Figure 1215

Photomicrograph of the wall of a medium-sized vein. This photomicrograph shows the three tunics in a section through the wall of a medium-sized vein. The tunica intima consists of endothelium and a very thin subendothelial layer of connective tissue containing some smooth muscle cells. The tunica media contains circularly and spirally arranged smooth muscle cells with collagen and elastic fibers. Note the tunica adventitia, which contains an abundance of collagen and some elastic fibers. The few nuclei seen in this layer belong to fibroblasts. x360.

• The tunica media of medium-sized veins is much thinner than the same layer in medium-sized arteries. It contains several layers of circularly arranged smooth muscle cells with interspersed collagen and elastic fibers. In addition, longitudinally arranged smooth muscle cells may be present just beneath the tunica adventitia

• The tunica adventitia is typically thicker than the tunica media and consists of collagen fibers and networks of elastic fibers (Fig. 12.15).

Large Veins

In large veins, the tunica media is relatively thin, and the tunica adventitia relatively thick

Veins with a diameter greater than 10 mm are classified as large veins. The tunica intima of these veins (Fig. 12.16) consists of an endothelial lining with its basal lamina, a small amount of subendothelial connective tissue, and some smooth muscle cells. Often, the boundary between the tunica intima and tunica media is not clear, and it is not always easy to decide if the smooth muscle cells close to the intimal endothelium belong to the tunica intima or to the tunica media.

The tunica media is relatively thin and contains circum-ferentially arranged smooth muscle cells, collagen fibers, and some fibroblasts. In some animals, but not in humans, cardiac muscle cells extend into the tunica media of the venae cavae and the pulmonary veins, near their junction with the heart.

The tunica adventitia of large veins (e.g., the subclavian veins and the venae cavae) is the thickest layer of the ves-

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