Osmolality is a measure of the number of osmotically active solute particles dissolved in a kilogram of solvent (water in biological systems). These osmotically active substances increase osmolality of a fluid and cause solvent (water) to move across membranes. Osmolarity is the number of particles in a litre of fluid. For clinical purposes, osmolality and osmolarity values are approximately the same and used interchangeably….Osmolality is a measure of the number of osmotically active solute particles dissolved in a kilogram of solvent (water in biological systems). These osmotically active substances increase osmolality of a fluid and cause solvent (water) to move across membranes. Osmolarity is the number of particles in a litre of fluid. For clinical purposes, osmolality and osmolarity values are approximately the same and used interchangeably. Unlike osmolarity, osmolality is unchanged by temperature. Serum osmolality is an important stimulus for maintaining water balance by controlling the amount of water excreted in the urine and by regulating water intake through the sensation of thirst. In health, the osmolality of blood is very tightly regulated. Osmotic sensors in the body detect changes in the amount of particles in water in the bloodstream (osmolality). When blood osmolality increases, for example in dehydration, the hypothalamus promotes secretion of the hormone antidiuretic hormone (ADH) from the posterior pituitary. ADH signals for the kidneys to reabsorb and conserve water, resulting in formation of concentrated urine (which has a high osmolality). This retention of water dilutes the blood causing a decrease in osmolality back to normal levels. Increased osmolality also causes a sensation of thirst to promote increased fluid (water) intake which helps to return osmolality to normal levels. If blood osmolality decreases, for example following a large drink of water, then ADH secretion is suppressed and the kidneys excrete increased amounts of dilute urine (with low osmolality). Combined with decreased thirst and therefore decreased fluid (water) intake, this results in a decrease in the amount of water in the body, and so blood osmolality rises to normal. Urine osmolality is a measure of the kidney’s ability to concentrate urine; the more concentrated the urine is, the higher its osmolality. Urine osmolality is largely due to the presence of urea and creatinine. If serum and urine osmolality are not in keeping with each other this may indicate a problem with water balance which may manifest in abnormal sodium results. Osmolality can be measured directly in the laboratory using an osmometer, often by freezing point depression. See MoreSee Less
