Describe the counter-current mechanism.

(N/A) The counter-current mechanism is a specialized process in the kidneys that helps in concentrating the urine. It primarily involves the $Loop$ $of$ $Henle$ and the $Vasa$ $recta$.
$1$. The flow of filtrate in the two limbs of the $Loop$ $of$ $Henle$ is in opposite directions,creating a counter-current.
$2$. The flow of blood through the two limbs of the $Vasa$ $recta$ is also in a counter-current pattern.
$3$. The proximity between the $Loop$ $of$ $Henle$ and $Vasa$ $recta$,as well as the counter-current in them,helps in maintaining an increasing osmolarity towards the inner medullary interstitium (from $300 \ mOsmol/L$ in the cortex to about $1200 \ mOsmol/L$ in the inner medulla).
$4$. This gradient is mainly caused by $NaCl$ and $Urea$.
$5$. $NaCl$ is transported by the ascending limb of the $Loop$ $of$ $Henle$ which is exchanged with the descending limb of the $Vasa$ $recta$.
$6$. $NaCl$ is returned to the interstitium by the ascending portion of the $Vasa$ $recta$.
$7$. Similarly,small amounts of $Urea$ enter the thin segment of the ascending limb of the $Loop$ $of$ $Henle$ and are transported back to the interstitium by the collecting tubule.
$8$. This mechanism helps in water reabsorption from the collecting duct,thereby concentrating the urine.