Counter Current Mechanism – Class 11 | Chapter – 19 | Biology Short Notes Series PDF

Counter Current Mechanism: The counter current mechanism is a mechanism in which the exchange of two fluids can take place from one direction to another with their concentrations. The definition of counter-current mechanism for all mammals and fishes is the same but the mechanism may vary.

Counter Current Mechanism

Let us say there are 2 tubes, through which a solution of the same substance is flowing. There is a free exchange of the solution between the two tubes. There can be two kinds of flow through these tubes.

Concurrent flow

Here, the solutions in the two tubes flow in the same direction. If at one end, one of them starts at 0% concentration and the other starts at 100% concentration. By the time they reach the other end of the tubes, the concentrations in each tube will be roughly 50%, as shown in the figure.

Countercurrent flow

Here the solutions in the two tubes flow in opposite directions. In one tube 0% concentration of the solution starts to flow from one end, and in the other tube, 100% concentration of the solution starts to flow from the opposite end.

Due to the free exchange of the substances between the two tubes, by the time the solutions reach the end of the tube, it will have acquired a concentration equal to the other tube at that end. This will become clear from the figure.

Types of Counter Current Mechanism

  • Countercurrent exchange 

  • Current exchange and 

  • Contra-current exchange 

Steps of Counter Current Mechanism

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

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

Step 3: Due to osmosis of water out of the descending limb, the tubular fluid in the descending limb and the interstitial fluid quickly approach osmotic equilibrium.

Step 4: Additional fluid flow from the proximal tubule into the Henle loop, causing hyperosmotic fluid previously generated in the descending limb to flow into the ascending limb.

Step 5: More ions are pushed into the interstitium while water remains in the tubular fluid, resulting in a 200-mOsm/L osmotic gradient.

Step 6: As the hyperosmotic tubular fluid from the descending limb flows into the ascending limb, additional solute is constantly pushed out of the tubules and deposited into the medullary interstitium.

Step 7: These stages are repeated over and over, with the net result of bringing more and more solute to the medulla in excess of water. Over time, this process traps solutes in the medulla and magnifies the concentration gradient generated by active pumping of ions out of the thick ascending limb. Eventually raising the interstitial fluid osmolarity to 1200- 1400 mOsm/L.

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By Team Learning Mantras