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Unformatted text preview: lar clefts are tight (i.e., have tight junctions); these capillaries have the lowest permeability. Fenestrated (found in exocrine glands, renal glomeruli, intestinal mucosa) perforations (fenestrae) in endothelium result in relatively high permeability. Discontinuous (found in liver, spleen, bone marrow) large intercellular gaps and gaps in basement membrane result in extremely high permeability. Large surface area and relatively high permeability (especially at intercellular clefts) to fluid and macromolecules make capillaries the primary site of exchange for fluid, electrolytes, gases, and macromolecules. 8 Pulmonary Circulation In some organs, precapillary sphincters (a circular band of smooth muscle at entrance to capillary) can regulate the number of perfused capillaries. Venules Small exchange vessels (10-50 ) composed of endothelial cells surrounded by basement membrane (smallest postcapillary venules) and smooth muscle (larger venules). Fluid and macromolecular exchange occur most prominently at venular...
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- Fall '07
- Hydrostatic Pressure