Question

give a brief account of the countercurrent mechanism

Answer 1

Mechanism

Step 1:  Assume that the loop of Henle is filled with a concentration of 300mOsm/L the same as that leaving the proximal tubules.

Step 2: The active ion pump of the thick ascending limb on the loop of Henle reduces the concentration inside the tubule and raises the interstitial concentration.

Step 3: The tubular fluid in the descending limb and the interstitial fluid quickly reach osmotic equilibrium because of osmosis of water out of the descending limb.

Step 4: The Additional flow of the fluid into the loop of Henle from the proximal tubule, which causes the hyperosmotic fluid previously formed in the descending limb to flow into the ascending limb.

Step 5: Additional ions pumped into the interstitium with water remaining in the tubular fluid, until a 200-mOsm/L osmotic gradient is established.

Step 6: Again, the fluid in the descending limb reaches equilibrium with the hyperosmotic medullary interstitial fluid and as the hyperosmotic tubular fluid from the descending limb flows into the ascending limb, still more solute is continuously pumped out of the tubules and deposited into the medullary interstitium.

Step 7: These steps are repeated over and over, with net effect of adding more and more solute to the medulla in excess of water, with sufficient time, this process gradually traps solutes in the medulla and multiplies the concentration gradient established by the active pumping of ions out of the thick ascending limb , eventually raising the interstitial fluid osmolarity to 1200- 1400 mOsm/L .