On the basis of evidence available from initial studies on body fluids and consensus:. Total body water (60% of body weight in male adolescents) is divided into extracellular fluid (ECF) and intracellular fluid (ICF). Blood volume (5% of body weight) is responsible for ensuring circulatory flow to organs. Sodium is primarily present in the ECF and potassium in the ICF, but serum concentrations of sodium or potassium do not reflect their respective total body content. Two major mechanisms regulate water metabolism: thirst and arginine vasopressin (also known as antidiuretic hormone). Sodium content, regulated by complex mechanisms (juxtaglomerular apparatus, renin-angiotensin-aldosterone system, glomerular filtration rate, and others), is responsible for maintaining the ECF volume. Serum osmolality is calculated using sodium, glucose, and blood urea nitrogen measurements. Hypernatremia or hyperosmolality is an indicator of relative deficiency of water to solute in the ECF. This free water deficit is corrected slowly to prevent seizure and cerebral edema. Hyponatremia or hypo-osmolality is an indicator of relative excess of water to solute in the ECF (dilutional or total body deficit of solute). Rapid correction causes osmotic shift of water from brain cells to the ECF and leads to central pontine myelinolysis.
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