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

The primary function of antidiuretic hormone (ADH), also known as vasopressin, is to retain water within the body. This hormone is produced in the hypothalamus and released from the posterior pituitary gland in response to high plasma osmolality or low blood volume. When ADH is released into the bloodstream, it acts on the kidneys, specifically on the collecting ducts, to increase their permeability to water. This allows more water to be reabsorbed back into the bloodstream rather than being excreted in urine, leading to concentrated urine and increased blood volume. As a result of this action, ADH plays a critical role in maintaining fluid balance and regulating blood pressure as part of the body's homeostatic mechanisms.

The other options describe functions that are not primarily attributed to ADH. For instance, while ADH can influence blood pressure indirectly by increasing blood volume, its main role is not to decrease blood pressure directly. Similarly, ADH does not increase heart rate or stimulate urine production; instead, it promotes water retention, which ultimately helps to decrease urine output.