Blood pressure is a measure of how much the blood presses against the walls of the arteries. This creates two forces: The first comes from when the heart pumps blood into the arteries; the second is the force of the arteries to resist the blood flow. When a person “takes a blood pressure,” he or she is taking a ratio: The higher number (called the systolic or top number) represents the pressure when the heart contracts to pump blood to the body; the lower number (called the diastolic or bottom number) represents the pressure when the heart relaxes between beats. For example, for a blood pressure of 120/76 (said, “120 over 76”), the systolic reading is 120 and the diastolic reading is 76.

When a nurse or doctor takes your blood pressure, the two numbers they read to you indicate systolic and diastolic pressures in your arteries.

In actuality, the numbers represent how high the blood’s pressure would force a column of mercury to rise in a tube. For example, a systolic reading of 120 means the mercury would rise 120 millimeters (usually labeled mm Hg, with Hg the symbol for mercury) in a tube. Based on the most recent information (and it keeps changing), a blood pressure below 120/80 is considered optimal for adults; 120 to 139 over 80 to 89 is considered “prehypertension.” Anything over 140/90 is considered hypertension, which includes three stages, with the highest hypertension reading, stage 3, being anything above 179/109.