Veins

Venules, joining with each other and growing larger, form veins. The walls of veins, like arteries, also have three membranes: internal, middle and external.

Taking into account the structure of the walls, there are two types of veins: amuscular and muscular veins. Amuscular veins are the veins of the dura mater and pia mater, the retina, bones, spleen and other organs of the immune system. In the walls of these veins, endothelial cells are adjacent to the basement membrane, which is covered on the outside with a thin layer of loose fibrous connective tissue. The walls of amuscular veins are fused with the connective tissue of the organs where these veins are located, so these veins retain their lumen, their walls do not collapse.

Muscular veins may have weakly, moderately, or strongly developed smooth muscle elements in their walls. Veins with weakly developed smooth muscle layers are located mainly in the upper parts of the body, in the neck and head area. As the caliber of the veins increases, circularly oriented myocytes appear in their walls. In medium-caliber veins, subendothelial connective tissue is located outside the basement membrane, in which individual elastic fibers are present. These veins do not have an internal elastic membrane. The middle layer is formed by 2-3 layers of circularly oriented myocytes, between which there are bundles of collagen and elastic fibers. The outer layer (adventitia) of medium-caliber veins is formed by loose fibrous connective tissue, in which blood vessels and nerve fibers (nerves) pass. In the walls of large veins, there is a small number of circularly oriented myocytes in the middle layer. The outer shell is thick, formed by connective tissue.

In the walls of veins with moderate development of smooth muscle elements (brachial vein, etc.) there is a basement membrane and a subendothelial layer. The internal elastic membrane is absent. The middle layer is formed by bundles of circularly oriented myocytes. The external elastic membrane is absent, the adventitia is well expressed.

Veins with a highly developed smooth muscle membrane are located in the lower half of the body, in the lower extremities. Smooth muscle cells are present in all three membranes of the vein walls, especially in the middle membrane.

Small, medium and some large veins have venous valves, flaps (valvulae venosae) - semilunar folds of the inner shell, which are usually located in pairs. Connective tissue fibers penetrate inside these folds. The veins of the lower extremities have the greatest number of valves. The valves allow blood to flow toward the heart and prevent its backflow. Both vena cava, veins of the head and neck, renal veins, portal, pulmonary veins do not have valves. The venous sinuses, into which blood flows from the brain, are located in the thickness (splitting) of the dura mater of the brain and have non-collapsing walls, ensuring unimpeded blood flow from the cranial cavity to the extracranial veins (internal jugular).

Depending on the topography and position of the veins in the body and organs, they are divided into superficial and deep. Superficial (subcutaneous) veins (venae superficiales), as a rule, follow independently. Deep veins (venae profundae) in double quantity (in pairs) are adjacent to the arteries of the extremities of the same name, therefore they are called accompanying veins (satellite veins). The names of the deep veins are similar to the names of the arteries to which the veins are adjacent (ulnar artery - ulnar vein, brachial artery - brachial vein). Unpaired deep veins are the internal jugular, subclavian, axillary, iliac (common, external, internal), femoral and some other large veins. Superficial veins connect with deep veins with the help of the so-called perforating veins, which act as fistulas - venous anastomoses. Adjacent veins are often connected to each other by numerous anastomoses, which together form venous plexuses (plexus venosus). These plexuses are clearly expressed on the surface or in the walls of some internal organs (bladder, rectum, esophagus). In general, the number of veins exceeds the number of arteries.

The largest veins of the systemic circulation are the superior and inferior vena cava. The hepatic veins and their tributaries flow into the inferior vena cava. The bypass flow of blood is carried out through veins through which venous blood flows away from the main path (collateral veins). The tributaries of one large (main) vein are connected to each other by intrasystemic venous anastomoses. Between the tributaries of various large veins (the superior and inferior vena cava, the portal vein) there are intersystemic venous anastomoses (cavo-caval, cavoportal, cavo-cavoportal), which are collateral pathways (vessels) of venous blood flow bypassing the main veins.