What action is promoted by vasopressin?

Vasopressin, also known as antidiuretic hormone (ADH), primarily promotes the reabsorption of water in the kidneys. It acts on the collecting ducts of the nephron, making them more permeable to water. This allows more water to be reabsorbed back into the bloodstream, reducing urine output and helping to concentrate the urine. This mechanism is crucial for regulating the body's hydration status and maintaining blood pressure.

While sodium reabsorption is a process that occurs in the kidneys, vasopressin primarily does not directly promote sodium reabsorption; instead, it focuses on water reabsorption. Similarly, tubular secretion of sodium and red blood cell production are not actions associated with vasopressin. Red blood cell production is more related to erythropoietin, another hormone produced by the kidneys in response to low oxygen levels.

Thus, the key action promoted by vasopressin is indeed the reabsorption of water, which is essential for maintaining fluid balance within the body.