An arteriole differs from an artery in that its wall has only one layer of muscle cells, thanks to which it performs a regulatory function. Arteriole proceeds directly into the precapillary, in which muscle cells are scattered and do not constitute a continuous layer. The precapillary differs from arterioles in the fact that it is not accompanied by a venule.

Numerous capillaries depart from the precapillary.
Capillaries are the thinnest vessels that perform the exchange function. In this regard, their wall consists of a single layer of flat endothelial cells, permeable to substances and gases dissolved in a liquid. Anastomozirovaya widely between themselves, the capillaries form a network (capillary network), turning into postcapillary, built similarly to precapillary. Postcapillary continues into the venula, accompanying arterios. Venules form thin initial segments of the venous bed that make up the roots of the veins and pass into the veins.

Veins (lat. Vena, Greek phlebs; hence phlebitis – inflammation of the veins) carry blood in the opposite direction to the arteries, from organs to the heart. Their walls are arranged according to the same plan as the walls of the arteries, but they are much thinner and have less elastic and muscular tissue, due to which empty veins fall, and the lumen of the arteries in the transverse section gapes; veins, merging with each other, form large venous trunks – veins that flow into the heart.

Veins widely anastomose among themselves, forming venous plexus.
The movement of blood through the veins is due to the activity and the suctioning action of the heart and chest cavity, in which negative pressure is created during inhalation due to the pressure difference in the cavities and also due to the reduction of the skeletal and visceral muscles of the organs and other factors.

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