The tissues of the body are permeated by a huge number of capillaries, in which a direct exchange of metabolites and oxygen is carried out. Blood is delivered to the capillaries by arterioles, to which it is directed by larger muscle-type arteries. Together with the transitional and elastic vessels, they make up the arterial bed of the circulatory system.
Types of arterial vessels
In the human body there are several types of arteries, which differ in the structure of the vessel wall. The elastic arteries, the aorta, the iliac, carotid, subclavian and renal arteries withstand the strongest pressure and carry blood at a speed of about 60 cm/sec. Due to their phenomenal elastic qualities, their wall perfectly transmits the pulse wave generated by cardiac output.
Gradually decreasing in diameter, the elastic arterial vessels pass into muscular-elastic ones. In their middle shell, the number of elastic fibers decreases,the number of muscle cells increases. These vessels are considered transitional from the elastic type to the muscular type and are located between them. Their task is to maintain blood pressure at some distance from the heart, which, with a decrease in diameter, requires the presence of muscle cells in the middle membrane of the arterial wall.
Transitional arteries, such as the femoral, brachial, mesenteric, internal and external carotid, celiac trunk and others similar in diameter, gradually become muscular. More precisely, there is no clear line between them, just in their middle shell the number of smooth muscle cells increases significantly. They are necessary in order to maintain a weakening pulse wave and push blood with the same blood pressure as in the elastic arteries.
Structure of the arterial wall
All arteries of the muscular type, as well as elastic vessels and capillaries, have a three-layer structure. From the inside, they are lined with a single-layer epithelium, an inner membrane located on a connective tissue membrane. The latter limits the inner shell from the middle, in which there are elastic fibers or muscle cells. On top of the middle shell is another connective tissue layer that provides the mechanical strength of the artery. In large vessels, for example, in the arteries of the muscular-elastic type or in the aorta, the outer membrane is very strong, and in the pulmonary capillaries it is practically absent.
Histological structure
All the sheaths of muscular type arteries retain a commonplan of the structure of blood vessels. In particular, from the inside there is a single-layer epithelium on a connective tissue membrane. It is covered with a middle shell with a large number of muscle cells and sparse elastic fibers. Outside, there is a connective tissue membrane, moderately expressed in vessels of this type. And in each of these layers there are identical cells, which is the case with elastic arteries or capillaries. Only the strength of the vessel, its caliber and the presence of pores in the endothelium differ.
All muscular arteries, as well as elastic and transient vessels, have a solid endothelial lining. This means that the inner epithelium, lining the wall from the inside in the place of direct contact with the blood, consists of cells that are in close contact with each other. But in the capillaries between the epithelial cells there are gaps through which the transition of leukocytes to the tissues and back occurs, the transport of substances and gas exchange occur. This means that muscle-type arteries, arterioles and vessels of larger diameter are needed not for direct metabolism, but only for transport.
Arterioles
Arterioles are thin muscular arteries. These are small blood vessels, from which multiple capillaries depart. These are one of the most distant parts of the arterial bed from the heart, which is why the provision of pulsation and a high level of blood pressure is achieved due to the muscle cells of the middle membrane. For example, the afferent arteriole of the nephron is capable ofmaintain a pressure indicator of 120 mmHg, despite the fact that the pulsation from the heart is practically not transmitted to it. Such an artery itself generates a pulse due to sympathetic innervation, and not stretching and compression, as is observed in the vessels of the elastic and transitional type.
Fundamentals of vascular pathologies
There is a possibility that certain substances will get under the inner shell, while a return return to the vessel cavity is practically impossible. Therefore, the penetration of cholesterol under the endothelium in the elastic and transitional vessels, as well as in the arteries of the muscular type, causes chronic macrophage inflammation with the development of atherosclerosis and stenosis. In capillaries and arterioles, a similar process is excluded, since these vessels quickly regenerate, and substances can be removed from under their endothelium either into the interstitial fluid or directly into the blood.
