Blood pressure is the amount of pressure that your blood exerts against your blood vessels.  “Systolic blood pressure, that’s the number everybody talks about and focuses on,” says Wexler, “is when your heart contracts. The diastolic blood pressure is actually a little more labile, and that’s when your heart relaxes or is filling up.” The top number in your blood pressure reading is your systolic blood pressure and the bottom number is the diastolic. It’s measured in millimeters of mercury (mm Hg), a unit of pressure.