Try Magic Notes and save time.Try it free
Try Magic Notes and save timeCrush your year with the magic of personalized studying.Try it free

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

Get a hint
How is the kidney able to produce urine that's more or less conc. than body fluids?
Click the card to flip 👆
1 / 10
1 / 10
Terms in this set (10)
How is the kidney able to produce urine that's more or less conc. than body fluids?
Urine osmolarity & flow depend on the body's hydration state & vertical osmotic gradient in the renal medulla, which has an osmotic range from 300 mOsm/L (cortex) to 1200 mOsm/L (medulla/renal pelvis junction)
Image: How is the kidney able to produce urine that's more or less conc. than body fluids?
Where & how is the vertical osmotic gradient established?
A countercurrent multiplication system involving juxtamedullary nephrons & the surrounding medullary interstitium establish the vertical osmotic gradient which is then maintained by the countercurrent exchange system involving the vasa recta
Image: Where & how is the vertical osmotic gradient established?
Countercurrent multiplier
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
Urine formation in water excess
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.
Urine formation in water deficit
High conc. of ADH makes CD highly permeable to water.

Results in a small volume of highly concentrated urine.