Regulation of the Circulation During Exercise

Oxygen consumption and cardiac output (direct Fick) have been measured in normal dogs at rest and during graded exercise on the treadmill up to a work intensity of 5 mph and 10°. Systemic and pulmonary artery pressures have also been recorded. The changes in cardiac output produced ‘at rest’ by excitement were frequently as large as those induced by moderate exercise. A short bout of exercise followed by a rest period was far more efficacious in producing lower and more uniform results during rest and subsequent exercise than a prolonged rest period alone. Under such conditions the ‘steady state’ was reached in 3 minutes or less of exercise. The linear relation between oxygen consumption and cardiac output during exercise in the dog is similar to that observed in man, and in the horse. The possible significance of this similarity is discussed and it is suggested that the data are consistent with the hypothesis that the increase in blood flow during exercise is largely the increase in muscle flow with a constant arteriovenous oxygen difference of approximately 14 vol. %.