Explain the mechanism of concentration of the filtrate.

(N/A) The mechanism of concentration of the urine takes place in the kidney.
- Mammals and birds have the ability to produce a concentrated urine; for this,they have developed a counter-current mechanism.
- Henle's loop and vasa recta play a significant role in this process.
- The flow of filtrate in the two limbs of Henle's loop is in opposite directions,thus forming 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,helps in maintaining an increasing osmolarity towards the inner medullary interstitium,i.e.,from $300 \ mOsmol \ L^{-1}$ in the cortex to about $1200 \ mOsmol \ L^{-1}$ in the inner medulla.
- This gradient is mainly caused by $NaCl$ and urea.
- $NaCl$ is transported by the ascending limb of Henle's loop,which is exchanged with the descending limb of vasa recta.
- Small amounts of urea enter the thin segment of the ascending limb of Henle's loop,which is transported back to the interstitium by the collecting tubule.
- Such transport of substances facilitated by the special arrangement of Henle's loop and vasa recta is called the counter-current mechanism.
- This mechanism helps to maintain a concentration gradient in the medullary interstitium.
- The presence of such an interstitial gradient helps in the easy passage of water from the collecting tubule,thereby concentrating the filtrate (urine).
- Human kidneys can produce urine which is nearly four times more concentrated than the initial filtrate formed.