One heartbeat—a cardiac cycle—consists of two parts called systole and diastole.

Diastole
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During diastole, the atria relax as oxygenated blood from the lungs fills the left atrium and deoxygenated (venous) blood from other parts of the body fills the right atrium. At the end of the diastole, the atria contract, forcing blood into the corresponding ventricles.

Systole
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During systole the ventricles contract, forcing the blood into the pulmonary artery to be reoxygenated in the lungs, and into the aorta for systemic distribution of oxygenated blood. The blood pressure in the aorta during systole normally rises to approximately 120 mm Hg, whereas during diastole it falls to about 80 mm Hg. The resting heart has approximately 60 to 100 beats per minute, and cardiac output typically ranges from 2.5 to 3.6 L/min/m² of body surface area.

Measurement of ventricular pressure and cardiac output are the standards for assessing cardiac function.