Plasma acid-base balance (Strong Ion Difference) (SID)

Plasma acid-base balance via the Strong Ion Difference (SID) is a physicochemical framework used to describe the regulation of hydrogen ion concentration in blood (Stewart, P. A., 1983, Canadian Journal of Physiology and Pharmacology). Developed by Peter Stewart, this approach identifies SID—the difference between the sum of strong cations (e.g., Na+, K+, Ca2+, Mg2+) and strong anions (e.g., Cl-, lactate)—as one of three independent variables determining plasma pH, alongside pCO2 and total weak acids (Kellum, J. A., 2000, Critical Care). Changes in SID necessitate a shift in the dissociation of water molecules to maintain electrical neutrality, which directly alters the concentration of H+ ions (Morgan, T. J., 2005, Clinical Biochemist Reviews). This model is particularly useful in critical care for diagnosing complex acid-base disorders that the traditional bicarbonate-centric model may oversimplify (StatPearls, 2023, "Metabolic Acidosis"). While SID itself is a mathematical construct rather than a single molecular target, it is therapeutically manipulated through the administration of intravenous fluids and medications that alter electrolyte concentrations (PubChem, 2024). Understanding SID is crucial for managing conditions like sepsis and renal failure, where electrolyte shifts significantly impact systemic pH.

Other names

Strong Ion DifferenceStewart approachPhysicochemical acid-base theorySIDaSIDe

Modulation of the net charge difference between strong cations and strong anions in plasma to shift the water dissociation equilibrium and alter hydrogen ion concentration.