Physiological Viewpoints on Clinical Acid-Base Diagnostics

Abstract

Acid-base physiology is concerned with sources, extent, and control of hydrogen ion donation in the body, at the organ-physiological as well as the molecular level of study. With the introduction of Van Slyke's methods for quantitative carbon dioxide measurements in biological fluids, one important source of hydrogen ion donation became identifiable; and these and derived methods have permitted of fairly precise quantitative descriptions of transport and pulmonary elimination of carbon dioxide. However, the inevitable operational concept of non-carbonic (non-volatile) contributions to the titratable acidity of the body fluids has been a cause of considerable methodological and conceptual difficulties; and whereas it is now possible by means of the micro-equilibration technique to make accurate assessments of the concentration of non-volatile titratable acid (base) in blood, the question of the physiological relevance of the concept of ‘base excess’ remains open. In particular, the concept of non-carbonic acid does not possess a specific relevance with respect to the acid-base physiology of kidney, bone, and gastro-intestinal tract comparable to the ‘substrate-specificity’ of carbon dioxide with respect to the lung.

Our studies indicate that a subdivision of the titratable non-carbonic acid of any biological medium in two subcomponents will provide an improvement of specificity, adequate for a system physiological approach at the organ level. Thus, a distinction should be made between (1) processes of hydrogen ion donation, reversible by endogenous metabolic means (quantitated in terms of the component MA = metabolizable non-carbonic acid) and (2) processes of hydrogen ion donation associated with gastrointestinal, skeletal, and renal transport, storage, and control of non-metabolizable non-carbonic acid (NA). Some implications of this distinction for acid-base physiology and acid-base diagnostics are discussed.