Figure 1.

Regulation of water balance by arginine-vasopressin (AVP). Osmoreceptors residing in the anteroventral third ventricle region of the hypothalamus detect decreases in serum osmolality, thereby stimulating the production of AVP. Baroreceptors located in the left atrium, carotid sinus, and aortic arch detect arterial underfilling and stimulate neurons in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) to produce AVP. (The atrial receptors are mediated by the vagus nerve rather than blood pressure.) The neurons of the SON and PVN project into the posterior pituitary gland, where AVP is initially stored and then released into the circulation. V_1a receptors, located in the vascular smooth muscle, sense increased levels of AVP and cause vasoconstriction. AVP also stimulates V₂ receptors, located in the collecting duct of the kidney, causing free water absorption. (Adapted from Sanghi P, Uretsky BF, Schwarz ER. Vasopressin antagonism: a future treatment option in heart failure. Eur Heart J. 2005; 26:538-43, by permission of Oxford University Press.)¹²