The Role of Peripheral Resistance in Controlling Cardiac Output During Exercise*

Cardiac output during exercise is directly related to the level of metabolic activity in the exercising muscles.!:' The transition from rest to moderate exercise in untrained subjects results in an increase in cardiac output due primarily to an increase in heart rate."However, if heart rate is artificially controlled during exercise at a given treadmill speed, cardiac output is independent of the heart rate setting from heart rates of 70 per "minute to 240 per minute in a dog running at three miles per hour (m.p.h.)."If the treadmill speed is increased to 4 m.p.h., cardiac output promptly increases.These observations clearly suggest that cardiac output is regulated by a closed-loop mechanism during exercise.In 1931 Koch" proposed that the cardiac output is regulated by means of arterial pressure receptors to whatever level is required to maintain a constant arterial pressure in the face of a changing peripheral resistance.It is the purpose of this paper to present an explicit theory of cardiac output regulation derived from a modification of this concept, and to describe experiments performed to test the key element of this theory.