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Counter-current multiplication

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Physiology 3120
Tom Stavraky

Human Physiology Monday, December 7, 2009 “Renal VII” Counter-Current Multiplier System • High osmolarity of the medulla is essential for concentrating urine • Mechanism  Initially, all osmolarities are 300  At the same time, water leaves the descending loop, and salt leaves the ascending loop, so the interstitium is more concentrated than normal  As the fluid flows, water from descending loop leaves, and salt from the ascending loop leaves, which forms a gradient • Arrangement of vasa recti  Less abundant than peritubular capillaries  Run in parallel to each other, and anti-parallel to the loop of Henle  Run in hairpin loop, which really maintains concentration gradient  As it moves through the medulla, the blood becomes more concentrated, but never equalizes with the interstitium because the plasma moves too fast (except at the bottom of the loop • Recycling of urea  Don’t worry about complicated explanation  Urea = organic ion that is a waste product of the body  50% excreted in urine; 50% is kept in the medulla of the kidney, and contributes to medullary osmolarity  During low protein intake, decreased ability to concentrate urine (can’t make urea) • Tubule  Epithelial cells in the medulla are in a highly hyper-osmotic solution, so how do they stand these conditions  Cells make osmolytes  Molecules such as inositol are specialized that accumulate in the cell; artificially increase the cell osmolarity to match the surrounding  Prevents cell from shrivelling • Not all mammals are the same 1. Kangaroo rat  Desert dwelling, so must be able to conserve a lot of water
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