Plasma osmolality, a water absorption force of solute molecules through a semi-permeable membrane, depends on the number of solute particles and is independent of properties such as the molecular weight radius of the solute. Since the number of crystalline solutes in plasma is much larger than the number of colloids, plasma osmolality is mainly composed of crystal osmolality. Plasma colloid osmotic pressure consists mainly of protein molecules, of which plasma albumin has a smaller molecular weight and a larger number, which determines the amount of plasma colloid osmotic pressure, so plasma osmotic pressure consists of crystal osmotic pressure and colloid osmotic pressure. The water balance inside and outside the blood vessels and inside and outside the cells is regulated by the colloidal osmolality and crystalloid osmolality.