To quote Wikipedia (with slight editing): "With each heartbeat, blood pressure varies between the systolic and diastolic pressures. Systolic pressure is the peak pressure in the arteries, which occurs near the end of the cardiac cycle when the ventricles are contracting (i.e. the heart is pumping blood). Diastolic pressure is the minimum pressure in the arteries, which occurs near the beginning of the cardiac cycle when the ventricles are filling with blood (i.e. before the heart begins to pump blood). An example of normal measured values for a resting, healthy adult human is 120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg, and spoken as "one–twenty over eighty")."
The words 'systolic' and 'diastolic' refer to the functions of the heart: systole is the contraction of the heart muscle and diastole is the enlargement (technically known as dilatation) of the heart as it fills with blood. The same words are used in prosody (the scientific study of poetic meters and versification): systole is the shortening of a syllable that's normally long, and diastole is the opposite. Diastole typically occurs before a pause or at the ictus (a stressed syllable).
© Haydn Thompson 2021