Interaction of physiological mechanisms during exercise.

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Abstract
The effects of work intensity and duration on the metabolic circulatory and ventilatory response to exercise are quantitated in healthy male subjects during cycle ergo-meter exercise. There is a well-ordered relationship between work rate and oxidative energy sources. Steady-state time for O2 consumption (Vo2) is dependent on work intensity (the time arterial blood laotate concentration stops increasing). At moderate work, anaerobic metabolism is a very small part of credit oxidation, while at very heavy work, the pyruvate-lactate mechanism is the major creditor. The increase in gas exchange ratio (R) reflecting the production of CO2 from bicarbonate (buffering of lactic aid) is transient. After the steady state is reached, R decreases. The heart rate increase during constant-load exercise parallels the increase in V02. Physiological deal space/tidal volume ratio decreases from. 33 to. 17 during exercise. The reduction is independent of work duration and only slightly reduced as work intensity increases. Arterial O2 tension does not decrease during exercise at sea level even at maximal rates of O2 transport. Arterial-end tidal CO2 tension differences suggest a fluctuation in pulmonary capillary CO2 tension of approximately 8 mm Hg during the respiratory cycle of the exercising subject. Changes in minute ventilation are best predicted from the rate of CO2 production and the extent of respiratory compensation for metabolic acidosis.

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