The deeper regions of the medulla, around the tips of the loops of juxtamedullary nephrons, reach a concentration of 1,200 to 1,400 mOsm. In order to reach this high a concentration, the salt pumped out of the ascending limb must accumulate in the interstitial fluid of the medulla. This occurs because of the properties of the descending limb, to be discussed next, and because blood vessels around the loop do not carry back all of the extruded salt to the general circulation. The capillaries in the medulla are
Filtrate (tubular lumen)
Apical membrane 2 Cl-
Apical membrane 2 Cl-
Na+ ATP ADP
■ Figure 17.15 The transport of ions in the ascending limb. In the thick segment of the ascending limb of the loop, Na+ and K+ together with two Cl- enter the tubule cells. Na+ is then actively transported out into the interstitial space and Cl- follows passively. The K+ diffuses back into the filtrate, and some also enters the interstitial space.
uniquely arranged to trap NaCl in the interstitial fluid, as will be discussed shortly.
The descending limb does not actively transport salt, and indeed is believed to be impermeable to the passive diffusion of salt. It is, however, permeable to water. Since the surrounding interstitial fluid is hypertonic to the filtrate in the descending limb, water is drawn out of the descending limb by osmosis and enters blood capillaries. The concentration of tubular fluid is thus increased, and its volume is decreased, as it descends toward the tips of the loops.
As a result of these passive transport processes in the descending limb, the fluid that "rounds the bend" at the tip of the loop has the same osmolality as that of the surrounding interstitial fluid (1,200 to 1,400 mOsm). There is, therefore, a higher salt concentration arriving in the ascending limb than there would be if the descending limb simply delivered isotonic fluid. Salt transport by the ascending limb is increased accordingly, so that the "saltiness" (NaCl concentration) of the interstitial fluid is multiplied (fig. 17.16).
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