Final answer:
ADH, also known as vasopressin, is the hormone that stimulates the thirst mechanism by increasing water reabsorption in the kidneys when there is a significant loss of blood volume or increased osmolarity of tissue fluid.
Step-by-step explanation:
The hormone that stimulates the thirst mechanism is antidiuretic hormone (ADH). ADH is secreted by the cells in the hypothalamus and transported to the posterior pituitary, where it is stored until its release is triggered by an increasing osmolarity of tissue fluid, usually in response to significant loss of blood volume. This hormone increases the reabsorption of water by the kidneys, which helps to restore blood volume and pressure, and thereby activates the thirst mechanism as part of the body's attempt to maintain homeostasis. Notably, while aldosterone does contribute to water retention through the retention of sodium, it works in a different capacity than ADH and does not directly trigger thirst. ADH's action reduces urine output and stimulates thirst, leading to increased water intake.