What is the effect of vasopressin on urine concentration?

Vasopressin, also known as antidiuretic hormone (ADH), plays a crucial role in the regulation of water balance in the body. Its primary function is to promote water reabsorption in the kidneys, specifically in the collecting ducts. When vasopressin is released into the bloodstream, it binds to receptor sites in the renal collecting ducts, which triggers the insertion of aquaporin water channels into the cell membranes. This process enhances the permeability of the collecting ducts to water, allowing more water to be reabsorbed back into the bloodstream rather than being excreted in the urine.

As a consequence of this increased reabsorption, the volume of urine produced decreases, while the concentration of solutes in the remaining urine increases. Therefore, vasopressin effectively increases urine concentration, which is why the answer is correct. This physiological mechanism is particularly important in conditions of dehydration or high plasma osmolality, where the body needs to conserve water and produce concentrated urine to maintain hydration levels and electrolyte balance.