Explain the role of anti-diuretic hormone in osmotic homeostasis.

Anti-diuretic hormone (ADH) is produced by the hypothalamus and stored in the posterior pituitary gland. When the hypothalamus identifies the bloods osmotic pressure as being outside the homeostatic range, this ADH is released from the posterior pituitary gland into the bloodstream. When ADH reaches the kidneys it induces the insertion of aquaporins into the membranes of the collecting tubules/ducts. The increased density of aquaporins in these regions increases their water permeability and increases water re-absorption into the bloodstream, hence reducing the osmotic pressure back to within the homeostatic range.