Materials are exchanged between blood and tissue in three ways. Describe each of these ways. Explain the advantages of "filtration" and how this process is accomplished.
First, materials are exchanged by diffusion: from the blood into the cells of the capillary walls and, then, into the surrounding cells. Second, materials are exchanged by vesicles in the cells of capillary walls that pick up materials from the blood, transport them across the capillary walls, and expel them into the surrounding cells. Third, materials are exchanged by the filtration of water and all dissolved molecules except proteins, being too large, through gaps between the endothelial cells of the capillary walls. Such filtration allows blood both to deliver dissolved substances to cells and to pick up wastes from cells. The mechanism of filtration works in the following manner. As the blood passes from arterioles into capillary beds, the hydrostatic pressure in the capillaries is higher than the opposing osmotic pressure in the cells of the surrounding tissue. Such a pressure gradient causes water and dissolved substances to move out of the capillaries. By the time the blood passes from the capillaries into the venules, the hydrostatic pressure has fallen lower than the osmotic pressure of the surrounding cells. The new pressure gradient forces water and dissolved wastes to move back into the capillaries. Thus, water filtered out at the
