Why do mammals reabsorb most of the water filtered by their kidneys?

Mammals reabsorb the vast majority of water filtered by their kidneys to maintain essential physiological equilibrium, including proper osmotic pressure, acid-base balance, and overall water content.1

Every 24 hours, the human kidneys filter approximately 170 litres of water from the bloodstream into the renal tubules. If this volume were not reabsorbed, the body would rapidly lose critical fluids, leading to severe dehydration and death. By reabsorbing about 99 percent of this filtrate, the kidneys ensure that valuable substances—such as glucose and essential salts—are returned to the blood while nitrogenous wastes are concentrated for excretion.12

This process is highly regulated:
* Efficiency: The loop of Henle allows mammals to produce urine that is far more concentrated than blood, which is a vital adaptation for conserving water, especially in arid environments.34
* Hormonal Control: The hormone antidiuretic hormone (ADH) adjusts the permeability of the collecting ducts, allowing the body to increase or decrease water reabsorption based on hydration levels and blood volume.4