Which chemical messenger is released in response to both low blood volume and high plasma osmolarity?

The chemical messenger released in response to both low blood volume and high plasma osmolarity is antidiuretic hormone (ADH), also known as vasopressin. ADH is produced in the hypothalamus and stored in the posterior pituitary gland.

When blood volume decreases (as in dehydration or significant blood loss), baroreceptors detect this change and stimulate the release of ADH. The hormone acts primarily on the kidneys, promoting water reabsorption in the collecting ducts, which helps to concentrate urine and conserve water, thereby increasing blood volume.

Conversely, when plasma osmolarity is high (indicating dehydration or increased solute concentration in the blood), osmotic receptors in the hypothalamus detect this condition and trigger ADH release. Once in circulation, ADH facilitates the reabsorption of water from the urine back into the bloodstream, diluting the plasma and normalizing osmolarity.

Thus, ADH serves as a crucial regulator of both fluid balance and blood osmolarity, responding effectively to conditions of low volume and high osmotic pressure.