Why can dehydration cause the kidneys to produce less urine?

When the body becomes dehydrated, the kidneys produce less urine as part of a vital feedback mechanism designed to maintain water balance and blood volume.12

This process is primarily regulated by the hypothalamus, which contains specialized osmoreceptors sensitive to changes in the concentration of dissolved particles in the blood (serum osmolality). When dehydration occurs, serum osmolality increases, triggering the release of vasopressin—also known as antidiuretic hormone (ADH)—from the posterior pituitary gland.13

Vasopressin travels to the kidneys and acts on the collecting ducts, where it stimulates the insertion of water channels called aquaporins into the cell membranes. This makes the ducts permeable to water, allowing it to be reabsorbed back into the bloodstream rather than being excreted. Consequently, the volume of urine decreases, and the urine becomes more concentrated. Additionally, if dehydration causes a significant drop in blood volume, baroreceptors detect the change and further stimulate vasopressin release to help restore blood pressure.13