ADH or antidiuretic hormone is produced in the hypothalamus and secreted from the neurohypophysis (posterior pituitary gland). ADH increases the reabsorption of water from the urine in the renal system back into the blood. This increases intravascular fluid volume and decreases the amount of urine voided out of the body. Thus ADH conserves water in the kidneys and returns that fluid back to the general circulation. This conservation of urine acts to increase blood pressure because total intravascular fluid volume is increased.
In a condition called diabetes insipidus, the hormone ADH is secreted in smaller amounts (hyposecreted), thus causing an increase in urine volume voided out of the body. People with DI may void thousands of ml/cc in urine volume. This of course causes other serious problems like hypotension.