Differentiating Electrolyte and Fluid Disorders

Vasopressin or anti-diuretic hormone (ADH) is a nonapeptide that is synthesized in the hypothalamus and plays an important role in the control of the body’s osmotic balance, blood pressure regulation and kidney function. ADH induces expression of water transport proteins in the late distal tubule and collecting duct to increase water re-absorption. ADH is produced by neurons within the supraoptic nuclei of the hypothalamus, stored in secretary vesicles and transported through the hypothalamic-hypophyseal tract and released in the posterior pituitary. The secreted hormones then enter nearby fenestrated capillaries where they enter the body’s systemic circulation. These neurons express osmo receptors that are responsive to blood osmolarity and respond to changes as little as 2 mOsm/L.

Therefore slight elevations in osmolarity result in secretion of ADH. ADH acts in the kidneys to increase water re-absorption, thus returning the osmolarity to baseline. ADH is also secreted in times of hypovolaemia. Decreased arterial blood volume is sensed by bar receptors in the left atrium, carotid artery and aortic arch and the signal transported to the vagus nerve which directly stimulates the release of ADH. ADH then promotes water re-absorption in the kidneys to increase effective arterial blood volume and increase blood pressure to maintain tissue perfusion. Osmolarity and volume status are the two greatest factors that affect ADH secretion. There are three pathologic states related to ADH.