Sodium (Na) reabsorption along the mammalian nephron. About 25 moles of Na in 180 L of fluid daily is delivered into the glomerular filtrate of a normal person. About 60% of this load is reabsorbed along the proximal tubule (PROX), indicated in dark blue; about 25% along the loop of Henle (LOH), including the thick ascending limb indicated in light blue; about 5% to 7% along the distal convoluted tubule (DCT), indicated in dark gray; and 3% to 5% along the collecting duct (CD) system, indicated in light gray. All Na transporting cells along the nephron express the ouabain-inhibitable sodium-potassium adenosine triphosphatase (Na-K ATPase) pump at their basolateral (blood) cell surface. (The pump is not shown here for clarity.) Unique pathways are expressed at the luminal membrane that permit Na to enter cells. The most quantitatively important of these luminal Na entry pathways are shown here. These pathways are discussed in more detail in Figures 2-15 to 2-19. CA—carbonic anhydrase; Cl—chloride; CO2—carbon dioxide; H—hydrogen; H2CO3—carbonic acid; HCO3—bicarbonate; K—potassium; OH—hydroxyl ion.

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