Urinary concentrating ability of kidney increases if there is: September 2008
Correct Answer: Contraction in extracellular fluid volume
Description: Ans. D: Contraction in extracellular fluid volumeThe descending limb of the loop of Henle is permeable to H2O, so H20 diffuses out into the surrounding fluids. Because the loop is impermeable to Na+ and Cl" and because these ions are not pumped out by active transpo, Na+ and Cl" remain inside the loop.As the fluid continues to travel down the descending limb of the loop, it becomes more and more concentrated, as water continues to diffuse out. Maximum concentration occurs at the bottom of the loop.The ascending limb of the loop of Henle is impermeable to water, but Na+ and Cl" are pumped out into the surrounding fluids by active transpo.As fluid travels up the ascending limb, it becomes less and less concentrated because Na+ and Cl" are pumped out. At the top of the ascending limb, the fluid is only slightly less concentrated than at the top of the descending limb. In other words, there is little change in the concentration of the fluid in the tubule as a result of traversing the loop of Henle.In the fluid surrounding the loop of Henle, however, a gradient of salt (Na+, Cl") is established, increasing in concentration from the top to the bottom of the loop.Fluid at the top of the collecting duct has a concentration of salts about equal to that at the beginning of the loop of Henle (some water is reabsorbed in the DCT).As the fluid descends the collecting duct, the fluid is exposed to the surrounding salt gradient established by the loop of Henle.Two outcomes are possible:If water conservation is necessary, as in contraction of the extra cellular fluid volume, ADH stimulates the opening of water channels in the collecting duct, allowing H20 to diffuse out of the duct and into the surrounding fluids. The result is concentrated urine.If water conservation is not necessary, ADH is not secreted and the duct remains impermeable to H20. The result is dilute urine.
Category:
Physiology
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