The respiratory center, its localization, structure and regulation of activity. Respiratory center Reflex influences on breathing from vascular reflexogenic zones

Breathing regulation - this is the coordinated nervous control of the respiratory muscles, which sequentially carry out respiratory cycles consisting of inhalation and exhalation.

Respiratory center - this is a complex multi-level structural and functional formation of the brain that carries out automatic and voluntary regulation of breathing.

Breathing is an automatic process, but it is subject to voluntary regulation. Without such regulation, speech would be impossible. At the same time, breathing control is built on reflex principles: both unconditioned reflex and conditioned reflex.

Breathing regulation is based on the general principles of automatic regulation that are used in the body.

Pacemaker neurons (neurons are “rhythm creators”) provide automatic the occurrence of excitation in the respiratory center even if the respiratory receptors are not irritated.

Inhibitory neurons provide automatic suppression of this excitation after a certain time.

The respiratory center uses the principle reciprocal (i.e. mutually exclusive) interaction of two centers: inhalation And exhalation . Their arousal is inversely proportional. This means that the excitation of one center (for example, the inhalation center) inhibits the second center associated with it (the exhalation center).

Functions of the respiratory center
- Providing inspiration.
- Providing exhalation.
- Ensuring automatic breathing.
- Ensuring adaptation of breathing parameters to environmental conditions and body activity.
For example, when the temperature rises (both in the environment and in the body), breathing becomes more frequent.

Respiratory center levels

1. Spinal (in the spinal cord). The spinal cord contains centers that coordinate the activity of the diaphragm and respiratory muscles - L-motoneurons in the anterior horns of the spinal cord. Diaphragmatic neurons are in the cervical segments, intercostal neurons are in the thoracic segments. When the pathways between the spinal cord and brain are cut, breathing is disrupted because spinal centers do not have autonomy (i.e. independence) And do not support automation breathing.

2. Bulbar (in the medulla oblongata) - main department respiratory center. In the medulla oblongata and the pons there are 2 main types of neurons of the respiratory center - inspiratory(inhalation) and expiratory(exhalatory).

Inspiratory (inhalation) - are excited 0.01-0.02 s before the start of active inspiration. During inhalation, their pulse frequency increases and then immediately stops. They are divided into several types.

Types of inspiratory neurons

By influence on other neurons:
- inhibitory (stop inhalation)
- facilitating (stimulating inhalation).
By time of excitation:
- early (a few hundredths of a second before inhalation)
- late (active throughout the entire inhalation process).
By connections with expiratory neurons:
- in the bulbar respiratory center
- in the reticular formation of the medulla oblongata.
In the dorsal nucleus, 95% are inspiratory neurons, in the ventral nucleus - 50%. The neurons of the dorsal nucleus are connected to the diaphragm, and the ventral nucleus is connected to the intercostal muscles.

Expiratory (exhalation) - excitation occurs a few hundredths of a second before the start of exhalation.

There are:
- early,
- late,
- expiratory-inspiratory.
In the dorsal nucleus, 5% of neurons are expiratory, and in the ventral nucleus - 50%. In general, there are significantly fewer expiratory neurons than inspiratory neurons. It turns out that inhalation is more important than exhalation.

Automatic breathing is ensured by complexes of 4 neurons with the obligatory presence of inhibitory ones.

Interaction with other brain centers

Respiratory inspiratory and expiratory neurons have output not only to the respiratory muscles, but also to other nuclei of the medulla oblongata. For example, when the respiratory center is excited, the swallowing center is reciprocally inhibited and at the same time, on the contrary, the vasomotor center for regulating cardiac activity is excited.

At the bulbar level (i.e. in the medulla oblongata) it is possible to distinguish pneumotaxic center , located at the level of the pons, above the inspiratory and expiratory neurons. This center regulates their activity and provides a change in inhalation and exhalation. Inspiratory neurons provide inspiration and, at the same time, excitation from them enters the pneumotaxic center. From there, the excitation runs to the expiratory neurons, which are excited and provide exhalation. If you cut the paths between the medulla oblongata and the pons, the frequency of respiratory movements will decrease, due to the fact that the activating effect of the PTDC (pneumotaxic respiratory center) on inspiratory and expiratory neurons is reduced. This also leads to a lengthening of inspiration due to the long-term preservation of the inhibitory effect of expiratory neurons on inspiratory neurons.

3. Suprapontial (i.e. "above-pontine") - includes several areas of the diencephalon:
Hypothalamic region - when irritated, causes hyperpnea - an increase in the frequency of respiratory movements and depth of breathing. The posterior group of hypothalamic nuclei causes hyperpnea, the anterior group acts in the opposite way. It is through the respiratory center of the hypothalamus that breathing responds to ambient temperature.
The hypothalamus, together with the thalamus, ensures changes in breathing during emotional reactions.
Thalamus - provides changes in breathing during pain.
Cerebellum - adapts breathing to muscle activity.

4. Motor and premotor cortex cerebral hemispheres. Provides conditioned reflex regulation of breathing. In just 10-15 combinations you can develop a conditioned breathing reflex. Due to this mechanism, for example, athletes experience hyperpnea before a start.
Asratyan E.A. in his experiments he removed these areas of the cortex from animals. During physical activity, they quickly developed shortness of breath - dyspnea, because... they lacked this level of breathing regulation.
The respiratory centers of the cortex enable voluntary changes in breathing.

Regulation of the activity of the respiratory center
The bulbar section of the respiratory center is the main one; it provides automatic breathing, but its activity can change under the influence humoral And reflex influences

Humoral influences on the respiratory center
Frederick's Experience (1890). He cross-circulated the two dogs - each dog's head received blood from the other dog's body. In one dog, the trachea was clamped, consequently, the level of carbon dioxide increased and the level of oxygen in the blood decreased. After this, the other dog began to breathe rapidly. Hyperpnea occurred. As a result, the level of CO2 in the blood decreased and the level of O2 increased. This blood flowed to the first dog's head and inhibited its respiratory center. Humoral inhibition of the respiratory center could lead this first dog to apnea, i.e. stopping breathing.
Factors that humorally affect the respiratory center:
Excess CO2 - hypercarbia, causes activation of the respiratory center.
Lack of O2 - hypoxia, causes activation of the respiratory center.
Acidosis - accumulation of hydrogen ions (acidification), activates the respiratory center.
Lack of CO2 - inhibition of the respiratory center.
Excess O2 - inhibition of the respiratory center.
Alkolosis - +++inhibition of the respiratory center
Due to their high activity, the neurons of the medulla oblongata themselves produce a lot of CO2 and locally influence themselves. Positive feedback (self-reinforcing).
In addition to the direct effect of CO2 on the neurons of the medulla oblongata, there is a reflex effect through the reflexogenic zones of the cardiovascular system (Reimans reflexes). With hypercarbia, chemoreceptors are excited and from them excitation flows to the chemosensitive neurons of the reticular formation and to the chemosensitive neurons of the cerebral cortex.
Reflex effect on the respiratory center.
1. Constant influence.
Gehling-Breuer reflex. Mechanoreceptors in the tissues of the lungs and airways are excited when the lungs expand and collapse. They are sensitive to stretching. From them, impulses along the vagus (vagus nerve) go to the medulla oblongata to the inspiratory L-motoneurons. Inhalation stops and passive exhalation begins. This reflex ensures the change of inhalation and exhalation and maintains the activity of the neurons of the respiratory center.
When the vacus is overloaded and cut, the reflex is canceled: the frequency of respiratory movements decreases, the change in inhalation and exhalation is carried out abruptly.
Other reflexes:
stretching of the lung tissue inhibits subsequent inhalation (expiratory facilitation reflex).
Stretching of the lung tissue during inhalation beyond the normal level causes an additional sigh (Head's paradoxical reflex).
Heymans reflex - arises from the chemoreceptors of the cardiovascular system to the concentration of CO2 and O2.
Reflex influence from the propreoreceptors of the respiratory muscles - when the respiratory muscles contract, a flow of impulses arises from the propreoreceptors to the central nervous system. According to the feedback principle, the activity of inspiratory and expiratory neurons changes. With insufficient contraction of the inspiratory muscles, a respiratory-facilitating effect occurs and inhalation increases.
2. Fickle
Irritant - located in the respiratory tract under the epithelium. They are both mechano- and chemoreceptors. They have a very high irritation threshold, so they work in extraordinary cases. For example, when pulmonary ventilation decreases, lung volume decreases, irritant receptors are excited and cause a forced inhalation reflex. As chemoreceptors, these same receptors are excited by biologically active substances - nicotine, histamine, prostaglandin. There is a feeling of burning, tickling and in response - a protective cough reflex. In case of pathology, irritant receptors can cause spasm of the airways.
in the alveoli, juxta-alveolar and juxta-capillary receptors respond to lung volume and biologically active substances in the capillaries. Increases breathing rate and contracts bronchi.
On the mucous membranes of the respiratory tract there are exteroceptors. Coughing, sneezing, holding your breath.
The skin contains heat and cold receptors. Breath holding and breathing activation.
Pain receptors - short-term holding of breath, then intensification.
Enteroreceptors - from the stomach.
Propreoreceptors - from skeletal muscles.
Mechanoreceptors - from the cardiovascular system.

The rhythmic sequence of inhalation and exhalation, as well as changes in the nature of respiratory movements depending on the state of the body (rest, work of varying intensity, emotional manifestations, etc.) are due to the presence of a respiratory center located in the medulla oblongata (Fig. 27). The respiratory center is a set of neurons that ensure the activity of the respiratory apparatus and its adaptation to changing conditions of the external and internal environment.

Of decisive importance in determining the localization of the respiratory center and its activity were the studies of the domestic physiologist N.A. Mislavsky, who in 1885 showed that the respiratory center in mammals is located in the medulla oblongata on two fourth ventricles in the region of the reticular formation. The respiratory center is a paired, symmetrically located formation, which includes inhalation and exhalation parts.

The results of N. A. Mislavsky’s research formed the basis of modern ideas about the localization, structure and function of the respiratory center. They were confirmed in experiments using microelectrode technology and the removal of biopotentials from various structures of the medulla oblongata. It has been shown that in the respiratory center there are two groups of neurons - inspiratory (inhalation) and expiratory (exhalation). Some peculiarities in the functioning of the respiratory center have been discovered. During quiet breathing, only a small part of the respiratory neurons are active and, therefore, in the respiratory center there is a reserve of neurons, which is used when the body’s increased need for oxygen. It has been established that there are functional relationships between the inspiratory and expiratory neurons of the respiratory center. They are expressed in the fact that when the inspiratory neurons that provide the inhalation phase are excited, the activity of the expiratory nerve cells is inhibited and vice versa. Thus, one of the reasons for the rhythmic, automatic activity of the respiratory center is the interconnected functional relationships between inspiratory and expiratory neurons.

There are other ideas about the localization and organization of the respiratory center, which are supported by a number of Soviet and foreign physiologists. It is believed that the centers of inhalation, exhalation and convulsive breathing are localized in the medulla oblongata. In the upper part of the pons of the brain (pons) there is a pneumotactic center, which controls the activity of the lower inhalation and exhalation centers and ensures the correct alternation of cycles of respiratory movements.

The respiratory center, located in the medulla oblongata, sends impulses to the spinal cord motor neurons that innervate the respiratory muscles. The diaphragm is innervated by axons of motor neurons located at the level of the III-IV cervical segments of the spinal cord. Motor neurons, the processes of which form the intercostal nerves that innervate the intercostal muscles, are located in the anterior horns of the thoracic segments of the spinal cord (III-XII).

Regulation of the activity of the respiratory center

Regulation of the activity of the respiratory center is carried out humorally, due to reflex effects and nerve impulses coming from the overlying parts of the brain.

According to I.P. Pavlov, the activity of the respiratory center depends on the chemical properties of the blood and on reflex influences, primarily from the lung tissue.

Humoral influences. A specific regulator of the activity of neurons in the respiratory center is carbon dioxide, which acts on respiratory neurons directly and indirectly. During the activity of the neurons of the respiratory center, metabolic products (metabolites) are formed in them, including carbon dioxide, which has a direct effect on the inspiratory nerve cells, exciting them. Chemoreceptors sensitive to carbon dioxide were found in the reticular formation of the medulla oblongata near the respiratory center. With an increase in carbon dioxide tension in the blood, chemoreceptors become excited and transmit these excitations to inspiratory neurons, which leads to an increase in their activity. The laboratory of M.V. Sergievsky obtained data indicating that carbon dioxide increases the excitability of neurons in the cerebral cortex. In turn, the cells of the cerebral cortex stimulate the activity of neurons in the respiratory center. In the mechanism of the stimulating effect of carbon dioxide on the respiratory center, an important place belongs to the chemoreceptors of the vascular bed. In the area of ​​the carotid sinuses and aortic arch, chemoreceptors were found that are sensitive to changes in the tension of carbon dioxide and oxygen in the blood.

It has been shown that washing the carotid sinus or aortic arch, which is humorally isolated but with preserved nerve connections, with a liquid containing a high carbon dioxide content is accompanied by stimulation of respiration (Heymans reflex). In similar experiments it was found that an increase in oxygen tension inhibits the activity of the respiratory center.

Cross-circulation experiment (Frederick's experience). The influence of blood gas composition on the activity of neurons in the respiratory center has been proven in an experiment with cross-circulation (Frederick's experiment). To do this, the carotid arteries and jugular veins of two anesthetized dogs are cut and cross-connected (Fig. 28). As a result of the operation, the head of the first dog received blood from the body of the second, and the head of the second dog - from the body of the first. After cross-circulation is established, the trachea of ​​the first dog is clamped, i.e., it is suffocated. As a result, this dog has respiratory arrest, and the second one has severe shortness of breath.

The established facts are related to the fact that an excess amount of carbon dioxide accumulates in the blood of the first dog, which, traveling through the blood to the head of the second dog, stimulates the activity of the neurons of the respiratory center, as a result of which shortness of breath is observed. Due to hyperventilation, the second dog's blood contains an increased amount of oxygen and a decreased amount of carbon dioxide. Entering the head of the first dog, the blood of the second dog, rich in oxygen and poor in carbon dioxide, inhibits the activity of the neurons of the respiratory center, and the first dog experiences respiratory arrest.

From Frederick’s experience it follows that the activity of the respiratory center is stimulated when there is an excess of carbon dioxide in the blood and is inhibited when the oxygen tension increases. Opposite shifts in the activity of the respiratory center are observed with a decrease in the concentration of carbon dioxide and a decrease in oxygen tension in the blood.

The mechanism of the effect of carbon dioxide on the activity of neurons in the respiratory center is complex. Carbon dioxide has a direct effect on respiratory neurons (excitation of cells of the cerebral cortex, neurons of the reticular formation), as well as a reflex effect due to irritation of special chemoreceptors of the vascular bed. Consequently, depending on the gas composition of the internal environment of the body, the activity of the neurons of the respiratory center changes, which is reflected in the nature of respiratory movements.

With optimal levels of carbon dioxide and oxygen in the blood, respiratory movements are observed, reflecting a moderate degree of excitation of the neurons of the respiratory center. These breathing movements of the chest are called eipnea.

Excessive carbon dioxide and lack of oxygen in the blood increase the activity of the respiratory center, which causes frequent and deep respiratory movements - hyperpnea. An even greater increase in the amount of carbon dioxide in the blood leads to disruption of the breathing rhythm and the appearance of shortness of breath - dyspnea. A decrease in the concentration of carbon dioxide and excess oxygen in the blood inhibit the activity of the respiratory center. In this case, breathing becomes shallow, rare and may stop - apnea..

Periodic breathing is a type of breathing in which groups of respiratory movements alternate with pauses. The duration of pauses ranges from 5 to 20 s or even more. With periodic breathing of the Cheyne-Stokes type, after a pause, weak respiratory movements appear, subsequently intensifying. When the maximum is reached, a weakening of breathing is observed again, and then it stops - a new pause occurs. At the end of the pause, the cycle repeats again. Cycle duration is 30-60 s. With a decrease in the excitability of the respiratory center due to a lack of oxygen, other types of periodic breathing are observed.

Causes of a newborn's first breath. In the mother's body, gas exchange in the fetus occurs through the umbilical vessels, which are in close contact with the mother's placental blood. After the birth of the child and its separation from the placenta, this connection is broken. Metabolic processes in the body of a newborn lead to the formation and accumulation of carbon dioxide, which humorally stimulates the respiratory center. In addition, a change in the child’s living conditions leads to excitation of extero- and proprioceptors, which is also one of the mechanisms involved in the occurrence of the first breath.

Reflex influences on the activity of neurons in the respiratory center. The activity of neurons in the respiratory center is strongly influenced by reflex effects. There are constant and non-permanent (episodic) reflex influences on the respiratory center.

Constant reflex influences arise as a result of irritation of the receptors of the alveoli (Hering-Breuer reflex), the root of the lung and pleura (pulmothoracic reflex), chemoreceptors of the aortic arch and carotid sinuses (Heymans reflex), mechanoreceptors of these vascular areas, proprioceptors of the respiratory muscles.

The most important reflex of this group is the Hering-Breuer reflex. The alveoli of the lungs contain stretch and collapse mechanoreceptors, which are sensitive nerve endings of the vagus nerve. Stretch receptors are excited during normal and maximum inspiration, i.e., any increase in the volume of the pulmonary alveoli excites these receptors. Collapse receptors become active only under pathological conditions (with maximum alveolar collapse).

In experiments on animals, it was found that when the volume of the lungs increases (blowing air into the lungs), a reflex exhalation is observed, while pumping air out of the lungs leads to a rapid reflex inhalation. These reactions did not occur during transection of the vagus nerves. Consequently, nerve impulses enter the central nervous system through the vagus nerves.

The Hering-Breuer reflex refers to the mechanisms of self-regulation of the respiratory process, ensuring a change in the acts of inhalation and exhalation. When the alveoli are stretched during inspiration, nerve impulses from stretch receptors travel along the vagus nerve to expiratory neurons, which, when excited, inhibit the activity of inspiratory neurons, which leads to passive exhalation. The pulmonary alveoli collapse, and nerve impulses from the stretch receptors no longer reach the expiratory neurons. Their activity decreases, which creates conditions for increasing the excitability of the inspiratory part of the respiratory center and active inspiration. In addition, the activity of inspiratory neurons increases with an increase in the concentration of carbon dioxide in the blood, which also contributes to the act of inhalation.

Thus, self-regulation of breathing is carried out on the basis of the interaction of the nervous and humoral mechanisms of regulation of the activity of neurons of the respiratory center.

The pulmothoracic reflex occurs when receptors located in the lung tissue and pleura are excited. This reflex appears when the lungs and pleura are stretched. The reflex arc closes at the level of the cervical and thoracic segments of the spinal cord. The final effect of the reflex is a change in the tone of the respiratory muscles, resulting in an increase or decrease in the average volume of the lungs.

Nerve impulses from the proprioceptors of the respiratory muscles constantly flow to the respiratory center. During inhalation, the proprioceptors of the respiratory muscles are excited and nerve impulses from them travel to the inspiratory neurons of the respiratory center. Under the influence of nerve impulses, the activity of inspiratory neurons is inhibited, which promotes the onset of exhalation.

Fickle reflex influences on the activity of respiratory neurons are associated with the excitation of extero- and interoreceptors of various functions.

Non-constant reflex effects that influence the activity of the respiratory center include reflexes that arise from irritation of receptors in the mucous membrane of the upper respiratory tract, nose, nasopharynx, temperature and pain receptors of the skin, proprioceptors of skeletal muscles, interoreceptors. For example, when suddenly inhaling vapors of ammonia, chlorine, sulfur dioxide, tobacco smoke and some other substances, irritation of the receptors in the mucous membrane of the nose, pharynx, and larynx occurs, which leads to a reflex spasm of the glottis, and sometimes even the muscles of the bronchi and a reflex holding of breath.

When the epithelium of the respiratory tract is irritated by accumulated dust, mucus, as well as ingested chemical irritants and foreign bodies, sneezing and coughing are observed. Sneezing occurs when receptors in the nasal mucosa are irritated, and coughing occurs when receptors in the larynx, trachea, and bronchi are stimulated.

Coughing and sneezing begin with a deep breath, which occurs reflexively. Then a spasm of the glottis occurs and at the same time active exhalation. As a result, the pressure in the alveoli and airways increases significantly. The subsequent opening of the glottis leads to the release of air from the lungs into the respiratory tract and out through the nose (when sneezing) or through the mouth (when coughing). Dust, mucus, and foreign bodies are carried away by this stream of air and expelled from the lungs and respiratory tract.

Coughing and sneezing under normal conditions are classified as protective reflexes. These reflexes are called protective because they prevent harmful substances from entering the respiratory tract or promote their removal.

Irritation of the temperature receptors of the skin, in particular Cold receptors, leads to a reflex holding of breath. Excitation of skin pain receptors is usually accompanied by increased respiratory movements.

Excitation of proprioceptors of skeletal muscles causes stimulation of the act of breathing. The increased activity of the respiratory center in this case is an important adaptive mechanism that provides the body with increased oxygen needs during muscular work.

Irritation of interoreceptors, for example mechanoreceptors of the stomach during its distension, leads to inhibition of not only cardiac activity, but also respiratory movements.

When the mechanoreceptors of vascular reflexogenic zones (aortic arch, carotid sinuses) are excited, shifts in the activity of the respiratory center are observed as a result of changes in blood pressure. Thus, an increase in blood pressure is accompanied by a reflex holding of breath, a decrease leads to stimulation of respiratory movements.

Thus, the neurons of the respiratory center are extremely sensitive to influences that cause excitation of extero-, proprio- and interoreceptors, which leads to a change in the depth and rhythm of respiratory movements in accordance with the living conditions of the body.

The influence of the cerebral cortex on the activity of the respiratory center. The regulation of breathing by the cerebral cortex has its own qualitative characteristics. Experiments with direct stimulation of individual areas of the cerebral cortex by electric current showed a pronounced effect on the depth and frequency of respiratory movements. The results of research by M.V. Sergievsky and his colleagues, obtained by direct stimulation of various parts of the cerebral cortex with electric current in acute, semi-chronic and chronic experiments (implanted electrodes), indicate that cortical neurons do not always have a clear effect on breathing. The final effect depends on a number of factors, mainly on the strength, duration and frequency of stimulation used, the functional state of the cerebral cortex and the respiratory center.

Important facts were established by E. A. Asratyan and his colleagues. It was found that animals with the cerebral cortex removed had no adaptive reactions of external respiration to changes in living conditions. Thus, muscle activity in such animals was not accompanied by stimulation of respiratory movements, but led to prolonged shortness of breath and incoordination of breathing.

To assess the role of the cerebral cortex in the regulation of breathing, data obtained using the method of conditioned reflexes are of great importance. If in humans or animals the sound of a metronome is accompanied by inhalation of a gas mixture with a high content of carbon dioxide, this will lead to an increase in pulmonary ventilation. After 10-15 combinations, isolated activation of the metronome (conditioned signal) will cause stimulation of respiratory movements - a conditioned respiratory reflex has been formed to a selected number of metronome beats per unit of time.

The increase and deepening of breathing that occurs before the start of physical work or sports competitions is also carried out through the mechanism of conditioned reflexes. These changes in respiratory movements reflect shifts in the activity of the respiratory center and have adaptive significance, helping to prepare the body for work that requires a lot of energy and increased oxidative processes.

According to M.E. Marshak, cortical regulation of breathing ensures the necessary level of pulmonary ventilation, the rate and rhythm of breathing, and the constancy of the level of carbon dioxide in the alveolar air and arterial blood.

The adaptation of breathing to the external environment and changes observed in the internal environment of the body is associated with extensive nervous information entering the respiratory center, which is pre-processed, mainly in the neurons of the pons (pons), midbrain and diencephalon, and in the cells of the cerebral cortex .

Thus, the regulation of the activity of the respiratory center is complex. According to M.V. Sergievsky, it consists of three levels.

First level of regulation represented by the spinal cord. The centers of the phrenic and intercostal nerves are located here. These centers cause contraction of the respiratory muscles. However, this level of breathing regulation cannot ensure a rhythmic change in the phases of the respiratory cycle, since a huge number of afferent impulses from the respiratory apparatus, bypassing the spinal cord, are sent directly to the medulla oblongata.

Second level of regulation associated with the functional activity of the medulla oblongata. Here is the respiratory center, which receives a variety of afferent impulses coming from the respiratory apparatus, as well as from the main reflexogenic vascular zones. This level of regulation ensures a rhythmic change in the phases of breathing and the activity of spinal motor neurons, the axons of which innervate the respiratory muscles.

Third level of regulation- These are the upper parts of the brain, including cortical neurons. Only in the presence of the cerebral cortex is it possible to adequately adapt the reactions of the respiratory system to the changing conditions of the organism's existence.

Breathing during physical work

Physical activity is accompanied by significant changes in the activity of organs and physiological systems of the body. Increased energy expenditure is ensured by an increase in oxygen utilization, which leads to an increase in the content of carbon dioxide in body fluids and tissues. Shifts in the chemical composition of the internal environment of the body cause an increase in the functional activity of the respiratory system. Thus, in trained people during intense muscular work, the volume of pulmonary ventilation increases to 5·10 -2 m 3 and even to 1·10 -1 m 3 (50 and even 100 l/min) compared to 5·10 -3 -8 ·10 -3 m 3 (5-8 l/min) in a state of relative physiological rest.

An increase in minute volume of breathing during physical activity is associated with an increase in the depth and frequency of respiratory movements. At the same time, in trained people, the depth of breathing mainly changes, in untrained people, the frequency of respiratory movements changes.

Shifts in the functional activity of the respiratory system during physical activity are determined by nervous and humoral mechanisms. During physical activity, the concentration of carbon dioxide and lactic acid in the blood and tissues increases, which stimulate the neurons of the respiratory center both humorally and through nerve impulses coming from vascular reflexogenic zones. In addition, the neurons of the respiratory center are stimulated by nervous influences coming from the proprioceptors of the respiratory and skeletal muscles. Finally, the activity of the neurons of the respiratory center is ensured by the flow of nerve impulses coming from the cells of the cerebral cortex, which are highly sensitive to a lack of oxygen and excess carbon dioxide.

Simultaneously with changes in the respiratory system during physical activity, adaptive reactions occur in the cardiovascular system. The frequency and strength of heart contractions increase, blood pressure rises, vascular tone is redistributed - the vessels of working muscles dilate and the vessels of other areas narrow. In addition, an additional number of capillaries open in working organs and blood is released from the depot.

The cerebral cortex plays a significant role in coordinating the functions of organs and physiological systems during physical activity. Thus, in the pre-start state, athletes experience an increase in the strength and frequency of heart contractions, pulmonary ventilation increases, and blood pressure rises. Consequently, the conditioned reflex mechanism is one of the most important nervous mechanisms of adaptation of the body to changing environmental conditions.

The respiratory system provides the body with increased oxygen needs. The circulatory and blood systems, being reconstructed to a new functional level, promote the transport of oxygen to the tissues and carbon dioxide to the lungs.

For the normal course of tissue metabolism, the O content is especially important 2 and CO 2 in arterial blood.

Regulation of external respiration

Ventilation is the process of updating the gas composition of alveolar air, which ensures the supply of oxygen and the removal of carbon dioxide. This process is carried out by the rhythmic work of the respiratory muscles, changing the volume of the chest. The intensity of ventilation is determined by the depth of inspiration and breathing frequency. Thus, the minute volume of respiration is an indicator of pulmonary ventilation, which should ensure the gas homeostasis that is necessary in a specific situation (rest, physical work). Regulation of external respiration is the process of changing the minute volume of respiration in various conditions to ensure the optimal gas composition of the internal body environment.

In the second half of the 19th century, a hypothesis appeared that the main factors in the regulation of respiration are the partial pressure of oxygen and carbon dioxide in the alveolar air and, consequently, in the arterial blood. Experimental evidence that enrichment of arterial blood with carbon dioxide and depletion of oxygen enhances ventilation of the lungs as a result of the resulting excitation of the respiratory center was obtained in Frederick's classic experiment with cross-circulation in 1890 (Figure 13). In two anesthetized dogs, the carotid arteries and separately the jugular veins were cut and connected. After such connection and ligation of the vertebral arteries, the head of the first dog was supplied with blood from the second and vice versa. If the trachea was blocked in the first dog and asphyxia was caused in this way, then the second dog developed hyperpnea- increased pulmonary ventilation. In the first dog, despite the increase in carbon dioxide tension in the blood and the decrease in oxygen tension, after some time there occurred apnea- cessation of breathing movements. This is explained by the fact that the carotid artery of the first dog receives blood from the second dog, in which, as a result of hyperventilation, the carbon dioxide content in the arterial blood decreases. Even then it was established that the regulation of breathing occurs through feedback: deviations in the gas composition of arterial blood lead, by influencing the respiratory center, to such changes in breathing that reduce these deviations.

Figure 13. Schematic of Frederick's cross-circulation experiment.

Tracheal compression in Dog A causes shortness of breath in Dog B. Shortness of breath in Dog B causes breathing to slow and stop in Dog A

At the beginning of the 19th century, it was shown that in the medulla oblongata at the bottom of the fourth ventricle there are structures, the destruction of which by a needle prick leads to the cessation of breathing and the death of the body. This small area of ​​the brain in the lower corner of the rhomboid fossa was called the respiratory center.

Numerous studies have established that changes in the gas composition of the internal environment affect the respiratory center not directly, but by influencing special chemosensitive receptors located in the medulla oblongata - central (medullary) chemoreceptors and in vascular reflexogenic zones - peripheral (arterial) chemoreceptors.

During evolutionary development, the main function in stimulating the respiratory center moved from peripheral chemoreceptors to central ones. We are talking primarily about bulbar chemosensitive structures that respond to changes in the concentration of hydrogen ions and CO tension 2 in the extracellular fluid of the brain. Behind peripheral, arterial chemoreceptors, which are also excited when CO tension increases 2 , and with a decrease in oxygen tension in the blood washing them, only an auxiliary role in stimulating breathing remained.

Therefore, let us first consider the central chemoreceptors, which have a more pronounced effect on the activity of the respiratory center.

The main humoral stimulator of the respiratory center is excess carbon dioxide in the blood, as demonstrated in the experiments of Frederick and Holden.

Frederick's experience on two dogs with cross circulation. In both dogs (first and second), the carotid arteries are cut and cross-connected. The same is done with the jugular veins. The vertebral arteries are ligated. As a result of these operations, the head of the first dog receives blood from the second dog, and the head of the second dog from the first. The first dog's trachea is blocked, which causes hyperventilation (fast and deep breathing) in the second dog, whose head receives blood from the first dog, depleted in oxygen and enriched in carbon dioxide. The first dog has apnea; blood enters its head with a lower CO2 voltage and approximately the usual, normal content of 02 - hyperventilation washes out CO2 and has virtually no effect on the content of 02 in the blood, since hemoglobin is saturated

0 2 almost completely and without hyperventilation.

The results of Frederick's experiment indicate that the respiratory center is excited either by an excess of carbon dioxide or by a lack of oxygen.

In Holden's experiment in a closed space from which CO 2 is removed, breathing is weakly stimulated. If CO2 is not removed, shortness of breath is observed - increased and deepening of breathing. Later it was proven that an increase in CO 2 content in the alveoli by 0.2% leads to an increase in lung ventilation by 100%. An increase in the content of CO 2 in the blood stimulates respiration both due to a decrease in pH and the direct effect of CO 2 itself.

The effect of CO 2 and H + ions on respiration is mediated mainly by their effect on special structures of the brain stem that are chemosensitive (central chemoreceptors). Chemoreceptors that respond to changes in the gas composition of the blood are found externally in the walls of blood vessels in only two areas - in the aortic arch and the sinocarotid region.

The role of aortic and sinocarotid chemoreceptors in the regulation of respiration has been shown experimentally with voltage reduction 0 2 in arterial blood (hypoxemia) below 50-60 mm Hg. Art. - at the same time, ventilation of the lungs increases within 3-5 s. Such hypoxemia can occur when climbing to a height, with cardiopulmonary pathology. Vascular chemoreceptors are also excited under normal blood gas tension; their activity increases greatly during hypoxia and disappears when breathing pure oxygen. Stimulation of respiration when voltage decreases 0 2 is mediated exclusively by peripheral chemoreceptors. Carotid chemoreceptors are secondary - these are bodies synaptically connected to the afferent fibers of the carotid nerve. They are excited by hypoxia, a decrease in pH and an increase in Pco 2, while calcium enters the cell. Their mediator is dopamine.

The aortic and carotid bodies are also excited when the CO2 voltage increases or when the pH decreases. However, the effect of CO 2 from these chemoreceptors is less pronounced than the effect of 0 2 .

Hypoxemia (decreased partial pressure of oxygen in the blood) stimulates breathing much more if it is accompanied hypercapnia, which is observed during very intense physical work: hypoxemia increases the response to CO 2. However, with significant hypoxemia, due to a decrease in oxidative metabolism, the sensitivity of central chemoreceptors decreases. Under these conditions, a decisive role in stimulating respiration is played by vascular chemoreceptors, the activity of which increases, since for them an adequate stimulus is a decrease in 0 2 tension in the arterial blood (emergency mechanism for stimulating respiration).

Thus, vascular chemoreceptors respond predominantly to a decrease in oxygen levels in the blood, central chemoreceptors - to changes in the blood and cerebrospinal fluid pH and PCO g

The importance of pressoreceptors of the carotid sinus and aortic arch. An increase in blood pressure increases afferent impulses in the sinocarotid and aortic nerves, which leads to some depression of the respiratory center and a weakening of pulmonary ventilation. On the contrary, breathing increases somewhat with a decrease in blood pressure and a decrease in afferent impulses into the brain stem from vascular pressoreceptors.