Microcirculatory bed

The microcirculatory bed is not a mechanical sum of various vessels, but a complex anatomical and physiological complex consisting of 7 links (5 blood vessels, lymphatic and interstitial) and providing the main vital process of the body – metabolism. Therefore, V. V. Kupriyanov regards it as a microcirculation system.

The structure of the microvasculature has its own characteristics in different organs, corresponding to their structure and function. Thus, in the liver there are wide capillaries – hepatic sinusoids, which receive arterial and venous (from the portal vein) blood. In the kidneys there are arterial capillary glomeruli. Special sinusoids are characteristic of the bone marrow, etc.

Microcirculation fluid is not limited to microscopic blood vessels. The human body is 70% composed of water, which is contained in cells and tissues and makes up the bulk of blood and lymph. Only xls of the whole fluid is in the vessels, and the remaining 4/5 of it is contained in the plasma of cells and in the extracellular medium. Microcirculation of the fluid is carried out, in addition to the circulatory system, also in the tissues, in the serous and other cavities, and in the path of transport of lymph.

From the microvasculature, blood flows through the veins, and lymph through the lymphatic vessels, which ultimately fall into the cardiac veins. Venous blood containing lymph that has joined it, flows into the heart, first into the right atrium, and from it into the right ventricle. From the latter, venous blood enters the lungs through the small (pulmonary) circulation.

Circulation of blood

Blood circulation begins in the tissues, where the metabolism takes place through the walls of the capillaries (blood and lymph). The capillaries constitute the main part of the microcirculatory bed, in the colum microcirculation of blood and lymph occurs.

Lymphatic capillaries and interstitial spaces also belong to the microcirculatory bed. Microcirculation is the movement of blood and lymph in the microscopic part of the vascular bed.

Microcirculatory bed, according to V. V. Kupriyanov, includes 5 links:

1) arterioles as the most distal parts of the arterial system,
2) precapillaries, or precapillary arterioles, which are intermediate between arterioles and true capillaries;
3) capillaries;
4) postcapillaries, or postcapillary venules,
5) venules, which are the roots of the venous system.

All these links are equipped with mechanisms that ensure the permeability of the vascular wall and the regulation of blood flow at the microscopic level. The microcirculation of blood is regulated by the work of the arterial muscles and arteriol, as well as the special muscle sphincters, the existence of which was predicted by I. Sechenov and called them “cranes”. Such sphincters are in pre- and postcapillaries. Some vessels of the microvasculature (arterioles) perform predominantly distribution function, and the rest (precapillaries, capillaries, postcapillaries and venules) predominantly trophic (exchange).

At each given moment only a part of the capillaries (open capillaries) functions, and the other remains in reserve (closed capillaries).

In addition to these vessels, the Soviet anatomists proved that they belong to the microcirculatory channel of arterio-venous anastomoses, which are present in all organs and represent the paths of the shortened flow of arterial blood into the venous channel, bypassing the capillaries. These anastomoses are divided into true anastomoses, or shunts (with locking devices capable of blocking the flow of blood, and without them), and inter-arterioles, or half-shunts. Due to the presence of arteriovenous anastomoses, the terminal blood flow is divided into two ways of blood flow: 1) transcapillary, which serves for metabolism, and 2) extracapillary juxtacillary (from lat. Juxta – near, beside) flow of blood necessary for regulation of hemodynamic balance; the latter is due to the presence of direct connections (shunts) between the arteries and veins (arteriovenous anastomoses) and arterioles and venules (arteriolenovenular anastomoses).

Due to the extracapillary blood flow, if necessary, the capillary bed is unloaded and the blood transport in the organ or a given area of ​​the body is accelerated. It is like a special form of a roundabout, collateral, blood circulation.

Muscle contraction

The contraction of the muscular layer of the veins is also important, which in the veins of the lower half of the body, where the conditions for venous outflow is more complicated, is more developed than in the veins of the upper body. The reverse flow of venous blood is prevented by special devices of the veins – valves that make up the peculiarities of the venous wall. Venous valves consist of a fold of endothelium containing a layer of connective tissue. They face the free edge towards the heart and therefore do not prevent the blood from flowing in this direction, but keep it from returning back. Arteries and veins usually go together, with small and medium arteries accompanied by two veins, and large – one. Except for some deep veins, this rule excludes mainly superficial veins reaching the subcutaneous tissue and almost never accompanying arteries.

The walls of blood vessels have their own serving thin arteries and veins, vasa vasorum. They depart either from the same trunk, the wall of which is supplied with blood, or from the neighboring one and pass in the connective tissue layer surrounding the blood vessels and more or less closely connected with their outer sheath; This layer is called the vascular vagina, vagina vasorum. In the wall of arteries and veins there are numerous nerve endings (receptors and effectors) connected with the central nervous system, due to which the nervous regulation of blood circulation is carried out by the mechanism of reflexes. Blood vessels are extensive reflexogenic zones that play a large role in the neuro-humoral regulation of metabolism. Accordingly, the functions and the structure of the various departments and the features of innervation all the blood vessels in recent times have been divided into 3 groups:

1) the heart vessels that begin and end both circles of circulation, the aorta and the pulmonary trunk (i.e., elastic arteries), hollow and pulmonary veins;
2) the great vessels that serve for the distribution of blood throughout the body. These are large and medium sized extraorgan arteries of the muscular type and extraorgan veins;
3) organ vessels providing exchange reactions between the blood and the parenchyma of organs. These are intraorgan arteries and veins, as well as links in the microvasculature.

Arteriole

An arteriole differs from an artery in that its wall has only one layer of muscle cells, thanks to which it performs a regulatory function. Arteriole proceeds directly into the precapillary, in which muscle cells are scattered and do not constitute a continuous layer. The precapillary differs from arterioles in the fact that it is not accompanied by a venule.

Numerous capillaries depart from the precapillary.
Capillaries are the thinnest vessels that perform the exchange function. In this regard, their wall consists of a single layer of flat endothelial cells, permeable to substances and gases dissolved in a liquid. Anastomozirovaya widely between themselves, the capillaries form a network (capillary network), turning into postcapillary, built similarly to precapillary. Postcapillary continues into the venula, accompanying arterios. Venules form thin initial segments of the venous bed that make up the roots of the veins and pass into the veins.

Veins (lat. Vena, Greek phlebs; hence phlebitis – inflammation of the veins) carry blood in the opposite direction to the arteries, from organs to the heart. Their walls are arranged according to the same plan as the walls of the arteries, but they are much thinner and have less elastic and muscular tissue, due to which empty veins fall, and the lumen of the arteries in the transverse section gapes; veins, merging with each other, form large venous trunks – veins that flow into the heart.

Veins widely anastomose among themselves, forming venous plexus.
The movement of blood through the veins is due to the activity and the suctioning action of the heart and chest cavity, in which negative pressure is created during inhalation due to the pressure difference in the cavities and also due to the reduction of the skeletal and visceral muscles of the organs and other factors.

Circulatory system.

Arteries. The wall of the arteries. Capillaries Veins. The circulatory system consists of a central organ – the heart – and closed tubes of various calibers in conjunction with it, called blood vessels (Latin vas, Greek angeion – vessel; hence, angiology). With its rhythmic contractions, the heart drives the whole mass of blood, contained in the vessels.

Arteries. The blood vessels that go from the heart to the organs and carry blood to them are called arteries (aeg – air, tereo – contain; arteries on corpses are empty, which is why they used to be considered as air tubes).

The wall of the arteries consists of three shells. Inner shell, tunica intima. lined on the lumen of the vessel by the endothelium, under which the subendothelium and the inner elastic membrane lie; medium, tunica media, built from loose muscle fibers, myocytes, alternating with elastic fibers; the outer sheath, tunica externa, contains connective tissue fibers. The elastic elements of the arterial wall form a single elastic frame that acts as a spring and determines the elasticity of the arteries.

As they move away from the heart, arteries divide into branches and grow smaller and smaller. The arteries closest to the heart (the aorta and its large branches) perform mainly the function of conducting blood. In them to the fore the anti-stretching of a mass of blood, which is thrown out by a heart beat, comes forward. Therefore, in their wall structures of a mechanical nature are relatively more developed, i.e., elastic fibers and membranes. Such arteries are called elastic type arteries. In medium and small arteries, in which the inertia of the cardiac impulse weakens and requires its own contraction of the vascular wall for further blood advancement, the contractile function prevails. It is provided with a relatively large development in the vascular wall of muscle tissue. Such arteries are called muscular arteries. Individual arteries supply blood to entire organs or parts of them.

In relation to the organ, there are arteries that go beyond the organ, before entering it – extraorgan arteries, and their continuations that branch out inside it – intraorgan, or ingraorganic arteries. Lateral branches of the same stem or branches of different trunks can be connected to each other. Such a combination of vessels before disintegration of them into the capillaries is called anastomosis, or fistula (stoma – mouth). The arteries that form the anastomoses are called anastomosing (most of them). Arteries that do not have anastomoses with adjacent trunks before moving into the capillaries (see below) are called end arteries (for example, in the spleen). The terminal, or terminal, arteries are more easily blocked with a blood stopper (thrombus) and predispose to the formation of a heart attack (local organ death). The last branchings of the arteries become thin and small and therefore stand out under the name of arterioles.