BSIP VEM / SCIENCE PHOTO LIBRARY BSIP VEM / SCIENCE PHOTO LIBRARY
Heart cycle. Illustration of four hearts in cross section, showing the stages of a heartbeat. Top (diastole): the two atria (red, top) fill with blood while the heart rests between beats. Right (atrial systole): the atria contract, squeezing blood into the ventricles (red, bottom). Bottom (ventricular systole): the ventricles contract, squeezing blood into the aorta (arched artery at top) and pulmonary artery (forked artery). Left (diastole): the empty heart rests until the atria fill again. The human heart beats continuously throughout life, contracting around 2500 million times in the average lifetime.
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