Heart Rate, Stroke Volume and Vasomotor Regulation in the Cat during Exertion and Postural Changes*

9bstract. The cat, when its peripheral metabolic demands are augmented, has a limited capacity to increase its heart rate, but often responds with a considerable rise in stroke volume: stroke volume can be increased by active changes of p t u r e from lying to standing, by a short or prolonged bout of fighting, and by mild exercise on the treadmill. In all these conditions stroke volume attains values larger than those measured during recumbent rest. It appears therefore that an increase in stroke volume is a mechanism ordinarily called upon in the cat when natural behaviour requires an increase in cardiac output, while in man it is called upon in extraordinary conditions only. However, the fact that a stroke volume regulating mechanism is ordinarily operative in the cat shorn that it has a physiologi-cal significance, and supports the idea that in man also exer-tion is accompanied by a masked stimulation of myocardial contractility. A description is also presented of the vasomotor changes produced by active changes in posture