Systole

Systole (/ˈsɪstəli/ 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.

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.

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 and send blood to the ventricles. This flow fills the ventricles with blood, and the resulting pressure closes the valves to the atria. The ventricles now perform isovolumetric contraction, which is contraction while all valves are closed. This contraction ends the first stage of systole. The second stage proceeds immediately, pumping oxygenated blood from the left ventricle through the aortic valve and aorta to all body systems, and simultaneously pumping oxygen-poor blood from the right ventricle through the pulmonic valve and pulmonary artery to the lungs. Thus, the pairs of chambers (upper atria and lower ventricles) contract in alternating sequence to each other. First, atrial contraction feeds blood into the ventricles, then ventricular contraction pumps blood out of the heart to the body systems, including the lungs for resupply of oxygen.

Cardiac systole is the contraction of the cardiac muscle in response to an electrochemical stimulus to the heart's cells (cardiomyocytes).

Cardiac output is the volume of blood pumped by the ventricles in one minute. The ejection fraction is the volume of blood pumped divided by the total volume of blood in the left ventricle.

Atrial systole occurs late in ventricular diastole and represents the contraction of myocardium of the left and right atria. The sharp decrease in ventricular pressure that occurs during ventricular diastole allows the atrioventricular valves (or mitral and tricuspid valves) to open and causes the contents of the atria to empty into the ventricles. The atrioventricular valves remain open while the aortic and pulmonary valves remain closed because the pressure gradient between the atrium and ventricle is preserved during late ventricular diastole. Atrial contraction confers a minor-fraction addition to ventricular filling, but becomes significant in left ventricular hypertrophy, or thickening of the heart wall, as the ventricle does not fully relax during its diastole. Loss of normal electrical conduction in the heart—as seen during atrial fibrillation, atrial flutter, and complete heart block—may eliminate atrial systole completely.

Contraction of the atria follows depolarization, represented by the P wave of the ECG. As both atrial chambers contract—from the superior region of the atria toward the atrioventricular septum—pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular valves. At the start of atrial systole, during ventricular diastole, the ventricles are normally filled to about 70–80 percent of capacity by inflow from the atria. Atrial contraction also referred to as the "atrial kick," contributes the remaining 20–30 percent of ventricular filling. Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole.