Although large and medium veins have three layers— also designated tunica intima, tunica media, and tunica adventitia—these layers are not as distinct as they are in arteries. Largeand medium-sized veins usually travel with largeand medium-sized arteries; arterioles and muscular venules also sometimes travel together, thus allowing comparison in histologic sections. Typically, veins have thinner walls than their accompanying arteries, and the lumen of the vein is larger than that of the artery. The arteriole lumen
FIGURE 13.23 • Diagram of microcirculation. This schematic diagram shows a metarteriole (initial segment of a thoroughfare channel) giving rise to capillaries. The precapillary sphincters of the arteriole and metarteriole control the entry of blood into the capillaries. The distal segment of the thoroughfare channel receives capillaries from the microcirculatory bed, but no sphincters are present where the afferent capillaries enter the thoroughfare channels. Blind-ended lymphatic vessels are shown in association with the capillary bed. Note the presence of anchoring filaments and the valve system within the lymphatic capillaries.
is usually patent; that of the vein is often collapsed. Many veins, especially those that convey blood against gravity, such as those of the limbs, contain valves that allow blood to flow in only one direction, back toward the heart. The valves are semilunar flaps consisting of a thin connective tissue core covered by endothelial cells.
Postcapillary venules collect blood from the capillary network and are characterized by the presence of pericytes.
Postcapillary venules possess an endothelial lining with its basal lamina and pericytes (Plate 35, page 438). The endothelium of postcapillary venules is the principal site of action of vasoactive agents such as histamine and serotonin.
Response to these agents results in extravasation of ﬂuid and emigration of white blood cells from the vessel during inﬂammation and allergic reactions. Postcapillary venules of lymph nodes also participate in the transmural migration of lymphocytes from the vascular lumen into the lymphatic tissue. Pericytes form umbrella-like connections with the endothelial cells. The relationship between endothelial cells and pericytes promote their mutual proliferation and survival. Both synthesize and share the basal lamina (see Fig. 13.21), synthesize growth factors, and communicate with each other through tight and gap junctions. Pericyte coverage is more extensive in the postcapillary venules than capillaries.
The postcapillary venules in the lymph nodes are also called high endothelial venules (HEVs) because of the prominent cuboidal appearance of their endothelial cells and their ovoid nuclei.
Muscular venules are distinguished from postcapillary venules by the presence of a tunica media.
Muscular venules are located distal to the postcapillary venules in the returning venous network and have a diameter of as much as 0.1 mm. Whereas postcapillary venules have no true tunica media, the muscular venules have one or two layers of smooth muscle that constitute a tunica media. These vessels also have a thin tunica adventitia. Usually, pericytes are not found in muscular venules.
Medium veins have a diameter of as much as 10 mm. Most deep veins that accompany arteries are in this category (e.g., radial vein, tibial vein, popliteal vein). Valves are a characteristic feature of these vessels and are most numerous in the inferior portion of the body, particularly the lower limbs, to prevent retrograde movement of blood because of gravity.