Countercurrent multiplication of the NaCl concentration is the mechanism that contributes most to the hypertonicity of the interstitial fluid in the medulla. However, urea, a waste product of amino acid metabolism (chapter 5; see fig. 5.15), also contributes significantly to the total osmolality of the interstitial fluid.
The role of urea was inferred from experimental evidence showing that active transport of Na+ occurs only in the thick segments of the ascending limbs. The thin segments of the ascending limbs, which are located in the deeper regions of the medulla, are not able to extrude salt actively. But since salt does indeed leave the thin segments, a diffusion gradient for salt must exist, despite the fact that the surrounding interstitial fluid has the same osmolality as the tubular fluid. Investigators therefore concluded that molecules other than salt—specifically urea— contribute to the hypertonicity of the interstitial fluid.
It was later shown that the ascending limb of the loop of Henle and the terminal portion of the collecting duct in the inner medulla are permeable to urea. Indeed, the region of the collecting duct in the inner medulla has specific urea transporters that permit a very high rate of diffusion into the surrounding interstitial fluid. Urea can thus diffuse out of this portion of the collect ing duct and into the ascending limb (fig. 17.18). In this way, a certain amount of urea is recycled through these two segments of the nephron. The urea is thereby trapped in the interstitial fluid where it can contribute significantly to the high osmolality of the medulla. This relates to the ability to produce a concentrated urine, as will be described in the next section.
The transport properties of different tubule segments are summarized in table 17.2.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.