Systole – When the heart muscle contracts; squeezes
(Note: Yes, it really is pronounced the same as sisterly)
Systole is when the heart muscle contracts.
A nun gives a sisterly (systole) hug to an orphaned kid then tickles (ventricles) him and squeezes him tightly (contract).
Systole is generally used to refer to when the ventricles of the heart contract, forcing blood out of the heart through the arteries. When the ventricles contract during the systolic phase, both tricuspid and mitral (bicuspid) valves (atrioventricular valves) shut in order to prevent backflow of blood into the atria. At this time both the pulmonary and aortic valves (semilunar valves) are open, directing blood out of the heart.
Another way to remember the action of systole is:
In systole the blood is going into the systemic and pulmonary circuits, and the only way is to squeeze it in.