What is the function of antidiuretic hormone (ADH)?

Antidiuretic hormone (ADH), also known as vasopressin, primarily functions to regulate water balance in the body. It promotes water reabsorption in the kidneys by increasing the permeability of the renal collecting ducts, which leads to the reabsorption of water back into the bloodstream. This action helps to concentrate urine and maintain proper hydration levels within the body. When the body is dehydrated or when blood volume is low, the secretion of ADH increases, which in turn conserves water and reduces urine output.

In contrast, the other answer choices do not accurately represent the primary role of ADH. Regulating heart rate is primarily the function of the autonomic nervous system and hormones like adrenaline, while stimulating insulin secretion is associated with pancreatic function in response to glucose levels. Increasing sodium excretion typically involves different hormones like atrial natriuretic peptide (ANP) and is part of sodium regulation, not directly affected by ADH. Thus, the correct answer highlights the critical role of ADH in maintaining fluid balance in the body.