Circulatory and Respiratory Adaptation during Prolonged Exercise of Moderate Intensity in the Sitting Position

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Abstract
Circulatory and respiratory adaptation during prolonged exercise of moderate intensity in the sitting position were studied with heart catheterization technique in 6 healthy young men of ordinary, physical fitness. The work load was chosen so that they worked at a level of 57.3 per cent of the rate of work they could perform at a pulse rate of 170 beats/min. This load gav a mean heart rate of 126.7 beats/min after 10 min and 138.5 beatslmin after 60 min of work. The circulatory changes under these experimental conditions were characterized by a constant cardiac output with a continuous increase in heart rate and a continuously falling stroke volume. The systemic arterial mean blood pressure fell slightly but significantly and the mean pulmonary arterial pressure also decreased significantly from 10 to 40 min work. The total blood volume decreased 7.2 per cent from rest to 10 min work and then remained unchanged. The respiratory response was characterized by a numerical increase (P<0.1) in total ventilation, dead space ventilation and an unchanged alveolar ventilation. The respiratory quotient increased from rest to the beginning of the exercise and then decreased significantly. The observed changes during prolonged exercise may be explained by a decreased vasomotor tone with a change in the compliance of the capacity vessels and the distribution of the blood volume. The relative level of work intensity seems to be more important than the body position to maintain a steady state.