Which of the following help in maintaining an increasing osmolarity towards the inner medullary interstitium i.e., from 300 mOsmolL^-1  in the cortex to about 1200 mOsmolL^-1 in the inner medulla?

(a) The proximity between loop of Henle and vasa recta.

(b) Counter current in Henle’s loop and vasa recta.

(c) NaCl and urea in the Bowman’s capsule.

NaCl and urea concentration helps in maintaining an increasing osmolarity towards the inner medullary interstitium.

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 mOsmolL^–1 in the cortex to about 1200 mOsmolL^–1 in the inner
medulla. This gradient is mainly caused by NaCl and urea. Na⁺, Cl⁻, and urea diffuse from the blood into interstitial fluid, and water diff uses from interstitial fluid back into the vasa recta. The long nephron loop establishes the osmotic gradient in the renal medulla by countercurrent multiplication, but the vasa recta maintains the osmotic gradient in the renal medulla by countercurrent exchange.