Figure 212

Carbonic anhydrase-mediated Na+/H+ exchange in proximal convoluted tubule. Na+/H+ exchange across apical cell membranes is shown by the open circle. Carbonic anhydrase (CA) is present in a membrane-bound form in the apical membrane and a soluble form within the cytoplasm. The Na+/K+-ATPase is shown by the filled circle at the basolateral membrane.

continue to absorb the remaining Na+. Second, Na+ that escapes proximal tubular transport is delivered to more distal nephron segments, where compensatory reabsorption reduces the impact of diminished upstream Na+ recovery. Hence, although most Na+ is reabsorbed by proximal tubules, diuretics inhibiting its transport in this nephron segment have only a modest effect in reducing overall Na+ reabsorption.

Most of the K+ that is filtered at the glomerulus is reabsorbed by proximal tubules. K+ appearing in the voided urine was secreted by distal and terminal nephron segments (discussed later).

Another significant feature of the proximal tubule is that it is the site of organic acid transport. This is important in understanding both the pharmacokinetics of many of the diuretics, most of which are weak organic acids, and also certain of the side effects induced by these drugs. For instance, uric acid, which is the end product of purine metabolism in humans, is both reabsorbed and secreted by the organic acid transport pathway (see Chapter 37).

An important functional characteristic of the proximal tubule is that fluid reabsorption is isosmotic; that is, proximal reabsorbed tubular fluid has the same osmotic concentration as plasma. Solute and water are transported in the same proportions as in the plasma because of the high water permeability of the proximal tubule. Thus, the total solute concentration of the fluid in the proximal convoluted tubule does not change as the fluid moves toward the descending loop of Henle. The corollary of this high water permeability is that unabsorbable or poorly permeable solutes in the luminal fluid retard fluid absorption by proximal tubules. This is an important consideration for understanding the actions of osmotic diuretics.

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