systole
Systole is the push. It is the part of the heartbeat when the ventricular muscle tightens and squeezes blood out toward the body and lungs, the way you would squeeze a water balloon to make a jet.
It begins when the ventricles start to contract. Pressure inside them rises sharply until it exceeds the pressure in the aorta and pulmonary artery; the outlet valves then open and blood is ejected. Systole ends when the ventricles begin to relax and those outlet valves snap shut.
When clinicians say systole without qualifying it, they usually mean ventricular systole, because that is what generates the pulse and the systolic blood pressure. The atria have their own brief systole slightly earlier, which tops off ventricular filling.
The strength and timing of systole depend on how full the ventricle was (preload), how much resistance it pushes against (afterload), and the muscle's intrinsic vigor (contractility). Disease in any of these blunts the force of ejection.
The higher number in a blood pressure reading, such as the 120 in 120/80, is the systolic pressure — the peak arterial pressure during ventricular systole.
Systolic blood pressure reflects the pressure peak of each ventricular contraction.
Systole has two sub-phases: isovolumetric contraction (all valves closed, pressure building) followed by ejection (outlet valves open, blood leaving). Only ejection actually moves blood out of the heart.