Peculiarities of the nervous regulation of blood circulation (baroreceptors, chemoreceptors, alpha-adrenergic receptors and beta-adrenergic receptors). The role of baroreceptors in the regulation of blood pressure Baroreceptors of the aortic arch and carotid sinus

In addition to the significant rise in blood pressure during exercise and stress, the autonomic nervous system provides continuous control over blood pressure levels through multiple reflex mechanisms. Almost all of them operate on the principle of negative feedback.

The most studied neural control mechanism blood pressure is the baroreceptor reflex. The baroreceptor reflex occurs in response to irritation of stretch receptors, which are also called baroreceptors or pressoreceptors. These receptors are located in the wall of some large arteries large circle blood circulation. An increase in blood pressure leads to stretching of the baroreceptors, signals from which enter the central nervous system... The feedback signals are then directed to the centers of the autonomic nervous system, and from them to the vessels. As a result, the pressure drops to normal levels.

Baroreceptors are branched nerve endings located in the wall of arteries. They get excited when stretched. A number of baroreceptors are found in the wall of almost every major artery in the chest and neck. However, especially many baroreceptors are found: (1) in the wall of the internal carotid artery near the bifurcation (in the so-called carotid sinus); (2) in the wall of the aortic arch.

Signals from the carotid baroreceptors are carried along Hering's very fine nerves to the glossopharyngeal nerve in the upper neck, and then along the bundle of a solitary tract to the medullary part of the brainstem. Signals from the aortic baroreceptors located in the aortic arch are also transmitted along the fibers of the vagus nerve to the bundle of the solitary tract of the medulla oblongata.

Reaction of baroreceptors to pressure changes. Different levels of blood pressure affect the frequency of impulses traveling along Hering's carotid sinus nerve. Synocarotid baroreceptors are not excited at all if the pressure is from 0 to 50-60 mm Hg. Art. When the pressure changes above this level, the impulse in nerve fibers increases progressively and reaches its maximum frequency at a pressure of 180 mm Hg. Art. The aortic baroreceptors form a similar response, but begin to be excited at a pressure of 30 mm Hg. Art. and higher.

The slightest deviation of blood pressure from the normal level (100 mm Hg) is accompanied by a sharp change in impulses in the fibers of the carotid sinus nerve, which is necessary to return blood pressure to normal levels. Thus, the baroreceptor feedback mechanism is most effective in the pressure range in which it is needed.

Baroreceptors respond extremely quickly to changes in blood pressure. The frequency of generation of impulses in fractions of a second increases during each systole and the decrease in arteries causes a reflex decrease in blood pressure both by reducing peripheral resistance and by decreasing cardiac output... Conversely, with a decrease in blood pressure, an opposite reaction occurs, aimed at increasing blood pressure to normal levels.

The ability of baroreceptors to maintain relatively constant blood pressure in the upper torso is especially important when a person stands up after being in a horizontal position for a long time. Immediately after standing up, blood pressure in the vessels of the head and upper torso decreases, which could lead to loss of consciousness. However, a decrease in pressure in the baroreceptor region immediately induces a sympathetic reflex response that prevents a decrease in blood pressure in the vessels of the head and upper body.

7) Vasopressin... Vasopressin, or the so-called antidiuretic hormone, is a vasoconstrictor hormone. It is formed in the brain, in the nerve cells of the hypothalamus, then transported along the axons of the nerve cells to the posterior lobe of the pituitary gland, where it is secreted into the blood as a result.

Vasopressin could have a significant effect on circulatory function. However, a very small amount of vasopressin is normally secreted, so most physiologists believe that vasopressin does not play a significant role in the regulation of blood circulation. However, experimental studies have shown that the concentration of vasopressin in the blood after severe blood loss increases so much that it causes an increase in blood pressure of 60 mm Hg. Art. and practically brings it back to normal.

An important function of vasopressin is to enhance the reabsorption of water from the renal tubules into the bloodstream, or, in other words, to regulate the volume of fluid in the body, which is why the hormone has a second name - antidiuretic hormone.

8) Renin-angiotensin system (RAS) or the renin-angiotensin-aldosterone system (RAAS) is a hormonal system in humans and mammals that regulates blood pressure and blood volume in the body.

Renin is formed in the form of grenin and is secreted in the juxtaglomerular apparatus (JGA) (from the Latin words juxta - about, glomerulus - glomerulus) of the kidneys by myoepithelioid cells of the bringing arteriole of the glomerulus, called juxtaglomerular cells (JGC). The structure of the YUGA is shown in Fig. 6.27. In the YUGA, in addition to the JGC, the part of the distal tubule of the nephron adjacent to the arterioles is also included, the multilayered epithelium of which forms a dense spot here - the macula densa. Renin secretion in the SGC is regulated by four main influences. First, the magnitude of the blood pressure in the bringing arteriole, i.e., the degree of its stretching. Decrease in stretch activates and increase suppresses renin secretion. Secondly, the regulation of renin secretion depends on the sodium concentration in the urochadistal tubule, which is perceived by the macula densa, a kind of Na receptor. The more sodium is in the urine of the distal tubule, the higher the level of renin secretion. Thirdly, the secretion of renin is regulated by sympathetic nerves, the branches of which end in the JGC; the norepinephrine mediator through beta-adrenergic receptors stimulates the secretion of renin. Fourthly, the regulation of renin secretion is carried out according to the mechanism of negative feedback, which is switched on by the level in the blood of other components of the system - angiotensin and aldosterone, as well as their effects - the content of sodium, potassium in the blood, blood pressure, the concentration of prostaglandins in the kidney formed under the influence of angiotensin.

In addition to the kidneys, renin formation occurs in the endothelium. blood vessels many tissues, myocardium, brain, salivary glands, the glomerular zone of the adrenal cortex.

Renin secreted into the blood causes the splitting of plasma alpha globulin - angiotensinogen, which is formed in the liver. In this case, an inactive decapeptide angiotensin-I is formed in the blood (Fig. 6.1-8), which in the vessels of the kidneys, lungs and other tissues is exposed to the action of a converting enzyme (carboxycatepsin, kininase-2), which cleaves two amino acids from angiotensin-1. The resulting octapeptide angiotensin II possesses a large number various physiological effects, including the stimulation of the glomerular zone of the adrenal cortex, secreting aldosterone, which gave reason to call this system renin-angiotensin-aldosterone.

Angiotensin II, in addition to stimulating the production of aldosterone, has the following effects:

Causes constriction of arterial vessels,

It activates the sympathetic nervous system both at the level of the centers and by promoting the synthesis and release of norepinephrine at the synapses,

Increases myocardial contractility,

Increases sodium reabsorption and weakens glomerular filtration in the kidneys,

Promotes thirst and drinking behavior.

Thus, the renin-angiotensin-aldosterone system is involved in the regulation of systemic and renal circulation, circulating blood volume, water-salt metabolism and behavior.

Nervous regulation of blood circulation is carried out in the cardiovascular center of blood circulation, which is located in the medulla oblongata. It includes the pressor (vasoconstrictor) and depressor (vasodilator) sections. It is mainly influenced by impulses from reflexogenic zones located in the carotid sinus, aortic arch, thyrocarotid and cardiopulmonary regions. Here are the receptors that sense changes in blood pressure - baroreceptors and the chemical composition of blood - chemoreceptors.

By their chemical structure, receptors are composed of proteins, nucleic acids, and other compounds. Receptors are located on the outer surface of the cell membrane; they transfer information from the environment into the cell.

In cardiology, the most studied alpha adrenergic receptors and beta-adrenergic receptors... Epinephrine and norepinephrine act on alpha-adrenergic receptors and cause vasoconstriction and increase. Adrenaline can cause excitation and beta-adrenergic receptors of some vessels, for example, the vessels of skeletal muscles, and causes their expansion. Excitation of beta-adrenergic receptors of the myocardium with adrenaline and norepinephrine increases the frequency and strength of heart contractions. Many pharmacological drugs have the ability to block the action of agents that stimulate alpha-adrenergic receptors and beta-adrenergic receptors. These drugs are called adrenergic blockers.

The carotid sinus is located at the beginning of the internal carotid artery. The nerve endings located in it are sensitive to stretching of the arterial wall when the pressure in the vessel rises. These baroreceptors are stretch receptors. Similar baroreceptors are found in the aortic arch, in the pulmonary artery and its branches, in the chambers of the heart. Impulses from baroreceptors inhibit the sympathetic and excite the parasympathetic centers. As a result, the tone of sympathetic vasoconstrictor fibers decreases. There is a decrease in the pulse rate, a decrease in the strength of the heart contractions, and a decrease in peripheral vascular resistance, which causes a decrease in blood pressure.

In the area of \u200b\u200bbifurcation carotid arteries chemoreceptors are located - the so-called aortic bodies, which are a reflexogenic zone that reacts to chemical composition blood - the partial pressure of oxygen and carbon dioxide. These chemoreceptors are especially sensitive to a lack of oxygen in the blood, hypoxia. Hypoxia increases their activity, this is accompanied by a reflex deepening of breathing, an increase in the heart rate, an increase in the minute volume of blood circulation.

Fibers of the sympathetic nerves with the help of mediators - adrenaline and norepinephrine - mainly cause vasoconstriction and an increase in blood pressure. The fibers of the parasympathetic nerves with the help of the neurotransmitter acetylcholine mainly cause vasodilation and a decrease in blood pressure. The density of the innervation of the arteries is higher than that of the veins.

Peripheral chemoreceptors - aortic and carotid sinus bodies, respond to ↓ PO2, РСО2 (↓ pH). Impulses → to the respiratory and circulatory centers of the medulla oblongata. Excitation of chemoreceptors \u003d\u003e ↓ HR (through the circulatory center) and HR (through the respiratory center), vasoconstriction (prevail over the change in HR) \u003d\u003e art. pressure. A similar effect occurs with ↓ blood flow in the receptor area.

Receptors in the central nervous system - the centers of the medulla oblongata, the surface of the brainstem (react to extracells.).

Baroreceptors.

Baroreceptors - in the walls of large intrathoracic and cervical arteries ( area of \u200b\u200bthe arch and carotid sinus). Fibers from them are included in nn.glossopharyngeus et vagus. React to transmur. pressure (wall tension). The pulse frequency is higher at higher blood pressure. + react to the rate of increase in blood pressure (impulses are proportional to the rate of increase in blood pressure).

Afferents - to cardio-inhibitory and vasomotor. centers of the protracted brain \u003d\u003e inhibition of symp. nerves, excitement of parasympathetic. \u003d\u003e ↓ Symptom tone vasoconstrictor fibers. The reflex also manifested itself under normal conditions. levels of blood pressure. Result: resistive expansion. vessels \u003d\u003e ↓ total periphery. resistance; expansion of capacitive \u003d\u003e roof capacity. channel. All together \u003d\u003e ↓ BP (including and due to ↓ central venous pressure \u003d\u003e ↓ stroke volume and due to negative foreign and chronotropic effects from baroreceptors).

Effects on other parts of the central nervous system: impulses from baroreceptors \u003d\u003e inhibition of some departments \u003d\u003e surface. breath, ↓ mouse. tone, ↓ mouse impulses. spindles through γ-fibers, ↓ monosyn. reflexes, EEG changes (strong stretching \u003d\u003e weak signs of falling asleep).

Effect on blood volume: BP \u003d\u003e ↓ vasomotor. tone \u003d\u003e vasodilation \u003d\u003e effect. capillary pressure \u003d\u003e fluid filtration rate in the interstices. space.

Heart strain receptors... In the atria: A-type (responsive to muscle contraction \u003d\u003e agitation in systole) and B-type (responsive to pressures - to pass. stretching). Impulses - according to n.vagus in the circulator. center extended brain. Effect - brake. synaptic and arousal. parasymp. divisions of the circulator. nerve. centers; impulses to the center of osmoregulation in the hypothalamus \u003d\u003e decrease in blood volume with vasopressin... In addition, B-type receptors \u003d\u003e soil vasoconstriction. vessels. In the ventricles: the receptors impulse only in the isovolum phase. abbreviations \u003d\u003e neg. chronotropic effect under strong stretching.

Buffer function of the baroreceptor blood pressure regulation system.

Since the baroreceptor system resists both an increase and a decrease in blood pressure, it called a pressure control buffer system, and the nerves coming from the baroreceptors are called buffer nerves.
In conclusion, we can say that the main task of the arterial baroreceptor system is a continuous, minute-by-minute decrease in blood pressure fluctuations by about 1/3 compared to those fluctuations that occur in the absence of a baroreceptor mechanism.

What is the role of baroreceptors in the long-term regulation of blood pressure?

Despite the fact that arterial baroreceptors continuously control blood pressure, their importance for long-term pressure regulation remains controversial... The reason why many physiologists consider this mechanism ineffective for long-term regulation of blood pressure is the ability of baroreceptors to rebuild after 1-2 days and get used to a new level of pressure... So, if blood pressure increases from the normal level of 100 mm Hg. Art. up to 160 mm Hg. Art., the frequency of impulses coming from baroreceptors initially increases.

Over the next few minutes, the frequency of pulse generation decreases markedly; then a gradual decrease in frequency continues for another 1-2 days, and by the end of this period, the frequency of pulse generation practically returns to the initial normal level, despite the fact that the mean arterial pressure is still equal to 160 mm Hg. Art. And vice versa, if the pressure drops to a very low level, initially the impulse from the baroreceptors disappears, but then gradually, within 1-2 days, the frequency of impulses coming from the baroreceptors returns to the initial level.

This "readjustment" of receptors, obviously, makes the baroreceptor mechanism unable to correct changes in blood pressure if they persist for more than a few days. Experimental studies, however, suggest that a complete readjustment of baroreceptors does not occur, and they may be involved in long-term regulation of blood pressure, mainly due to the effect on the activity of the sympathetic nerves of the kidneys.

For example, with prolonged increases in blood pressure, baroreceptor reflexes can decrease the activity of the sympathetic nerves of the kidneys, which leads to increased secretion of sodium and water by the kidneys. This, in turn, contributes to a decrease in blood volume and a return of blood pressure to normal levels. Thus, long-term regulation of mean arterial pressure with the participation of baroreceptors occurs when this mechanism interacts with the system of renal control over pressure and the amount of fluid in the body (which includes special nervous and humoral mechanisms).

Regulation is divided into short-term(aimed at changing the minute blood volume, total peripheral vascular resistance and maintaining the level of blood pressure. These parameters can change within a few seconds) and long-term. With physical exertion, these parameters should change rapidly. They change quickly if bleeding occurs and the body loses some of the blood. Long-term regulation is aimed at maintaining the magnitude of the blood volume and the normal distribution of water between blood and tissue fluid. These indicators cannot appear and change within minutes and seconds.

The spinal cord is a segmental center... It leaves the sympathetic nerves that innervate the heart (upper 5 segments). The rest of the segments take part in the innervation of the blood vessels. The spinal centers are unable to provide adequate regulation. The pressure drops from 120 to 70 mm. rt. pillar. These sympathetic centers need constant influx from the centers of the brain to ensure normal regulation of the heart and blood vessels.

Under natural conditions - a reaction to pain, temperature irritations, which are closed at the level of the spinal cord.

Vasomotor center

The main center will be vasomotor center which lies in the medulla oblongata and the discovery of this center was associated with the name of our physiologist - Ovsyannikov.

He performed brain stem transections in animals and found that as soon as the brain incisions passed below the lower tubercles of the quadruple, there was a decrease in pressure. Ovsyannikov found that in some centers there was a constriction, and in others, vasodilation.

The vasomotor center includes:

- vasoconstrictor zone - depressor - anteriorly and laterally (now it is designated as a group of C1 neurons).

The second is located posteriorly and medially vasodilator zone.

The vasomotor center lies in reticular formation ... The neurons of the vasoconstrictor zone are in constant tonic excitation. This zone is connected by descending paths with the lateral horns of the gray matter of the spinal cord. Arousal is transmitted using a mediator glutamate... Glutamate transmits excitation to the neurons of the lateral horns. Further impulses go to the heart and blood vessels. It is excited periodically if impulses come to it. The impulses arrive at the sensitive nucleus of a single tract and from there to the neurons of the vasodilator zone, and it is excited.

It has been shown that the vasodilator zone is in an antagonistic relationship with the vasoconstrictor.

Vasodilator zone also includes nucleus of the vagus nerve - double and dorsal the core from which efferent pathways to the heart. Seam core - they produce serotonin. These nuclei have an inhibitory effect on the sympathetic centers of the spinal cord. It is believed that the nuclei of the seam are involved in reflex reactions, are involved in the processes of arousal associated with stressful reactions of the emotional plan.

Cerebellum affects the regulation of the cardiovascular system during exercise (muscle). Signals go to the tent nuclei and the cortex of the cerebellar worm from the muscles and tendons. The cerebellum increases the tone of the vasoconstrictor area... Receptors of cardio-vascular system - aortic arch, carotid sinuses, hollow veins, heart, vessels of the small circle.

The receptors that are located here are subdivided into baroreceptors. They lie directly in the vascular wall, in the aortic arch, in the carotid sinus region. These receptors sense pressure changes to monitor the pressure level. In addition to baroreceptors, there are chemoreceptors, which lie in the glomeruli on the carotid artery, the aortic arch and these receptors respond to changes in the oxygen content in the blood, ph. The receptors are located on the outer surface of the vessels. There are receptors that perceive change in blood volume. - currency receptors - perceive the change in volume.

Reflexes are divided into depressor - lowering pressure, pressor - increasinge, accelerating, decelerating, interoceptive, exteroceptive, unconditioned, conditional, proper, conjugate.

The main reflex is the reflex to maintain the pressure level. Those. reflexes aimed at maintaining the pressure level from the baroreceptors. Baroreceptors of the aorta, carotid sinus perceive the level of pressure. They perceive the magnitude of pressure fluctuations during systole and diastole + average pressure.

In response to an increase in pressure, baroreceptors stimulate the activity of the vasodilator zone. At the same time, they increase the tone of the nuclei of the vagus nerve. In response, reflex reactions develop, reflex changes occur. The vasodilator zone suppresses the vasoconstrictor tone. Vascular dilation occurs and the tone of the veins decreases. Arterial vessels are dilated (arterioles) and veins dilate, the pressure will decrease. The sympathetic influence decreases, the wandering increases, the rhythm frequency decreases. High blood pressure returns to normal. Dilation of arterioles increases blood flow in the capillaries. Part of the fluid will pass into the tissues - the blood volume will decrease, which will lead to a decrease in pressure.

With chemoreceptors arise pressor reflexes... An increase in the activity of the vasoconstrictor zone along the descending pathways stimulates the sympathetic system, while the vessels are narrowed. The pressure rises through the sympathetic centers of the heart, the heart will work faster. The sympathetic system regulates the release of hormones by the adrenal medulla. Blood flow in the pulmonary circulation will increase. The respiratory system reacts with increased breathing - the release of blood from carbon dioxide. The factor that caused the pressor reflex leads to the normalization of the blood composition. In this pressor reflex, a secondary reflex to a change in the work of the heart is sometimes observed. Against the background of an increase in pressure, a strain of the heart is observed. This change in the work of the heart is in the nature of a secondary reflex.

Mechanisms of reflex regulation of the cardiovascular system.

Among the reflexogenic zones of the cardiovascular system, we attributed the mouths of the hollow veins.

Bainbridge injected into the venous part of the mouth 20 ml of physical. Solution or the same volume of blood. After that, there was a reflex increase in the heart rate, followed by an increase in blood pressure. The main component in this reflex is an increase in the frequency of contractions, and the pressure rises only a second time. This reflex occurs when blood flow to the heart increases. When the blood flow is greater than the outflow. In the area of \u200b\u200bthe mouth of the genital veins - sensitive receptors that respond to an increase in venous pressure. These sensory receptors are the endings of the afferent fibers of the vagus nerve, as well as the afferent fibers of the posterior spinal roots. Excitation of these receptors leads to the fact that impulses reach the nuclei of the vagus nerve and cause a decrease in the tone of the nuclei of the vagus nerve, while the tone of the sympathetic centers increases. There is an increase in the work of the heart and blood from the venous part begins to be pumped into the arterial. The pressure in the vena cava will decrease.

Under physiological conditions, this condition can increase with physical activity, when blood flow increases and with heart defects, blood stasis is also observed, which leads to an increase in the heart's work.

An important reflexogenic zone will be the zone of the vessels of the pulmonary circulation.

In the vessels of the pulmonary circulation, they are located in receptors that respond to an increase in pressure in the pulmonary circulation. With an increase in pressure in the small circle of blood circulation, a reflex arises, which causes the expansion of the vessels of the large circle, at the same time the work of the heart is strained and the volume of the spleen increases. Thus, a kind of unloading reflex arises from the small circle of blood circulation. This reflex was discovered by V.V. Parin... He worked a lot in the development and research of space physiology, headed the Institute for Biomedical Research. Increased pressure in the pulmonary circulation is a very dangerous condition, because it can cause pulmonary edema... Because the hydrostatic pressure of the blood increases, which helps to filter the blood plasma and, due to this state, the liquid enters the alveoli.

The heart itself is a very important reflexogenic zone. in the circulatory system. In 1897, scientists Doggel it was found that the heart has sensory endings, which are mainly concentrated in the atria and to a lesser extent in the ventricles. Further studies showed that these endings are formed by sensory fibers of the vagus nerve and fibers of the posterior spinal roots in the upper 5 thoracic segments.

Sensory receptors in the heart are found in the pericardium and it is noted that an increase in fluid pressure in the pericardial cavity or blood entering the pericardium when injured reflexively slows down the heart rate.

A slowdown in heart contraction is also observed with surgical interventionswhen the surgeon sips the pericardium. Irritation of pericardial receptors - slowing down of the heart, and with more severe irritation, temporary cardiac arrest is possible. Turning off the sensitive endings in the pericardium caused an increase in heart rate and an increase in pressure.

The increase in pressure in the left ventricle induces the typical depressor reflex, i.e. there is a reflex vasodilation and a decrease in peripheral blood flow and, at the same time, an increase in the heart. A large number of sensitive endings are located in the atrium and it is the atrium that contains stretch receptors, which are sensitive fibers vagus nerves. Hollow veins and the atria belong to the low pressure zone, because the pressure in the atria does not exceed 6-8 mm. rt. Art. Because the atrial wall is easily stretched, then an increase in atrial pressure does not occur and atrial receptors respond to an increase in blood volume. Studies of the electrical activity of atrial receptors have shown that these receptors are divided into 2 groups -

- Type A. In type A receptors, arousal occurs at the moment of contraction.

-TypeB. They are excited when the atria are filled with blood and when the atria are stretched.

Reflex reactions occur from the atrial receptors, which are accompanied by a change in the secretion of hormones and from these receptors the volume of circulating blood is regulated. Therefore, atrial receptors are called Valium receptors (responsive to changes in blood volume). It was shown that with a decrease in the excitation of atrial receptors, with a decrease in volume, parasympathetic activity reflexively decreased, i.e. the tone of the parasympathetic centers decreases, and vice versa, the excitation of the sympathetic centers increases. Excitation of sympathetic centers has a vasoconstrictor effect, and especially on renal arterioles.

Which causes a decrease in renal blood flow. A decrease in renal blood flow is accompanied by a decrease in renal filtration, and sodium excretion decreases. And the formation of renin increases, in the juxta-glomerular apparatus. Renin stimulates the formation of anthyotenism 2 from angiotensinogen. This causes vasoconstriction. Further, angiotensin 2 stimulates the formation of aldostrone.

Angiotensin 2 also increases thirst and increases the secretion of antidiuretic hormone, which will help the kidneys reabsorb water. Thus, an increase in the volume of fluid in the blood will occur and this decrease in receptor irritation will be eliminated.

If the blood volume is increased and the atrial receptors are excited at the same time, then reflexive inhibition and release of antidiuretic hormone occurs. Consequently, less water will be absorbed in the kidneys, diuresis will decrease, then the volume will be normalized. Hormonal shifts in organisms arise and develop within a few hours, therefore, the regulation of circulating blood volume refers to the mechanisms of long-term regulation.

Reflex reactions in the heart can occur when spasm of the coronary vessels. It causes pain areas of the heart, and the pain is felt behind the sternum, strictly along the midline. The pains are very severe and are accompanied by screams of death. These pains are different from tingling pains. At the same time, painful sensations spread to the left arm and scapula. Along the zone of distribution of sensitive fibers of the upper thoracic segments. Thus, reflexes of the heart are involved in the mechanisms of self-regulation of the circulatory system and they are aimed at changing the frequency of heart contractions, changes in the volume of circulating blood.

In addition to reflexes that arise from reflexes of the cardiovascular system, reflexes may occur that occur when irritated from other organs are called conjugate reflexes in an experiment on the tops, the scientist Goltz discovered that sipping of the stomach, intestines or light beating of the intestines in a frog is accompanied by a slowdown in the work of the heart, up to a complete stop. This is due to the fact that from the receptors impulses arrive at the nuclei of the vagus nerves. Their tone rises and the work of the heart is inhibited, or even its arrest.

There are also chemoreceptors in the muscles, which are excited with an increase in potassium ions, hydrogen protons, which leads to an increase in the minute blood volume, narrowing of the vessels of other organs, an increase in average pressure and an increase in the work of the heart and respiration. Locally, these substances contribute to the expansion of the vessels of the skeletal muscles themselves.

Superficial pain receptors increase the heart rate, constrict blood vessels, and increase average blood pressure.

Exciting deep pain receptors, visceral and muscle pain receptors leads to bradycardia, vasodilation and decreased pressure. In the regulation of the cardiovascular system the hypothalamus is of great importance, which is connected by descending paths with the vasomotor center of the medulla oblongata. Through the hypothalamus during defensive defensive reactions, during sexual activity, during food, drinking reactions and with joy, the heart beat faster. The posterior nuclei of the hypothalamus lead to tachycardia, vasoconstriction, increased blood pressure and increased blood levels of adrenaline and norepinephrine. When the anterior nuclei are excited, the work of the heart slows down, the vessels expand, the pressure drops and the anterior nuclei affect the centers parasympathetic system... When the ambient temperature rises, the minute volume increases, blood vessels in all organs, except for the heart, contract and the vessels of the skin expand. Increased blood flow through the skin - greater heat release and maintenance of body temperature. Through the hypothalamic nuclei, the influence of the limbic system on blood circulation is carried out, especially during emotional reactions, and emotional reactions are realized through the nuclei of the Suture, which produce serotonin. From the nuclei of the Seam there are paths to the gray matter of the spinal cord. The cerebral cortex also takes part in the regulation of the circulatory system and the cortex is connected with the centers of the diencephalon, i.e. the hypothalamus, with the centers of the midbrain and it was shown that irritation of the motor and premature cortex zones led to a narrowing of the cutaneous, celiac and renal vessels .. This caused the expansion of the vessels of the skeletal muscles, while the expansion of the vessels of the skeletal muscles is realized through a descending effect on the sympathetic, cholinergic fibers ... It is believed that it is the motor areas of the cortex that trigger skeletal muscle contraction that simultaneously activate vasodilating mechanisms that promote large muscle contraction. The participation of the cortex in the regulation of the heart and blood vessels is proved by the development of conditioned reflexes. In this case, you can develop reflexes to change the state of blood vessels and to change the heart rate. For example, the combination of a sound signal of a bell with temperature stimuli - temperature or cold, leads to vasodilation or vasoconstriction - we apply cold. The ring tone is pre-given. Such a combination of an indifferent ringing sound with thermal irritation or cold, leads to the development of a conditioned reflex, which caused either vasodilation or constriction. A conditioned eye-cardiac reflex can be developed. The heart harnesses the work. There were attempts to develop a reflex to cardiac arrest. They turned on the bell and irritated the vagus nerve. In life, we don't need cardiac arrest. The body reacts negatively to such provocations. Conditioned reflexes are developed if they are of an adaptive nature. As a conditioned reflex reaction, you can take - prelaunch state athlete. His heart becomes more frequent, blood pressure rises, blood vessels narrow. The signal for such a reaction will be the situation itself. The body is already preparing in advance and mechanisms are activated that increase the blood supply to muscles and blood volume. During hypnosis, you can achieve a change in the work of the heart and vascular tone, if you suggest that a person is doing hard physical work. In this case, the heart and blood vessels react in the same way as if it were in reality. When acting on the centers of the cortex, cortical effects on the heart and blood vessels are realized.

Regulation of regional blood circulation.

The heart receives blood from the right and left coronary arteries, which extend from the aorta, at the level of the superior edges of the semilunar valves. The left coronary artery is divided into the anterior descending artery and the circumflex artery. Coronary arteries usually function as annular arteries. And between the right and left coronary arteries, the anastomoses are very poorly developed. But if there is a slow closure of one artery, then the development of anastomoses between the vessels begins and which can pass from 3 to 5% from one artery to another. This is when the coronary arteries are slowly closed. Rapid overlap leads to heart attack and is not compensated from other sources. The left coronary areria supplies the left ventricle, the anterior half of the interventricular septum, and the left and partially right atrium. The right coronary artery feeds the right ventricle, right atrium, and the posterior half of the interventricular septum. Both coronary arteries are involved in the blood supply to the cardiac conduction system, but in humans, the right one is larger. The outflow of venous blood occurs through veins that run parallel to the arteries and these veins flow into the coronary sinus, which opens into the right atrium. Through this path, from 80 to 90% of the venous blood flows out. Venous blood from the right ventricle in the interatrial septum flows through the smallest veins into the right ventricle and these veins are called veins of tibesium, which directly remove venous blood into the right ventricle.

200-250 ml flows through the coronary vessels of the heart. blood per minute, i.e. this is 5% of the minute volume. For 100 g. Myocardium, 60 to 80 ml flows per minute. The heart extracts 70-75% of oxygen from arterial blood, therefore there is a very large arterio-venous difference in the heart (15%) In other organs and tissues - 6-8%. In the myocardium, capillaries densely entwine each cardiomyocyte, which creates best condition for maximum blood extraction. The study of coronary blood flow is very difficult because it changes from the cardiac cycle.

Increased coronary blood flow in diastole, systole, decreased blood flow, due to compression of blood vessels. Diastole accounts for 70-90% of coronary blood flow. The regulation of coronary blood flow is primarily regulated by local anabolic mechanisms, responds quickly to a decrease in oxygen. A decrease in the level of oxygen in the myocardium is a very powerful signal for vasodilation. A decrease in oxygen content leads to the fact that cardiomyocytes secrete adenosine, and adenosine is a powerful vasodilator. It is very difficult to assess the effect of the sympathetic and parasympathetic systems on blood flow. Both the vagus and sympathicus change the work of the heart. It has been established that irritation of the vagus nerves slows down the heart, increases the continuation of diastole, and the direct release of acetylcholine will also cause vasodilation. Sympathetic influences promote the release of norepinephrine.

There are 2 types of adrenoceptors in the coronary vessels of the heart - alpha and beta adrenoceptors. In most people, the predominant type is beta-adrenergic receptors, but some have a predominance of alpha receptors. Such people will feel a decrease in blood flow when worried. Epinephrine causes an increase in coronary blood flow, due to an increase in oxidative processes in the myocardium and an increase in oxygen consumption and due to the effect on beta-adrenergic receptors. Thyroxin, prostaglandins A and E have an expanding effect on coronary vessels, vasopressin narrows coronary vessels and reduces coronary blood flow.

Cerebral circulation

It has many features in common with coronary, for the brain is characterized by high metabolic processes, increased oxygen consumption, the brain has a limited ability to use anaerobic glycolysis, and cerebral vessels react poorly to sympathetic influences. Cerebral blood flow remains normal with wide ranges of blood pressure changes. From 50-60 minimum to 150-180 maximum. The regulation of the centers is especially well expressed. brain stem... Blood enters the brain from 2 pools - from the internal carotid arteries, vertebral arteries, which then form on the basis of the brain velisian circle, and from it 6 arteries depart, blood-consuming the brain. For 1 minute, the brain receives 750 ml of blood, which is 13-15% of the minute blood volume and cerebral blood flow depends on cerebral perfusion pressure (the difference between mean arterial pressure and intracranial pressure) and the diameter of the vascular bed. Normal pressure cerebrospinal fluid - 130 ml. water column (10 ml. Hg. column), although in humans it can range from 65 to 185.

For normal blood flow, the perfusion pressure should be above 60 ml. Otherwise ischemia is possible. Self-regulation of blood flow is associated with the accumulation of carbon dioxide. If there is oxygen in the myocardium. With a partial pressure of carbon dioxide above 40 mm Hg. Also, the cerebral vessels expand the accumulation of hydrogen ions, adrenaline, and to an increase in potassium ions, to a lesser extent, the vessels respond to a decrease in oxygen in the blood and the reaction is observed to decrease oxygen below 60 mm. RT Art. Depending on the work of different calving of the brain, local blood flow can increase by 10-30%. Cerebral circulation does not respond to humoral substances due to the presence of the blood-brain barrier. The sympathetic nerves do not cause vasoconstriction, but they affect the smooth muscle and the endothelium of the blood vessels. Hypercapnia is a decrease in carbon dioxide. These factors cause the expansion of blood vessels by the self-regulation mechanism, and also reflexively increase the mean pressure, followed by a slowdown in the work of the heart, through the excitation of baroreceptors. These changes in the systemic circulation - cushing's reflex.

BP is maintained at the required working level with the help of reflex control mechanisms that function on the basis of the feedback principle.

 Baroreceptor reflex... One of the well-known neural mechanisms of blood pressure control is the baroreceptor reflex. Baroreceptors are found in the wall of almost all large arteries in the chest and neck, especially in the carotid sinus and in the aortic arch wall. Baroreceptors of the carotid sinus (see Fig. 25-10) and the aortic arch do not respond to blood pressure in the range from 0 to 60-80 mm Hg. An increase in pressure above this level causes a response that progressively increases and reaches a maximum at a BP of about 180 mm Hg. Normal blood pressure (its systolic level) ranges from 110–120 mm Hg. Small deviations from this level increase the excitation of baroreceptors. Baroreceptors respond to changes in blood pressure very quickly: the frequency of impulses increases during systole and just as quickly decreases during diastole, which occurs within fractions of a second. Thus, baroreceptors are more sensitive to pressure changes than to a stable level.

 Enhanced impulse from baroreceptorscaused by the rise in blood pressure, enters the medulla oblongata, inhibits the vasoconstrictor center of the medulla oblongata and excites the center of the vagus nerve... As a result, the lumen of arterioles expands, the frequency and strength of heart contractions decreases. In other words, the excitation of baroreceptors reflexively leads to a decrease in blood pressure due to a decrease in peripheral resistance and cardiac output.

 Low blood pressure has the opposite effect, which leads to its reflex increase to a normal level. A decrease in pressure in the area of \u200b\u200bthe carotid sinus and the aortic arch inactivates baroreceptors, and they cease to have an inhibitory effect on the vasomotor center. As a result, the latter is activated and causes an increase in blood pressure.

 Orthostatic collapse... The baroreceptor reflex is involved in maintaining blood pressure when changing from a horizontal position to a vertical one. Immediately after the adoption of the vertical position, blood pressure in the head and upper body decreases, which can cause loss of consciousness (which happens in some cases when the baroreceptor reflex is insufficient - this condition is called orthostatic syncope). The drop in pressure in the baroreceptor region immediately activates a reflex that stimulates the sympathetic system and minimizes the pressure drop in the upper torso and head.

 Chemoreceptors of the carotid sinus and aorta... Chemoreceptors - chemosensitive cells that respond to a lack of oxygen, an excess of carbon dioxide and hydrogen ions - are located in the carotid corpuscles and in the aortic corpuscles. Chemoreceptor nerve fibers from the bodies, together with baroreceptor fibers, go to the vasomotor center of the medulla oblongata. With a decrease in blood pressure below the critical level, chemoreceptors are stimulated, since a decrease in blood flow reduces O 2 and increases the concentration of CO 2 and H +. Thus, impulses from chemoreceptors excites the vasomotor center and increases blood pressure.

 Reflexes with pulmonary artery and atria... There are stretch receptors (low pressure receptors) in the wall of both the atria and the pulmonary artery. Low pressure receptors perceive changes in volume that occur simultaneously with changes in blood pressure. Excitation of these receptors induces reflexes in parallel with baroreceptor reflexes.

 Reflexes from the atria, activating kidneys... Stretching of the atria causes reflex expansion of afferent (bringing) arterioles in the glomeruli of the kidneys. At the same time, a signal is sent from the atrium to the hypothalamus, decreasing the secretion of ADH. The combination of two effects - an increase in glomerular filtration and a decrease in fluid reabsorption - helps to reduce blood volume and return it to normal levels.

 Atrial reflex that controls heart rate... The increase in pressure in the right atrium causes a reflex increase in heart rate (Bainbridge reflex). The atrial stretch receptors, which trigger the Bainbridge reflex, transmit afferent signals through the vagus nerve to the medulla oblongata. The excitement then returns back to the heart through the sympathetic pathways, increasing the frequency and strength of the heart. This reflex prevents the overflow of blood in the veins, atria and lungs.

 Direct effects on the vasomotor center... If blood circulation in the brain stem area decreases, causing brain ischemia, then the excitability of neurons in the vasomotor center increases significantly, leading to a maximum rise in systemic blood pressure. This effect is caused by the local accumulation of CO 2, lactic acid and other acidic substances and their stimulating effect on the sympathetic part of the vasomotor center. The ischemic response of the central nervous system to blood circulation is unusually high: within 10 minutes, the average blood pressure can sometimes rise to 250 mm Hg. The ischemic response of the central nervous system is one of the most powerful activators of the sympathetic vasoconstrictor system. This mechanism occurs when blood pressure drops to 60 mm Hg. and lower, which happens with large blood loss, circulatory shock, collapse. It is the response of a life-saving pressure control system that prevents blood pressure from falling further to lethal levels.

 ReflexCushing's(Cushing's reaction) - ischemic reaction of the central nervous system in response to an increase in intracranial pressure. If intracranial pressure rises and becomes equal to blood pressure, then arteries in the cranial cavity are compressed and ischemia occurs. Ischemia causes an increase in blood pressure, and blood enters the brain again, overcoming the compressive effect of increased intracranial pressure. Simultaneously with an increase in pressure, the heart rate and respiration rate become less frequent due to the excitation of the center of the vagus nerve.

 Renin-angiotensin systemdiscussed in Chapter 29.