Blood vessels

The blood vessels are comparable to the pipes of a duct filled with liquid (the blood) and connected to a pump (the heart). The pressure generated in the heart allows an adequate flow of blood to each section of the duct.

The set of blood vessels forms the vascular system, preceded by the adjective cardio in the case in which the blood and the heart are also considered.

There are three types of blood vessels, respectively called arteries, capillaries and veins.

The vessels that carry blood from the heart to the periphery are called arteries, while the return to the heart muscle is entrusted to the veins; finally, the capillaries act as a bridge between the two types of vessels, and are responsible for the exchange of substances between blood and perfused tissues. Thanks to their very thin walls made up of a single layer of cells, the endothelium, and the low speed with which the blood circulates inside them, the capillaries can easily exchange respiratory gases, nutrients, enzymes, hormones and waste substances.

The walls of the arteries, thick and elastic, are made up of three layers: the innermost one (intimate tunic) is a layer of endothelial cells, the intermediate - called the media tunic - is made up of smooth muscle tissue, while the outermost (tunic external or adventitia) is formed by connective tissue very rich in elastic fibers.

The presence of muscular and elastic tissue allows the arteries to accumulate, expanding, the energy impressed on the blood mass by the contraction of the heart; when it relaxes between one contraction and the next, the energy accumulated by the arteries is slowly transferred to the blood column direct to the periphery; in this way the arteries help to transform the intermittent blood flows, coming from the heart, into a continuous (laminar) flow essential to allow normal exchanges at the capillary level.

Like arteries, veins are made up of three layers, but their walls are less extensible and thick than arteries of the same caliber; this allows the transit of large quantities of blood without opposing a great resistance. Along some veins, particularly in the larger ones located at the level of the lower limbs, special valves are located - called crescent or dovetail valves - which ensure the unidirectional flow of blood in a centripetal sense (from the periphery to the heart).

In the "man," the largest artery - the aorta - has a diameter of about 2.5 cm, while in the smallest capillary the caliber is reduced to 5 µm, then reaching 3 cm in the largest vein, the vein In fact, the vascular system begins with large arteries that branch out gradually into smaller and branched arteries, then into even smaller ones (called arterioles) that continue in a network of very small blood vessels, the aforementioned capillaries After giving up the necessary and having accumulated the waste, the blood passes from the capillaries to very small veins (the venules), then into larger veins through which it returns to the heart. Arterioles, capillaries and venules form the so-called microcirculation.

The blood vessels - thanks to the precapillary sphincters - have the ability to vary their tone by directing greater blood flow to the organs that perform more intense work and vice versa.