Veins

Venules, connecting with each other and becoming larger, form veins. Walls of veins, like arteries, also have three shells: inner, middle and outer.

Given the structure of the walls, there are two types of veins: the muscle and the muscle. The veins of the jawless type are the veins of the hard and soft meninges, the retina of the eye, bones, spleen and other organs of the immune system. In the walls of these veins, endotheliocytes adhere to the basal membrane, which is covered externally with a thin layer of loose fibrous connective tissue. The walls of the jawless 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.

The veins of the muscular type can have weak, medium and highly developed smooth muscle elements in their walls. Veins with a weak development of the smooth muscle are mainly located in the upper parts of the trunk, in the neck and head. As the caliber of veins increases, circularly oriented myocytes appear in their walls. In middle-vein veins, outside the basal membrane, there is a subendent connective tissue in which individual elastic fibers are present. There is no internal elastic membrane in these veins. The middle shell is formed by 2-3 layers of circularly oriented myocytes, between which there are bundles of collagen and elastic fibers. The outer membrane (adventitia) of medium-sized veins is formed by a loose fibrous connective tissue, in which blood vessels and nerve fibers (nerves) pass. In the walls of large veins in the middle shell there is a small number of circularly oriented myocytes. Outer shell thick, formed by connective tissue.

In the walls of the veins with an average development of smooth muscle cells (brachial vein, etc.), there is a basal membrane and a subendothelial layer. There is no internal elastic membrane. The middle shell is formed by bundles of circularly oriented myocytes. The outer elastic membrane is absent, the adventitia is well pronounced.

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

Small, medium and some large veins have venous valves, valves (valvulae venosae) - semilunar folds of the inner membrane, which are usually located in pairs. Inside these folds penetrate the fibers of connective tissue. The greatest number of valves have veins of the lower extremities. Valves flow blood towards the heart and prevent it from flowing backward. Both hollow veins, veins of the head and neck, renal veins, portal, pulmonary veins do not have valves. Venous sinuses, in which the blood flows from the brain, are located in the thickness (splittings) of the hard shell of the brain and have disintegrating walls that provide unimpeded flow of blood 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 subdivided into superficial and deep. Superficial (subcutaneous) veins (venae superficiales), as a rule, follow independently. Deep veins (venae profundae) are doubled (in pairs) adjacent to the same arteries of the limbs, therefore they are called accompanying veins (companion veins). The names of deep veins are similar to the names of arteries to which the veins adjoin (ulnar artery - ulnar vein, brachial artery - brachial vein). Unpaired deep veins are the internal jugular, subclavian, axillary, iliac (general, external, internal), femoral and some other large veins. The superficial veins are connected to the deep veins with the help of the so-called perforating veins, which serve as the joints - venous anastomoses. Neighboring veins are often joined together by numerous anastomoses, forming in aggregate the venous plexus (plexus venosus). These plexuses are well 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 great circle of circulation are the upper and lower hollow veins. The hepatic veins with their tributaries enter the inferior vena cava. A roundabout flow of blood is carried out through the veins along which the venous blood flows away from the main path (collateral veins). Inflows of one large (main) vein are interconnected by intra-systemic venous anastomoses. Between the inflows of various large veins (upper and lower hollow veins, portal vein) there are inter-systemic venous anastomoses (cava-caval, cavaaportal, and cavaocapoital), which are the collateral veins of venous blood flow around the main veins.