Heart Cycle in Systole : There are two phases of the cardiac cycle: systole and diastole. Systole is the phase during which the heart contracts, pushing blood out of the left and right ventricles, into the systemic and pulmonary circulation respectively. The ventricles fill with more and more blood until the pressure is great enough against the semilunar valves that they open, allowing the blood to enter the aorta and pulmonary trunk. Systolic pressure is the blood pressure felt in your arteries when your heart beats. Blood pressure is denoted as a fraction, with the systolic pressure being the top number. Blood pressure higher than the average of 120/80 enters the range of hypertension.

Systole /ˈsɪstəl/ is an ancient medical term first understood as a gathering of blood and later contraction of the heart. The period of ventricular contraction of the cardiac cycle is called, systole. The term "systole" originates from New Latin, from Ancient Greek συστολή (sustolē), from συστέλλειν (sustellein, "to contract"), from σύν (syn, "together") + στέλλειν (stellein, "send").

The mammalian heart has 4 chambers: the left atrium, the left ventricle, the right atrium and the right ventricle.

When the smaller, upper atria chambers contract in late diastole, they send blood down to the larger, lower ventricle chambers. When the lower chambers are filled and the valves to the atria are closed, the ventricles undergo isovolumetric contraction (contraction of the ventricles while all valves are closed), marking the first stage of systole. The second phase of systole sends blood from the left ventricle to the aorta and body extremities, and from the right ventricle to the lungs. Thus, the atria and ventricles contract in alternating sequence. The left and right atria feed blood, at the same time, into the ventricles. Then, the left and right ventricles contract simultaneously as well.

Cardiac systole is the contraction of the cardiac muscle in response to an electrochemical stimulus to the heart's cells (cardiomyocytes).

The cardiac output (CO) is the volume of blood pumped by the left ventricle in one minute. The ejection fraction (EF) is the volume of blood pumped divided by the total volume of blood in the left ventricle.

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