Antidiuretic Hormone (ADH)
It is also called vasopressin. ADH aids in retention of water. It is synthesised in the neurosecretory cells of the hypothalamus and stored in the neurohypophysis. ADH increases the permeability of the collecting ducts, possibly by the enlargement of cell pores. This results in the movement of water out of the tubules and increase in the concentration of salts in the surrounding fluids and blood capillaries. The effect of ADH on the urine passing out of the kidney is to make it more hypertonic.
The urine enters the collecting duct as hypertonic to blood and leaves it as hypotonic. The role of ADH in the retention of water is supported by adrenocortical hormones. Recently it has been discovered that another set of hormones called prostaglandins, produced in the kidney function antagonistic to ADH.