Osmolality
Osmolality, in biology and chemistry, is a measure of moless of solute per kg of water. (Osmolarity is similar but measures moles per litre of solvent.) Osmolality increases with dehydration and decreases with overhydration.In normal people, increased osmolality in the blood will stimulate secretion of ADH (antidiuretic hormone). This will result in increased water reabsorption, more concentrated urine, and less concentrated plasma. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.
Normal osmolality in blood plasma is about 385 mosm/L, this is contributed to mostly by sodium, chloride, potassium, urea and glucose, as well as other ions and substances in the blood.
An increase in any one of these osmotically active compounds will cause an increase in osmolality of the blood. For instance, a diabetic whose glucose is 25 mmol/L instead of the normal 5 mmol/L, will have an osmolality 20 mosm/L higher than normal.