A huge number of capillaries penetrate the body's tissues, in which the metabolites and oxygen are exchanged directly. Arterioles deliver blood to the capillaries, to which larger muscle arteries direct it. Together with 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 that differ in the structure of the vessel wall. The elastic arteries, aorta, iliac, carotid, subclavian and renal arteries withstand the strongest pressure and carry blood at a speed of about 60 cm / sec. Due to the phenomenal elastic qualities, their wall perfectly conveys the pulse wave generated by the cardiac output.
Gradually decreasing in diameter, the elastic arterial vessels pass into the muscular-elastic. In their middle shell, the number of elastic fibers decreases, the number of muscle cells increases. These vessels are considered transitional from elastic to muscle 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 other similar in diameter, gradually turn into muscle. More precisely, there is no clear line between them, just in their middle membrane the number of smooth muscle cells increases significantly. They are needed in order to maintain a weakening pulse wave and push blood with the same blood pressure as in elastic arteries.
Arterial wall structure
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 the 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, providing mechanical strength of the artery. In large vessels, for example, in arteries of the muscular-elastic type or in the aorta, the outer membrane is very strong, and practically absent in the pulmonary capillaries.
Histological structure
All membranes of muscle-type arteries preserve the general plan of the structure of blood vessels. In particular, a monolayer epithelium on the connective tissue membrane is located inside. It is covered by the middle shell with a large number of muscle cells and rare elastic fibers. Outside is the connective tissue membrane, moderately expressed in vessels of this type. And in each of these layers there are identical cells, as is the case with elastic arteries or capillaries. The only difference is the strength of the vessel, its caliber and the presence of pores in the endothelium.
All arteries of the muscular type, as well as elastic and transitional vessels, have a single endothelial lining. This means that the internal epithelium, lining the wall from the inside at the site of direct contact with 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 leukocytes pass into the tissues and vice versa, there is a transport of substances and gas exchange. This means that muscle type arteries, arterioles and larger vessels are not needed for direct metabolism, but only for transport.
Arterioles
Arterioles are called thin muscle arteries. These are small-sized blood vessels from which multiple capillaries depart. These are some of the sections of the arterial bed most distant from the heart, which is why pulsation and a high level of blood pressure are achieved due to the muscle cells of the middle membrane. For example, the nephron-bearing arteriole is able to maintain a pressure indicator of 120 mmHg, despite the fact that pulsation from the heart is practically not transmitted to it. Such an artery generates a pulse itself due to sympathetic innervation, rather than stretching and compression, as is observed in vessels of an elastic and transitional type.
Basics of Vascular Pathologies
There is a likelihood of certain substances entering the inner shell, while the return to the cavity of the vessel is virtually eliminated. Therefore, the ingress of cholesterol under the endothelium in the elastic and transitional vessels, as well as in arteries of the muscle 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.