Why is the osmolarity of the medullary fluid in the kidney greater than the osmolarity of the blood plasma?

This is because the loop of Henle that is found in the kidney functions as a countercurrent multiplier and contributes solutes to the interstitial fluid. Urea is recycled from the collecting duct and is transported to the interstitial fluid.

Explanation:

Osmolarity can be defined as the determination of the quantity of the solute that present in a given quantity of water( 1 liter ). It is used to measure the concentration of solute.

Osmolarity of the medullary fluid in the kidney greater than the osmolarity of the blood plasma and this is because the loop of Henle that is found in the kidney uses energy to generate an osmotic gradient that enables it to reabsorb water from the tubular fluid and produce concentrated urine a process that is referred to as countercurrent multiplication and contributes solutes to the interstitial fluid. Urea is recycled from the collecting duct and is transported to the interstitial fluid.

Hence, osmolarity of the medullary fluid in the kidney is set at a high limit of as 1500 mOsm/L while that of plasma osmolarity is 280 to 295 mOsm/L and it must not exceed the upper limit of 310 mOsm/L.