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Capillary Exchange

Capillary Exchange - sometimes shape or charge of the...

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Capillary Exchange Exchange of solutes between the blood and surrounding tissues only occurs across the endothelial walls of the capillaries. There are three primary routes across the endothelial cells that form the walls of the capillaries: intercellular clefts , fenestrations and directly through the cytoplasm of the endothelial cells. The major mechanisms involved include diffusion , transcytosis , filtration and reabsorption . Of these mechanisms, diffusion is by far the most important. Lipid soluble substances diffuse easily through the cellular membranes of endothelial cells while water soluble substances pass through membrane channels and pores, fenestrations or intercellular clefts. Larger particles do not fit through these pores and fenestrations and must be transported via transcytosis. Very large particles are not transported across the capillary walls and remain within the blood. (See Figure 10) Figure 10 Transport of substances out of the blood across capillary walls is determined by the size (and
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Unformatted text preview: sometimes shape or charge) of the solute. Thus, at the level of the capillary, blood is "filtered" and small solutes (and water) move out of the blood into the tissues. At the arterial end of a capillary bed, the relatively high blood pressure (enough to override the osmotic pressure driving fluid into the blood) drives additional fluid out of the blood. This tendency for water movement out of the blood is balanced somewhat by the high colloid osmotic pressure of the blood. The blood contains impermeable proteins (albumins) that maintain the osmolarity of the blood and regulate the loss and reabsorption of water. At the venous end of the capillary bed, blood pressure is lower and the COP is the same. The high blood COP (due to albumins) will cause water to be reabsorbed and move into the blood by osmosis . Thus, there is a net flow of water back into the capillaries. (See Figure 11) Figure 11...
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