Requirements for Excreting a Concentrated Urine—High ADH Levels and Hyperosmotic Renal Medulla
The basic requirements for forming a concentrated urine are (1) a high level of ADH, which increases the permeability of the distal tubules and collecting ducts to water, thereby allowing these tubular segments to avidly reabsorb water, and (2) a high osmolarity of therenal medullary interstitial fluid, which provides theosmotic gradient necessary for water reabsorption to occur in the presence of high levels of ADH.
The renal medullary interstitium surrounding the collecting ducts normally is very hyperosmotic, so that when ADH levels are high, water moves through the tubular membrane by osmosis into the renal intersti-tium; from there it is carried away by the vasa recta back into the blood. Thus, the urine concentrating ability is limited by the level of ADH and by the degree of hyperosmolarity of the renal medulla. We discuss the factors that control ADH secretion later, but for now, what is the process by which renal medullary interstitial fluid becomes hyperosmotic? This process involves the operation of the countercur-rent mechanism.
The countercurrent mechanism depends on the special anatomical arrangement of the loops of Henle and the vasa recta, the specialized peritubular capillar-ies of the renal medulla. In the human, about 25 percent of the nephrons are juxtamedullary nephrons, with loops of Henle and vasa recta that go deeply into the medulla before returning to the cortex. Some of the loops of Henle dip all the way to the tips of the renal papillae that project from the medulla into the renal pelvis. Paralleling the long loops of Henle are the vasa recta, which also loop down into the medulla before returning to the renal cortex. And finally, the collecting ducts, which carry urine through the hyper-osmotic renal medulla before it is excreted, also play a critical role in the countercurrent mechanism.
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