Osmoreceptors in the hypothalamus sense changes in the osmotic pressure of blood. When the osmotic pressure of blood changes, becoming either more dilute or more concentrated, water diffusion into and out of osmoreceptor cells occurs. The cells expand when the blood plasma is more dilute and contract with higher concentration. This causes a neural signal to be sent from the hypothalamus, which then increases or decreases vasopressin secretion from the posterior pituitary to return blood concentration (serum osmolality) to normal.
