Mechanisms of capillary exchange (transcytosis & bulk flow)

■ This method of capillary exchange is mainly used to transport small amounts of large, lipid-insoluble (water soluble) molecules, such as large proteins. ■ Substances, packaged in vesicles, move through endothelial cells via endocytosis and exocytosis. ■ This method of exchange is the movement of large volumes of ions, molecules or particles between blood and interstitial fluid. ■ Bulk flow occurs when a pressure gradient pushes water and dissolved solutes out of or into the capillary. ■ The solution of substances pass across fenestrations and intercellular clefts. ■ Fenestrated capillaries allow more bulk flow. ■ Continuous capillaries with extensive tight junctions at their intercellular clefts allow little bulk flow. ■ During bulk flow, most solutes cross with water. However, formed elements in the blood (such as a red blood cell) and most proteins do not cross.

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