Give a brief account of the counter current mechanism.

(N/A) The counter current mechanism operating inside the kidney is the main adaptation for the conservation of water.
There are two counter current mechanisms inside the kidneys: Henle's loop and vasa recta.
Henle's loop is a $U$-shaped part of the nephron. Blood flows in the two limbs of the tube in opposite directions,which gives rise to counter currents.
The vasa recta is a fine capillary network that runs parallel to Henle's loop and is also $U$-shaped.
Blood flows in opposite directions in the two limbs of the vasa recta. As a result,blood entering the renal medulla in the descending limb comes in close contact with the outgoing blood in the ascending limb.
The osmolarity increases from $300 \, mOsmol \, L^{-1}$ in the cortex to $1200 \, mOsmol \, L^{-1}$ in the inner medulla due to the counter current mechanism.
This helps in maintaining a concentration gradient,which facilitates the easy movement of water from the collecting tubules.
The gradient is primarily a result of the movement of $NaCl$ and urea.