Question

The kidneys primary function is to remove waste from the blood while maintaining its

balance of water and solutes. Essential to this function is the osmolarity gradient between

the renal cortex and renal medulla. Use the simplest language you can to explain how the

kidney maintains this gradient and then uses it to perform these tasks.

Answer 1

The osmolarity gradient between the renal cortex and the renal medulla of the kidney required for the production of a concentrated urine is known as Corticopapillary Osmotic Gradient and is present in the renal interstitial fluid between the mentioned two areas of the kidney. Sodium and urea are the osmolytes that maintain this gradient. Furthermore, the antidiuretic hormone regulates the size of the gradient (bigger size, more concentrated urine). The gradient is 300 mOsm/L at the cortical terminus and 600-1200 mOsm/L at the papillary terminus.

There are two major processes to the generation of the Corticopapillary Osmotic Gradient, these are -

• Countercurrent Mechanism: This process takes place in the loop of Henle, which are renal tubes/limbs parallel and opposite in direction. The descending tube of the loop of Henle being impermeable to solutes but permeable to water causes the movement of water into the medullary space. The hypertonic (water potential lower) filtrate then travels to the ascending tube. Unlike the descending limb, the ascending limb is permeable to solutes (like Na⁺, Cl⁻, and K⁺) but impermeable to water, making the interstitium hypertonic/ salty and the filtrate hypotonic (having a higher water potential). When such a cycle multiplies in effect it leads to a countercurrent flow inside the descending and ascending tubes and forms the osmotic gradient.
• Urea Recycling: This refers to the amount of urea resorption and the regulation of the concentration of urine by the antidiuretic hormone during the countercurrent mechanism.