Page 32, CV Review, Essentials, Develop. Physiol., Dr. D. Penney


Cardiac Cycle (cont...)

The cardiac cycle can also be set up along the lines of a Carnot cycle; i.e. ventricle volume is plotted against ventricle pressure. 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 semilunar valve opening, ejection occurs, with falling ventricular volume. With semilunar 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, 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.

Not all of the blood in the heart is ejected during a cardiac cycle. Even utilizing the systolic reserve volume as during enhanced inotropicity, there remains a residual volume, which cannot be ejected. The portion of blood ejected during one cardiac cycle is referred to as the ejection fraction. It is obtained by dividing stroke volume by end-diastolic volume. Two-thirds (0.67) is a normal value of Ef. Likewise, the heart can be filled to a greater extent by making use of the diastolic reserve volume, as is done in the Frank-Starling mechanism. Making use of both systolic reserve and diastolic reserve volumes, maximal stroke volume is achieved. Heart function is similar to lung function in this regard. By increasing both stroke volume and heart rate over rest values, cardiac output in excess of that at rest ("Cardiac reserve") can be called upon (Figure 1.13), (Figure 1.14).

Animated cardiac cycle cartoons are also available.


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