As discussed in chapter 1, epithelial membranes cover all body surfaces and line the cavities of all hollow organs. Therefore, in order for a molecule or ion to move from the external environment into the blood (and from there to the body organs), it must first pass through an epithelial membrane. The transport of digestion products (such as glucose) across the intestinal epithelium into the blood is called absorption. The transport of molecules out of the urinary filtrate (originally derived from blood) back into the blood is called reabsorption.
The cotransport of Na+ and glucose described in the last section can serve as an example. The cotransport carriers for Na+ and glucose are located in the apical (top) plasma membrane of the epithelial cells, which faces the lumen of the intestine or kidney tubule. The Na+/K+ pumps, and the carriers for the facilitated diffusion of glucose, are on the opposite side of the epithelial cell (facing the location of blood capillaries). As a result of these active and passive transport processes, glucose is moved from the lumen, through the cell, and then to the blood (fig. 6.19).
The membrane transport mechanisms described in this section move materials through the cytoplasm of the epithelial cells, a process termed transcellular transport. However, diffusion and osmosis may also occur to a limited extent in the very tiny spaces between epithelial cells, a process termed paracellu-lar transport. Such passive transport processes that do occur are limited by the tight junctions (regions where the plasma membranes of adjacent cells are fused all the way around) and desmosomes (buttonlike points of fusion of plasma membranes) between epithelial cells of the intestine and kidney tubules.
It should be noted that there are additional processes that cause movements of materials across various epithelial membranes. For example, the epithelial cells that comprise the walls of many blood capillaries (the thinnest of blood vessels) have pores between them that can be relatively large, permitting filtration of water and dissolved molecules out of the capillaries through the paracellular route. In the capillaries of the brain, however, such filtration is prevented by tight junctions, so molecules must be transported transcellularly. This involves the cell transport mechanisms previously described, as well as the processes of endocyto-sis and exocytosis, as described in the next section.
Bft Severe diarrhea is responsible for almost half of all deaths worldwide of children under the age of ^ 4 (amounting to about 4 million deaths per year). Because rehydration through intravenous therapy is often not practical, the World Health Organization (WHO) developed a simpler, more economical treatment called oral rehydration therapy. The therapy is effective because (1) the absorption of water by osmosis across the intestine is proportional to the absorption of Na+ and (2) the intestinal epithelium cotransports Na+ and glucose. The WHO provides those in need with a mixture (which can be diluted with tap water in the home) containing both glucose and Na+ as well as other ions. The glucose in the mixture promotes the cotransport of Na+ and the Na+ transport promotes the osmotic movement of water from the intestine into the blood. It has been estimated that oral rehydration therapy saves the lives of more than a million small children each year.
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