Systole ( /ˈsɪstəliː/) is an ancient medical term first understood as a gathering and later contraction of the heart. More recently it is understood as a force that drives blood out of the heart. Without qualifiers, it usually means the contraction of the left ventricle. The term "systole" originates from New Latin, from Ancient Greek συστολή (sustolē), from συστέλλειν (sustellein, “to contract”), from σύν (sun, “together”) + στέλλειν (stellein, “send”).
When the smaller, upper atria chambers contract in the first phase of systole, 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 contract in the second phase, sending blood from the left ventricle to the aorta and body, and from the right ventricle to the lungs. Thus, the atria and ventricles contract in sequence (the atria feeding blood into the ventricles), while the left and right ventricles contract at the same time.
Cardiac systole is the contraction of the specialized muscle tissue of the heart 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 1 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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