Regulation of breathing called the process of controlling pulmonary ventilation, aimed at maintaining the respiratory constants of the internal environment of the body and adapting breathing to changing conditions of the external and internal environment.
In the process of regulating breathing, its frequency, depth, minute volume and blood circulation adapt to the changing needs of metabolism and to the implementation of some other body functions (speech, crying, screaming, coughing, swallowing).
It was previously noted that the start of each respiratory cycle is carried out by the inspiratory section of the respiratory center, which sends a stream of nerve impulses to the spinal cord and from it to the inspiratory muscles. The frequency of respiratory cycles is determined by the frequency of sending nerve impulses. The depth of breathing, or tidal volume, is determined by the force of contraction respiratory muscles, which depends on the number of nerve impulses in a separate series (packet) of impulses sent by the respiratory center to start the respiratory cycle. Thus, the regulation of frequency, depth of breathing and ventilation of the lungs ultimately comes down to changes in the activity of neurons of the respiratory center and its parts and is carried out by one of the functional systems of the body.
The activity of the functional system of respiratory regulation is aimed at achieving the final useful result - maintaining the respiratory constants of the internal environment of the body at the proper level. Its simplified diagram is shown in Fig. 1. These constants are the oxygen tension in arterial blood(р0 2), the carbon dioxide tension in it (рС0 2) and the pH of arterial blood and cerebrospinal fluid. Normal level p0 2 of arterial blood with hemoglobin oxygenation of 94-98% is 95-100 mm Hg. Art., рС0 2 - 35-45 mm Hg. art., arterial blood plasma pH - 7.36-7.44 (in erythrocytes - 7.25-7.30), cerebrospinal fluid pH - 7.35-7.40.
Rice. 1. Scheme of the functional system for regulating oxygen tension, carbon dioxide and the acid-base state of the internal environment: 1, 2, 3 - signaling from extero-, intero- and proprioceptors; MNGR - mechanisms neurohumoral regulation
Thus, the breathing regulation system controls three indicators at once. These systems in cybernetics are called multi-parametric interconnected control systems and are considered very complex. The main structural components of the functional system of respiratory regulation are chemoreceptors, the respiratory center, mechanisms of neurohumoral regulation of breathing, and executive (effector) mechanisms. They contribute to the impact on the gas composition and pH, mechanisms feedback, with the help of which the effectiveness of breathing regulation is assessed (Fig. 1).
Rice. Regulation external respiration(per minute volume of respiration) a - effect of pCO2 - hypercapnic stimulus, b - pH indicator; c — pO2 — hypoxic stimulus
Chemoreceptors, designed to assess the tension of oxygen, carbon dioxide, pH of arterial blood and cerebrospinal fluid, are located in the vessels and in the medulla oblongata. They send information about the gas composition to the respiratory, vasomotor centers and other structures of the central nervous system. The respiratory center is represented various groups neurons located predominantly in the medulla oblongata and pons. Some of these neurons have the ability to spontaneously excite rhythmically and form a flow of efferent nerve impulses that set a certain frequency and depth of breathing. The activity of neurons in the respiratory center is modulated by the flow of afferent signals entering the respiratory center from chemoreceptors and other receptors of the body, as well as from neurons in the cortex, limbic and other areas of the brain. As a result, a different pattern of activity of the neurons of the respiratory center is formed, adapting breathing to the nature of the current functional activity and the changing metabolic needs of the body.
Table. Major chemoreceptors
Effector tissues and mechanisms in the functional system of respiratory regulation are respiratory muscles, providing external respiration, heart, smooth myocytes of the walls of blood vessels and bronchi, blood, mechanisms of formation and destruction of red blood cells and hemoglobin, buffer systems and mechanisms for the release of acidic or alkaline products kidneys and gastrointestinal tract, metabolism in cells and tissues. The effectiveness of adaptive changes in breathing is assessed using feedback mechanisms.
Breathing is one of the vegetative functions, which has arbitrary regulation. Each person can arbitrarily change the rhythm and depth of breathing, hold it for a certain time (from 20-60 to 240 s). The possibility of voluntary changes in breathing indicates the regulatory influence of the cortex cerebral hemispheres for this function.
Vivid evidence of cortical regulation of breathing was obtained by the method of conditioned reflexes. The conditioned breathing reflex can be developed to the action of any external stimulus, if combined with some unconditioned breathing reflex.
G.P. Conradi and Z.P. Grandmothers used inhalation of a gas mixture with a high content of carbon dioxide as an unconditional stimulus (this increases pulmonary ventilation). Inhalation of the mixture was preceded by the sound of a metronome for 5-10 s. After
10-15 combinations of inhalation of the mixture and the sound of a metronome, one sound of the metronome (without inhalation of the mixture) caused an increase in pulmonary ventilation.
Pre-start changes in breathing in athletes are also an indicator of its conditioned reflex regulation. Its significance in this case lies in the body’s adaptation to increased physical activity, requiring increased gas exchange. Pre-start change (increase) in the depth and frequency of breathing (simultaneously with a change in activity cardiovascular system) ensures faster delivery of oxygen to working muscles and removal of carbon dioxide from the blood.
The regulation of breathing was formed in humans during the process of evolution in connection with the formation of speech. Pronunciation is carried out while exhaling, therefore, in order to speak, it is necessary to change the depth and rhythm of breathing, thanks to which one can achieve recitation, singing, etc.
Regulation of breathing is the adaptation of pulmonary ventilation to the needs of the body. Breathing regulation is carried out reflexively and includes several mechanisms.
The main role belongs to the respiratory center, which is a collection of cells located in different departments central nervous system and providing coordinated rhythmic respiratory muscle activity to adapt breathing to changes in the external and internal environment of the body.
Rice. 2. Nervous and humoral regulation breathing
The respiratory center of the brain is represented by the inspiratory center (a group of nerve cells that control inhalation), the expiratory center (exhalation center) and the pneumotaxic center, which regulates the functioning of the inspiratory and expiratory centers. The centers of inhalation and exhalation are located in the medulla oblongata, and the pneumotaxic center is in the upper part of the pons of the midbrain.
Nerve impulses arising in the inspiratory center of the medulla oblongata are transmitted to subordinate motor centers spinal cord or motor centers of the vagus and facial nerves. During normal breathing, regulatory impulses from the center of inspiration enter the intercostal muscles and the diaphragm, causing them to contract, which leads to an increase in volume chest and the entry of air into the lungs. An increase in lung volume excites stretch receptors located in the walls of the lungs, impulses from them travel along the centripetal nerves to the center of exhalation. Irritation of the neurons of this center suppresses the activity of the neurons of the inhalation center, and the flow of nerve impulses to the respiratory muscles stops. The intercostal muscles relax, the volume of the chest cavity decreases and air from the lungs is forced out.
It plays an important role in the regulation of breathing, especially during behavioral acts. For example, the hypothalamic effect on the respiratory center is manifested by activation of breathing during painful stimulation, during physical work, with emotional excitement.
The activity of the respiratory center is also influenced by signals coming from the upper respiratory tract. Receptors in the nasal passages are innervated by the olfactory and trigeminal cranial nerves, and they are sensitive to various chemicals as well as mechanical stimuli. Responses to their stimulation range from apnea to sneezing. The pharyngeal zone is innervated by a branch of the glossopharyngeal nerve. Stimulation of this area causes sharp inhalations. The larynx and trachea contain receptors of various types that respond to chemical and mechanical stimulation. They are innervated primarily by branches of the vagus nerve. Their stimulation has different effects. When inhaling, the incoming air flow irritates the receptors of the nasal mucosa, impulses from the receptors are sent to the brain along the fibers trigeminal nerve and have a weak inhibitory effect on the respiratory center.
The lungs have three types of receptors innervated by the vagus nerve, the so-called pulmonary stretch receptors.
Respiration is also influenced by arterial receptors. Thus, mechanoreceptors are localized in the arterial and venous systems of the systemic circulation, and when excited, various reactions occur. If blood pressure increases, irritation of the pressor receptors of the carotid sinus and aortic arch increases, which is accompanied by a slight inhibition of the activity of the respiratory center and a decrease in ventilation of the lungs. When decreasing blood pressure, due to the weakening of the irritation of these receptors, ventilation of the lungs, on the contrary, increases.
A certain importance in the act of breathing is given to stretch proprioceptors, which lie in the muscles of the diaphragm, abdominal wall, intercostal muscles, as well as irritant receptors located in the epithelium and subepithelial layer of all airways.
Adaptation of breathing to the external environment and changes observed in the internal environment of the body is associated with a variety of nervous information, entering the inspiratory center, which is preliminarily analyzed in the neurons of the pons, midbrain and diencephalon, as well as in the cells of the cerebral cortex.
The determining factor influencing the level of respiratory movements in the body is blood carbon dioxide concentration. An increase in CO content increases the excitability of the structures of the respiratory and pneumotaxic centers, resulting in increased breathing. The first breath in newborns is also associated with an increase in the concentration of CO 2 in the blood after separation from the umbilical cord. The concentration of C0 2, having reached a threshold value, activates nerve structures respiratory center, and the newborn begins to breathe atmospheric air.
The stimulating effect of the increased content of carbon dioxide in the blood is due not only to its direct effect on the cells of the respiratory center, but also to the indirect reflex effect on the respiratory rhythm from the chemoreceptors of the reflexogenic zones.
There are two groups of chemoreceptors that regulate breathing: peripheral (arterial) and central (medullary). Arterial chemoreceptors are located in the carotid sinuses and aortic arch. They are located in special small bodies, abundantly supplied with arterial blood.
Carotid chemoreceptors are most important in the regulation of breathing. Aortic chemoreceptors have little effect on breathing; they are primarily involved in the regulation of blood circulation.
Chemoreceptors of the carotid and aortic bodies react sensitively to a decrease in oxygen levels in the blood by sending afferent signals. In addition, the afferent influences of chemoreceptors increase with an increase in the content of carbon dioxide and the concentration of hydrogen ions in the arterial blood.
The functional activity of chemoreceptors is under the control of the nervous system. Thus, when the effector parasympathetic fibers are irritated, the sensitivity of the chemoreceptors decreases, and when the sympathetic ones are irritated, it increases. It is the chemoreceptors that signal to the respiratory center about the levels of oxygen and carbon dioxide in the blood. Central chemoreceptors are located in the medulla oblongata. They respond to changes in the pH of the cerebrospinal fluid. Central chemoreceptors have a stronger effect on the activity of the respiratory center than peripheral ones.
Even lower, but with physical activity it increases 5-10 times compared to the resting state.
Regulating breathing is very complex. The acts of inhalation and exhalation, the depth of breathing change automatically depending on the person’s condition. Muscles performing breathing movements, work closely together. This relationship is regulated by neural and humoral pathways.
Coordinated respiratory movements are controlled from the respiratory center in the medulla oblongata. It consists of two halves connected to each other by jumpers. Each half coordinates the corresponding half of the chest. This can be proven experimentally on a cat by splitting its medulla oblongata along the midline. Then the right and left halves of the chest begin to breathe independently and with a special rhythm.
The respiratory center sends impulses to the respiratory muscles not directly, but through the corresponding centers of the spinal cord.
Associated with the respiratory center are: motor nerves - the facial nerve for the wings of the nose, nerves for the bronchi and larynx (the vagus narrows the bronchi, the sympathetic dilates them), spinal nerves: phrenic (from the cervical spinal cord) and costal for pectoral muscles(Fig. 73).
The sensory nerves involved in the act of breathing come from the nose and nasopharyngeal space in the trigeminal and olfactory nerves, from the trachea, bronchi and deep parenchyma of the lungs themselves in the vagus nerve.
Currently, the respiratory center distinguishes between areas whose irritation stimulates inhalation (the so-called inhalation center) and areas that stimulate exhalation (the so-called exhalation center).
Nerve impulses from the breathing center in the medulla oblongata arrive every 4-5 seconds. V nerve centers, regulating the movements of the diaphragm and intercostal muscles, which are located in the cervical and thoracic spinal cord, and cause their excitation. This excitation, transmitted along nerve fibers, moves the diaphragm and intercostal muscles. In this way it is carried out automatic regulation processes of inhalation and exhalation.
The higher center, which regulates breathing, is located in the cerebral cortex. With the participation of this higher center, a person can voluntarily hold his breath for a certain time, however excess accumulation carbon dioxide as a result of holding the breath causes strong stimulation of the respiratory center in the medulla oblongata and breathing automatically resumes.
The higher respiratory center coordinates the frequency and depth of respiratory movements during various states person, that is, while talking, singing, performing physical exercise, walking. Under the influence of emotions - anger, fear, etc. - breathing quickens, and in case of fear or pain it may even stop. In the higher center of the cerebral cortex, conditioned breathing reflexes are formed.
The Hering-Breuer reflex is one of the most important reflexes providing self-regulation respiratory process, which ensures a change in the acts of inhalation and exhalation. Sensitive fibers of the vagus nerve branch in the lung parenchyma; they participate in building the correct breathing rhythm. These are two types of fibers. Some become irritated when the lungs expand during inhalation, their excitation reaches the medulla oblongata and inhibits the center of inhalation, exhalation occurs. Other fibers become excited when the lungs collapse and reflexively cause inhalation.
The activity of the respiratory center is also affected by irritation of the receptors of the sinocarotid node and the aortic zone as a result of excess carbon dioxide or lack of oxygen, which causes a deepening and some increase in breathing.
Reflexes from the airways are of great importance for the normal functioning of the respiratory system. In the upper respiratory tract, the air is warmed, saturated with water vapor and cleared of dust and bacteria. This is facilitated by the narrowness of these pathways and constant hyperemia of the mucous membrane. A reindeer, forced to breathe deeply and forcefully when running fast, has a special device in the trachea in the form of nodules made of blood vessels that significantly warm cold air.
The nasal mucosa is very sensitive. The sensitivity in it is varied - thermal, pain, tactile, pressure, etc. and is higher than on the skin. When the nasal mucosa is irritated, a number of secretory, vascular, and motor reflexes are evoked. Mechanical irritation of the nasal mucosa leads to a sneezing reflex, but severe irritation can lead to respiratory arrest. Reflexes that arise when the nasal mucosa is irritated have a great influence on the body, since it is unimpeded, free nasal breathing provides normal course many processes.
Of great importance are reflexes coming from the larynx, the sensitive nerve of which is the anterior laryngeal nerve. The mucous membrane of the respiratory tract is lined with ciliated epithelium, which transports particles that accidentally enter there to the larynx. Irritation of the larynx by coarse particles causes a cough reflex - a strong exhalation while simultaneously narrowing the glottis. When coughing, a strong stream of air removes irritating particles from the trachea.
The experience with cross-circulation is indicative in this regard. The carotid arteries of two dogs are cut and connected to each other so that the blood from the body of the first dog flows into the head of the other, and the blood of the second into the head of the first. If you now create an obstacle to the normal breathing of the first dog (for example, by squeezing the trachea), then carbon dioxide will accumulate in its blood. This will increase the second dog's breathing. As a result of increased breathing, the carbon dioxide content in her blood will decrease, which can lead the first dog into a state of apnea.
Hypercapnia is an increase in carbon dioxide levels in the blood.
The irritant of the respiratory center is a shift in the blood reaction to the acidic side, which occurs when there is a lack of oxygen or when there is an excess of carbon dioxide in the blood - hypercapnia.
Hypercapnia leads to excitation of the respiratory center in the medulla oblongata, resulting in increased breathing. Material from the site
An excellent example of the effect of hypercapnia on the respiratory center is the mechanism of the first breath. The fetus in the womb is in a state of apnea, since carbon dioxide does not accumulate in its blood, but continuously passes into the mother’s blood. The cessation of placental circulation leads to the first breath, the root cause of which is the lack of oxygen in the newborn’s blood and the accumulation of carbon dioxide in it.
Hypercapnia may occur if large number people will be in a room with closed doors and windows for a long time, as a result of which the carbon dioxide content in the air will increase. When breathing this air, the content of carbon dioxide in the blood of students will increase, which will lead to strong stimulation of the respiratory center and increased breathing. If the classroom is not immediately ventilated, students may experience dizziness, drowsiness, yawning, general weakness, shortness of breath and other undesirable phenomena.
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, to regulate the change in inhalation and exhalation higher value have receptors sensitive to lung stretch (mechanical irritation).
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 combined common name respiratory center (RC). The neurons of the respiratory center of the medulla oblongata are 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 arrive through the motor neurons of the spinal cord, the axons that make up the phrenic, external intercostal and intercartilaginous nerves that innervate the main inspiratory muscles. 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 it 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 various types 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 the rhythm of external respiration through a nervous (reflex) mechanism. However, it is known that the intensity of respiration 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 has in full the possibility of adapting the breathing intensity to changing living conditions will remain, which will be carried out according to an unconditional reflex type with the participation of the 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 value 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. The leading importance in determining the amount of pulmonary ventilation is the tension of carbon dioxide in the arterial blood; it, as it were, creates 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 decreased ventilation and then 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 vertebral artery zone (PA). 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, having a 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 CO 2 diffusion from the blood into cerebrospinal fluid and further into the brain tissue it takes more time. 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.
The lungs are located in the chest cavity. Muscle movements that change the volume of this cavity cause air to move into and out of the lungs, alternately increasing or decreasing the volume of the chest cavity. This is due to the rhythmic contractions of the respiratory muscles, as a result of which inhalation and exhalation are carried out - the entry and removal of air from the lungs, their ventilation.
When inhaling the intercostal muscles raise the ribs, and the diaphragm, contracting, becomes less convex, as a result, the volume of the chest increases, the lungs expand, the air pressure in them becomes lower than atmospheric pressure and the air rushes into the lungs - a calm inhalation occurs. When you inhale deeply, in addition to the external intercostal muscles and the diaphragm, the muscles of the chest and shoulder girdle simultaneously contract.
When exhaling the intercostal muscles and the diaphragm relax, the ribs descend, the convexity of the diaphragm increases, as a result the volume of the chest decreases, the lungs compress, the pressure in them becomes higher than atmospheric pressure and air rushes out of the lungs - a calm exhalation occurs. Deep exhalation is caused by contraction of the internal intercostal and abdominal muscles.
Thus, the rhythmic increase or decrease in the volume of the chest cavity acts as a mechanical pump, forcing air into and out of the lungs.
The speed and strength of breathing movements are extremely finely regulated nervous system throughout a person’s life: from the moment of his birth until his death. The consistency, coordination, and rhythmicity of contractions and relaxations of the respiratory muscles are determined by impulses arriving through the nerves from the respiratory center of the medulla oblongata.
I.M. Sechenov established in 1882 that approximately every 4 seconds, excitations automatically arise in the respiratory center, ensuring the alternation of inhalation and exhalation. The respiratory center not only regulates the rhythmic alternation of inhalation and exhalation, but is also able to change the frequency and depth of respiratory movements, adapting pulmonary ventilation to the needs of the body, thereby ensuring the optimal content of gases in the blood.
The nervous mechanisms of self-regulation of breathing are manifested in the fact that inhalation reflexively causes exhalation, and exhalation causes inhalation. This happens because during inhalation when stretching lung tissue in the nerve receptors located in it, excitation occurs, which is transmitted to the medulla oblongata and causes activation of the exhalation center and inhibition of the inhalation center, forming the respiratory center.
The contraction of the respiratory muscles stops, they relax, and exhalation occurs. When you exhale, the flow of impulses from the receptors stops, the exhalation center ceases to be activated, the inhalation center disinhibits, becomes more active and inhalation occurs.
Humoral regulation of breathing consists in the fact that an increase in the concentration of carbon dioxide in the blood excites the respiratory center - the frequency and depth of breathing increase. A decrease in carbon dioxide content in the blood reduces the excitability of the respiratory center - the frequency and depth of breathing decreases.
Breathing is very closely related to blood circulation. Increasing your breathing rate can promote blood circulation. The deeper the breath, the more the pressure in the chest cavity decreases. This drop in pressure not only forces air into the lungs, but also causes blood to flow back to the heart from the veins located in the lungs. various parts bodies. Sitting or standing still for a long time can cause a deep and involuntary sigh, causing large amounts of blood to flow to the heart and thus promoting blood circulation.
Forms of respiratory activity are sneezing and coughing. They are regulated by protective breathing reflexes.
Sneezing- this is a strong and very fast reflex exhalation through the nostrils, resulting from irritation of the receptors of the mucous membrane of the nasal cavity. When sneezing, substances that irritate (dust, substances with pungent odor etc.).
Cough- a sharp reflex exhalation through the mouth, resulting from irritation of the receptors of the larynx.
The vital capacity of the lungs consists of tidal volume, inspiratory reserve volume and expiratory reserve volume. Tidal volume is the amount of air entering the lungs in one breath. At rest, it is approximately 0.5 liters and corresponds to the volume of exhaled air during one exhalation. If, after a calm inhalation, you take a strong additional breath, then another 1.5 liters (1500 cm 3) of air can enter the lungs, which constitutes the inspiratory reserve volume. After a calm exhalation, you can exhale another 1.5 liters of air at maximum tension. This amount is called expiratory reserve volume.
Thus, tidal volume (0.5 l) + inspiratory reserve volume (1.5 l) + expiratory reserve volume (1.5 l) constitute the vital capacity of the lungs. Its indicators range from 3.5 l to 4.8 l in men and from 3.0 l to 3.5 l in women.
The largest amount of air that a person can exhale after taking the deepest breath is called the vital capacity of the lungs.
In physically healthy, trained people, the vital capacity of the lungs reaches 6.0-7.0 liters. Vital capacity is measured using a spirometer.
Artificial respiration is used when providing first aid to drowned people, in case of injury electric shock, lightning, poisoning carbon monoxide and other accidents. Artificial respiration allows you to resume the activity of the respiratory center and save a person from death. To do this, it is necessary to ensure permeability respiratory tract, clearing the mouth and throat of foreign bodies.
For example, when rescuing a drowned person, you first need to remove water from his respiratory tract. To do this, the rescuer, standing on one knee, places the victim on his thigh so that his head and upper part their bodies hung down. Next, they open the mouth of the drowning person and, patting him on the back, remove water from the respiratory tract.
Then the victim needs to be laid on his back, on a hard horizontal surface, freed from pressing parts of clothing and artificial respiration performed, which is best done together.
There are several methods of artificial respiration:
Insufflation is performed at intervals of 4–5 s, that is, 12–16 times per minute. The duration of exhalation should be twice as long as inhalation. Simultaneously with artificial respiration perform a heart massage if it stops.
To do this, the massager places his palm on the lower third of the sternum, places the other palm on top at a right angle, and makes jerky pressure on the sternum. The massage rate is 60 compressions per minute for an adult, 70–80 for children under 12 years of age.
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. At normal conditions breathing movements occur automatically, without control from our will. But when the nerves to the diaphragm (phrenic nerves) and intercostal muscles are cut or damaged (for example, 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 for it to cause any harm 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 nerve cells located in these walls, 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. Severe 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 safety 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”; thanks to this 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. The 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 you place it in the chamber, in addition, chemical substance, capable of quickly absorbing the released carbon dioxide so that its 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 given air to breathe normal amount oxygen, but with an increased content of 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 internal carotid artery (arteria carotid) there is a small swelling called the carotid sinus, which contains receptors that are sensitive to changes chemical composition blood. 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 ordinary 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. It's more effective way achieving the same goal
According to metabolic needs respiratory system provides gas exchange of O2 and CO2 between environment and the body. This vital function is regulated by 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 conditions external environment. The structures necessary for the emergence of the 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 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 studies have shown 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 departments The CNS 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 neural pathways and function in harmony. 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 breath control is a very 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 smooth muscles 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 narcotic substances, tobacco smoke, water causes constriction of the bronchi, bradycardia, decreased cardiac output, 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 centers of the brain 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 the regulation of respiration 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 carry out central mechanism breathing. 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.
