A quantitative physicochemical approach to acid-base physiology.

An approach to acid-base physiology based on quantitative relationships between systems is presented. Key elements in the approach advocated by Stewart are, first, that changes in H+ or protons and bicarbonate are not considered "primary," or initiating disturbances, but rather as changes that are "dependent" on the interactions between several systems. Secondly, the Henderson-Hasselbalch equation is not used as a control equation; [H%] is not controlled by changes in PCO2 and/or [HCO3]. Thirdly, the "independent" variables within the systems may be used to describe their interaction in aqueous solutions, using a series of equations easily solved by computer. In plasma, the systems and variables are strong ions (strong ion difference, SID); weak acids, or buffers (ion equivalence of total protein concentration, AtOt); and carbon dioxide (PCO2, as influenced by alveolar ventilation). This system provides a quantitative description of the interplay between systems, and it makes it much simpler to understand the effects of ion movements between cells and plasma, renal control of acid-base, the influence of metabolism, and management of acid-base disorders.