Systolic refers to systole, when the heart contracts and pumps blood out; diastolic refers to diastole, when the heart relaxes and fills with blood. On a blood pressure reading, the systolic number comes first (top), the diastolic number second (bottom): for example, 120/80 means a systolic pressure of 120 and a diastolic pressure of 80.
The core difference
Systolic describes the heart contracting; diastolic describes the heart relaxing and filling.
- systolic — relating to systole, the contracting, blood-pumping phase of the heartbeat: his systolic blood pressure reading was slightly higher than normal.
- diastolic — relating to diastole, the relaxing, blood-filling phase of the heartbeat: her diastolic pressure stayed steady even as the systolic figure rose.
How to tell them apart
Both words come from the two-beat rhythm of the heart, so a simple order trick helps: systolic comes first in the cycle and first in a reading, diastolic comes second in both. Think of systolic as the squeeze and diastolic as the settle.
The mix-up usually shows up on blood pressure readings and monitor displays, where the two numbers sit one above the other and it is easy to forget which represents which phase of the heartbeat.
