Question

Describe how the kidney is able to regulate independently the volume, the concentration, and the composition of urine.

Answer 1

The kidney is able to regulate the volume, composition and concentration of urine because of countercurrent multiplication process in the kidney.

Countercurrent multiplication in the kidneys is a process in which energy is used to generate an osmotic gradient across renal tubule which causes reabsorption of water from the tubular fluid and produce concentrated urine. Countercurrent multiplication is created and maintained by loop of Henle in the nephron. The descending limb of loop of Henle is permeable to water and impermeable to solutes. The diluted urine coming from proximal tubule passes through descending limb where majority of the water is reabsorbed and then urine passes to the ascending limb of loop of Henle. Ascending limb is premeable to solute and impermeable to water. Here Na+ and Cl- ions are reabsorbed into interstitial fluid which increases the osmolarity of interstitial fluid. The increased osmolarity of interstitial fluid drives water from the tubule. The water reabsorbed at ascending limb is returned to the circulatory system via the peritubular capillaries (vasa recta). Because the blood flow through these capillaries is very slow, any solutes that are reabsorbed into the bloodstream have time to diffuse back into the interstitial fluid, which maintains the solute concentration gradient in the medulla. This passive process is known as countercurrent exchange.