What is the primary action of antidiuretic hormone (ADH)?

The primary action of antidiuretic hormone (ADH), also known as vasopressin, is to decrease water loss in urine. ADH is produced in the hypothalamus and released from the posterior pituitary gland in response to factors such as increased plasma osmolality or decreased blood volume. When released into the bloodstream, ADH acts primarily on the kidneys, specifically the collecting ducts, to increase their permeability to water. This allows more water to be reabsorbed back into the bloodstream, thereby reducing the amount of water excreted in the urine.

This mechanism is crucial for maintaining fluid balance in the body, especially under conditions of dehydration or increased salt intake. If ADH levels are insufficient or the kidneys are unable to respond to it properly, it can lead to a condition known as diabetes insipidus, characterized by excessive urination and thirst due to the inability to concentrate urine.

Understanding the role of ADH helps to clarify the importance of hormonal regulation in fluid balance and the body's response to varying levels of hydration.