In dehydration, which mechanism is likely engaged to maintain water homeostasis in the kidneys?

• A. ADH activity in proximal tubular cells
• B. K-H exchange in distal convoluted tubule
• C. Urea reabsorption in medullary collecting ducts
• D. Water reabsorption in the ascending limb of the loop of Henle

Answer: (C)

In the context of dehydration, the kidneys work to conserve water and maintain homeostasis through several mechanisms. The engagement of urea reabsorption in the medullary collecting ducts plays a crucial role in this process.

When the body is dehydrated, the secretion of antidiuretic hormone (ADH) increases. ADH enhances the permeability of the collecting ducts to water, allowing more water to be reabsorbed back into the bloodstream, thus concentrating the urine. In addition to this, the reabsorption of urea in the medullary collecting ducts contributes to the osmotic gradient in the medulla. Higher concentrations of urea in the interstitial fluid of the renal medulla help to draw water out of the collecting ducts, further promoting water retention in the body and leading to the production of more concentrated urine. This mechanism is particularly important during states of dehydration, as it helps the body to retain water more effectively.

While other mechanisms play roles in renal function and water balance, they are less relevant in the specific context of dehydration and the immediate response to conserve water. Therefore, urea reabsorption becomes a critical component for maintaining water homeostasis in such situations.