ADH and Diabetes Insipidus

Antidiuretic hormone (ADH), also called vasopressin, is the kidney's main water-conservation signal. Osmoreceptors in the hypothalamus continuously monitor blood volume and ionic concentration; when they detect excessive fluid loss or a rise in blood pressure, they stimulate the neurohypophysis (posterior pituitary) to release ADH - a classic example of positive feedback in this context. ADH acts on the distal convoluted tubule and collecting duct by increasing the number of aquaporin water channels inserted into their cell membranes, which lets more water move out of the tubule into the surrounding interstitial fluid and back into the blood, so less is lost as urine. Conversely, drinking a large volume of water dilutes the blood, switches off the osmoreceptors, suppresses ADH release (negative feedback), and causes the aquaporins to be withdrawn back into the cytoplasm - making the collecting duct impermeable to water again, so the excess fluid is excreted as a large volume of dilute urine. When this control system fails - either because ADH cannot be secreted or because its receptors do not respond to it - the result i …