Physiology of Water and Electrolyte BalanceTop
1. Electroneutrality of body fluids: Body fluids are electrically neutral in all body water compartments, which means that in each body fluid the sum of anions (negative charges) equals the sum of cations (positive charges). In the extracellular fluid (ECF), the key cation is Na+, while the key anions are Cl− and HCO3−. In the intracellular fluid (ICF), the key cation is K+, while the key anions are proteinates and phosphates.
2. Isomolality (iso-osmolality) of body fluids: The osmotic pressure of body fluids in all water compartments is equal. An increase or a decrease in the concentration of effective osmolytes (osmotically active substances that do not readily pass through cell membranes) in one water compartment causes a shift of water between the compartments and stabilization of the osmotic pressure at a new level. Fluids with effective osmolality below the physiologic level are called hypotonic, while those with effective osmolality above the physiologic level are called hypertonic. The normal osmolality of body fluids ranges from 280 to 290 mmol/kg H2O. The osmolality of serum can be calculated on the basis of sodium, glucose, and urea levels (all in mmol/L) using the following formula:
|Serum osmolality = 2 × [Na+] + [glucose] + [urea]|
Osmolar gap is a difference between measured serum osmolality and osmolality calculated using the formula above. Under physiologic conditions, the osmolar gap is ≤10. Causes of a small osmolar gap (10-15) include ketoacidosis, lactic acidosis, and severe kidney disease without dialysis. A larger osmolar gap >15 suggests the presence of substances that are effective osmolytes in serum, such as ethanol, methanol, isopropanol, ethylene glycol, propylene glycol, or acetone.