Osmolality

Osmolality, in biology and chemistry, is a measure of moless of solute per kg of water. 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.