Aerobic Metabolism and the Steady-State Concept

Abstract

This chapter contains an analysis of the steady-state concept, as it is applied during light exercise. In this case, oxygen consumption increases upon exercise onset to attain a steady level, which can be maintained for a long period of time. The steady-state oxygen consumption is proportional to the exerted mechanical power. Under these circumstances, there is neither accumulation of lactate in blood nor changes in muscle phosphocreatine concentration: aerobic metabolism sustains the entire energy requirement of the exercising body. Once the steady state has been attained, the flow of oxygen is the same at all levels along the respiratory system. The quantitative relations determining the flow of oxygen across the alveoli and in blood are discussed. Special attention is given to the effects of ventilation-perfusion inequality and to the diffusion-perfusion interaction equations. The cardiovascular responses at exercise steady state are analysed in the context of the equilibrium between systemic oxygen delivery and systemic oxygen return. The relationship between oxygen consumption and power is discussed, along with the distinction between external and internal work during cycling. The concepts of mechanical efficiency of exercise and energy cost of locomotion are analysed. Concerning the latter, the distinction between aerodynamic work and frictional work is introduced. The roles of the cross-sectional surface area on the frontal plane and of air density in aerodynamic work are discussed. To end with, an equation linking ventilation, circulation and metabolism at exercise in a tight manner is developed, around the notion that the homeostasis of the respiratory system at exercise is maintained around given values of the constant oxygen return. This equation tells that, as long as we are during steady-state exercise in normoxia, any increase in the exercise metabolic rate requires an increase in ventilation that is proportional to that in oxygen consumption only if the pulmonary respiratory quotient stays invariant does not change, and an increase in cardiac output that is not proportional to the corresponding increase in oxygen consumption. At intense exercise, when lactate accumulation also occurs and hyperventilation superimposes, a new steady state would be attained only at P A CO 2 values lower than 40 mmHg: the homeostasis of the respiratory system would be modified. This new steady state, however, is never attained in fact, for reasons that are discussed in Chap. 3.