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.