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Each loop represents changes in ventricular pressure and volume over the course of one cardiac cycle, or one heartbeat, which includes both ventricular systole, or contraction, and diastole, or relaxation.
The lower right hand corner is the end-diastolic point, and it’s the point in the cardiac cycle when diastole is over. Αt this point, the mitral valve between the left atrium and the left ventricle, closes, leaving the left ventricle filled with the maximum volume of blood, called the end-diastolic volume.
And then, systole begins, which is when the left ventricle contracts to push that blood into the aorta. Ventricular contraction makes the pressure shoot up, but for a brief period of time, both the mitral and aortic valves are closed, so left ventricular volume doesn’t change.
This phase is isovolumetric contraction, but it doesn’t last long, because eventually the pressure inside the left ventricle exceeds aortic pressure, making the aortic valve pop open, and that starts the ejection phase.
The cardiac cycle, also called the stroke work, is the work performed by the heart's left ventricle during an ejection of a blood volume. It is the performance of the heart from the beginning of one heartbeat to the beginning of the next, and equals to the product of the mean aortic pressure and stroke volume, which is the amount of blood pumped by the left ventricle in one beat. Cardiac work done in a full minute will be referred to as cardiac minute work. It is equal to the product of mean aortic pressure and cardiac output, since the cardiac output equals heartbeats in one minute times the stroke volume.
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