Page 401 - Atlas of Histology with Functional Correlations

against gravity, have valves. Because of the low blood pressure in the veins,

blood flow to the heart in the veins is slow and can even back up. The presence
of valves in veins assists venous blood flow toward the heart by preventing
backflow. When blood flows toward the heart, pressure in the veins forces the
valves to open. As the blood begins to flow backward, the valve flaps close the
lumen and prevent backflow of blood. Venous blood between the valves in the

extremities flows toward the heart because of the contraction of surrounding
muscles, contractions between muscles, or contractions of organs that have some
muscle such as the spleen. However, valves are absent in veins of the central

nervous system (CNS), the inferior and superior venae cavae, and the viscera.

The walls of the veins, like the arteries, also exhibit three layers or tunics;
however, the muscular layer is much thinner and less prominent. The tunica
intima in veins exhibits an endothelium and subendothelial connective tissue. In
contrast to arteries, the muscular tunica media is thin in the veins, and the

smooth muscles intermix with connective tissue fibers. The tunica adventitia is
the thickest and best-developed layer of the three tunics. Longitudinal bundles of
smooth muscle fibers are common in the connective tissue of this layer (Fig.
10.2). The structure of the venous walls allows flexibility and the

accommodation of a large blood volume. As a result, veins contain most of the
blood in the body.

FIGURE 10.2 ■ Large vein.

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