Salt (NaCl) is actively extruded from the ascending limb into the surrounding interstitial fluid. This is not accomplished, however, by the same process that occurs in the proximal tubule. Instead, Na+ diffuses from the filtrate into the cells of the thick portion of the ascending limb, accompanied by the secondary active transport of K+ and Cl-. This occurs in a ratio of 1 Na+ to 1 K+ to 2 Cl-. The Na+ is then actively transported across the basolat-eral membrane to the interstitial fluid by the Na+/K+ pumps. Cl-follows the Na+ passively because of electrical attraction, and K+ passively diffuses back into the filtrate (fig. 17.15).
The ascending limb is structurally divisible into two regions: a thin segment, nearest the tip of the loop, and a thick segment of varying lengths, which carries the filtrate outward into the cortex and into the distal convoluted tubule. It is currently believed that only the cells of the thick segments of the ascending limb are capable of actively transporting NaCl from the filtrate into the surrounding interstitial fluid.
Although the mechanism of NaCl transport is different in the ascending limb than in the proximal tubule, the net effect is the same: salt (NaCl) is extruded into the surrounding fluid. Unlike the epithelial walls of the proximal tubule, however, the walls of the ascending limb of the loop of Henle are not permeable to water. The tubular fluid thus becomes increasingly dilute as it ascends toward the cortex, whereas the interstitial fluid around the loops of Henle in the medulla becomes increasingly more concentrated. By means of these processes, the tubular fluid that enters the distal tubule in the cortex is made hypotonic (with a concentration of about 100 mOsm), whereas the interstitial fluid in the medulla is made hypertonic.
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