In vitro characterization of hemoglobin oxygen dissociation curves and electrolyte shifts in human blood under varying PCO2.

Background: Efficient oxygen transport depends on hemoglobin (Hb) affinity for O2, which is modulated by factors like PCO2, as described by the Bohr effect. This in vitro study explored how varying PO2 and PCO2 influence hemoglobin oxygen saturation (HbO2) and plasma electrolyte concentrations in whole human blood. Methods: Blood from six healthy volunteers was equilibrated at 37°C with gas mixtures spanning PO2 and PCO2 ranges. A total of 346 samples were analyzed for blood gases, HbO2, and electrolytes. The HbO2 dissociation curve was modeled using a Gompertz function within a non-linear mixed-effects framework, while electrolyte dynamics were assessed via polynomial models. Results: HbO2 saturation ranged from 1.4 to 99.6%. Increasing PCO2 shifted the dissociation curve rightward, steepening its slope and raising the inflection point-hallmarks of the Bohr effect-without affecting maximal HbO2. Electrolyte analysis revealed that chloride decreased with PCO2 and increased with HbO2, consistent with the erythrocyte chloride shift. Sodium increased with PCO2, and a significant interaction between HbO2 and PCO2 was observed. Strong ion difference (SID) decreased linearly with HbO2 and increased quadratically with PCO2, suggesting a compensatory role in CO2-induced acid-base changes. Conclusion: These findings, validated against external datasets, underscore the tight coupling between respiratory gas exchange and electrolyte homeostasis. The study provides novel insights into how CO2 modulates both oxygen delivery and plasma ionic composition, with implications for understanding acid-base physiology and its regulation in health and disease.