Mechanisms of capillary exchange

■ The primary role of capillaries is to permit the exchange of nutrients and wastes between the blood and tissue cells (via interstitial fluid). ■ Oxygen and nutrients move from the blood to the cells. ■ Carbon dioxide and other wastes move from the cells to the blood. The three basic mechanisms for transport of a substance across a cell layer during capillary exchange are: ■ Diffusion. ■ Transcytosis (endocytosis coupled with exocytosis). ■ Bulk flow. Diffusion ■ Diffusion is the most important method of capillary exchange. ■ Smaller, lipid-soluble molecules move easily across endothelial cell membranes. ■ Medium-sized, lipid-insoluble (water-soluble) substances, such as glucose and amino acids, move across intercellular clefts and fenestrations. Transcytosis • 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. Bulk flow • 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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