Pregnancy and lactation are accompanied by an increase in circulating blood volume secondary to a 10 mOsmol/kgH20 decrease in plasma osmolality, decrease in the osmotic threshold for thirst and arginine vasopressin (AVP) release, prolactin-induced AVP, oxytocin and aldosterone release, as well as increased water intake and retention. The increased blood volume as a result of increased thirst; drinking and fluid retention could be beneficial for milk production and secretion during lactation. Furthermore, AVP can directly initiate milk ejection similar to oxytocin by interacting with both vasopressin and oxytocin receptors located in myoepithelial cells of the mammary gland. This review explores how osmotic equilibrium is maintained during lactation through changes in thirst, AVP release and plasma osmolality; and highlights the potential role of AVP in milk secretion
