Systole

Systole /ˈsɪstəliː/ is the part of the cardiac cycle when the ventricles contract. 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.

Cardiac electrical systole is staged and first derived from sympathetic charge from the sinoatrial node (SA node). Subsequent physiologic discharge from the SA node then finds its way through the atrial mass, eventually meeting at the atrioventricular node to be gated through the available channels from the atria to the ventricles to allow ventricular systole or [LVEF] + [RVEF].

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