23.5: Physiology of the Genitourinary System III: Urine Concentration and Dilution

The kidneys concentrate or dilute urine to maintain water and electrolyte balance. Nephrons, particularly the loop of Henle, play a crucial role in this process through the countercurrent multiplication system. This system establishes a high osmolarity in the renal medulla, which is essential for water reabsorption. In the loop of Henle’s descending limb, water is reabsorbed into the surrounding medulla due to its permeability to water. In contrast, the ascending limb actively transports sodium, potassium, and chloride ions into the medulla but is impermeable to water, helping to maintain the osmotic gradient.

This medullary osmotic gradient sets the stage for hormonal regulation by antidiuretic hormone (ADH). In response to dehydration or increased plasma osmolality, ADH levels rise, promoting the insertion of aquaporin water channels into the collecting ducts. ADH acts on the principal cells of the collecting ducts, enabling water reabsorption and concentrating the urine to conserve water. The vasa recta, a network of capillaries surrounding the loop of Henle, helps maintain this gradient by a countercurrent exchange mechanism, which prevents solutes from being washed away while allowing water and solutes to equilibrate.

When the body is well-hydrated, ADH levels drop, and the collecting ducts remain impermeable to water. Consequently, the kidney produces dilute urine to excrete excess water, maintaining osmotic balance. Urine dilution begins in the loop of Henle, where ions are reabsorbed without water. By the time the filtrate reaches the collecting ducts, it is already dilute, and without ADH, it remains so.

ADH also plays a role in urea recycling within the inner medulla. By increasing the permeability of the inner medullary collecting ducts to urea, ADH promotes urea reabsorption, which enhances the medullary osmolarity. This mechanism further allows the kidney to concentrate urine more effectively during water deprivation.