1. The medullary interstitium has a concentration >4 times that of its surrounding fluid, and must be both generated and maintained.
The medullary interstitium has a concentration >4 times that of its surrounding fluid, and must be both generated and maintained. The countercurrent multiplier, composed of a hairpin tubule loop with a water-permeable descending limb juxtaposed against an impermeable ascending limb with a highly active Na-K-2Cl pump, generates the concentration gradient. A separate hairpin loop within the tubular capillary system allows shunting of water from the descending limb to the ascending limb preventing the dilution of the medullary gradient. This process, countercurrent exchange, maintains the medullary concentration.
John Danziger, and Mark L. Zeidel CJASN 2015;10:
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