How does vasopressin help the kidneys conserve water?

Vasopressin, also known as antidiuretic hormone (ADH), helps the kidneys conserve water by regulating the permeability of the collecting ducts.12

When the body requires water retention—triggered by high serum osmolality or low blood volume—vasopressin is released from the posterior pituitary gland. It travels to the kidneys and binds to receptors on the cells of the collecting ducts. This binding stimulates vesicles containing aquaporins—specialized water channels—to fuse with the cell membrane exposed to the urine.13

By inserting these channels into the membrane, vasopressin makes the collecting ducts freely permeable to water. This allows water to move out of the ducts and be reabsorbed back into the circulation, effectively concentrating the urine and reducing its total volume. Conversely, in the absence of vasopressin, these channels are not present, rendering the ducts impermeable to water and resulting in the excretion of dilute urine.12