The cardiovascular system contains osmoreceptors and baroreceptors to detect osmolarity changes and pressure changes in the blood. When there’s an increase in osmolarity or decrease in pressure, one of the body’s methods of returning to homeostasis is to release ADH from the pituitary. It works on the kidneys to increase fluid absorption aka you retain more water and either raise your blood pressure or decrease your blood’s osmolarity.
Fun fact about ADH: Alcohol inhibits the release of ADH which is one of the reasons you lose so much water while drinking. So stay hydrated ;)