Die Legende steht unten im Notitzfeld Figure 38-3 Stepwise generation of a high interstitial osmolality by a countercurrent multiplier. This example illustrates in a stepwise fashion how a countercurrent-multiplier system in the loop of Henle increases the osmolality of the medullary interstitium. Heavy boundaries of ascending limb and early DCT indicate that these nephron segments are rather impermeable to water, even in the presence of AVP. The numbers refer to the osmolality (mOsm) of tubule fluid and interstitium. The top panel shows the starting condition (step 0), with isosmotic fluid (∼300 mOsm) throughout the ascending and descending limbs and in the interstitium. Each cycle comprises two steps. Step 1 is the "single effect": NaCl transport from the lumen of the ascending limb to the interstitium, which instantaneously equilibrates with the lumen of the descending limb (steps 1, 3, 5, and 7). Step 2 is an "axial shift" of tubule fluid along the loop of Henle (steps 2, 4, and 6), with an instantaneous equilibration between the lumen of the descending limb and the interstitium. Beginning with the conditions in step 0, the first single effect is NaCl absorption across the rather water-impermeable ascending limb. At each level, we assume that this single effect creates a 200-mOsm difference between the ascending limb (which is water impermeable) and a second compartment: the combination of the interstitium and descending limb (which is water permeable). Thus, the osmolality of the ascending limb falls to 200 mOsm, whereas the osmolality of the interstitium and descending limb rise to 400 mOsm (step 1). The shift of new isosmotic fluid (∼300 mOsm) from the proximal tubule in the cortex into the descending limb pushes the column of tubule fluid along the loop of Henle, thus decreasing osmolality at the top of the descending limb and increasing osmolality at the bottom of the ascending limb. Through instantaneous equilibration, the interstitium-with an assumed negligible volume-acquires the osmolality of the descending limb, thereby diluting the top of the interstitium (step 2). A second cycle starts with net NaCl transport out of the ascending limb (step 3), again generating an osmotic gradient of 200 mOsm-at each transverse level-between the ascending limb on the one hand and the interstitium and descending limb on the other. After the axial shift of tubule fluid and instantaneous equilibration of the descending limb with the interstitium (step 4), osmolality at the bottom of the ascending limb exceeds that of the preceding cycle. With successive cycles, interstitial osmolality at tip of the loop of Henle rises progressively from 300 (step 0) to 400 (step 1) to 500 (step 3) to 550 (step 5) and then to 600 (step 7). Thus, in this example, the kidney establishes a longitudinal osmotic gradient of 300 mOsm from the cortex (300 mOsm) to the papilla (600 mOsm) by iterating (i.e., multiplying) a single effect that is capable of generating a transepithelial osmotic gradient of only 200 mOsm. Step 7A adds the collecting duct and shows the final event of urine concentration: allowing the fluid in the collecting duct to equilibrate osmotically with the hyperosmotic interstitium, producing a concentrated urine. (Based on a model by Pitts RF: Physiology of the Kidney and Body Fluids. Chicago, Year Book, 1974.) Simplifications of the Countercurrent-Multiplier Model in Figure 38-3