endocrine and metabolic disorders
Water and Sodium Balance
Osmotic forces
The concentration of combined solutes in water is osmolarity (amount of solute per L of solution), which, in body fluids, is similar to osmolality (amount of solute per kg of solution). Plasma osmolality can be measured in the laboratory or estimated according to the formula Estimated plasma osmolality in conventional units (mOsm/kg )= where serum sodium (Na) is expressed in mEq/L, and glucose and bloodureanitrogen (BUN) are expressed in mg/dL. Estimated plasma osmolality in SI units is 2[serum Na] + glucose +ureawhere all values are expressed in mmol/L. Osmolality of body fluids is normally between 275 and 290 mOsm/kg (275 and 290 mmol/kg). Sodium is the major determinant of plasma osmolality. Apparent changes in calculated osmolality may result from errors in the measurement of sodium, which can occur in patients with hyperlipidemia or extreme hyperproteinemia because the lipid or protein occupies space in the volume of serum taken for analysis; the concentration of sodium in serum itself is not affected. Newer methods of measuring serum electrolytes with direct ion-selective electrodes circumvent this problem. An osmolar gap is present when measured osmolality exceeds estimated osmolality by≥10 mOsm/kg ( ≥ 10 mmol/kg). It is caused by unmeasured osmotically active substances present in the plasma. The most common are alcohols (ethanol, methanol, isopropanol, ethylene glycol),mannitol, andglycine. Water crosses cell membranes freely from areas of low solute concentration to areas of high solute concentration. Thus, osmolality tends to equalize across the various body fluid compartments, resulting primarily from movement of water, not solutes. Solutes such asureathat freely diffuse across cell membranes have little or no effect on water shifts (little or no osmotic activity), whereas solutes that are restricted primarily to one fluid compartment, such as sodium and potassium, have the greatest osmotic activity. Tonicity, or effective osmolality, reflects osmotic activity and determines the force drawing water across fluid compartments (the osmotic force). Osmotic force can be opposed by other forces. For example, plasma proteins have a small osmotic effect that tends to draw water into the plasma; this osmotic effect is normally counteracted by vascular hydrostatic forces that drive water out of the plasma. 