Transport In The Thin And Thick Ascending Limb

When the tubular fluid turns the bend at the tip of the loop of Henle, it enters the thin ascending limb of the loop of Henle. At this point, the epithelial transport properties change dramatically. Both the thin and the thick regions of the ascending limb of the loop of Henle are water impermeable because of the absence of aquaporin water channels, and there is no significant transepithelial water flow. On the other hand, these regions avidly reabsorb NaCl and can remove 20-25% of the filtered load of Na+, which is more than two-thirds of the amount delivered to the loop of Henle from the proximal tubule.

Although differences exist in the transport characteristics of the thin and thick ascending limbs of the loop of Henle, and between the medullary and cortical regions of the thick ascending limb, this discussion concentrates on their common ability to reabsorb salt while leaving water behind in the tubular fluid, resulting in dilution of the urine. This is the first region of the nephron in which water and solute transport are dissociated so that the tubular fluid can have an osmolality that is significantly different from the surrounding interstitial fluid.

Because the urine becomes concentrated in the thin descending limb of the loop of Henle, the NaCl concentration of the tubular fluid is higher than that of the medullary interstitium, about 575 mmol/L compared with 300 mmol/L in the interstitium, as shown in Fig. 2. (Chapter 28 examines how the medullary interstitium is made hyperosmotic and how the osmolality gradient is maintained.) Thus, a large concentration gradient favors passive diffusion of NaCl from the lumen into the interstitial fluid. Because the thin ascending limb is permeable to Na+ and Cl", this diffusion occurs rapidly, allowing an equilibration of the NaCl concentration between the tubular fluid and the adjacent medullary interstitium. There is also a gradient for urea that favors its entry into the ascending limb of the loop of Henle because of its higher concentration in the medullary interstitium; however, because this segment is relatively impermeable to urea, there is little entry.

As NaCl diffuses from the lumen of the thin ascending limb to the medullary interstitium, the tubular fluid becomes dilute compared with the interstitium. This osmolality difference would favor water movement out of the tubule, but this does not occur because of the low water permeability of this segment. The low water permeability, which is different from what is normally observed for most cell membranes and epithelia, results in a tubular fluid that is dilute with respect to the surrounding interstitium and becomes progressively more dilute as the urine flows into the less hyperosmotic regions of the medullary interstitium.

At the junction between the inner and outer medulla, the characteristics of the tubular epithelium change again. The thinner cells of the thin ascending limb (which contain relatively few mitochondria) disappear, the cells contain numerous mitochondria, and the epithelial cell layer thickens, giving the name to the

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