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 niicrocirculatory bed, but no sphincters are present where the afferent capillaries enter the thoroughfare channels. Blind-ending lymphatic vessels are shown in association with the capillary bed. Note the presence of anchoring filaments and the valve system within the lymphatic capillaries.
The tunics of veins are not as distinct or well defined as the tunics of arteries. Traditionally, veins are divided into three types on the basis of size:
• Small veins or venules, further subclassified as postcapillary and muscular venules
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. Large- and medium-sized veins usually travel with large-and 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 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.
Muscular venules are distinguished from postcapillary venules by the presence of a tunica media
Postcapillary venules receive blood from capillaries and have a diameter as small as 0.2 mm or slightly larger. They possess an endothelial lining with its basal lamina and pericytes. 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. 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 located distal to the postcapillary venules in the returning venous network and have a diameter up to 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.
Medium veins have a diameter of up to 10 mm. Most named veins are in this category. Valves are a characteristic feature of these vessels and are most numerous in the inferior portion of the body, particularly the legs, to prevent retrograde movement of blood because of gravity. The three tunics of the venous wall are most evident in medium-sized veins (Fig. 12.14):
• 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.
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