1. AnatomySystole ( SIST-ə-lee) is the part of the cardiac cycle during which some chambers of the heart contract after refilling with blood. Its contrasting phase is diastole, the relaxed phase of the cardiac cycle when the chambers of the heart are refilling with blood. == Etymology == The term originates, via Neo-Latin, from Ancient Greek συστολή (sustolē), from συστέλλειν (sustéllein 'to contract'; from σύν sun 'together' + στέλλειν stéllein 'to send'), and is similar to the use of the English term to squeeze. == Terminology, general explanation == The mammalian heart has four chambers: the left atrium above the left ventricle (lighter pink, see graphic), which two are connected through the mitral (or bicuspid) valve; and the right atrium above the right ventricle (lighter blue), connected through the tricuspid valve. The atria are the receiving blood chambers for the circulation of blood and the ventricles are the discharging chambers. In late ventricular diastole, the atrial chambers contract... from wikipedia.org