Systole is the phase of the cardiac cycle in which the heart muscles are contracted. Systole can be divided into atrial systole in which the atria contract, and ventricular systole when the ventricles contract. The other phase of the cardiac cycle is diastole, when all the muscles relax. Atrial systole moves blood from the atria into the ventricles, while ventricular systole moves blood out of the heart into the pulmonary and systemic circulations. Systole will end as diastole begins, and allows blood to fill the heart again. The following diagram shows systole in the context of the entire cardiac cycle.
Note that during systole, the ventricular pressure and arterial pressure spike. This coincides with the systolic pressure in the measurement of blood pressure. In a blood pressure measurement, the top number refers to the pressure in the arteries during systole. The bottom number is the diastolic pressure, or pressure of the arteries when the heart is relaxed. Systole can also be seen in an electrocardiogram, which is shown on the graph above. The electrocardiogram measure the electrical activity of the heart. Large changes in the electrical activity represent destabilization of the nerve membrane, which causes muscles to contract. The “QRS” peaks on the electrocardiogram represent the start of systole. Atrial systole starts just before this, caused by the destabilization of “P” on the electrocardiogram. This destabilization occurs in the sinoatrial node in the mammalian heart, and controls the pulse.