Page 30, CV Review, Essentials, Develop. Physiol., Dr. D. Penney

Cardiac Cycle

The cardiac cycle (Figure 1.03) has been divided into six phases: atrial systole (contraction), isovolumic ventricular systole, ventricular ejection, isovolumic (or isovolumetric) ventricular relaxation, rapid ventricular filling, and diastasis ("resting"). Some authors go further, splitting the ejection phase into two parts: early ejection and late ejection. There is some justification for this, i.e. the early versus late phase differences in blood flow and velocity and the mid-phase ventricular-aortic pressure flip.

Note that we are using the term isovolumic, not isometric, since while some muscle fibers may be contracting isometrically, others are contracting isotonically, and others may even be lengthening. The cardiac cycle for the right heart and left heart are similar, although not identical. For one, ventricular and arterial pressures are 3-4 times greater in the left heart than in the right. There are also important differences in timing between the two pumps. These are discussed next.

Atrial systole serves to "top off" ventricular filling. It is of minor importance at rest. As the ventricle begins to contract, the AV valve closes; the tricuspid valve for the right heart, the mitral (bicuspid) valve for the left heart. Pressure then increases rapidly and the shape of the ventricle changes; however, volume remains constant. When ventricular pressure exceeds aortic pressure the semilunar valve opens, the pulmonic valve for the right heart, the aortic valve for the left heart. Flow now begins from the ventricle to the aorta, very rapid during early ejection, and much slower later during ejection. Ventricular volume falls rapidly. Also, although pressure in the ventricle soon falls below aortic, ejection continues to occur, indicating that pressure is not the only form of energy responsible for blood flow. Kinetic (velocity) energy acts in this case to keep the valve open during late ejection.

Animated cardiac cycle cartoons are also available.

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