29.18: Formation of Concentrated Urine

There is a gradient of solutes in the interstitial fluid from the renal cortex through the medulla, known as the medullary osmotic gradient. The juxtamedullary nephrons establish and maintain this gradient using countercurrent mechanisms with loops extending deep into the medulla. These nephrons also use countercurrent mechanisms to regulate urine volume and concentration. The interaction between the descending and ascending limbs of the nephron loop creates an osmotic gradient through countercurrent multiplication. This gradient facilitates water reabsorption from the tubular fluid in the descending limb and the active transport of solutes like sodium in the ascending limb.

In contrast, countercurrent exchange occurs in the limbs of the vasa recta capillaries surrounding the nephron loop. These capillaries exchange solutes and water between the blood and the interstitial fluid, maintaining the osmotic gradient.

Dehydration induces the posterior pituitary to release antidiuretic hormone (ADH), resulting in the reabsorption of water in the late distal tubule and collecting ducts. As a result, urine is produced in small volumes, about four times more concentrated than blood plasma.