Venous System

Venous System

Venules are microscopic vessels that link capillaries to veins, which carry blood back to the atria. Vein walls aresimilar but not identical to arteries but have poorly devel-oped middle layers. Because they have thinner walls and are less elastic than arteries, their lumens have a greater diameter. Veins contain more fibrous tissue than arteries, and most veins in the extremities have valves.

Venules

The capillaries join to form venules. These vessels range between 8 and 100 μm in diameter, with the smallest or postcapillary venules made up only of endothelium, surrounded by pericytes or contractilecells. The larger venules have a thin tunica externa andone to two layers of smooth muscle cells making up their tunica media. Venules combine to form veins.

Veins

Many veins have flap-like valves projecting inward from their linings. These valves often have two structures that close if blood begins to back up in the vein. They aid in returning blood to the heart, opening if blood flow is toward the heart and closing if it reverses.

Veins mostly have three tunics, but these have thin-ner walls and larger lumens then the arteries to which they correspond. In a vein the tunica media has little smooth muscle or elastin. Even in larger veins the tunica media is relatively thin, with the heaviest wall layer being the tunica externa. As a result, veins can contain large amounts of blood. They are also called blood reser-voirs andcapacitance vessels. Veins can hold nearly 65%of the blood supply of the body at any given moment but are usually only partially filled (­FIGURE 19-5). Veins are usually in no danger of bursting because their blood pressure is relatively low. Veins have specialized struc-tures that help them to return blood to the heart at the same rate it was pumped into the circulation via the arteries. These structures include their larger lumens, through which blood passes with little resistance.

Veins also act as reservoirs for blood in certain conditions, such as during arterial hemorrhage. Resulting venous constrictions help to maintain blood pressure by returning more blood to the heart, ensuring an almost normal blood flow even when up to one-fourth of the blood volume is lost. TABLE 19-1 summarizes the blood vessel characteristics.

Venous Valves

The valves of veins prevent blood from flowing backward. Venous valves form from folds of the tunica intima (FIGURE 19-6). In appearance, they are similar to the heart’s semilunar valves and have sim-ilar ­functions. More venous valves exist in the limb veins. Here, gravity opposes upward blood flow. In the abdominal and thoracic body cavities, the veins mostly do not contain valves. Damage to the venous valves can lead to decreased venous return of blood to the heart.