The term exchange vessel is often used instead of capillary by microvascular physiologists to denote the fact that exchange processes occur on both sides of the anatomic capillary bed, with oxygen readily diffusing across metarterioles and proteins leaking across the smallest venules. Nonetheless, a substantial component of fluid and solute exchange does indeed take place across capillaries; our discussion of this process will be largely confined to this component of the microcirculation. The microstructure of the capillary wall varies between tissues, with capillaries classified into three ultrastructural types based on unique characteristics of the endothelial cell and its underlying basement membrane: continuous, fenestrated, and discontinuous. Some of the characteristics shared by all three types of capillaries include a single layer of highly attenuated endothelial cells that are less than 0.2 ^m thick (except in the region of their nuclei); thin slits (clefts) that are created by the close apposition of adjacent endothelial cells; a basement membrane lying beneath the endothelial cells, which is composed of fine fibrillar material capable of retarding the passage of macromolecules; and micro-pinocytotic vesicles that are formed on the luminal and abluminal surfaces of the endothelial cells and found within their cytoplasm.
Continuous capillaries (Fig. 2a), the most abundant and widespread type, are found in muscle, skin, lung, connective tissue, and the nervous system. Lipid-soluble (lipophilic) solutes, including dissolved oxygen and carbon dioxide, can take the transcellular route, which involves simple diffusion through the two cell membranes that make up the capillary wall. Hydrophilic solutes and water can cross the capillary wall through intercellular junctions (clefts), which in most capillary beds have a width of approximately 60 A. These clefts can be easily traversed by water, ions, and small organic solutes dissolved in the plasma; however, albumin (with a 70-A diameter) and other plasma proteins have a difficult time penetrating the clefts. Continuous capillaries in the nervous system exhibit a functionally closed cleft between adjacent endothelial cells, giving rise to vessels that are impermeable to passive diffusion of solutes as small as glucose and some electrolytes.
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