Arteries

From the aorta (or from its branches) all the arteries of the great circle of circulation begin. Depending on the thickness (diameter) of the artery conditionally divided into large, medium and small. Each artery is distinguished by the main trunk and its branches.

Arteries, blood supplying the walls of the body, are called parietal (parietal), arteries of internal organs - visceral (internal). Among the arteries, also the extraorganic, carrying blood to the organ, and intraorganic branches within the organ and supplying it with separate parts (lobes, segments, lobules) are also isolated. Many arteries derive their name from the name of the organ that they supply (renal artery, splenic artery). Some arteries have their name in connection with the level of their departure from the larger vessel (the superior mesenteric artery, the inferior mesenteric artery); by the name of the bone to which the vessel is due (radial artery); in the direction of the vessel (the medial artery surrounding the thigh), as well as in the depth of the location (superficial or deep artery). Small vessels that do not have special names are designated as branches (rami).

On the way to the organ or in the organ itself, the arteries branch into smaller vessels. Distinguish the main type of branching of the arteries and loose. At the main type, there is a main trunk - the main artery and the lateral branches departing from it. As the lateral branches branch off from the main artery, its diameter gradually decreases. The loose type of branching of the artery is characterized by the fact that the main trunk (artery) is immediately divided into two or more end branches, the general branching plan of which resembles the crown of a deciduous tree.

There are also arteries, providing a roundabout flow of blood, bypassing the main path, - collateral vessels. If the movement on the main (main) artery is difficult, the blood can flow along the collateral bypass vessels, which (one or several) begin either from the source common to the main vessel or from different sources and end in a common vascular network.

Collateral vessels that connect (anastomosing) with the branches of other arteries serve as interararterial anastomoses. Distinguish inter-system inter-arterial anastomoses - joints (anastomoses) between different branches of different large arteries, and intra-system interarterial anastomoses - connections between branches of one artery.

The wall of each artery consists of three shells: inner, middle and outer. The inner tunica (intima) is formed by a layer of endothelial cells (endotheliocytes) and a subendothelial layer. Endotheliocytes lying on a thin basal membrane are flat thin cells connected to each other by intercellular contacts (nexus). The peri-nuclear zone of endotheliocytes is thickened, protruding into the lumen of the vessel. The basal part of the cytolemma of endotheliocytes forms numerous small branched processes directed toward the subendothelial layer. These processes perforate the basal and internal elastic membranes and form nexus with smooth myocytes of the middle shell of the artery (myoepithelial contacts). The subepithelium layer in small arteries (muscle type) is thin, consists of the main substance, as well as collagen and elastic fibers. In larger arteries (muscular-elastic type), the subendothelial layer is better developed than in small arteries. The thickness of the subendothelial layer at the arteries of the elastic type reaches 20% of the thickness of the walls of the vessels. This layer in large arteries consists of thin-fibrillar connective tissue containing little-specialized star cells. Sometimes longitudinally oriented myocytes are found in this layer. In the intercellular substance, glycosaminoglycans and phospholipids are found in large numbers. In middle-aged and elderly people, the subendothelial layer reveals cholesterol and fatty acids. Outside the subendothelial layer, on the border with the middle shell, the arteries have an internal elastic membrane formed by densely interwoven elastic fibers and constituting a thin continuous or interrupted (final) plate.

The median tunica media is formed by smooth muscle cells of a circular (spiral) direction, as well as by elastic and collagen fibers. In different arteries, the structure of the middle shell has its own characteristics. Thus, in small arteries of the muscular type up to 100 μm in diameter, the number of smooth muscle cells does not exceed 3-5. Myocytes of the middle (muscular) membrane are located in the elastin containing main substance that produces these cells. At the arteries of the muscle type in the middle shell there are intertwined elastic fibers, due to which these arteries retain their lumen. In the middle shell of the arteries of the muscular-elastic type, smooth myocytes and elastic fibers are distributed approximately equally. In this shell there are also collagen fibers and single fibroblasts. Arteries of muscular type up to 5 mm in diameter. The middle shell is thick, formed by 10-40 layers of spirally oriented smooth myocytes, which are connected to each other by interdigitation.

At arteries of elastic type the thickness of an average cover reaches 500 microns. It is formed by 50-70 layers of elastic fibers (elastic final membranes), 2-3 μm thick each fiber. Between the elastic fibers are located relatively short spindle-shaped smooth myocytes. They are oriented spirally, connected with each other by tight contacts. Around the myocytes are thin elastic and collagen fibers and amorphous substance.

On the border of the middle (muscular) and outer membranes there is a fenestrated outer elastic membrane, which is absent in small arteries.

The outer shell, or adventitia (tunica externa, s.adventicia), is formed by a loose fibrous connective tissue passing into the connective tissue of the adjacent organs of the arteries. In adventitia, vessels that feed the walls of the arteries (vessels of the vessels, vasa vasorum) and nerve fibers (nerves of the vessels, nervi vasorum) pass through the adventitia.

In connection with the peculiarities of the structure of the walls of arteries of different calibers, arteries of elastic, muscular and mixed types are distinguished. Large arteries, in the middle shell of which elastic fibers predominate over muscle cells, are called arteries of the elastic type (aorta, pulmonary trunk). The presence of a large number of elastic fibers counteracts the excessive stretching of the blood vessel during the contraction (systole) of the ventricles of the heart. The elastic forces of the walls of arteries filled with blood under pressure also contribute to the movement of blood through the vessels during the relaxation (diastole) of the ventricles. Thus, a continuous movement is ensured-the circulation of blood through the vessels of the large and small circles of the circulation. Part of the arteries of the middle and all small-sized arteries are arteries of the muscle type. In their middle shell, the muscle cells predominate over the elastic fibers. The third type of arteries is an artery of mixed type (muscular-elastic), most middle arteries (carotid, subclavian, femoral, etc.) belong to them. In the walls of these arteries, the muscle and elastic elements are distributed approximately equally.

It should be borne in mind that as the caliber of arteries decreases, all of their membranes become thinner. Reduces the thickness of the subepithelial layer, the inner elastic membrane. The number of smooth myocytes of elastic fibers decreases in the middle shell, the outer elastic membrane disappears. In the outer shell, the number of elastic fibers decreases.

The topography of arteries in the human body has certain regularities (P. Flessgaft).

1. Arteries are sent to the organs along the shortest path. So, on the extremities, the arteries follow a shorter flexural surface, and not along a longer extensor.
2. The main significance is not the final position of the organ, but the place of its laying in the embryo. For example, a branch of the abdominal part of the aorta, the ovarian artery, is guided along the shortest path to the testicle, which is laid in the lumbar region. As the testicle descends into the scrotum, the artery that feeds it, the origin of which in the adult person is at a great distance from the testicle, also descends with it.
3. Arteries approach the organs on their inner side, facing the source of the blood supply - the aorta or other large vessel, and the artery or its branches in most cases enter through its gates.
4. Between the structure of the skeleton and the number of main arteries there are certain correspondences. The vertebral column accompanies the aorta, the collarbone - one subclavian artery. On the shoulder (one bone) there is one brachial artery, on the forearm (two bones - ray and elbow) - two arteries of the same name.
5. On the way to the joints from the main arteries, collateral arteries depart, and return arteries meet them from the underlying sections of the main arteries. Anastomizing the joints along the circumference, the arteries form articular arterial nets, which ensure a continuous blood supply to the joint during movements.
6. The number of arteries entering the organ and their diameter depend not only on the size of the organ, but also on its functional activity.
7. The patterns of branching of the arteries in the organs are determined by the shape and structure of the organ, the distribution and orientation of bundles of connective tissue in it. In organs having a lobed structure (lung, liver, kidney), the artery enters the gates and then branches correspondingly to the segments, segments and segments. To organs that are laid, in the form of a tube (for example, intestines, uterus, fallopian tubes), feeding arteries approach from one side of the tube, and their branches have an annular or longitudinal direction. Entering the organ, the arteries branch repeatedly to the arterioles.

The walls of the blood vessels have abundant sensory (afferent) and motor (efferent) innervation. In the walls of some large vessels (the ascending part of the aorta, the aortic arch, the bifurcation - the branch of the common carotid artery into the outer and inner, upper hollow and jugular veins, etc.), there are especially many sensitive nerve endings, in connection with which these regions are called reflexogenic zones. Virtually all blood vessels have abundant innervation, which plays an important role in regulating vascular tone and blood flow.

[1], [2], [3], [4], [5], [6], [7], [8], [9]