Often deep veins of lower limbs are the site of thrombus (blood clot) formation, a condition known as deep venous thrombosis (DVT). DVT is associated with immobilization of the lower limbs due to prolonged bed rest (after surgery or hospitalization), orthopedic casts, or restricted movement during long-haul ﬂights. DVT can be a life-threatening condition because of the potential for development of pulmonary embolism (blockage of the pulmonary arteries) by a dislodged blood clot originating from deep veins.
The three tunics of the venous wall are most evident in medium-sized veins (Fig. 13.24).
 The tunica intima consists of an endothelium with its basal lamina, a thin subendothelial layer with occasional smooth muscle cells scattered in the connective tissue elements, and, in some cases, a thin internal elastic membrane.
 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. 13.24b).
In large veins, the tunica media is relatively thin, and the tunica adventitia is relatively thick.
Veins with a diameter greater than 10 mm are classified as large veins.
 The tunica intima of these veins (Fig. 13.25 and Plate 34, page 436) 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 whether the smooth muscle cells close to the intimal endothelium belong to the tunica intima or to the tunica media.
FIGURE 13.24 • Schematic diagram and photomicrograph of a medium-sized vein. a. The cellular and extracellular components are labeled. Note that the tunica media contains a few layers of circularly arranged smooth muscle cells with interspersed collagen and elastic fibers. Also, longitudinally arranged smooth muscle cells are present at the junction with the tunica adventitia. b. This photomicrograph shows a section through the wall of a medium-sized vein in routine H&E preparation. The tunica intima consists of endothelium and a very thin subendothelial layer of connective tissue containing some smooth muscle cells. The tunica media contains a few layers of circularly and spirally arranged smooth muscle cells with collagen and elastic fibers. Note that the thickest layer is the tunica adventitia, which contains an abundance of collagen and some elastic fibers. The few nuclei seen in this layer belong to fibroblasts. 360.
 The tunica media is relatively thin and contains circumferentially 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 both superior and inferior vena cava and the pulmonary veins, near their junction with the heart.
 The tunica adventitia of large veins (e.g., the subclavian veins, portal vein, and the venae cavae) is the thickest layer of the vessel wall. Along with the usual collagen and elastic fibers and fibroblasts, the tunica adventitia also contains longitudinally disposed smooth muscle cells (Fig. 13.26).
In several locations in the body, blood vessels—both arteries and veins with an atypical structure are present. These include the following.