Question

Discuss how the counter current multiplier functions

Answer 1

As the name suggests, countercurrent mechanism or multiplication of entities is a phenomenon in which distribution of molecules occurs due to flow of medium in two opposite directions. This is particularly called multiplication because in a pair of tubes/tubules which maintain countercurrent, a continuous formation of osmotic/pressure gradient in turn promotes formation of countercurrent gradient within the system.In biological context, countercurrent multiplication is very efficiently realised in urine concentration or process of urine formation in kidneys. In order to understand this process, some points should be taken care of:

1. The head part of loop of Henle contains parallely arranged tubules (renal tubules) in which solvent/liquid (containing urine and other materials) in mutually opposite direction.

2. The descending limbic part of loop of Henle is freely permeable of water and impermeable to solutes (ions.)

3. The ascending limbic part of loop of Henle is freely permeable to solutes but impermeable to water.

Detailed process: As the whole solution (containing water and solvents) flows through lower or descending loop of Henle, the water is removed out of it making the urine inside the tubule very concentrated OR making the urine hypertonic as compared to surrounding solution. Now as the hypertonic urine ascends through the loop of Henle, it becomes impermeable to water and solutes are removed out of it. This makes the urine hypotonic as compared to surrounding medium. Now what comes out of this process is a gradient of ions and solvent moving through the membrane and tubule. This generates an osmotic concentration gradient within the tubule and in turn again promotes the formation of countercurrent gradient formation in the tubules. Thus, in total the initial countercurrent gradient a multiplicaiton of osmotic gradient is formed within the tubules maintaining countercurrent.