Question

Explain in depth the countercurrent multiplication and countercurrent exchange mechanisms. Describe fully their roles in urine production. Please make reference to osmolality changes that occur throughout the nephron and also make note of the loop of Henle.

Answer 1

The countercurrent multiplication and countercurrent exchange are the mechanisms by which the urine is concentrated in the nephrons.

Te countercurrent mechanisms involves the loop of henle and the vasa recta that forms a network of blood vessels around the loop of Henle.

In the countercurrent multiplier mechanism,a medullary interstitial gradient is created for the reabsorption of the water in the thin descending loop of the henle.The thin descending loop is permeable to water but impermeable for the passive movements of the ions.To maintain the concentration gradient,there occurs active transport of the sodium chloride into the medullary interstitium.This concentration gradient allows the passive flow of water into the medulla thereby concentrating the urine in the loop.The thick ascending limbs of the loop of henle are permeable to the solutes but impermeable to the water.This dilutes the urine.

In countercurrent exchange mechanism the concentration gradient that was created by the thin descending loop of Henle is maintained by the vasa recta.The vasa recta absorbs the water that is reabsorbed back into the blood stream meanwhile it secretes the solutes into the medullary interstitium to maintain the gradient.In the region of the thin ascending limb of the loop of henle,the descending loop of the vasa recta absorbs the solutes that are reabsorbed by the vasa recta.

As we move deep to the medulla,the osmolarity of the urine increases from 300 to 1200 mOsmol/L.