. Thirdly, changes occur in the cardiovascular and respiratory systems. The heart of a trained athlete is slightly enlarged and contracts more slowly at rest. During muscular work, it pumps a larger volume of blood, not so much due to increased contractions, but due to the greater force of each contraction. In addition, the athlete breathes slower and deeper than a common person, and during physical activity, the amount of air passing through his lungs increases mainly not due to increased breathing, but due to an increase in its depth. This is a more efficient way to achieve the same goal
According to metabolic needs respiratory system provides gas exchange of O2 and CO2 between environment and the body. This vital important function regulates a network of numerous interconnected neurons of the central nervous system, located in several parts of the brain and combined into a complex concept "respiratory center". When its structures are exposed to nervous and humoral stimuli, the respiratory function adapts to changing environmental conditions. The structures necessary for the emergence of a respiratory rhythm were first discovered in the medulla oblongata. Transection of the medulla oblongata in the area of the bottom of the fourth ventricle leads to cessation of breathing. Therefore, the main respiratory center is understood as a set of neurons of the specific respiratory nuclei of the medulla oblongata.
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 02 and CO2. 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.
Localization and functional properties of respiratory neurons
In the anterior horns of the spinal cord at the C3 - C5 level there are motor neurons that form the phrenic nerve. Motor neurons innervating the intercostal muscles are located in the anterior horns at levels T2 - T10 (T2 - T6 - motor neurons of inspiratory muscles, T8-T10 - expiratory muscles). It has been established that some motor neurons regulate predominantly the respiratory, while others regulate predominantly the postnotonic activity of the intercostal muscles.
The neurons of the bulbar respiratory center are located at the bottom of the IV ventricle in the medial part of the reticular formation of the medulla oblongata and form the dorsal and ventral respiratory groups. Respiratory neurons whose activity causes inspiration or expiration are called inspiratory and expiratory neurons, respectively. There is a reciprocal relationship between groups of neurons that control inhalation and exhalation. Excitation of the expiratory center is accompanied by inhibition in the inspiratory center and vice versa. Inspiratory and expiratory neurons, in turn, are divided into “early” and “late”. Each respiratory cycle begins with the activation of “early” inspiratory neurons, then the “late” inspiratory neurons are excited. Also, “early” and “late” expiratory neurons are sequentially excited, which inhibit inspiratory neurons and stop inhalation. Modern research showed that in the medulla oblongata there is no clear division into inspiratory and expiratory sections, but there are clusters of respiratory neurons with a specific function.
Spontaneous activity of neurons in the respiratory center begins to appear towards the end of the period of intrauterine development. Excitation of the respiratory center in the fetus appears due to the pacemaker properties of the network of respiratory neurons in the medulla oblongata. As synaptic connections of the respiratory center with various parts of the central nervous system are formed, the pacemaker mechanism of respiratory activity gradually loses its physiological significance.
The pons contains the nuclei of respiratory neurons that form the pneumotaxic center. It is believed that the respiratory neurons of the pons are involved in the mechanism of change between inhalation and exhalation and regulate the amount of tidal volume. The respiratory neurons of the medulla oblongata and the pons are interconnected by ascending and descending nerve pathways and function in concert. Having received impulses from the inspiratory center of the medulla oblongata, the pneumotaxic center sends them to the expiratory center of the medulla oblongata, exciting the latter. Inspiratory neurons are inhibited. Destruction of the brain between the medulla oblongata and the pons lengthens the inspiratory phase. The hypothalamic nuclei coordinate the connection between breathing and blood circulation.
Certain zones of the cerebral cortex carry out voluntary regulation of breathing in accordance with the peculiarities of the influence of environmental factors on the body and the associated homeostatic shifts.
Thus, we see that breathing control is a complex process carried out by many neural structures. In the process of breathing control, a clear hierarchy of various components and structures of the respiratory center is carried out.
Reflex regulation of breathing
Neurons of the respiratory center have connections with numerous mechanoreceptors of the respiratory tract and alveoli of the lungs and receptors of vascular reflexogenic zones. Thanks to these connections, a very diverse, complex and biologically important reflex regulation of breathing and its coordination with other functions of the body is carried out.
There are several types of mechanoreceptors: slow adapting lung stretch receptors, irritant fast adapting mechanoreceptors and J-receptors - “juxtacapillary” receptors of the lungs.
Slowly adapting lung stretch receptors located in the smooth muscles of the trachea and bronchi. These receptors are excited during inhalation, and impulses from them travel through the afferent fibers of the vagus nerve to the respiratory center. Under their influence, the activity of inspiratory neurons of the medulla oblongata is inhibited. Inhalation stops and exhalation begins, during which the stretch receptors are inactive. The inspiratory inhibition reflex when stretching the lungs is called the Hering-Breuer reflex. This reflex controls the depth and frequency of breathing. It is an example of feedback regulation. After cutting the vagus nerves, breathing becomes rare and deep.
Irritant, rapidly adapting mechanoreceptors, localized in the mucous membrane of the trachea and bronchi, they are excited by sudden changes in lung volume, by stretching or collapsing of the lungs, or by the action of mechanical or chemical irritants on the mucous membrane of the trachea and bronchi. The result of irritation of irritant receptors is rapid, shallow breathing, a cough reflex, or a bronchoconstriction reflex.
J-receptors - "juxtacapillary" receptors of the lungs are located in the interstitium of the alveoli and respiratory bronchi close to the capillaries. Impulses from J-receptors with increased pressure in the pulmonary circulation, or an increase in the volume of interstitial fluid in the lungs (pulmonary edema), or embolism of small pulmonary vessels, as well as with the action of biologically active substances (nicotine, prostaglandins, histamine) along the slow fibers of the vagus nerve enter the respiratory center - breathing becomes frequent and shallow (shortness of breath).
Important biological significance, especially due to deteriorating environmental conditions and air pollution, have protective respiratory reflexes - sneezing and coughing.
Sneezing. Irritation of the receptors of the nasal mucosa, for example, by dust particles or gaseous drugs, tobacco smoke, or water causes constriction of the bronchi, bradycardia, decreased cardiac output, and narrowing of the lumen of blood vessels in the skin and muscles. Various mechanical and chemical irritations of the nasal mucosa cause deep strong exhalation - sneezing, which contributes to the desire to get rid of the irritant. The afferent pathway of this reflex is the trigeminal nerve.
Cough occurs when the mechano- and chemoreceptors of the pharynx, larynx, trachea and bronchi are irritated. In this case, after inhalation, the expiratory muscles contract strongly, the intrathoracic and intrapulmonary pressure increases sharply (up to 200 mm Hg), the glottis opens, and air from the respiratory tract is released out under high pressure and removes the irritating agent. The cough reflex is the main pulmonary reflex of the vagus nerve.
Reflexes from proprioceptors of respiratory muscles
From the muscle spindles and Golgi tendon receptors located in the intercostal muscles and abdominal muscles, impulses enter the corresponding segments of the spinal cord, then to the medulla oblongata, the brain centers that control the state of skeletal muscles. As a result, the strength of contractions is regulated depending on the initial length of the muscles and the resistance of the respiratory system.
Reflex regulation of breathing is also carried out peripheral And central chemoreceptors, which is outlined in the section on humoral regulation.
Humoral regulation of respiration
The main physiological stimulus of the respiratory centers is carbon dioxide. Regulation of breathing determines the maintenance of normal CO2 content in the alveolar air and arterial blood. An increase in CO2 content in the alveolar air by 0.17% causes a doubling of MOR, but a decrease in O2 by 39-40% does not cause significant changes in MOR.
When the CO2 concentration in closed hermetically sealed cabins increased to 5–8%, the subjects observed an increase in pulmonary ventilation by 7–8 times. At the same time, the concentration of CO2 in the alveolar air did not increase significantly, since the main sign of breathing regulation is the need to regulate the volume of pulmonary ventilation, maintaining the constancy of the composition of the alveolar air.
The activity of the respiratory center depends on the composition of the blood entering the brain through the common carotid arteries. In 1890, this was demonstrated by Frederick in experiments with cross-circulation. The carotid arteries and jugular veins were cut and cross-connected in two anesthetized dogs. In this case, the head of the first dog was supplied with blood from the second dog and vice versa. If in one of the dogs, for example in the first, the trachea was blocked and in this way asphyxia was caused, then hyperpnea developed in the second dog. In the first dog, despite an increase in CO2 tension in the arterial blood and a decrease in O2 tension, apnea developed, since the blood of the second dog entered its carotid artery, in which, as a result of hyperventilation, the CO2 tension in the arterial blood decreased.
Carbon dioxide, hydrogen ions and mild hypoxia cause increased respiration. These factors enhance the activity of the respiratory center, influencing peripheral (arterial) and central (modular) chemoreceptors that regulate breathing.
Arterial chemoreceptors located in the carotid sinuses and aortic arch. They are located in special bodies, abundantly supplied with arterial blood. Aortic chemoreceptors have little effect on breathing and are more important for the regulation of blood circulation.
Arterial chemoreceptors are unique receptor structures that are stimulated by hypoxia. The afferent influences of carotid bodies also increase with an increase in the carbon dioxide tension and the concentration of hydrogen ions in the arterial blood. The stimulating effect of hypoxia and hypercapnia on chemoreceptors is mutually enhanced, while under conditions of hyperoxia the sensitivity of chemoreceptors to carbon dioxide sharply decreases. Arterial chemoreceptors inform the respiratory center about the tension of O2 and CO2 in the blood going to the brain.
After transection of arterial (peripheral) chemoreceptors in experimental animals, the sensitivity of the respiratory center to hypoxia disappears, but the respiratory response to hypercapnia and acidosis is completely preserved.
Central chemoreceptors located in the medulla oblongata lateral to the pyramids. Perfusion of this area of the brain with a solution with a reduced pH sharply increases breathing, and at a high pH, breathing weakens, up to apnea. The same thing happens when this surface of the medulla oblongata is cooled or treated with anesthetics. Central chemoreceptors, having a strong influence on the activity of the respiratory center, significantly change the ventilation of the lungs. It was found that a decrease in cerebrospinal fluid pH by only 0.01 is accompanied by an increase in pulmonary ventilation by 4 l/min.
Central chemoreceptors respond to changes in CO2 tension in arterial blood later than peripheral chemoreceptors, since it takes more time for CO2 to diffuse from the blood into the cerebrospinal fluid and further into the brain tissue. Hypercapnia and acidosis stimulate, and hypocapnia and alkalosis inhibit central chemoreceptors.
To determine the sensitivity of central chemoreceptors to changes in the pH of the extracellular fluid of the brain, to study the synergism and antagonism of respiratory gases, and the interaction of the respiratory system and the cardiovascular system, the rebreathing method is used. When breathing in a closed system, exhaled CO2 causes a linear increase in the concentration of CO2 and at the same time the concentration of hydrogen ions in the blood, as well as in the extracellular fluid of the brain, increases.
The set of respiratory neurons should be considered as a constellation of structures that implement the central mechanism of respiration. Thus, instead of the term “respiratory center”, it is more correct to talk about the system of central regulation of breathing, which includes the structures of the cerebral cortex, certain zones and nuclei of the intermediate, mesencephalon, medulla oblongata, pons, neurons of the cervical and thoracic spinal cord, central and peripheral chemoreceptors, as well as mechanoreceptors of the respiratory organs.
The uniqueness of the external respiration function is that it is both automatic and voluntarily controlled.
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