Arteries

All arteries of the systemic circulation originate from the aorta (or its branches). Depending on their thickness (diameter), arteries are conventionally divided into large, medium, and small. Each artery has a main trunk and its branches.

Arteries that supply blood to the walls of the body are called parietal, arteries of the internal organs are called visceral. Among the arteries, there are also extraorgan arteries, which carry blood to an organ, and intraorgan arteries, which branch within the organ and supply its individual parts (lobes, segments, lobules). Many arteries are named after the organ they supply (renal artery, splenic artery). Some arteries are named according to the level at which they branch off (begin) from a larger vessel (superior mesenteric artery, inferior mesenteric artery); according to the name of the bone to which the vessel is adjacent (radial artery); according to the direction of the vessel (medial artery surrounding the thigh), and also according to the depth of their 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, arteries branch into smaller vessels. A distinction is made between the main type of arterial branching and the scattered type. In the main type, there is a main trunk - the main artery and lateral branches extending from it. As the lateral branches extend from the main artery, its diameter gradually decreases. The scattered type of arterial branching is characterized by the fact that the main trunk (artery) immediately divides into two or more terminal branches, the general branching plan of which resembles the crown of a deciduous tree.

There are also arteries that provide a roundabout blood flow, bypassing the main route - collateral vessels. When movement along the main (trunk) artery is difficult, blood can flow through collateral bypass vessels, which (one or more) start either from a common source with the main vessel, or from different sources and end in a common vascular network for them.

Collateral vessels that connect (anastomose) with branches of other arteries act as interarterial anastomoses. A distinction is made between intersystemic interarterial anastomoses - connections (mouths) between different branches of different large arteries, and intrasystemic interarterial anastomoses - connections between branches of one artery.

The wall of each artery consists of three tunics: the inner, middle, and outer. The inner tunic (tunica intima) is formed by a layer of endothelial cells (endotheliocytes) and a subendothelial layer. Endothelial cells lying on a thin basement membrane are flat thin cells connected to each other by intercellular contacts (nexuses). The perinuclear zone of endothelial cells is thickened and protrudes into the lumen of the vessel. The basal part of the cytolemma of endothelial cells forms numerous small branched processes directed toward the subendothelial layer. These processes pierce the basal and internal elastic membranes and form nexuses with smooth myocytes of the middle tunic of the artery (myoepithelial contacts). The subepithelial layer in small arteries (muscular type) is thin, consists of the ground 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 in arteries of the elastic type reaches 20% of the thickness of the vessel walls. In large arteries, this layer consists of fine-fibrillar connective tissue containing poorly specialized stellate cells. Sometimes longitudinally oriented myocytes are found in this layer. Glycosaminoglycans and phospholipids are found in large quantities in the intercellular substance. In middle-aged and elderly people, cholesterol and fatty acids are found in the subendothelial layer. Outside the subendothelial layer, on the border with the middle layer, the arteries have an internal elastic membrane formed by densely intertwined elastic fibers and representing a thin continuous or discontinuous (finite) plate.

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

In elastic arteries, the thickness of the middle layer reaches 500 μm. It is formed by 50-70 layers of elastic fibers (elastic fenestrated membranes), each fiber is 2-3 μm thick. Between the elastic fibers are relatively short spindle-shaped smooth myocytes. They are oriented spirally, connected to each other by tight contacts. Around the myocytes are thin elastic and collagen fibers and an amorphous substance.

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

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

Due to the structural features of the walls of arteries of different calibers, arteries of the elastic, muscular and mixed types are distinguished. Large arteries, in the middle layer of which elastic fibers prevail over muscle cells, are called arteries of the elastic type (aorta, pulmonary trunk). The presence of a large number of elastic fibers counteracts excessive stretching of the vessel by blood during 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 relaxation (diastole) of the ventricles. Thus, continuous movement is ensured - circulation of blood through the vessels of the systemic and pulmonary circulation. Some arteries of medium caliber and all arteries of small caliber are arteries of the muscular type. In their middle layer, muscle cells prevail over elastic fibers. The third type of arteries are mixed arteries (muscular-elastic), which include most of the middle arteries (carotid, subclavian, femoral, etc.). In the walls of these arteries, the muscular and elastic elements are distributed approximately equally.

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

The topography of arteries in the human body has certain patterns (P. Flesgaft).

1. Arteries are directed to the organs along the shortest path. Thus, in the extremities, arteries go along the shorter flexor surface, and not along the longer extensor surface.
2. The final position of the organ is not of primary importance, but the place where it is laid in the embryo. For example, a branch of the abdominal part of the aorta, the testicular artery, goes 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 descends with it, the beginning of which in an adult is located at a large distance from the testicle.
3. Arteries approach organs from their inner side, facing the source of blood supply - the aorta or another large vessel, and the artery or its branches in most cases enter the organ through its gate.
4. There are certain correspondences between the structure of the skeleton and the number of main arteries. The spinal column is accompanied by the aorta, the clavicle - by one subclavian artery. On the shoulder (one bone) there is one brachial artery, on the forearm (two bones - the radius and the ulna) - two arteries of the same name.
5. On the way to the joints, collateral arteries branch off from the main arteries, and recurrent arteries branch off from the lower sections of the main arteries to meet them. By anastomosing with each other around the joints, the arteries form articular arterial networks that provide continuous blood supply to the joint during movements.
6. The number of arteries entering an organ and their diameter depend not only on the size of the organ, but also on its functional activity.
7. The patterns of arterial branching in organs are determined by the shape and structure of the organ, the distribution and orientation of connective tissue bundles in it. In organs with a lobular structure (lung, liver, kidney), the artery enters the gate and then branches according to the lobes, segments and lobules. For organs that are laid down in the form of a tube (for example, the intestine, uterus, fallopian tubes), the feeding arteries approach from one side of the tube, and their branches have a ring-shaped or longitudinal direction. Having entered the organ, the arteries repeatedly branch to arterioles.

The walls of blood vessels have abundant sensory (afferent) and motor (efferent) innervation. In the walls of some large vessels (ascending aorta, aortic arch, bifurcation - the place where the common carotid artery branches into the external and internal, superior vena cava and jugular veins, etc.) there are especially many sensory nerve endings, which is why these areas are called reflexogenic zones. In fact, all blood vessels have abundant innervation, which plays an important role in the regulation of vascular tone and blood flow.

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