mOsmol/kg H2O

Ascending Vasa Recta

1200 mOsmol/kg H2O

FIGURE 12 Countercurrent exchange in the vasa recta. The plasma flowing down the descending limb of the vasa recta becomes concentrated primarily because of solute entry. However, in the ascending limb of the loop of Henle, solute diffuses back outward. Note that the water movements are much less significant for osmotic equilibration than the solute movements.

extremely long loop of Henle. Early studies of the renal medulla of various mammals revealed the gradient of osmolality from most concentrated at the tip of the papilla to isosmotic at the corticomedullary junction. The final step in demonstrating where the medullary solute must come from and where the urine is concentrated occurred when micropuncture experiments showed that tubular fluid in the early distal convolution (the distal convoluted tubule) was always hypo-osmotic to plasma. Some samples were as low as 100 mOsm/kg H2O, and most fell in the range of 100-150 mOsm/kg H2O. This hypo-osmolality was observed regardless of whether the final urine osmolality was maximally dilute or maximally concentrated.

These micropuncture results demonstrated that in antidiuresis the final urine was concentrated by water reabsorption along the connecting tubule and the collecting duct. The results also showed that solute was added to the medullary interstitium in excess of water by the loop of Henle because the proximal tubular fluid samples were always isosmotic, whereas the samples from the early distal convolution were always hypo-osmotic. In vitro experiments with isolated perfused segments of the thick ascending limb also showed that active NaCl absorption could produce a tubular fluid that was dilute compared with the peritubular bathing solution. Nevertheless, even at slow flow rates, the maximal osmolality difference that could be generated was about 200 mOsm/kg H2O. Therefore, how does the kidney manage to develop a total osmolality difference of 900 mOsm/kg H2O from the corticomedullary junction to the tip of the papilla?

The development of this large osmolality gradient is a consequence of the hairpin arrangement of the descending and ascending limbs of the loop of Henle, much like the vasa recta. The effect of the counter-current flow together with active NaCl absorption in the thick ascending limb of the loop of Henle can be best understood from the classic sequence of diagrams of Pitts, shown in Fig. 13. For the moment, consider the effects only in a short loop of Henle in which the hairpin turn occurs at the junction between the inner and the outer medulla. In this loop of Henle, there is no thin ascending limb, so the effects only of the actively absorbing thick ascending limb are significant. Figure 13 presents what occurs during the development of medullary hyperosmolality from a starting point with an isosmotic fluid in the loop of Henle (step 1) and in the interstitium. Due to active NaCl absorption, the osmolality of the fluid in the thick ascending limb will fall by approximately 100 mOsm/kg H2O, and the interstitial fluid will rise by an equivalent amount to give a transepithelial osmolality gradient of 200 mOsm/ kg H2o. Because of the high water permeability of the

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