The vessels divide. General characteristics of blood vessels. Arteries of mixed, or muscular-elastic type

Arteries are blood vessels through which blood flows from the heart to organs and parts of the body. Arteries have thick walls consisting of three layers. The outer layer is represented by a connective tissue membrane and is called adventitia. The middle layer, or media, consists of smooth muscle tissue and contains connective tissue elastic fibers. The inner layer, or intima, is formed by the endothelium, under which there is a subendothelial layer and an internal elastic membrane. The elastic elements of the arterial wall form a single frame that works like a spring and determines the elasticity of the arteries. Depending on the organs and tissues supplied with blood, arteries are divided into parietal (parietal), which supply blood to the walls of the body, and visceral (visceral), which supply blood to internal organs. Before an artery enters an organ, it is called extraorgan; after entering an organ, it is called intraorgan, or intraorgan.

Depending on the development of the various layers of the wall, arteries of the muscular, elastic or mixed type are distinguished. Arteries of the muscular type have a well-developed middle tunica, the fibers of which are arranged spirally like a spring. These vessels include small arteries. Mixed arteries have approximately equal numbers of elastic and muscle fibers in their walls. These are the carotid, subclavian and other arteries of medium diameter. Elastic arteries have a thin outer shell and a thicker inner shell. They are represented by the aorta and pulmonary trunk, into which blood flows under high pressure. Lateral branches of one trunk or branches of different trunks can connect to each other. This connection of arteries before they break up into capillaries is called anastomosis, or anastomosis. The arteries that form anastomoses are called anastomosing (they are the majority). Arteries that do not have anastomoses are called terminal (for example, in the spleen). Terminal arteries are more easily clogged by a thrombus and are predisposed to the development of a heart attack.

After the birth of a child, the circumference, diameter, wall thickness and length of the arteries increase, and the level of departure of the arterial branches from the great vessels also changes. The difference between the diameter of the main arteries and their branches is small at first, but increases with age. The diameter of the main arteries grows faster than their branches. With age, the circumference of the arteries also increases, their length increases in proportion to the growth of the body and limbs. The levels of branches from the main arteries in newborns are located more proximally, and the angles at which these vessels depart are greater in children than in adults. The radius of curvature of the arcs formed by the vessels also changes. In proportion to the growth of the body and limbs and the increase in the length of the arteries, the topography of these vessels changes. As age increases, the type of branching of arteries changes: mainly from scattered to main. The formation, growth, and tissue differentiation of vessels of the intraorgan bloodstream in various human organs proceed unevenly during ontogenesis. The wall of the arterial section of intraorgan vessels, in contrast to the venous section, already has three membranes at the time of birth. After birth, the length and diameter of intraorgan vessels, the number of anastomoses, and the number of vessels per unit volume of the organ increase. This occurs especially intensively before the age of one year and from 8 to 12 years.

The smallest branches of the arteries are called arterioles. They differ from arteries in the presence of only one layer of muscle cells, thanks to which they perform a regulatory function. The arteriole continues into the precapillary, in which the muscle cells are scattered and do not form a continuous layer. The precapillary is not accompanied by a venule. Numerous capillaries extend from it.

At the points of transition of one type of vessel to another, smooth muscle cells are concentrated, forming sphincters that regulate blood flow at the microcirculatory level.

Capillaries are the smallest blood vessels with a lumen from 2 to 20 microns. The length of each capillary does not exceed 0.3 mm. Their number is very large: for example, there are several hundred capillaries per 1 mm2 of tissue. The total lumen of the capillaries of the whole body is 500 times larger than the lumen of the aorta. In the resting state of the organ, most of the capillaries do not function and the blood flow in them stops. The capillary wall consists of a single layer of endothelial cells. The surface of the cells facing the lumen of the capillary is uneven and folds form on it. This promotes phagocytosis and pinocytosis. There are feeding and specific capillaries. Feeding capillaries provide the organ with nutrients, oxygen and remove metabolic products from the tissues. Specific capillaries help the organ perform its functions (gas exchange in the lungs, excretion in the kidneys). Merging, the capillaries pass into postcapillaries, which are similar in structure to the precapillary. Postcapillaries merge into venules with a lumen of 4050 µm.

Veins are blood vessels that carry blood from organs and tissues to the heart. They, like arteries, have walls consisting of three layers, but contain fewer elastic and muscle fibers, therefore they are less elastic and collapse easily. Veins have valves that open as the blood flows, allowing blood to flow in one direction. The valves are semilunar folds of the inner membrane and are usually located in pairs at the confluence of two veins. In the veins of the lower limb, blood moves against the force of gravity, the muscular layer is better developed and valves are more common. They are absent in the vena cava (hence their name), the veins of almost all internal organs, the brain, head, neck and small veins.

Arteries and veins usually go together, with large arteries supplied by one vein, and medium and small ones by two companion veins that anastomose with each other many times. As a result, the total capacity of the veins is 10-20 times greater than the volume of the arteries. Superficial veins running in the subcutaneous tissue do not accompany the arteries. Veins, together with the main arteries and nerve trunks, form neurovascular bundles. According to their function, blood vessels are divided into pericardial, main and organ. The pericardium begins and ends both circles of blood circulation. These are the aorta, pulmonary trunk, vena cava and pulmonary veins. The great vessels serve to distribute blood throughout the body. These are large extraorgan arteries and veins. Organ vessels provide exchange reactions between blood and organs.

By the time of birth, the vessels are well developed, and the arteries are larger than the veins. The structure of blood vessels changes most intensively between the ages of 1 and 3 years. At this time, the middle shell is intensively developing, the final shape and size of the blood vessels are formed by 1418. Starting from 40-45 years, the inner membrane thickens, fat-like substances are deposited in it, and atherosclerotic plaques appear. At this time, the walls of the arteries become sclerotic, and the lumen of the vessels decreases.

General characteristics of the respiratory system. Fetal breathing. Pulmonary ventilation in children of different ages. Age-related changes in the depth, frequency of breathing, vital capacity of the lungs, regulation of breathing.

The respiratory organs provide the body with oxygen necessary for oxidation processes and the release of carbon dioxide, which is the end product of metabolic processes. The need for oxygen is more important for humans than the need for food or water. Without oxygen, a person dies within 57 minutes, while without water he can live up to 710 days, and without food - up to 60 days. Cessation of breathing leads to the death of first of all nerve cells and then other cells. There are three main processes in breathing: the exchange of gases between the environment and the lungs (external respiration), the exchange of gases in the lungs between alveolar air and blood, and the exchange of gases between blood and interstitial fluid (tissue respiration).

The inhalation and exhalation phases make up the respiratory cycle. The volume of the thoracic cavity changes due to contractions of the inspiratory and expiratory muscles. The main inspiratory muscle is the diaphragm. During a quiet inhalation, the dome of the diaphragm lowers by 1.5 cm. The inspiratory muscles also include the external oblique intercostal and intercartilaginous muscles, with the contraction of which the ribs rise, the sternum moves forward, and the lateral parts of the ribs move to the sides. With very deep breathing, a number of auxiliary muscles are involved in the act of inhalation: sternocleidomastoid, scalenes, pectoralis major and minor, serratus anterior, as well as muscles that extend the spine and fix the shoulder girdle (trapezius, rhomboid, levator scapula).

With active exhalation, the muscles of the abdominal wall (oblique, transverse and rectus) contract, as a result the volume of the abdominal cavity decreases and the pressure in it increases, it is transmitted to the diaphragm and raises it. Due to the contraction of the internal oblique and intercostal muscles, the ribs descend and move closer together. Accessory expiratory muscles include the spinal flexor muscles.

The respiratory tract is formed by the nasal cavity, nasal and oropharynx, larynx, trachea, bronchi of various calibers, including bronchioles.

Blood vessels develop from mesenchyme. First, the primary wall is formed, which subsequently turns into the inner lining of the vessels. Mesenchyme cells, connecting, form the cavity of future vessels. The wall of the primary vessel consists of flat mesenchymal cells that form the inner layer of future vessels. This layer of flat cells belongs to the endothelium. Later, the final, more complex vessel wall is formed from the surrounding mesenchyme. It is characteristic that all vessels in the embryonic period are laid down and built as capillaries, and only in the process of their further development is the simple capillary wall gradually surrounded by various structural elements, and the capillary vessel turns into either an artery, a vein, or a lymphatic vessel.

The final formed walls of the vessels of both arteries and veins are not the same along their entire length, but both of them consist of three main layers (Fig. 231). Common to all vessels is a thin inner membrane, or intima (tunica intima), lined on the side of the vascular cavity with the thinnest, very elastic and flat polygonal endothelial cells. The intima is a direct continuation of the endothelium and endocardium. This inner lining with a smooth and even surface protects the blood from clotting. If the endothelium of a vessel is damaged by injury, infection, inflammatory or degenerative process, etc., then small blood clots (blood clots) form at the site of damage, which can increase in size and cause blockage of the vessel. Sometimes they break away from the site of formation, are carried away by the blood stream and, as so-called emboli, clog a vessel in some other place. The effect of such a thrombus or embolus depends on where the vessel is blocked. Thus, blockage of a vessel in the brain can cause paralysis; A blockage in the coronary artery of the heart deprives the heart muscle of blood flow, resulting in a severe heart attack and often leading to death. Blockage of a vessel leading to any part of the body or internal organ deprives it of nutrition and can lead to necrosis (gangrene) of the supplied part of the organ.

Outside the inner layer is the middle shell (media), consisting of circular smooth muscle fibers with an admixture of elastic connective tissue.

The outer shell of the vessels (adventitia) covers the middle one. In all vessels it is built of fibrous fibrous connective tissue, containing predominantly longitudinally located elastic fibers and connective tissue cells.

At the border of the middle and inner, middle and outer shells of blood vessels, elastic fibers form a kind of thin plate (membrana elastica interna, membrana elastica externa).

In the outer and middle membranes of blood vessels, the vessels that feed their wall (vasa vasorum) branch.

The walls of capillary vessels are extremely thin (about 2 μ) and consist mainly of a layer of endothelial cells that form the capillary tube. This endothelial tube is braided on the outside with a thin network of fibers on which it is suspended, thanks to which it moves very easily and without damage. The fibers extend from a thin, main film, with which special cells are also associated - pericytes, covering the capillaries. The capillary wall is easily permeable to leukocytes and blood; It is at the level of capillaries through their wall that exchange takes place between blood and tissue fluids, as well as between blood and the external environment (in the excretory organs).

Arteries and veins are usually divided into large, medium and small. The smallest arteries and veins that turn into capillaries are called arterioles and venules. The arteriole wall consists of all three membranes. The innermost is endothelial, and the next middle one is built from circularly arranged smooth muscle cells. When an arteriole passes into a capillary, only single smooth muscle cells are observed in its wall. With the enlargement of the arteries, the number of muscle cells gradually increases to a continuous annular layer - a muscle-type artery.

The structure of small and medium arteries differs in some other feature. Under the inner endothelial membrane there is a layer of elongated and stellate cells, which in larger arteries form a layer that plays the role of cambium (germ layer) for blood vessels. This layer is involved in the processes of regeneration of the vessel wall, i.e. it has the property of restoring the muscular and endothelial layers of the vessel. In arteries of medium caliber or mixed type, the cambial (germ) layer is more developed.

Large-caliber arteries (aorta and its large branches) are called elastic arteries. Elastic elements predominate in their walls; in the middle shell, strong elastic membranes are concentrically laid, between which lies a significantly smaller number of smooth muscle cells. The cambial layer of cells, well defined in small and medium-sized arteries, in large arteries turns into a layer of subendothelial loose connective tissue rich in cells.

Due to the elasticity of the walls of the arteries, like rubber tubes, they can easily stretch under the pressure of blood and do not collapse, even if the blood is released from them. All the elastic elements of the vessels together form a single elastic frame, which works like a spring, each time returning the vessel wall to its original state as soon as the smooth muscle fibers relax. Since arteries, especially large ones, have to withstand fairly high blood pressure, their walls are very strong. Observations and experiments show that arterial walls can withstand even such strong pressure as occurs in the steam boiler of a conventional locomotive (15 atm.).

The walls of veins are usually thinner than the walls of arteries, especially their tunica media. There is also significantly less elastic tissue in the venous wall, so the veins collapse very easily. The outer shell is made of fibrous connective tissue, which is dominated by collagen fibers.

A feature of the veins is the presence of valves in them in the form of semilunar pockets (Fig. 232), formed from doubling the inner membrane (intima). However, not all veins in our body have valves; The veins of the brain and its membranes, the veins of the bones, as well as a significant part of the veins of the viscera, lack them. Valves are more often found in the veins of the limbs and neck; they are open towards the heart, i.e. in the direction of blood flow. By blocking the backflow that can occur due to low blood pressure and the law of gravity (hydrostatic pressure), the valves facilitate blood flow.

If there were no valves in the veins, the entire weight of a column of blood more than 1 m high would put pressure on the blood entering the lower limb and thereby greatly impede blood circulation. Further, if the veins were inflexible tubes, the valves alone could not ensure blood circulation, since the entire column of liquid would still press on the underlying sections. Veins are located among large skeletal muscles, which, contracting and relaxing, periodically compress the venous vessels. When a contracting muscle compresses a vein, the valves located below the clamping point close, and those located above open; when the muscle relaxes and the vein is again free from compression, the upper valves in it close and retain the upper column of blood, while the lower ones open and allow the vessel to refill with blood coming from below. This pumping action of the muscles (or "muscle pump") greatly aids blood circulation; standing for many hours in one place, in which the muscles help little to move the blood, is more tiring than walking.

Blood vessels get their name depending on the organ they supply (renal artery, splenic vein), where they arise from a larger vessel (superior mesenteric artery, inferior mesenteric artery), the bone to which they are adjacent (ulnar artery), direction (medial artery surrounding the thigh), depth (superficial or deep artery), Many small arteries are called branches, and veins are called tributaries.

Arteries . Depending on the area of ​​branching, the arteries are divided into parietal (parietal), which supply blood to the walls of the body, and visceral (internal), which supply blood to the internal organs. Before an artery enters an organ, it is called organ; after entering an organ, it is called intraorgan. The latter branches within the organ and supplies its individual structural elements.

Each artery breaks down into smaller vessels. With the main type of branching, lateral branches arise from the main trunk - the main artery, the diameter of which gradually decreases. With the tree-like type of branching, the artery immediately after its origin is divided into two or several terminal branches, resembling the crown of a tree.

The artery wall consists of three membranes: inner, middle and outer. The inner shell is formed by the endothelium, subendothelial layer and internal elastic membrane. Endotheliocytes line the lumen of the vessel. They are elongated along its longitudinal axis and have slightly tortuous boundaries. The subendothelial layer consists of thin elastic and collagen fibers and poorly differentiated connective tissue cells. On the outside there is an internal elastic membrane. The medial layer of the artery consists of spirally arranged myocytes, between which there is a small amount of collagen and elastic fibers, and an outer elastic membrane formed by intertwining elastic fibers. The outer shell consists of loose fibrous unformed connective tissue containing elastic and collagen fibers.

Depending on the development of the various layers of the artery wall, they are divided into vessels of the muscular, mixed (muscle-elastic) and elastic types. In the walls of arteries of the muscular type, which have a small diameter, the middle membrane is well developed. The myocytes of the middle lining of the walls of muscular arteries regulate blood flow to organs and tissues through their contractions. As the diameter of the arteries decreases, all the membranes of the walls become thinner, and the thickness of the subendothelial layer and internal elastic membrane decreases.

Fig. 102. Scheme of the structure of the wall of an artery (A) and vein (B) of the medium-caliber muscular type / -inner membrane: 1-endothelium. 2-basal membrane, 3-subendothelial layer, 4-internal elastic membrane; // - tunica media and in it: 5- myocytes, b-elastic fibers, 7-collagen fibers; /// - outer shell and in it: 8- outer elastic membrane, 9- fibrous (loose) connective tissue, 10- blood vessels

The number of myocytes and elastic fibers in the middle shell gradually decreases. The number of elastic fibers in the outer shell decreases, and the outer elastic membrane disappears.

The thinnest arteries of the muscular type - arterioles - have a diameter of less than 10 microns and pass into capillaries. The walls of arterioles lack an internal elastic membrane. The middle shell is formed by individual myocytes, which have a spiral direction, between which there is a small number of elastic fibers. The outer elastic membrane is expressed only in the walls of the largest arterioles and is absent in small ones. The outer shell contains elastic and collagen fibers. Arterioles regulate blood flow into the capillary system. Mixed-type arteries include large-caliber arteries such as the carotid and subclavian. In the middle shell of their walls there is approximately an equal number of elastic fibers and myocytes. The internal elastic membrane is thick and durable. In the outer shell of the walls of mixed-type arteries, two layers can be distinguished: the inner layer, containing individual bundles of myocytes, and the outer layer, consisting mainly of longitudinally and obliquely located bundles of collagen and elastic fibers. The elastic type arteries expose the aorta and pulmonary trunk, into which blood flows under high pressure at high speed from the heart. ; On the walls of these vessels, the inner lining is thicker; the internal elastic membrane is represented by a dense plexus of thin elastic fibers. The middle shell is formed by elastic membranes located concentrically, between which myocytes lie. The outer shell is thin. In children, the diameter of the arteries is relatively larger than in adults. In a newborn, the arteries are predominantly of the elastic type; their walls contain a lot of elastic tissue. The arteries of the muscle phlegm are not yet developed.

The distal part of the cardiovascular system is the microcirculatory bed (Fig. 103), which ensures the interaction of blood and tissues. The microcirculatory bed begins with the smallest arterial vessel - the arteriole and ends with the venule.

The artery wall contains only one row of myocytes. Precapillaries extend from the arteriole, at the beginning of which there are smooth muscle precapillary sphincters that regulate blood flow. In the walls of precapillaries, unlike capillaries, single myocytes lie on top of the endothelium. True capillaries begin from them. True capillaries flow into postcapillaries (postcapillary venules). Postcapillaries are formed from the fusion of two or more capillaries. They have a thin adventitial membrane, their walls are extensible and have high permeability. As the postcapillaries merge, venules are formed. Their caliber varies widely and under normal conditions is 25-50 microns. Venules merge into veins. Within the microcirculatory bed there are vessels for the direct transfer of blood from the arteriole to the venule-arteriolo-venular anastomoses, in the walls of which there are myocytes that regulate blood discharge. The microvasculature also includes lymphatic capillaries.

Typically, an arterial type vessel (arteriole) approaches the capillary network, and a venule emerges from it. In some organs (kidney, liver) there is a deviation from this rule. Thus, an arteriole (afferent vessel) approaches the glomerulus of the renal corpuscle. An arteriole (an efferent vessel) also leaves the glomerulus. 8 of the liver, the capillary network is located between the afferent (interlobular) and efferent (central) veins. A capillary network inserted between two vessels of the same type (arteries, veins) is called a miraculous network.

Capillaries . Blood capillaries (hemocapillaries) have walls formed by one layer of flattened endothelial cells - endothelial cells, a continuous or discontinuous basement membrane and rare pericapillary cells - pericytes, or Rouget cells.

Endotheliocytes lie on the basement membrane (basal layer), which surrounds the blood capillary on all sides. The basal layer consists of fibrils intertwined with each other and an amorphous substance. Outside the basal layer lie Rouget cells, which are elongated multi-processed cells located along the long axis of the capillaries. It should be emphasized that each endothelial cell is in contact with pericyte processes. In turn, each pericyte is approached by the ending of the axon of the sympathetic neuron, which, as it were, extends into its plasmalemma. The pericyte transmits an impulse to the endothelial cell, causing the endothelial cell to swell or lose fluid. This leads to periodic changes in the lumen of the capillary.

The cytoplasm of endothelial cells may have pores, or fenestrae (porous endotheliocyte). Non-cellular component - the basal layer can be solid, absent or porous. Depending on this, three types of capillaries are distinguished:

1. Capillaries with continuous endothelium and basal layer. Such capillaries are located in the skin; striated (striated) muscles, including the myocardium, and non-striated (smooth); cerebral cortex.

2. Fenestrated capillaries, in which some areas of endothelial cells are thinned.

3. Sinusoidal capillaries have a large lumen, up to 10 microns. Their endothelial cells contain mora, and the basement membrane is partially absent (discontinuous). Such capillaries are located in the liver, spleen, and bone marrow.

Postcapillary venules with a diameter of 100-300 µm, which are the final link of the microvasculature, flow into collecting venules (with a diameter of 100-300 µm). which, merging with each other, become larger. The structure of postcapillary venules over a considerable extent is similar to the structure of the walls of capillaries, they only have a wider lumen and a larger number of pericytes. The collecting venules have an outer membrane formed by collagen fibers and fibroblasts. In the middle shell of the wall of larger venules there are 1-2 layers of smooth muscle cells, the number of their layers increases in the collecting foams,

Vienna . The vein wall also consists of three membranes. There are two types of veins: amuscular and muscular types. In amuscular veins, a basement membrane is adjacent to the endothelium on the outside, behind which there is a thin layer of loose fibrous connective tissue. Non-muscular veins include the veins of the dura and pia mater, retina, bones, spleen and placenta. They are tightly fused with the walls of the organs and therefore do not collapse.

Veins of the muscular type have a well-defined muscular layer formed by circularly arranged bundles of myocytes separated by layers of fibrous connective tissue. There is no outer elastic membrane. The outer connective tissue membrane is well developed. There are valves on the inner lining of most medium-sized and some large veins (Fig. 104). Superior vena cava, brachiocephalic, common iliac, veins of the heart, lungs. adrenal glands, brain and their membranes, parenchymal organs do not have valves. The valves are thin folds of the inner membrane, consisting of fibrous connective tissue, covered on both sides with endothelial cells. They allow blood to pass only towards the heart, prevent the reverse flow of blood in the veins and protect the heart from unnecessary energy expenditure to overcome the oscillatory movements of the blood that constantly occur in the veins. The venous sinuses of the dura mater, which drain blood from the brain, have non-collapsing walls that ensure unimpeded flow of blood from the cranial cavity into the extracranial veins (internal jugular).

The total number of veins is greater than the number of arteries, and the total size of the venous bed exceeds the arterial one. The speed of blood flow in the veins is less than in the arteries; in the veins of the torso and lower extremities, blood flows against gravity. The names of many deep veins of the extremities are similar to the names of the arteries that they accompany in pairs - companion veins (ulnar artery - ulnar veins, radial artery - radial veins).

Most veins located in body cavities are single. The unpaired deep veins are the internal jugular, subclavian, axillary, iliac (common, external and internal), femoral and some others. Superficial veins are connected to deep veins with the help of perforating veins, which act as anastomoses. Neighboring veins are also interconnected by numerous anastomoses, collectively forming venous plexuses, which are well expressed on the surface or in the walls of some internal organs (bladder, rectum).

The superior and inferior vena cava of the greater circulation drain into the heart. The system of the inferior hollow foam includes the portal vein and its tributaries. The roundabout flow of blood also occurs through collateral veins, but through them the blood flows out and bypasses the main path. The tributaries of one large (main) vein are connected to each other by intrasystemic venous anastomoses. Venous anastomoses are more common and better developed than arterial anastomoses.

The small, or pulmonary, circle of blood circulation begins in the right ventricle of the heart, from where the pulmonary trunk emerges, which is divided into the right and left pulmonary arteries, and the latter branch in the lungs into arteries that turn into capillaries. In the capillary networks entwining the alveoli, the blood gives off carbon dioxide and enriched with oxygen. Oxygen-enriched arterial blood flows from the capillaries into the veins, which, merging into four pulmonary veins (two on each side), flow into the left atrium, where the pulmonary (pulmonary) circulation ends.

The systemic, or bodily, circulation serves to deliver nutrients and oxygen to all organs and tissues of the body. It begins in the left ventricle of the heart, where arterial blood flows from the left atrium. The aorta emerges from the left ventricle, from which arteries extend to all organs and tissues of the body and branch in their thickness up to the arterioles and capillaries. The latter pass into venules and then into veins. Through the walls of capillaries, metabolism and gas exchange occurs between the blood and body tissues. The arterial crawl flowing in the capillaries releases nutrients and oxygen and receives metabolic products and carbon dioxide. Bens stick together into two large trunks - the superior and inferior vena cava, which flow into the right atrium of the heart, where the systemic circulation ends. In addition to the great circle, there is a third (cardiac) circle of blood circulation, serving the heart itself. It begins with the coronary arteries emerging from the aorta and ends with the veins of the heart. The latter stick together into the coronary sinus, which flows into the right atrium, and the remaining smallest veins open directly into the cavity of the right atrium and ventricle.

The course of the arteries and the blood supply to various organs depend on their structure, function and development and are subject to a number of laws. Large arteries are located according to the skeleton and nervous system. Thus, the aorta lies along the spinal column. On the limbs of the bone there is one main artery.

The arteries go to the corresponding organs along the shortest path, that is, approximately along a straight line connecting the main trunk with the organ. Therefore, each artery supplies blood to nearby organs. If an organ moves during the prenatal period, the artery, lengthening, follows it to the place of its final location (for example, diaphragm, testicle). The arteries are located on the shorter flexor surfaces of the body. Articular arterial networks are formed around the joints. Protection from damage and compression is provided by the bones of the skeleton, various grooves and channels formed by bones, mice, and fascia.

Arteries enter organs through the gate located on their bent medial or internal surface facing the source of blood supply. Moreover, the diameter of the arteries and the nature of their branching depend on the size and functions of the organ.

The human body is completely riddled with blood vessels. These peculiar highways ensure continuous delivery of blood from the heart to the most distant parts of the body. Thanks to the unique structure of the circulatory system, each organ receives a sufficient amount of oxygen and nutrients. The total length of blood vessels is about 100 thousand km. This is really so, although it is hard to believe. The movement of blood through the vessels is ensured by the heart, which acts as a powerful pump.

To understand the answer to the question: how the human circulatory system works, you need, first of all, to carefully study the structure of blood vessels. In simple terms, these are strong elastic tubes through which blood moves.

Blood vessels branch throughout the body, but ultimately form a closed circuit. For normal blood flow, there must always be excess pressure in the vessel.

The walls of blood vessels consist of 3 layers, namely:

• The first layer is epithelial cells. The fabric is very thin and smooth, providing protection against blood elements.
• The second layer is the densest and thickest. Consists of muscle, collagen and elastic fibers. Thanks to this layer, blood vessels have strength and elasticity.
• The outer layer consists of connective fibers with a loose structure. Thanks to this fabric, the vessel can be securely fixed to different parts of the body.

Blood vessels additionally contain nerve receptors that connect them to the central nervous system. Thanks to this structure, nervous regulation of blood flow is ensured. In anatomy, there are three main types of vessels, each of which has its own functions and structure.

Arteries

The main vessels that transport blood directly from the heart to the internal organs are called aortas. Very high pressure is constantly maintained inside these elements, so they must be as dense and elastic as possible. Doctors distinguish two types of arteries.

Elastic. The largest blood vessels that are located in the human body closest to the heart muscle. The walls of such arteries and the aorta are made of dense elastic fibers that can withstand continuous heartbeats and sudden surges of blood. The aorta can expand, filling with blood, and then gradually return to its original size. It is thanks to this element that the continuity of blood circulation is ensured.

Muscular. Such arteries are smaller in size compared to the elastic type of blood vessels. Such elements are removed from the heart muscle and are located near peripheral internal organs and systems. The walls of muscle arteries can contract strongly, allowing blood to flow even at low pressure.

The main arteries supply all internal organs with a sufficient amount of blood. Some circulatory elements are located around the organs, while others go directly into the liver, kidneys, lungs, etc. The arterial system is very branched, it can smoothly turn into capillaries or veins. Small arteries are called arterioles. Such elements can directly participate in the self-regulation system, since they consist of only one layer of muscle fibers.

Capillaries

Capillaries are the smallest peripheral vessels. They can freely penetrate any tissue, as a rule, they are located between larger veins and arteries.

The main function of microscopic capillaries is to transport oxygen and nutrients from the blood to the tissues. Blood vessels of this type are very thin, so they consist of only one layer of epithelium. Thanks to this feature, useful elements can easily penetrate through their walls.

There are two types of capillaries:

• Open – constantly involved in the blood circulation process;
• Closed ones are, as it were, in reserve.

1 mm of muscle tissue can accommodate from 150 to 300 capillaries. When muscles are under stress, they need more oxygen and nutrients. In this case, reserve closed blood vessels are additionally used.

Vienna

The third type of blood vessel is veins. Their structure is the same as arteries. However, their function is completely different. After the blood has given up all its oxygen and nutrients, it rushes back to the heart. At the same time, it is transported precisely through the veins. The pressure in these blood vessels is reduced, so their walls are less dense and thick, and their middle layer is less thin than in the arteries.

The venous system is also very branched. In the area of ​​the upper and lower extremities there are small veins, which gradually increase in size and volume towards the heart. The outflow of blood is ensured by back pressure in these elements, which is formed during contraction of muscle fibers and exhalation.

Diseases

In medicine, there are many pathologies of blood vessels. Such diseases can be congenital or acquired throughout life. Each type of vessel may have one or another pathology.

Vitamin therapy is the best prevention of diseases of the circulatory system. Saturating the blood with useful microelements allows you to make the walls of arteries, veins and capillaries stronger and more elastic. People at risk of developing vascular pathologies must additionally include the following vitamins in their diet:

• C and R. These microelements strengthen the walls of blood vessels and prevent capillary fragility. Contained in citrus fruits, rose hips, and fresh herbs. You can also additionally use Troxevasin medicinal gel.
• Vitamin B. To enrich your body with these microelements, include legumes, liver, cereal porridge, and meat in your menu.
• AT 5. Chicken meat, eggs, and broccoli are rich in this vitamin.

Eat oatmeal with fresh raspberries for breakfast, and your blood vessels will always be healthy. Dress salads with olive oil, and for drinks, give preference to green tea, rosehip infusion or fresh fruit compote.

The circulatory system performs the most important functions in the body - it delivers blood to all tissues and organs. Always take care of the health of your blood vessels, undergo regular medical examinations, and take all necessary tests.

Blood circulation (video)

Blood vessels are a closed system of branched tubes of different diameters that are part of the systemic and pulmonary circulation. This system distinguishes: arteries, through which blood flows from the heart to organs and tissues, veins- through them the blood returns to the heart, and the complex of blood vessels microvasculature, providing, along with the transport function, the exchange of substances between the blood and surrounding tissues.

Blood vessels are developing from mesenchyme. In embryogenesis, the earliest period is characterized by the appearance of numerous cellular accumulations of mesenchyme in the wall of the yolk sac - blood islands. Inside the islet, blood cells are formed and a cavity is formed, and the cells located along the periphery become flat, connect to each other using cell contacts and form the endothelial lining of the resulting tube. As they form, such primary blood tubes interconnect and form a capillary network. The surrounding mesenchymal cells develop into pericytes, smooth muscle cells, and adventitial cells. In the body of the embryo, blood capillaries are formed from mesenchymal cells around slit-like spaces filled with tissue fluid. When blood flow through the vessels increases, these cells become endothelial, and elements of the middle and outer membrane are formed from the surrounding mesenchyme.

The vascular system has a very large plasticity. First of all, there is significant variability in the density of the vascular network, since depending on the organ’s needs for nutrients and oxygen, the amount of blood brought to it varies widely. Changes in blood flow speed and blood pressure lead to the formation of new vessels and the restructuring of existing vessels. There is a transformation of a small vessel into a larger one with characteristic features of the structure of its wall. The greatest changes occur in the vascular system with the development of roundabout, or collateral, circulation.

Arteries and veins are built according to a single plan - three membranes are distinguished in their walls: internal (tunica intima), middle (tunica media) and external (tunica adventicia). However, the degree of development of these membranes, their thickness and tissue composition are closely related to the function performed by the vessel and hemodynamic conditions (blood pressure and blood flow velocity), which are not the same in different parts of the vascular bed.

Arteries. According to the structure of the walls, arteries of the muscular, muscular-elastic and elastic types are distinguished.

To elastic arteries include the aorta and pulmonary artery. In accordance with the high hydrostatic pressure (up to 200 mm Hg) created by the pumping activity of the ventricles of the heart, and the high speed of blood flow (0.5 - 1 m/s), these vessels have pronounced elastic properties, which ensure the strength of the wall when stretched and returning to its original position, and also contribute to the transformation of pulsating blood flow into a constant continuous one. The wall of elastic arteries is distinguished by its considerable thickness and the presence of a large number of elastic elements in the composition of all membranes.

Inner shell consists of two layers - endothelial and subendothelial. Endothelial cells that form a continuous internal lining have different sizes and shapes and contain one or more nuclei. Their cytoplasm contains few organelles and many microfilaments. Beneath the endothelium is the basement membrane. The subendothelial layer consists of loose, fine-fibrous connective tissue, which, along with a network of elastic fibers, contains poorly differentiated stellate-shaped cells, macrophages, and smooth muscle cells. The amorphous substance of this layer, which is of great importance for the nutrition of the wall, contains a significant amount of glycosaminoglycans. When the wall is damaged and a pathological process (atherosclerosis) develops, lipids (cholesterol and its esters) accumulate in the subendothelial layer. The cellular elements of the subendothelial layer play an important role in wall regeneration. At the border with the tunica media there is a dense network of elastic fibers.

Middle shell consists of numerous elastic fenestrated membranes, between which obliquely oriented bundles of smooth muscle cells are located. Through the windows (fenestrae) of the membranes, intrawall transport of substances necessary to nourish the cells of the wall occurs. Both the membranes and the cells of smooth muscle tissue are surrounded by a network of elastic fibers, which, together with the fibers of the inner and outer membranes, form a single framework that provides. high wall elasticity.

The outer shell is formed by connective tissue, which is dominated by bundles of collagen fibers oriented longitudinally. In this shell, vessels are located and branch, providing nutrition to both the outer shell and the outer zones of the middle shell.

Muscular arteries. Arteries of this type of different caliber include most of the arteries that deliver and regulate blood flow to various parts and organs of the body (brachial, femoral, splenic, etc.). Upon microscopic examination, the elements of all three shells are clearly visible in the wall (Fig. 5).

Inner shell consists of three layers: endothelial, subendothelial and internal elastic membrane. The endothelium has the appearance of a thin plate, consisting of cells elongated along the vessel with oval nuclei protruding into the lumen. The subendothelial layer is more developed in arteries of large diameter and consists of stellate or spindle-shaped cells, thin elastic fibers and an amorphous substance containing glycosaminoglycans. On the border with the middle shell lies internal elastic membrane, clearly visible on preparations in the form of a shiny, light pink, eosin-colored wavy stripe. This membrane is permeated with numerous holes that are important for the transport of substances.

Middle shell is built predominantly from smooth muscle tissue, the bundles of cells of which run in a spiral, however, when the position of the arterial wall changes (stretching), the location of the muscle cells can change. Contraction of the tunica media muscle tissue is important in regulating blood flow to organs and tissues according to their needs and maintaining blood pressure. Between the bundles of muscle tissue cells there is a network of elastic fibers, which, together with the elastic fibers of the subendothelial layer and the outer shell, form a single elastic frame that gives the wall elasticity when it is compressed. At the border with the outer shell in large arteries of the muscular type there is an outer elastic membrane, consisting of a dense plexus of longitudinally oriented elastic fibers. In smaller arteries this membrane is not expressed.

Outer shell consists of connective tissue in which collagen fibers and networks of elastic fibers are elongated in the longitudinal direction. Between the fibers there are cells, mainly fibrocytes. The outer shell contains nerve fibers and small blood vessels that supply the outer layers of the artery wall.

Rice. 5. Scheme of the structure of the wall of an artery (A) and vein (B) of the muscular type:

1 - inner shell; 2 - middle shell; 3 - outer shell; a - endothelium; b - internal elastic membrane; c - nuclei of smooth muscle tissue cells in the middle shell; d - nuclei of adventitia connective tissue cells; d - vessels of blood vessels.

Arteries of the muscular-elastic type According to the structure of the wall, they occupy an intermediate position between arteries of elastic and muscular types. In the middle shell, spirally oriented smooth muscle tissue, elastic plates and a network of elastic fibers are developed in equal quantities.

Vessels of the microvasculature. At the site of the transition of the arterial to venous bed in organs and tissues, a dense network of small precapillary, capillary and postcapillary vessels is formed. This complex of small vessels, which provides blood supply to organs, transvascular exchange and tissue homeostasis, is collectively called the microvasculature. It consists of various arterioles, capillaries, venules and arteriole-venular anastomoses (Fig. 6).

R
is.6. Diagram of microvasculature vessels:

1 - arteriole; 2 - venule; 3 - capillary network; 4 - arteriolo-venular anastomosis

Arterioles. As the diameter of muscular arteries decreases, all the membranes become thinner and they turn into arterioles - vessels with a diameter of less than 100 microns. Their inner shell consists of endothelium located on the basement membrane and individual cells of the subendothelial layer. Some arterioles may have a very thin internal elastic membrane. The tunica media contains one row of spirally arranged smooth muscle cells. In the wall of the terminal arterioles, from which the capillaries branch, smooth muscle cells do not form a continuous row, but are located separately. This precapillary arterioles. However, at the site of the branch from the arteriole, the capillary is surrounded by a significant number of smooth muscle cells, which form a kind of precapillary sphincter. Due to changes in the tone of such sphincters, blood flow in the capillaries of the corresponding area of ​​​​tissue or organ is regulated. There are elastic fibers between muscle cells. The outer shell contains individual adventitial cells and collagen fibers.

Capillaries- the most important elements of the microvasculature, in which the exchange of gases and various substances takes place between the blood and surrounding tissues. In most organs, branching structures are formed between arterioles and venules. capillary networks located in loose connective tissue. The density of the capillary network in different organs can be different. The more intense the metabolism in an organ, the denser the network of its capillaries. The most developed network of capillaries is in the gray matter of the nervous system, in the internal secretion organs, the myocardium of the heart, and around the pulmonary alveoli. In skeletal muscles, tendons, and nerve trunks, capillary networks are oriented longitudinally.

The capillary network is constantly in a state of restructuring. In organs and tissues, a significant number of capillaries do not function. Only blood plasma circulates in their greatly reduced cavity ( plasma capillaries). The number of open capillaries increases with the intensification of the organ's work.

Capillary networks are also found between vessels of the same name, for example, venous capillary networks in the liver lobules and adenohypophysis, arterial ones in the renal glomeruli. In addition to forming branched networks, capillaries can take the form of a capillary loop (in the papillary layer of the dermis) or form glomeruli (choroid glomeruli of the kidneys).

Capillaries are the narrowest vascular tubes. Their caliber on average corresponds to the diameter of an erythrocyte (7-8 µm), however, depending on the functional state and organ specialization, the diameter of the capillaries can be different. Narrow capillaries (4-5 µm in diameter) in the myocardium. Special sinusoidal capillaries with a wide lumen (30 microns or more) in the liver lobules, spleen, red bone marrow, and internal secretion organs.

The wall of blood capillaries consists of several structural elements. The internal lining is formed by a layer of endothelial cells located on the basement membrane, the latter containing cells - pericytes. Around the basement membrane there are adventitial cells and reticular fibers (Fig. 7).

Fig.7. Scheme of the ultrastructural organization of the wall of a blood capillary with a continuous endothelial lining:

1 - endotheliocyte: 2 - basement membrane; 3 - pericyte; 4 - pinocytotic microbubbles; 5 - contact zone between endothelial cells (Fig. Kozlov).

Flat endothelial cells elongated along the length of the capillary and have very thin (less than 0.1 μm) peripheral anucleate areas. Therefore, with light microscopy of a cross section of a vessel, only the area where the nucleus is located, 3-5 µm thick, is visible. The nuclei of endothelial cells are often oval in shape and contain condensed chromatin, concentrated near the nuclear membrane, which, as a rule, has uneven contours. In the cytoplasm, the bulk of organelles are located in the perinuclear region. The inner surface of endothelial cells is uneven, the plasmalemma forms microvilli, protrusions and valve-like structures of different shapes and heights. The latter are especially characteristic of the venous section of the capillaries. Along the inner and outer surfaces of endothelial cells there are numerous pinocytosis vesicles, indicating intensive absorption and transfer of substances through the cytoplasm of these cells. Endothelial cells, due to their ability to quickly swell and then, releasing fluid, decrease in height, can change the size of the lumen of the capillary, which, in turn, affects the passage of blood cells through it. In addition, electron microscopy revealed microfilaments in the cytoplasm that determine the contractile properties of endothelial cells.

basement membrane, located under the endothelium, is detected by electron microscopy and represents a plate 30-35 nm thick, consisting of a network of thin fibrils containing type IV collagen and an amorphous component. The latter, along with proteins, contains hyaluronic acid, the polymerized or depolymerized state of which determines the selective permeability of capillaries. The basement membrane also provides elasticity and strength to the capillaries. In the cleavages of the basement membrane, special branched cells are found - pericytes. They cover the capillary with their processes and, penetrating the basement membrane, form contacts with endothelial cells.

In accordance with the structural features of the endothelial lining and basement membrane, three types of capillaries are distinguished. Most capillaries in organs and tissues belong to the first type ( general type capillaries). They are characterized by the presence of a continuous endothelial lining and basement membrane. In this continuous layer, the plasma membranes of neighboring endothelial cells are as close as possible and form connections like tight contacts, which are impenetrable to macromolecules. There are also other types of contacts when the edges of neighboring cells overlap each other like tiles or are connected by jagged surfaces. According to the length of the capillaries, narrower (5 - 7 µm) proximal (arteriolar) and wider (8 - 10 µm) distal (venular) parts are distinguished. In the cavity of the proximal part, the hydrostatic pressure is greater than the colloid-osmotic pressure created by proteins in the blood. As a result, the liquid is filtered behind the wall. In the distal part, the hydrostatic pressure becomes less than the colloid osmotic pressure, which causes the transition of water and substances dissolved in it from the surrounding tissue fluid into the blood. However, the output flow of fluid is greater than the input, and the excess fluid, as part of the tissue fluid of the connective tissue, enters the lymphatic system.

In some organs in which the processes of absorption and release of fluid intensively occur, as well as rapid transport of macromolecular substances into the blood, the endothelium of the capillaries has rounded submicroscopic openings with a diameter of 60-80 nm or rounded areas covered by a thin diaphragm (kidneys, internal secretion organs). This capillaries with fenestraes(Latin fenestrae - windows).

Capillaries of the third type - sinusoidal, are characterized by a large diameter of their lumen, the presence of wide gaps between endothelial cells and a discontinuous basement membrane. Capillaries of this type are found in the spleen and red bone marrow. Not only macromolecules, but also blood cells penetrate through their walls.

Venules- the efferent section of the micropirculatory bed and the initial link of the venous section of the vascular system. They collect blood from the capillary bed. The diameter of their lumen is wider than in capillaries (15-50 microns). In the wall of venules, as well as in capillaries, there is a layer of endothelial cells located on the basement membrane, as well as a more pronounced outer connective tissue membrane. In the walls of the venules, which turn into small veins, there are individual smooth muscle cells. IN postcapillary venules of the thymus, lymph nodes, the eldothelial lining is represented by tall endothelial cells that promote selective migration of lymphocytes during their recycling. Due to the thinness of their walls, slow blood flow and low blood pressure, a significant amount of blood can be deposited in the venules.

Arteriolo-venular anastomoses. In all organs, tubes have been found through which blood from arterioles can be sent directly to venules, bypassing the capillary network. There are especially many anastomoses in the dermis of the skin, in the auricle, and in the crest of birds, where they play a certain role in thermoregulation.

Structurally, true arteriolovenular anastomoses (shunts) are characterized by the presence in the wall of a significant number of longitudinally oriented bundles of smooth muscle cells located either in the subendothelial layer of the intima (Fig. 8) or in the inner zone of the tunica media. In some anastomoses, these cells acquire an epithelial-like appearance. Longitudinal muscle cells are also found in the outer shell. There are not only simple anastomoses in the form of single tubes, but also complex ones, consisting of several branches extending from one arteriole and surrounded by a common connective tissue capsule.

Fig.8. Arteriolo-venular anastomosis:

1 - endothelium; 2 - longitudinally located epithelioid muscle cells; 3 - circularly located muscle cells of the middle shell; 4 - outer shell.

With the help of contractile mechanisms, anastomoses can reduce or completely close their lumen, as a result of which the flow of blood through them stops and blood enters the capillary network. Thanks to this, the organs receive blood depending on the need associated with their work. In addition, high arterial blood pressure is transmitted through anastomoses to the venous bed, thereby facilitating better blood movement in the veins. The role of anastomoses is significant in enriching venous blood with oxygen, as well as in regulating blood circulation during the development of pathological processes in organs.

Vienna- blood vessels through which blood from organs and tissues flows to the heart, into the right atrium. The exception is the pulmonary veins, which carry oxygen-rich blood from the lungs to the left atrium.

The wall of veins, like the wall of arteries, consists of three membranes: inner, middle and outer. However, the specific histological structure of these membranes in different veins is very diverse, which is associated with differences in their functioning and local (according to the location of the vein) blood circulation conditions. Most veins of the same diameter as arteries of the same name have a thinner wall and a wider lumen.

In accordance with the hemodynamic conditions - low blood pressure (15-20 mm Hg) and low blood flow velocity (about 10 mm / s) - the elastic elements in the vein wall are relatively poorly developed and there is less muscle tissue in the tunica media. These signs make it possible to change the configuration of the veins: when the blood supply is low, the walls of the veins become collapsed, and when the outflow of blood is difficult (for example, due to blockage), stretching of the wall and expansion of the veins easily occur.

Essential in the hemodynamics of venous vessels are valves located in such a way that, while allowing blood to flow towards the heart, they block the path for its reverse flow. The number of valves is greater in those veins in which blood flows in the direction opposite to gravity (for example, in the veins of the extremities).

According to the degree of development of muscle elements in the wall, veins of non-muscular and muscular types are distinguished.

Veins are of non-muscular type. Typical veins of this type include the veins of bones, the central veins of the hepatic lobules and the trabecular veins of the spleen. The wall of these veins consists only of a layer of endothelial cells located on the basement membrane and an outer thin layer of fibrous connective tissue. With the participation of the latter, the wall fuses tightly with the surrounding tissues, as a result of which these veins are passive in the movement of blood through them and do not collapse. The muscleless veins of the meninges and retina, when filled with blood, can easily stretch, but at the same time, the blood, under the influence of its own gravity, easily flows into larger venous trunks.

Muscular veins. The wall of these veins, like the wall of arteries, consists of three membranes, but the boundaries between them are less distinct. The thickness of the muscular membrane in the wall of veins of different locations is not the same, which depends on whether the blood moves in them under the influence of gravity or against it. Based on this, muscular-type veins are divided into veins with weak, medium and strong development of muscle elements. The veins of the first type include the horizontally located veins of the upper body of the body and the veins of the digestive tract. The walls of such veins are thin; in their middle shell, smooth muscle tissue does not form a continuous layer, but is located in bundles, between which there are layers of loose connective tissue.

Veins with strong development of muscle elements include large veins of the limbs of animals, through which blood flows upward, against gravity (femoral, brachial, etc.). They are characterized by longitudinally located small bundles of smooth muscle tissue cells in the subendothelial layer of the intima and well-developed bundles of this tissue in the outer shell. Contraction of the smooth muscle tissue of the outer and inner membranes leads to the formation of transverse folds of the vein wall, which prevents reverse blood flow.

The tunica media contains circularly arranged bundles of smooth muscle cells, the contractions of which help move blood to the heart. In the veins of the extremities there are valves, which are thin folds formed by the endothelium and subendothelial layer. The basis of the valve is fibrous connective tissue, which at the base of the valve leaflets may contain a number of smooth muscle cells. The valves also prevent the backflow of venous blood. For the movement of blood in the veins, the suction action of the chest during inhalation and the contraction of skeletal muscle tissue surrounding the venous vessels are essential.

Vascularization and innervation of blood vessels. The walls of large and medium arterial vessels are nourished both from the outside - through the vascular vessels (vasa vasorum), and from the inside - due to the blood flowing inside the vessel. Vascular vessels are branches of thin perivascular arteries running in the surrounding connective tissue. In the outer shell of the vessel wall, arterial branches branch, capillaries penetrate into the middle shell, the blood from which collects in the venous vessels of the vessels. The intima and inner zone of the middle tunic of the arteries do not have capillaries and are fed from the side of the lumen of the vessels. Due to the significantly lower strength of the pulse wave, the smaller thickness of the middle shell, and the absence of an internal elastic membrane, the mechanism of supply of the vein from the side of the cavity is not of particular importance. In the veins, the vasculature supplies arterial blood to all three membranes.

The narrowing and dilation of blood vessels and the maintenance of vascular tone occur mainly under the influence of impulses coming from the vasomotor center. Impulses from the center are transmitted to the cells of the lateral horns of the spinal cord, from where they enter the vessels through sympathetic nerve fibers. The terminal branches of the sympathetic fibers, which contain the axons of the nerve cells of the sympathetic ganglia, form motor nerve endings on the cells of smooth muscle tissue. Efferent sympathetic innervation of the vascular wall determines the main vasoconstrictor effect. The question of the nature of vasodilators has not been completely resolved.

It has been established that parasympathetic nerve fibers are vasodilators in relation to the vessels of the head.

In all three membranes of the vessel walls, the terminal branches of the dendrites of nerve cells, mainly the spinal ganglia, form numerous sensory nerve endings. In the adventitia and perivascular loose connective tissue, among the free endings of various shapes, encapsulated bodies are also found. Specialized interoreceptors that perceive changes in blood pressure and its chemical composition, concentrated in the wall of the aortic arch and in the area where the carotid artery branches into internal and external - aortic and carotid reflexogenic zones, are of particular physiological importance. It has been established that, in addition to these zones, there are a sufficient number of other vascular territories that are sensitive to changes in pressure and chemical composition of the blood (baro- and chemoreceptors). From the receptors of all specialized territories, impulses along the centripetal nerves reach the vasomotor center of the medulla oblongata, causing a corresponding compensatory neuroreflex reaction.