Vasopressin

Vasopressin (ADH, AVP) is the hormone which controls the production of urine in the bladder. It is secreted by the pituitary gland based on the salt concentration of the blood.

It is a negative feedback loop, which means the more concentrated the salt in the blood is, the more vasopressin is secreted. This, in turn, makes the kidney tubules more permeable, allowing more water to be reabsorbed into the blood and creating more concentrated urine, which can be less concentrated. It also triggers feelings of thirst.