Читать книгу Disorders of Fluid and Electrolyte Metabolism - Группа авторов - Страница 20

The most abundant and therefore the primary determinant of the tonicity of plasma is its concentration of sodium, as reflected in the following calculation:

where PNa is the plasma sodium concentration (in mmol/L), Glucose refers to the plasma glucose concentration (in mg/dL), BUN is the blood urea nitrogen (in mg/dL).

It is important to differentiate plasma tonicity from plasma osmolality. Plasma osmolality is determined by all the particles dissolved in plasma while plasma tonicity is limited to only those particles that exert an osmotic effect [2], a function of the permeability of the particle across cell membranes, which determines its ability to transport water across such a membrane. Thus, urea contributes to plasma osmolality (equation 1) but does not confer tonicity because its high permeability allows for rapid equilibration across cell membranes. Solutes that increase plasma osmolality without significantly altering tonicity include primarily alcohols such as ethanol, methanol, and ethylene glycol. In contrast, glucose in the absence of insulin does not enter cell readily. Thereby, in addition to contributing to the osmolality, glucose also exerts an osmotic force and partakes in the tonicity of plasma causing water to move from cells to extracellular fluid. Glucose concentration under normal conditions is around 90 mg/dL, contributing to only 5 mOsm/kg. Therefore, the main determinant of plasma tonicity is plasma sodium concentration where plasma tonicity is approximately twice the plasma sodium concentration.