Delayed pH Equilibration in Blood During Carbonic Anhydrase Inhibition

Abstract

The events that occur in a red cell as it passes through a lung capillary were well-summarized by Roughton in 1964¹, although a mechanism for pH equilibration between red cells and plasma was not included in that schema. The need for such a mechanism arises because, as blood exchanges gases in the pulmonary capillaries, [H⁺] changes intrace11u1ar1y because of the oxygenation of hemoglobin and carbon dioxide reactions that take place there, but plasma [H⁺] changes little. As a result, H⁺ must be transferred across the red cell membrane to reach complete equilibrium within blood. The rate of exchange of OH^- for C1^- is relatively slow₂ and is thought not to be important in this process of pH equilibration in vivo. Rather, in the presence of CO₂, it is the JacobsStewart cycle³ that effects the movement of H⁺’equivalents across the red cell membrane⁴. This cycle is rate-limited by the uncatalyzed extracellular hydration-dehydration reactions of CO₂.