What is the primary function of ADH in the body?

The primary function of antidiuretic hormone (ADH), also known as vasopressin, is to increase the re-absorption of water in the kidneys. When the body is dehydrated or when blood osmolality is high, ADH is released from the posterior pituitary gland. This causes the kidneys to hold onto water and concentrate the urine, thus conserving water in the body and helping to maintain proper hydration and blood pressure levels.

This function is crucial for regulating the body’s fluid balance, particularly during states of dehydration. By promoting water retention, ADH helps to prevent excessive loss of water through urine, ensuring that the body's hydration status is stable.

In contrast, the other functions listed do not pertain to the role of ADH. Increasing blood glucose is primarily managed by hormones such as glucagon, while stimulating milk production is primarily under the influence of prolactin. Metabolism regulation involves various hormones, including insulin and thyroid hormones, but is not a function of ADH.