Which of the following hormones is primarily responsible for regulating fluid balance in the body?

Anti-diuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating fluid balance in the body. This hormone is produced in the hypothalamus and stored in the posterior pituitary gland. ADH acts primarily on the kidneys, promoting water reabsorption in the collecting ducts. When the body is in a state of dehydration or when blood osmolarity increases, ADH is released, allowing the kidneys to conserve water and produce more concentrated urine. This helps to maintain blood volume and pressure, thus supporting overall fluid balance in the body.

While aldosterone also contributes to fluid balance by promoting sodium retention, its primary mechanism involves regulating sodium levels rather than directly influencing water retention. Cortisol, a glucocorticoid, primarily regulates metabolism and has effects on stress response rather than directly managing fluid balance. Dopamine, although involved in various physiological processes, does not play a direct role in the regulation of fluid balance in the body like ADH does.