Mechanism of Concentration of the Filtrate
வடிநீர் செறிவாக்கும் இயக்கமுறை
Concentrated urine is possible only because the kidney keeps its inner medulla salty — up to about 1200 mOsmolL-1. The counter-current mechanism exists to build and protect that gradient.
The idea
Mammals can produce concentrated urine. Henle's loop and the vasa recta play a significant role. The flow of filtrate in the two limbs of Henle's loop is in opposite directions, forming a counter current; the flow of blood through the two limbs of the vasa recta is also counter-current. The proximity between the loop of Henle and the vasa recta, together with the counter current in them, helps maintain an increasing osmolarity towards the inner medullary interstitium, 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. NaCl is transported by the ascending limb of Henle's loop and exchanged with the descending limb of the vasa recta; NaCl is returned to the interstitium by the ascending portion of the vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle's loop and are transported back to the interstitium by the collecting tubule. This special interstitial gradient helps easy passage of water from the collecting tubule, thereby concentrating the filtrate (urine). Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.
Why it matters
Land animals must save water; a salty medulla pulls water out of the collecting duct by osmosis, without spending energy on pumping water itself.
NEET lens
Key facts: counter current in Henle's loop and vasa recta; gradient 300 → 1200 mOsmolL-1 (cortex → inner medulla); caused mainly by NaCl and urea; urine can be about four times as concentrated as the initial filtrate. 'Maintaining hyperosmolarity towards the inner medullary interstitium' is the factor responsible for concentrated urine.
Clinical significance
Desert animals such as the kangaroo rat have very long loops of Henle and can make extremely concentrated urine. In humans, a damaged medullary gradient (or lack of ADH) leads to large volumes of dilute urine.
Introduction
The counter-current mechanism lets mammals make urine more concentrated than blood, saving water.
Core concept
Counter current: opposite flow in the two limbs of Henle's loop and in the two limbs of vasa recta. Gradient: 300 mOsmolL-1 (cortex) → ~1200 mOsmolL-1 (inner medulla), caused mainly by NaCl and urea. NaCl: transported out by ascending limb of Henle's loop → exchanged with descending limb of vasa recta → returned to interstitium by ascending vasa recta. Urea: small amounts enter thin ascending limb, carried back to interstitium by collecting tubule. Result: water leaves the collecting tubule easily → concentrated urine; human urine up to ~4 times as concentrated as initial filtrate.
Go deeper
Vasa recta are U-shaped, so the blood flowing through them does not wash away the medullary salt — it is a counter-current exchanger, while the loop of Henle is the counter-current multiplier.
From Class 10
When you drink little water, your urine becomes dark and concentrated. This is how the kidney does it.