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1.18_Countercurrent Exchange, Function of the Vasa Recta, & Micturition

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How is the kidney able to produce urine that's more or less conc. than body fluids?
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Terms in this set (10)
1. Active transport of ions out of the thick limb of the loop of Henle increases the osmolarity of the medullary interstitium
2. Tubular fluid in the descending limb reaches osmotic equilibrium w interstitial fluid by outward osmotic flow
3. Additional flow of 300 mOsm/L fluid into the descending limb from the PCT causes hyperomotic fluid previously formed in the descending limb to flow into the ascending limb, where active transport pushes additional ions into the interstitial fluid, increasing the osmolarity of the interstitium.
4. Water once again moves out of the descending limb until equilibrium is reached.

This process is repeated in a positive feedback cycle, multiplying the conc. gradient in the medullary interstitium. Urea reabsorbed from the CD contributes ~40-50% of medullary hyperosmolarity.
Image: Countercurrent multiplier
As the dilute fluid in the early distal tubule passes into the late DCT & CD, there's additional reabsorption of NaCl. In the absence of ADH, this portion of the tubule is impermeable to water. Tubular fluid becomes even more dilute, decreasing its osmolarity to as low as 50-100mOsm/L.

Results in a large volume of dilute urine.