Question

Explain the mechanism of formation of concentrated urine in mammals.

Answer 1

Countercurrent mechanism:

• Mammals produce concentrated urine which conserves water in terrestrial habitats.
• Henle's loop and vasa recta play an important role in the concentration of urine. When the filtrate flows in the two limbs o Henle's loop, the flow is in opposite directions, i.e.counter-current. Similarly, blood flows through the two limbs of the vasa recta in a countercurrent pattern.
• The Henle’s loop and vasa recta are nearer to each other. This proximity as well as the countercurrent direction of flow help in maintaining an increased osmolarity towards the inner medullary interstitium. The osmolarity of the cortex is \(300\)mOsmol/\(L\) while it is\(1200 mOsmol/L\) in the inner medulla.

NaCl and ureagradient:

• NaCl and urea are mainly responsible for the formation of the gradient. NaCl present in the ascending limb of Henle’s loop is exchanged with the descending limb of the vasa recta. Then the NaClis returned to the interstitium by the ascending portion of the vasa recta.
• Similarly, urea entering the thin segment of the ascending limb of Henle's loop is transported back to the interstitium by the collecting tubule. This countercurrent mechanism helps to maintain a concentration gradient in the medullary interstitium.
• This interstitial gradient is responsible for an easy passage of water from the collecting tubule thereby concentrating the filtrate (urine)
• The urine formed is up to four times more concentrated than the initial filtrate.