Page 6, Cardiac Cycle, Dr. D. Penney

The cardiac cycle can also be set up along the lines of a Carnot cycle; i.e. ventricle volume is plotted against ventricle pressure (Figure 1.03). Filling begins at end-systolic volume as the blood does work on the ventricle in stretching it to the end-diastolic volume. With AV valve closure, isovolumic contraction occurs and pressure increases. After semi-lunar valve opening, ejection occurs, with falling ventricular volume. With semi-lunar valve closing, isovolumic relaxation takes place and pressure falls, returning the trace to the point at which the AV valve opens.

Stroke volume is equal to end-diastolic volume minus end-systolic volume (see Definitions), and stroke work (force x distance) is equal to the loop area. Adding time as a factor, stroke volume multiplied by heart rate yields cardiac output.

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