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(Counter current mechanism) Mammals have the ability to produce a concentrated urine. The Henle's loop and vasa recta play a significant role in this. The flow of filtrate in the two limbs of Henle's loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of vasa recta is also in a counter current pattern. The proximity between the Henle's loop and vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from $300{\mathrm{mOsmoll}}^{-1}$ in the cortex to about $1200{\mathrm{mOsmoll}}^{-1}$ in the inner medulla. This gradient is mainly caused by $\mathrm{NaCl}$ and urea. Loop of Henle maintains the interstitial gradient of $\mathrm{NaCl}$. $\mathrm{NaCl}$ is transported by the ascending limb of Henle's loop which is exchanged with the descending limb of vasa recta. $\mathrm{NaCl}$ is returned to the interstitium by ascending part of vasa recta. The second solute urea is added to the interstitial medullary fluid in small amount by diffusing out of the collecting duct. Urea remaining in the collecting duct is eventually excreted out. Urea re enters in the ascending thin segment of the loop of Henle by diffusion. 171

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Updated on: Nov 8, 2022