What is the countercurrent mechanism?
The countercurrent mechanism is the process by which renal medullary interstitial fluid becomes hyperosmotic(increasing osmolality gradient).
The countercurrent system has two divisions:
- Countercurrent multiplier formed by a loop of Henle.
- Countercurrent exchanger formed by vasa recta.
- Step 1: At first, suppose that the loop of Henle is filled with fluid with a concentration of 300 mOsm/L, and the same as that leaving the proximal tubule.
- Step 2: Active absorption of NaCl by Thick Ascending Limb of the loop of Henle, establishes a 200-mOsm/L concentration gradient between the tubular fluid (200 mOsm/L) and the interstitial fluid (400 mOsm/L). This 200 mOsm/L is the maximum gradient that can be established by Thick Ascending Limb.
- Step 3: Absorption of water (Osmosis) in the Distal Tubule Limb established an osmotic equilibrium (400 mOsm/L) between Descending Thick Limb and interstitium.
- Step 4: Additional flow of fluid volume into the loop of Henle from the proximal tubule (300 mOsm/L), and which causes the hyperosmotic fluid previously formed in the descending limb (400 mOsm/L) to flow into the ascending limb of the loop of Henle.
- Step 5: Additional ions that are pumped into the interstitium until a 200 mOsm/L osmotic gradient is established or confirmed, with the interstitial fluid osmolarity rising to 500 mOsm/L.
- Step 6: Then, once again, the fluid in the descending limb reaches equilibrium with the hyperosmotic medullary interstitial fluid due to water absorption in Descending Thick Limb of the loop of Henle.
- Step 7: These steps are repeated several times, with the net effect of adding more and more solute to the medulla in excess of water; with sufficient time, and this process gradually traps solutes molecules in the medulla and multiplies the concentration gradient established by the active pumping of ions out of the thick ascending loop of Henle, and eventually increasing the interstitial fluid osmolarity to 1200 to 1400 mOsm/L.
- Plasma flowing down the descending limb of the vasa recta and becomes more hyperosmotic because of the diffusion of water molecules out of the blood and the diffusion of solutes from the renal interstitial fluid into the blood.
- In the ascending limb of the vasa recta, solutes molecules diffuse back into the interstitial fluid and water diffuses back into the vasa recta.
- Thus, although there is a large amount of fluid and solute exchange across the vasa recta, there is a small scale net dilution of the concentration of the interstitial fluid volume at each level of the renal medulla because of the U-shape of the vasa recta capillaries, which act as countercurrent exchanger.
- Thus, the vasa recta do not create hyperosmolarity in the medulla, but it does prevent it from being dissipated.