Which hormone's primary function is to regulate water balance in the body?

Antidiuretic Hormone (ADH), also known as vasopressin, is primarily responsible for regulating water balance in the body. It is produced in the hypothalamus and released from the posterior pituitary gland. When the body is dehydrated or when blood osmolarity increases (indicating a higher concentration of solutes in the blood), ADH secretion increases. This hormone acts primarily on the kidneys, promoting water reabsorption in the renal collecting ducts. As a result, less water is excreted in urine, helping to concentrate the urine and conserve water in the body, which is essential for maintaining hydration and proper fluid balance.

While aldosterone also plays a role in fluid balance, its primary function is to regulate sodium and potassium levels in the body, which indirectly influences water balance. Cortisol is involved in various metabolic processes and stress responses but does not primarily regulate water balance. Insulin primarily regulates glucose levels in the bloodstream and is not directly involved in fluid balance management. Therefore, the specific role of ADH in promoting water reabsorption makes it the key hormone in maintaining water balance within the body.