The heart is under constant action nervous system and humoral factors. The body is in different conditions of existence. The result of the work of the heart is the injection of blood into the systemic and pulmonary circulation.
Assessed by the minute volume of blood. In a normal state, in 1 minute - 5 liters of blood are pushed out by both ventricles. In this way we can appreciate the work of the heart.
Systolic blood volume and heart rate - minute volume of blood.
For comparison with different people- introduced cardiac index- how much blood per minute falls on 1 square meter of the body.
In order to change the value of the volume - you need to change these indicators, this happens due to the mechanisms of regulation of the heart.
Minute blood volume (MOV)=5l/min
Cardiac index \u003d IOC / Sm2 \u003d 2.8-3.6 l / min / m2
IVO=systolic volume*rate/min
Mechanisms of regulation of the heart
- Intracardiac (intracardiac)
- Extracardiac (Extracardiac)
To intracardiac mechanisms include the presence of tight contacts between the cells of the working myocardium, the conduction system of the heart coordinates the individual work of the chambers, intracardiac nerve elements, hydrodynamic interaction between individual chambers.
Extracardiac - nervous and humoral mechanism, which change the work of the heart and adapt the work of the heart to the needs of the body.
Nervous regulation of the heart is carried out by the autonomic nervous system. The heart receives innervation from parasympathetic(wandering) and sympathetic(lateral horns spinal cord T1-T5) nerves.
ganglia parasympathetic system lie inside the heart and there the preganglionic fibers switch to postganglionic. Preganglionic nuclei - medulla oblongata.
Sympathetic- are interrupted in the stellate ganglion, where the postganglionic cells that go to the heart will already be located.
Right vagus nerve- innervates the sino-atrial node, the right atrium,
Left vagus nerve to the atrioventricular node and right atrium
Right sympathetic nerve- to the sinus node, right atrium and ventricle
Left sympathetic nerve- to the atrioventricular nodes and to the left half of the heart.
In the ganglia, acetylcholine acts on N-cholinergic receptors
Sympathetic secrete norepinephrine, which acts on adrenergic receptors (B1)
Parasympathetic- acetylcholine at M-cholino receptors (muscarino)
Influence on the work of the heart.
- Chronotropic effect (on heart rate)
- Inotropic (on the strength of heart contractions)
- Bathmotropic effect (on excitability)
- Dromotropic (for conductivity)
1845 - Weber brothers - discovered the influence of the vagus nerve. They cut a nerve in his neck. When the right vagus nerve was irritated, the frequency of contractions decreased, but it could stop - negative chronotropic effect(suppression of automatic sinus node). If the left vagus nerve was irritated, conduction worsened. The atrioventricular nerve is responsible for delaying excitation.
vagus nerves reduce myocardial excitability and reduce the frequency of contractions.
Under the action of the vagus nerve - slowing down the diastolic depolarization of p - cells, pacemakers. Increases the release of potassium. Although the vagus nerve causes cardiac arrest, it cannot be done completely. There is a resumption of contraction of the heart - escape from the influence of the vagus nerve and the resumption of the work of the heart is due to the fact that the automation from the sinus node passes to the atrioventricular node, which returns the work of the heart with a frequency of 2 times less.
Sympathetic Influences- studied by the Zion brothers - 1867. When irritated sympathetic nerves Zions discovered that sympathetic nerves give positive chronotropic effect. Pavlov studied further. In 1887 he published his work on the influence of nerves on the functioning of the heart. In his research, he discovered that individual branches, without changing the frequency, increase the strength of contractions - positive inotropic effect. Further, the bamotropic and dromotropic effects were discovered.
Positive effects on the heart is due to the influence of norepinephrine on beta 1 adrenoreceptors, which activate adenylate cyclase, promote the formation of cyclic AMP, and increase the ion permeability of the membrane. Diastolic depolarization occurs at a faster rate and this causes a more frequent rhythm. Sympathetic nerves increase the breakdown of glycogen, ATP, thereby providing the myocardium with energy resources, and the excitability of the heart increases. The minimum duration of the action potential in the sinus node is set to 120 ms, i.e. theoretically, the heart could give us the number of contractions - 400 per minute, but the atrioventricular node is not able to conduct more than 220. The ventricles are maximally reduced with a frequency of 200-220. The role of mediators in the transmission of excitation to the hearts was established by Otto Levi in 1921. He used 2 isolated frog hearts, and these hearts were fed from the 1st cannula. In one heart, nerve conductors were preserved. When one heart was irritated, he observed what was happening in the other. When the vagus nerve was irritated, acetylcholine was released - through the liquid it influenced the work of another heart.
The release of norepinephrine increases the work of the heart. The discovery of this neurotransmitter excitation brought Levy the Nobel Prize.
The nerves of the heart are in a state of constant excitement - tone. At rest, the tone of the vagus nerve is especially pronounced. When transection of the vagus nerve, there is an increase in the work of the heart by 2 times. The vagus nerves constantly depress the automation of the sinus node. The normal frequency is 60-100 contractions. Switching off the vagus nerves (transection, cholinergic receptor blockers (atropine)) cause an increase in the work of the heart. The tone of the vagus nerves is determined by the tone of its nuclei. Excitation of the nuclei is maintained reflexively due to impulses that come from baroreceptors blood vessels into the medulla oblongata from the aortic arch and carotid sinus. Breathing also affects the tone of the vagus nerves. In connection with breathing - respiratory arrhythmia, when on exhalation there is an increase in the work of the heart.
The tone of the sympathetic nerves of the heart at rest is weakly expressed. If you cut the sympathetic nerves - the frequency of contractions decreases by 6-10 beats per minute. This tone increases with physical activity, increases with various diseases. The tone is well expressed in children, in newborns (129-140 beats per minute)
The heart is still subject to the action of the humoral factor- hormones (adrenal glands - adrenaline, noradarenaline, thyroid gland thyroxine and mediator acetylcholine)
Hormones have + influence on all 4 properties of the heart. The electrolyte composition of the plasma affects the heart and the work of the heart changes with changes in the concentration of potassium and calcium. Hyperkalemia- elevated potassium in the blood - a very dangerous condition, it can lead to cardiac arrest in diastole. hypokalimi I - a less dangerous condition on the cardiogram, a change in the PQ distance, a perversion of the T wave. The heart stops in systole. The body temperature also affects the heart - an increase in body temperature by 1 degree - an increase in the work of the heart - by 8-10 beats per minute.
Systolic volume
- Preload (the degree of stretching of cardiomyocytes before their contraction. The degree of stretching will be determined by the volume of blood that will be in the ventricles.)
- Contractility (Stretching of cardiomyocytes, where the length of the sarcomere changes. Usually, the thickness is 2 microns. The maximum contraction force of cardiomyocytes is up to 2.2 microns. This is the optimal ratio between the bridges of myosin and actin filaments, when their interaction is maximum. This determines the force of contraction, further stretching up to 2.4 reduces contractility.This adapts the heart to blood flow, with its increase - a greater force of contraction.The force of myocardial contraction can change without changing the amount of blood, due to the hormones adrenaline and norepinephrine, calcium ions, etc. - the force of contraction of the myocardium increases)
- Afterload (Afterload is the tension in the myocardium that must occur in systole to open the semilunar valves. The magnitude of afterload is determined by the systolic pressure in the aorta and pulmonary trunk)
Laplace's law
Stress degree of the ventricular wall = Intragastric pressure * radius / wall thickness. The greater the intraventricular pressure and the larger the radius (the size of the lumen of the ventricle), the greater the tension of the ventricular wall. The increase in thickness - affects inversely proportionally. T=P*r/W
The amount of blood flow depends not only on the minute volume, but it is also determined by the amount of peripheral resistance that occurs in the vessels.
Blood vessels have a powerful effect on blood flow. All blood vessels are lined with endothelium. Next is the elastic frame, and in the muscle cells there are also smooth muscle cells and collagen fibers. The vessel wall obeys Laplace's law. If there is intravascular pressure inside the vessel and the pressure causes tension in the vessel wall, then there is a state of tension in the wall. Also affects the radius of the vessels. The stress will be determined by the product of the pressure and the radius. In the vessels, we can distinguish the basal vascular tone. Vascular tone, which is determined by the degree of contraction.
Basal tone- determined by the degree of stretching
Neurohumoral tone- influence of nervous and humoral factors on vascular tone.
The increased radius puts more stress on the walls of the vessels than in the can, where the radius is smaller. In order to carry out normal blood flow and ensure adequate blood supply, there are mechanisms for regulating blood vessels.
They are represented by 3 groups
- Local regulation of blood flow in tissues
- Nervous regulation
- Humoral regulation
Tissue blood flow provides
Delivery of oxygen to cells
Delivery of nutrients (glucose, amino acids, fatty acids, etc.)
CO2 removal
Removal of H+ protons
Blood flow regulation- short-term (a few seconds or minutes as a result of local changes in tissues) and long-term (occurs over hours, days and even weeks. This regulation is associated with the formation of new vessels in the tissues)
The formation of new vessels is associated with an increase in tissue volume, an increase in the intensity of metabolism in the tissue.
Angiogenesis- the formation of blood vessels. This is under the influence of growth factors - vascular endothelial growth factor. Fibroblast growth factor and angiogenin
Humoral regulation of blood vessels
- 1. Vasoactive metabolites
A. Vasodilation provides - decrease in pO2, Increase - CO2, t, K + lactic acid, adenosine, histamine
b. vasoconstriction cause - an increase in serotonin and a decrease in temperature.
2. Influence of the endothelium
Endothelins (1,2,3). - constriction
Nitric oxide NO - expansion
Formation of nitric oxide (NO)
- Release of Ach, bradykinin
- Opening of Ca+ channels in the endothelium
- Binding of Ca+ to calmodulin and its activation
- Enzyme activation (nitric oxide synthetase)
- Conversion of Lfrginine to NO
Mechanism of actionNO
NO - activates guanylcyclase GTP - cGMP - opening of K channels - exit of K + - hyperpolarization - decrease in calcium permeability - expansion of smooth muscles and vasodilation.
It has a cytotoxic effect on bacteria and tumor cells when isolated from leukocytes
It is a mediator of the transmission of excitation in some neurons of the brain
Mediator of parasympathetic postganglionic fibers for penile vessels
Possibly involved in the mechanisms of memory and thinking
A.Bradikinin
B. Kallidin
Kininogen with VMV - bradykinin (with Plasma kallikrein)
Kininogen with YVD - kallidin (with tissue kallikrein)
Kinins are formed during the active activity of the sweat glands, salivary glands and pancreas.
It should be noted that one of important nitric oxide synthesis stimulators is the mechanical deformation of endothelial cells by blood flow - the so-called endothelial shear deformation.
In addition to nitric oxide, the endothelium produces other vasodilators: prostacyclin (prostaglandin I2), endothelial hyperpolarization factor, adrenomedulin, C-type natriuretic peptide. In the endothelium, the kallikrein-kinin system functions, producing the most powerful peptide dilator bradykinin (Kulikov V.P., Kiselev V.I., Tezov A.A., 1987).
Endothelium also produces vasoconstrictors: endothelins, thromboxane (prostaglandin A2), angiotensin II, prostaglandin H2. Endothelial 1 (ET1) is the most potent of all known vasoconstrictors.
endothelial factors affect adhesion and aggregation of platelets. Prostacycline is the most important antiplatelet agent, and thromboxane, on the contrary, stimulates platelet adhesion and aggregation.
Violation this balance is referred to as endothelial dysfunction, which plays important role in pathogenesis cardiovascular diseases. The most important laboratory markers of endothelial dysfunction are endothelins and von Willebrand factor.
Humoral-hormonal regulation. It is mainly carried out by balancing the activity of the pressor renin-angiotensin-aldosterone and depressor kallikrein-kinin blood systems. These systems are linked by angiotensin converting enzyme (ACE). ACE converts inactive angiotensin I into angiotensin II, which is a vasoconstrictor and stimulates the production of aldosterone in the adrenal cortex, which is accompanied by water retention in the body and contributes to the rise in blood pressure. At the same time, ACE is the main enzyme for the destruction of bradykinin and thus eliminates its depressant effect. Therefore, ACE inhibitors effectively reduce blood pressure in hypertension, changing the balance of systems towards kinin.
Neurogenic regulation. As already noted, the leading efferent link in the neurogenic control of vascular tone is the sympathetic nervous system. The so-called ischemic reaction of the CNS is known. With a significant decrease in systemic blood pressure, ischemia of the vasomotor center and activation of the sympathetic nervous system occur. The mediator of the latter is norepinephrine, which causes tachycardia (1-receptors) and an increase in vascular tone (1 and 2-receptors).
Afferent link of neurogenic regulation vascular tone is represented by baroreceptors and chemoreceptors located in the aortic arch and carotid sinus.
Baroreceptors respond to the degree and rate of stretching of the vascular wall. Chemoreceptors respond to changes in CO2 concentration in the blood. Sensitive fibers from baroreceptors and chemoreceptors of the aortic arch and carotid sinus pass through the carotid sinus nerve, branches of the glossopharyngeal nerve, and depressor nerve.
Neurogenic regulation provides constant (tonic) control over the resistive vessels of most vascular areas and emergency reflex regulation, for example, when taking an orthostatic position. In this and other cases, when the pressure in the carotid sinus and aortic arch drops sharply, the carotid baroreflex turns on, which, through the activation of baroreceptors and the sympathetic nervous system, constricts blood vessels, activates the heart and ensures a rise in blood pressure. The baroreceptor reflex, on the contrary, triggers an increase in blood pressure, which ensures its decrease through inhibition of sympathetic influences and activation of the vagus nerve. The chemoreceptor reflex provides an increase in blood pressure by activating sympathetic influences under conditions of hypoxia, when carbon dioxide accumulates in the blood.
Vascular tone- this is some constant pressure vascular walls, which determines the lumen of the vessel.
Regulation vascular tone is carried out local And systemic nervous and humoral mechanisms.
Thanks to automation some smooth muscle cells of the walls of blood vessels, blood vessels, even in the conditions of their denervation, have original(basal )tone , which is characterized self-regulation.
So, with an increase in the degree of stretching of smooth muscle cells basal tone increases(especially expressed in arterioles).
Superimposed on the basal tone tone, which is provided by nervous and humoral mechanisms of regulation.
The main role belongs to the nervous mechanisms, which reflexively regulate lumen of blood vessels.
Enhances basal tone constant tone of sympathetic centers.
Nervous regulation carried out vasomotors, i.e. nerve fibers that terminate in muscle vessels (with the exception of metabolic capillaries, where there are no muscle cells). IN azomotors refer to autonomic nervous system and subdivided into vasoconstrictors(vasoconstriction) and vasodilators(expand).
Sympathetic nerves are more often vasoconstrictors, since their transection is accompanied by vasodilatation.
Sympathetic vasoconstriction is referred to as a systemic mechanism for regulating the lumen of blood vessels, because it is accompanied by an increase in blood pressure.
The vasoconstrictive effect does not extend to the vessels of the brain, lungs, heart and working muscles.
When the sympathetic nerves are stimulated, the vessels of these organs and tissues expand.
TO vasoconstrictors relate:
1. Sympathetic adrenergic nerve fibers innervating blood vessels of the skin, organs abdominal cavity, parts of skeletal muscles (during the interaction norepinephrine with a- adrenoreceptors). Their centers located in all thoracic and three upper lumbar segments of the spinal cord.
2. Parasympathetic cholinergic nerve fibers leading to the vessels of the heart. Vasodilating nerves are often part of the parasympathetic nerves. However, vasodilating nerve fibers were also found in the composition of sympathetic nerves, as well as the posterior roots of the spinal cord.
TO vasodilators (there are fewer of them than vasoconstrictors) include:
1. Adrenergic sympathetic nerve fibers innervating blood vessels.
Parts of skeletal muscles (when interacting norepinephrine with b- adenoreceptors);
Hearts (when interacting norepinephrine with b 1 - adenoreceptors).
2. Cholinergic sympathetic nerve fibers innervating the vessels of some skeletal muscles.
3. Cholinergic parasympathetic fibers of the vessels of the salivary glands (submandibular, sublingual, parotid), tongue, gonads.
4. Metasympathetic nerve fibers, innervating the vessels of the genital organs.
5. Histaminergic nerve fibers (refer to regional or local mechanisms of regulation).
Vasomotor Center- This is a combination of structures of various levels of the central nervous system that provide regulation of blood supply.
Humoral regulation vascular tone is carried out by biologically active substances and metabolic products. Some substances expand, others constrict blood vessels, some have a dual effect.
1. Vasoconstrictor substances are produced in various cells of the body, but more often in transducer cells (similar to the chromaffin cells of the adrenal medulla). The most powerful substance that narrows arteries, arterioles and, to a lesser extent, veins, is angiotensin, produced in the liver. However, in the blood plasma, it is in an inactive state. It is activated by renin (renin-angiotensin system).
With a decrease in blood pressure, the production of renin in the kidney increases. By itself, renin does not constrict blood vessels; being a proteolytic enzyme, it cleaves plasma a2-globulin (angiotensinogen) and converts it into a relatively inactive decapeptide (angiotensin I). The latter, under the influence of angiotensinase, an enzyme fixed on the cell membranes of the capillary endothelium, turns into angiotensin II, which has a strong vasoconstrictive effect, including on coronary arteries(the mechanism of angiotensin activation is similar to membrane digestion). Angiotensin provides vasoconstriction also by activating the sympathetic-adrenal system. Vasoconstrictor action of angiotensin
on II exceeds the influence of nor-adrenaline by more than 50 times. With a significant increase in blood pressure, renin is produced in smaller quantities, blood pressure decreases - normalizes. In large quantities, angiotensin does not accumulate in the blood plasma, as it is quickly destroyed in the capillaries by angiotensinase. However, in some diseases of the kidneys, as a result of which their blood supply worsens, even with normal initial systemic blood pressure, the amount of renin ejected increases, develops hypertension renal origin.
Vasopressin(ADH - antidiuretic hormone) also constricts blood vessels, its effects are more pronounced at the level of arterioles. However, vasoconstrictive effects are well manifested only with a significant drop in blood pressure. In this case, a large amount of vasopressin is released from the posterior pituitary gland. With the introduction of exogenous vasopressin into the body, vasoconstriction is observed, regardless of baseline blood pressure. Under normal physiological conditions, its vasoconstrictor effect is not manifested.
Norepinephrine acts mainly on a-adrenergic receptors and constricts blood vessels, as a result, peripheral resistance increases, but the effects are small, since the endogenous concentration of norepinephrine is small. With exogenous administration of norepinephrine, blood pressure increases, resulting in reflex bradycardia, the work of the heart decreases, which inhibits the pressor effect.
Vascular center. Levels of central regulation of vascular tone (spinal, bulbar, hypothalomic cortical). Features of reflex and humoral regulation in the circulatory system in children
Vasomotor center - a set of neurons located at different levels of the central nervous system and regulating vascular tone.
The CNS contains next levels
:
spinal;
bulbar;
hypothalamic;
cortical.
2. The role of the spinal cord in the regulation of vascular tone Spinal cord plays a role in the regulation of vascular tone.
Neurons that regulate vascular tone: nuclei of sympathetic and parasympathetic nerves innervating blood vessels. The spinal level of the vasomotor center was discovered in 1870. Ovsyannikov. He cut the central nervous system at various levels and found that in a spinal animal, after the removal of the brain, blood pressure (BP) decreases, but then gradually recovers, although not to the initial level, and is maintained at a constant level.
The spinal level of the vasomotor center is not of great independent importance, it transmits impulses from the higher lying sections of the vasomotor center.
3. The role of the medulla oblongata in the regulation of vascular tone Medulla also plays a role in the regulation of vascular tone.
Bulbar department of the vasomotor center opened: Ovsyannikov and Ditegar(1871-1872). In a bulbar animal, the pressure almost does not change, i.e. in the medulla oblongata is the main center that regulates vascular tone.
Ranson and Alexander. Point irritation of the medulla oblongata, it was found that in the bulbar part of the vasomotor center there are pressor and depressor zones. The pressor zone is in the rostral region, the depressor zone is in the caudal region.
Sergievsky, Valdian. Modern views: the bulbar part of the vasomotor center is located at the level of neurons of the reticular formation of the medulla oblongata. The bulbar part of the vasomotor center contains pressor and depressor neurons. They are located diffusely, but there are more pressor neurons in the rostral region, and depressor neurons in the caudal region. The bulbar part of the vasomotor center contains cardioinhibitory neurons. There are more pressor neurons than depressor neurons. That. with excitation of the vasomotor center - a vasoconstrictor effect.
In the bulbar part of the vasomotor center there are 2 zones: lateral and medial
.
Lateral zone consists of small neurons that perform mainly an afferent function: it receives impulses from the receptors of the heart vessels, internal organs, exteroreceptors. They do not cause a response, but transmit impulses to the neurons of the medial zone.
Medial zone consists of large neurons that perform an efferent function. They do not have direct contacts with receptors, but receive impulses from the lateral zone and transmit impulses to the spinal section of the vasomotor center.
4. Hypothalamic level of regulation of vascular tone Consider the hypothalamic level of the vasomotor center.
When the anterior groups of the nuclei of the hypothalamus are excited, the parasympathetic nervous system is activated - a decrease in tone. Irritation of the posterior nuclei mainly produces a vasoconstrictive effect.
Features of hypothalamic regulation:
carried out as a component of thermoregulation;
the lumen of the vessels changes in accordance with changes in t environment.
The hypothalamic department of the vasomotor center provides the use of skin coloring in emotional reactions. The hypothalamic part of the vasomotor center is closely connected with the bulbar and cortical parts of the vasomotor center.
5. Cortical department of the vasomotor center Methods for studying the role of the cortical department of the vasomotor center.
Irritation method: it was found that the irritated parts of the cerebral cortex, when excited, change the vascular tone. The effect depends on the strength and is most pronounced with stimulation of the anterior central gyrus, frontal and temporal zones of the cerebral cortex.
Conditioned reflex method: it was found that the cerebral cortex provides the development of conditioned reflexes to both dilation and constriction of blood vessels.
Metronome > adrenaline > skin vasoconstriction.
Metronome > saline > skin vasoconstriction.
Conditioned reflexes are developed faster for contraction than for expansion. Due to the cortical section of the vasomotor center, the vascular reaction adapts to changes in environmental conditions.
IN childhood the functional state of nerve cells is very variable: the level of their excitability changes, and strong or prolonged excitation easily turns into inhibition. This feature of nerve cells explains the "instability of the rhythm of heart contractions, which is characteristic of children of early and preschool age." teeth and the duration of the intervals between individual teeth.Unstable and reflex changes in the work of the heart and blood vessels, in particular, their own reflexes circulatory system aimed at maintaining normal blood pressure.
In subsequent years, the stability of both the rhythm of heart contractions and reflex changes in the heart and blood vessels gradually increases. However, for a long time, often up to 15-17 years, increased excitability of the cardiovascular nerve centers persists. This explains the excessive severity of vasomotor and cardiac reflexes in children. They manifest themselves in blanching or, conversely, reddening of the skin of the face, a sinking heart or an increase in its contractions.
Vascular regulation- this is the regulation of vascular tone, which determines the size of their lumen. The lumen of the vessels is determined functional state their smooth muscles, and the lumen of the capillaries depends on the state of the endothelial cells and the smooth muscles of the precapillary sphincter.
Humoral regulation of vascular tone. This regulation is carried out due to those chemicals that circulate in the bloodstream and change the width of the lumen of the vessels. All humoral factors that affect vascular tone are divided into vasoconstrictor(vasoconstrictors) and vasodilating(vasodilators).
Vasoconstrictors include:
adrenaline - hormone of the adrenal medulla, narrows the arterioles of the skin, digestive organs and lungs, in low concentrations dilates the vessels of the brain, heart and skeletal muscles, thereby ensuring adequate redistribution of blood necessary to prepare the body to respond in a difficult situation;
norepinephrine - the hormone of the adrenal medulla is similar in its action to adrenaline, but its action is more pronounced and longer;
vasopressin - a hormone formed in the neurons of the supraoptic nucleus of the hypothalamus, a form in the cells of the posterior pituitary gland, acts mainly on arterioles;
serotonin - is produced by the cells of the intestinal wall, in some parts of the brain, and is also released during the decay platelets; .
The vasodilators are:
histamine - formed in the wall of the stomach, intestines, other organs, dilates arterioles;
acetylcholine - mediator of parasympathetic nerves and sympathetic cholinergic vasodilators, dilates arteries and veins;
bradykinin - isolated from extracts of organs (pancreas, submandibular salivary gland, lungs), formed by the breakdown of one of the blood plasma globulins, dilates the vessels of skeletal muscles, heart, spinal cord and brain, salivary and sweat glands;
prostaglandins - are formed in many organs and tissues, have a local vasodilating effect;
Nervous regulation of vascular tone. Nervous regulation of vascular tone is carried out by the autonomic nervous system. The vasoconstrictor effect is predominantly exerted by fibers sympathetic department autonomic (autonomous) nervous system, and vasodilating - parasympathetic and, in part, sympathetic nerves. The vasoconstrictive action of the sympathetic nerves does not extend to the vessels of the brain, heart, lungs, and working muscles. The vessels of these organs expand when the sympathetic nervous system is stimulated. It should also be noted that not all parasympathetic nerves are vasodilators, for example, fibers of the parasympathetic vagus nerve constrict the vessels of the heart.
Vasoconstrictor and vasodilating nerves are under the influence of vasomotor center. The vasomotor or vasomotor center is a set of structures located at different levels of the central nervous system and providing regulation of blood circulation. The structures that make up the vasomotor center are located mainly in the spinal and medulla oblongata, the hypothalamus, and the cerebral cortex. The vasomotor center consists of pressor and depressor departments.
Depressor department reduces the activity of sympathetic vasoconstrictor influences and, thereby, causes vasodilation, a drop in peripheral resistance and a decrease in blood pressure. Press department causes vasoconstriction, increased peripheral resistance and blood pressure.
The activity of neurons of the vasomotor center is formed nerve impulses, coming from the cerebral cortex, the hypothalamus, the reticular formation of the brain stem, as well as from various receptors, especially those located in the vascular reflexogenic zones.
Baroreceptors. Fluctuations in blood pressure are perceived by special formations located in the wall of blood vessels - baroreceptors , or pressoreceptors. Their excitation occurs as a result of stretching of the arterial wall with increasing pressure; therefore, by the principle of response, they are typical mechanoreceptors. In a light microscope, baroreceptors are visible as wide ramifications of pointed-type nerve endings, freely ending in the adventitia of the vascular wall.
Classification. There are two types of receptors based on their activity. Type A receptors in which the maximum impulse occurs at the time of atrial systole, and type B receptors the discharge of which falls on the time of diastole, i.e. when filling the atria with blood.
Physiological properties of baroreceptors. All baroreceptors have a number of physiological properties that allow them to perform their main function - monitoring blood pressure.
· Each baroreceptor or each group of baroreceptors perceives only its specific parameters of blood pressure changes. Three groups of baroreceptors are distinguished depending on the specifics of reactions to pressure changes.
· With a rapid pressure drop, baroreceptors respond with more pronounced changes in salvo activity than with a slow, gradual change in pressure. With a sharp increase in pressure, already by a small increase, the same increase in impulsation is observed, as with a smooth change in pressure by much larger values.
· Baroreceptors have the ability to increase impulsation exponentially by the same amount of increase in blood pressure, depending on its initial level.
Most baroreceptors perceive fluctuating pressure within their range. When exposed to constant pressure, which is observed with its persistent increase or decrease, they stop responding with an increase in impulses, i.e. adapt. As the pressure increases (0-140 mm Hg), the impulse frequency increases. However, with a persistent increase in the range from 140 to 200 mm Hg. the phenomenon of adaptation occurs - the frequency of impulses remains unchanged.
Vascular regulation- this is the regulation of vascular tone, which determines the size of their lumen. The lumen of the vessels is determined by the functional state of their smooth muscles, and the lumen of the capillaries depends on the state of the endothelial cells and smooth muscles of the precapillary sphincter.
Humoral regulation of vascular tone. This regulation is carried out due to those chemicals that circulate in the bloodstream and change the width of the lumen of the vessels. All humoral factors that affect vascular tone are divided into vasoconstrictor(vasoconstrictors) and vasodilating(vasodilators).
Vasoconstrictors include:
adrenaline - hormone of the adrenal medulla, narrows the arterioles of the skin, digestive organs and lungs, in low concentrations dilates the vessels of the brain, heart and skeletal muscles, thereby ensuring adequate redistribution of blood necessary to prepare the body to respond in a difficult situation;
norepinephrine - the hormone of the adrenal medulla is similar in its action to adrenaline, but its action is more pronounced and longer;
vasopressin - a hormone formed in the neurons of the supraoptic nucleus of the hypothalamus, a form in the cells of the posterior pituitary gland, acts mainly on arterioles;
serotonin - produced by the cells of the intestinal wall, in some parts of the brain, and also released during the breakdown of platelets; .
The vasodilators are:
histamine - formed in the wall of the stomach, intestines, other organs, dilates arterioles;
acetylcholine - mediator of parasympathetic nerves and sympathetic cholinergic vasodilators, dilates arteries and veins;
bradykinin - isolated from extracts of organs (pancreas, submandibular salivary gland, lungs), formed by the breakdown of one of the blood plasma globulins, dilates the vessels of skeletal muscles, heart, spinal cord and brain, salivary and sweat glands;
prostaglandins - are formed in many organs and tissues, have a local vasodilating effect;
Nervous regulation of vascular tone. Nervous regulation of vascular tone is carried out by the autonomic nervous system. The vasoconstrictor effect is predominantly exerted by the fibers of the sympathetic division of the autonomic (autonomous) nervous system, and the vasodilating effect is exerted by parasympathetic and, partially, sympathetic nerves. The vasoconstrictive action of the sympathetic nerves does not extend to the vessels of the brain, heart, lungs, and working muscles. The vessels of these organs expand when the sympathetic nervous system is stimulated. It should also be noted that not all parasympathetic nerves are vasodilators, for example, fibers of the parasympathetic vagus nerve constrict the vessels of the heart.
Vasoconstrictor and vasodilating nerves are under the influence of vasomotor center. The vasomotor or vasomotor center is a set of structures located at different levels of the central nervous system and providing regulation of blood circulation. The structures that make up the vasomotor center are located mainly in the spinal and medulla oblongata, the hypothalamus, and the cerebral cortex. The vasomotor center consists of pressor and depressor departments.
Depressor department reduces the activity of sympathetic vasoconstrictor influences and, thereby, causes vasodilation, a drop in peripheral resistance and a decrease in blood pressure. Press department causes vasoconstriction, increased peripheral resistance and blood pressure.
The activity of the neurons of the vasomotor center is formed by nerve impulses coming from the cerebral cortex, the hypothalamus, the reticular formation of the brain stem, as well as from various receptors, especially those located in the vascular reflex zones.
Baroreceptors. Fluctuations in blood pressure are perceived by special formations located in the wall of blood vessels - baroreceptors , or pressoreceptors. Their excitation occurs as a result of stretching of the arterial wall with increasing pressure; therefore, by the principle of response, they are typical mechanoreceptors. In a light microscope, baroreceptors are visible as wide ramifications of pointed-type nerve endings, freely ending in the adventitia of the vascular wall.
Classification. There are two types of receptors based on their activity. Type A receptors in which the maximum impulse occurs at the time of atrial systole, and type B receptors the discharge of which falls on the time of diastole, i.e. when filling the atria with blood.
Physiological properties of baroreceptors. All baroreceptors have a number of physiological properties that allow them to perform their main function - monitoring blood pressure.
· Each baroreceptor or each group of baroreceptors perceives only its specific parameters of blood pressure changes. Three groups of baroreceptors are distinguished depending on the specifics of reactions to pressure changes.
· With a rapid pressure drop, baroreceptors respond with more pronounced changes in salvo activity than with a slow, gradual change in pressure. With a sharp increase in pressure, already by a small increase, the same increase in impulsation is observed, as with a smooth change in pressure by much larger values.
· Baroreceptors have the ability to increase impulsation exponentially by the same amount of increase in blood pressure, depending on its initial level.
Most baroreceptors perceive fluctuating pressure within their range. When exposed to constant pressure, which is observed with its persistent increase or decrease, they stop responding with an increase in impulses, i.e. adapt. As the pressure increases (0-140 mm Hg), the impulse frequency increases. However, with a persistent increase in the range from 140 to 200 mm Hg. the phenomenon of adaptation occurs - the frequency of impulses remains unchanged.