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Distance from Glomerulus (mm)

FIGURE 4 Change in the TF/P ratio of inulin and osmolality with distance along the proximal tubule. The ratio of the inulin concentration or osmolality in tubular fluid (TF) samples to that in the plasma (P) is plotted as a function of the distance along the proximal tubule from the glomerulus.

FIGURE 5 Reabsorption of solutes and water in the proximal tubule. Approximately two-thirds of the filtered solute and water is reabsorbed along the proximal tubule. The remainder is passed to the descending limb of the loop of Henle.

fluid sample to that in the plasma rose as the distance of the sampling point from the glomerulus increased, whereas the osmolality remained nearly the same as the plasma; i.e., the reabsorption was isosmotic. However, micropuncture can be used to sample only from those regions of the proximal convoluted tubule that are accessible on the surface of the cortex just beneath the capsule of the kidney. Thus, it measures the operation of only superficial proximal tubules, and only the convoluted segments of these superficial nephrons. However, if one extrapolates the rise in inulin concentration in the superficial proximal convoluted tubules to the proximal straight tubule, and assumes similar rates of reabsorption in the juxtamedullary proximal tubules, it can be calculated that the (TF/P) concentration ratio for inulin reaches 3 by the end of the proximal tubule. Thus, from Eq. [2], one can calculate that the rate of fluid delivery to the loop of Henle is one-third of the rate of glomerular filtration and, thus, that two-thirds of the GFR must be reabsorbed in the proximal tubule.

Because the tubular fluid osmolality remains nearly the same as that of the plasma, the total amount of solute reabsorbed must be in proportion to the water reabsorbed. Considered from this perspective as shown in Fig. 5, the total rate of solute filtration at the glomerulus would be approximately 37.7 mOsm/min (290 mOsm/L • 130 mL/min), and the rate of reabsorption would be two-thirds of this, or 25.1 mOsm/min, leaving 12.6 mOsm/min to flow on into the loop of Henle, as shown schematically in Fig. 3. The next sections consider what solutes comprise the 25.1 mOsm/min reabsorbed and how these reabsorptive processes drive water (volume) reabsorption.

Transepithelial Voltage in the Proximal Tubule

As Na+ is actively reabsorbed along the proximal tubule, an equivalent amount of anions must accompany it to maintain electrical neutrality. In the early part of the proximal tubule, the active reabsorption of Na+ leads to the development of a lumennegative electrical potential difference—the lumen of the early proximal convoluted tubule is negative with respect to the interstitium by about —5 mV. This potential difference is relatively low because the electrical resistance of the junctional complexes in the proximal tubule epithelium is very low, which essentially ''short-circuits'' the transepithelial voltage. Nevertheless, even this small lumen-negative voltage across the epithelium serves as an effective driving force for passive Cl— reabsorption across the junctional complexes in the early proximal convoluted tubule.

In the more distal portions of the proximal tubule, the lumen-negative electrical potential decreases and becomes positive in the proximal straight tubule. This change is due to slower rates of Na+ reabsorption in the later regions of the proximal tubule and to the development of diffusion potentials as concentration differences of the anions develop. As discussed in subsequent sections, the concentration of Cl— rises along the proximal tubule because HCO— is preferentially reabsorbed. Therefore, there is a concentration gradient of Cl— from the lumen to the interstitium that imposes a diffusion potential that is lumen positive by as much as 5 mV, as shown in Fig. 6. Because of the change in the electrical potential difference from lumen negative to lumen positive, and because this potential difference is small, it is most convenient to regard the average electrical potential difference in the proximal

FIGURE 6 Change in the transepithelial voltage along the proximal tubule. In the early regions of the proximal tubule, the active, electrogenic transport of Na+ out of the lumen causes development of a lumen-negative voltage that favors passive Cl3 diffusion to accompany Na+. Owing to the preferential reabsorption of HCO3" along the proximal tubule, the HCO3 concentration in the tubular fluid falls, whereas the Cl3 concentration rises. Because Cl3 is more permeant than HCO33 , a lumen-positive diffusion potential develops in the late proximal tubule, but the concentration gradient for Cl3 favors its continued reabsorption.

FIGURE 6 Change in the transepithelial voltage along the proximal tubule. In the early regions of the proximal tubule, the active, electrogenic transport of Na+ out of the lumen causes development of a lumen-negative voltage that favors passive Cl3 diffusion to accompany Na+. Owing to the preferential reabsorption of HCO3" along the proximal tubule, the HCO3 concentration in the tubular fluid falls, whereas the Cl3 concentration rises. Because Cl3 is more permeant than HCO33 , a lumen-positive diffusion potential develops in the late proximal tubule, but the concentration gradient for Cl3 favors its continued reabsorption.

Water Reabsorption Driven by Solute Reabsorption tubule to be negligible. Using this assumption, the net passive movement of ions across the proximal tubule is determined simply by their concentration differences.

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