Final answer:
Dehydration leads to increased osmolarity, which triggers the release of ADH (antidiuretic hormone) from the posterior pituitary gland. ADH conserves water by reducing urine volume and helps to restore normal blood osmolarity.
Step-by-step explanation:
Dehydration triggers an increase in osmolarity and secretion of ADH/vasopressin. This happens because when the body is dehydrated, the blood volume decreases, and the osmolarity (the concentration of solutes in the blood) increases. The hypothalamus detects this increase in osmolarity and responds by signaling the posterior pituitary gland to release antidiuretic hormone (ADH). ADH then acts on the kidneys to retain water, reducing urine volume and helping to increase blood volume and lower osmolarity back to normal levels. This negative feedback mechanism ensures homeostasis within the body fluids.