Biomedical importance of water

Water is the predominant chemical component of living organisms. Its unique physical properties, including the ability to solvate a wide range of organic and inorganic molecules, derive from its bipolar structure and its exceptional ability to form hydrogen bonds. The way in which water interacts with a solvated biomolecule influences the structure of both, both the biomolecule and the water. An excellent nucleophile, water is either a reactant or a product in many metabolic reactions. Regulation of water balance depends on hypothalamic mechanisms that control thirst, antidiuretic hormone (ADH), retention or excretion of water by the kidneys, and evaporative loss. Nephrogenic diabetes insipidus, which involves the inability to concentrate urine or to adjust to subtle changes in extracellular fluid osmolarity, results from the unresponsiveness of the osmoreceptors in the renal tubules to ADH. Water has a slight propensity to dissociate into hydroxide ions and protons. The proton concentration, or acidity, of aqueous solutions is usually reported using the logarithmic pH scale. Bicarbonate and other buffers under normal circumstances maintain the pH of the extracellular fluid between 7.35 and 7.45. Suspected disturbances in acid-base balance are verified by measuring the pH of arterial blood and the CO2 content of venous blood. Causes of acidosis (blood pH <7.35) are diabetic ketosis and lactic acidosis. Alkalosis (pH >7.45) may occur after vomiting of acidic gastric contents.