systole

Imagine your heart as a powerful pump. Systole is the moment that pump squeezes. It is the phase when the heart muscle contracts to push blood out, especially from the ventricles into the arteries. It is one half of the heartbeat cycle, followed by diastole, the phase when the heart relaxes and fills again. You can feel the wave of pressure from this pumping phase as your pulse. Doctors also measure systolic blood pressure, which is the highest pressure reached during this part of the heartbeat.

The word systole comes from a Greek word meaning "contraction." It refers to the phase of the heartbeat when a chamber of the heart contracts. In most everyday and medical contexts, it usually points to ventricular systole, when the heart's main pumping chambers squeeze and send blood into the aorta and pulmonary artery. If you picture squeezing a water balloon, you have a simple image of what happens during this part of the cardiac cycle.

It makes the most sense when paired with its opposite, diastole. The cardiac cycle moves in a steady loop: contraction, then relaxation; pumping, then filling. Together, these phases create the rhythm that keeps blood moving through the body. In medical language, systolic blood pressure is the pressure in the arteries during this contraction phase. It is the higher number in a blood pressure reading, so in "120 over 80," the 120 is the systolic value. When that number stays too high over time, it can be a sign of increased risk for heart and blood vessel problems.

Beyond cardiology, systole is occasionally used metaphorically to describe any forceful contraction or rhythmic surge of energy. While less common, you might encounter it in poetic descriptions of powerful movements or the "pulse" of a city. However, its primary and most significant meaning remains firmly rooted in the study of the heart's rhythmic dance between systole and diastole.