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Module 1
CARDIAC CYCLE
Dr. D. Penney


Introduction:

The cardiac cycle 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") (Figure 1.01). 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 below.




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