Vorming van geconcentreerde urine

There is a medullary osmotic gradient of solutes in the interstitial fluid from the renal cortex through the medulla.

This gradient is established and maintained by the juxtamedullary nephrons, with loops extending deep into the medulla, using countercurrent mechanisms.

These mechanisms also regulate urine volume and concentration.

The countercurrent multiplication uses the osmolarity difference between the descending and ascending limbs of the nephron loop.

The water-permeable descending limb facilitates water reabsorption from the tubular fluid, while the ascending limb reabsorbs solutes like sodium and chloride, creating the medullary osmotic gradient.

The 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 large amounts of antidiuretic hormone or ADH.

This causes more filtered water to be reabsorbed in the late distal tubule and the collecting ducts.

As a result, urine is produced in small volumes, about four times more concentrated than blood plasma.