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FIGURE 13 Countercurrent multiplication in the loop of Henle. Structures are the thin descending limb (DLH), the medullary interstitium (MI), and the thick ascending limb (TAL). (Based on Pitts RF. Physiology of the kidney and body fluids, 3rd ed. Chicago: Yearbook Medical Publications, 1974, p126.)

thin descending limb, it has the same osmolality as the medullary interstitium, as shown in step 2. In step 3, the effects of fluid flow into the thin descending limb are shown. This flow shifts new isosmotic fluid into the thin descending limb and shifts the hyper-osmotic fluid around the hairpin turn. In step 4, active NaCl absorption again dilutes the osmolality of the fluid in the thick ascending limb and concentrates it in the medullary interstitium and the thin descending limb. However, in this case, there is a difference in osmolality along all of the structures due to the shift of isosmotic fluid into the descending limb.

The cycle is again completed in step 5 with an inflow of isosmotic fluid and in step 6 with the active transport process establishing a 200 mOsm/kg H2O gradient.

In the steady state that is established by this counter-current multiplication, one expects there to be a gradient of osmolality along the thin descending limb and the medullary interstitium from 300 mOsm/kg H2O at the corticomedullary junction to 700 mOsm/kg H2O at the tip of the loop for the example in Fig. 13. On the other hand, there is a gradient from 100-500 mOsm/kg H2O along the thick ascending limb. This example is shown in a stepwise manner in Fig. 13, assuming separate steps of flow into the descending limb and active transport out of the thick ascending limb. Obviously, these processes normally go on concurrently and continuously in the development and normal maintenance of the osmolality gradient. It can be seen from the figure that the longer the loop of Henle, the greater will be the difference in osmolality from the corticomedullary junction to the tip of the loop.

Countercurrent multiplication also occurs in the inner medulla by NaCl absorption from the thin ascending limb. The only difference is that, in the case of the thin ascending limb, the NaCl absorption is not active but occurs down a concentration gradient, as discussed in Chapter 27. This NaCl gradient develops because water is reabsorbed from the thin descending limb of the loop of Henle and because half of the osmolality of the medullary interstitium in the inner medulla is made up of urea, whereas most of the hyperosmolality of the fluid in the thin descending and thin ascending limb is due to NaCl. This results in an effective passive countercurrent multiplication in the inner medulla (Fig. 14), which depends ultimately on the maintenance of a high urea concentration in this region of the kidney.

Urea Recycling in the Medulla

The final component of the medullary hyperosmo-lality is urea. This solute is delivered passively to the medulla and kept there in high concentration by counter-current exchange. Figure 15 shows the primary permeability characteristics that allow urea to be concentrated in the inner medulla. As described in Chapter 26, approximately 50% of the filtered urea is reabsorbed passively in the proximal tubule. Because of the high urea concentration in the medulla, some urea diffuses passively into the thin descending limb of the loop of Henle. However, beyond the tip of the loop the thin and thick ascending limbs of the loop of Henle, the distal convoluted tubule, the connecting tubule, the cortical collecting tubule, and outer medullary collecting duct are all impermeable to urea. Consequently, as water is lost from the tubular fluid in the descending limb of the loop

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