Hydrogen ion balance during exercise

This paper discusses certain aspects of the metabolic acidosis which accompanies heavy exercise, concentrating on the factors influencing hydrogen ion production and the associated effects on carbon dioxide output and ventilation. Classical concepts of lactic acidosis in exercise were established many years ago (Hill, 1932; Margaria, Edwards & Dill, 1933) and continue to be widely accepted. In situations where oxygen delivery to muscles does not match the demands for oxidative phosphorylation of ADP through aerobic metabolism, the deficit in energy needs (the ‘oxygen deficit’) is met by anaerobic glycolysis with the production of lactic acid. Thus lactic acid production may occur at the onset of exercise, in the transition between one work load and one of higher power or when very heavy exercise is undertaken. The consequent rise in blood lactate concentration is accompanied by an equivalent fall in blood bicarbonate concentration, by an increase in carbon dioxide output by the lungs (Vco, ) which is relatiyely greater than the increase in oxygen uptake ( Vo2) and by a high respiratory exchange ratio (R = Vco,/Vo,). The concept of an ‘anaerobic threshold‘ has been introduced (Wasserman, Whipp, Koyal & Beaver, 1973); this may be defined practically during an exercise test, in which power is progressively increasing, by observing the point at which there is an. abrupt increase in ventilation (V,) for a given YO,; i.e. the point .at which the ventilatory equivalent for oxygen (V,/ VO,) increases. Although the acidosis of exercise is ascribed to imbalance between muscle oxygen demand and supply, it is difficult to establish a causal