Discuss the function of the countercurrent mechanism and its role in the production of concentrated urine
What will be an ideal response?
The function of the countercurrent mechanism is to conserve water when the body needs it. To reabsorb water, both an osmotic gradient and antidiuretic hormone (ADH) must be present in the collecting duct to reabsorb water. To establish an osmotic gradient, a countercurrent multiplier in the thick ascending limb of the nephron loop pumps NaCl from the filtrate into the interstitial fluid. Continued solute reabsorption, including urea recycling, occurs from the filtrate in the medullary collecting duct to help add to the osmotic gradient. Finally, the countercurrent exchanger of the vasa recta allows perfusion of the inner medulla while maintaining the medullary interstitial gradient. Since the interstitial fluid creates a gradient for water reabsorption, water is drawn into the interstitial fluid by osmosis when ADH and aquaporins are present. The resulting urine is concentrated, up to 1200 mOsm.
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