What the two numbers are
Blood pressure is the force the blood pushes against the walls of the arteries, measured in millimetres of mercury (mmHg). It is not one steady value — it rises and falls with every heartbeat. So we record two numbers. The higher one is the systolic pressure: the peak, reached when the heart contracts and ejects blood during systole. The lower one is the diastolic pressure: the trough, the resting pressure that remains while the heart relaxes and refills during diastole.
So a reading of 120/80 mmHg means the pressure peaks at 120 with each beat and never falls below 80 between beats. We say it aloud as “120 over 80.” Blood pressure is one of the core vital signs, reported alongside heart rate, breathing rate and temperature.
The gap between them: pulse pressure
The difference between the top and bottom numbers is the pulse pressure. For 120/80 that is 40 mmHg. It reflects how forcefully the heart ejects blood and how stiff or springy the large arteries are. Young, elastic arteries cushion each beat and keep the gap modest; with age, arteries stiffen and the gap often widens.
Pulse pressure = systolic − diastolic Example reading: 120 / 80 mmHg Systolic (peak, during systole) = 120 Diastolic (trough, during diastole) = 80 Pulse pressure = 120 − 80 = 40 mmHg A stiffer-arteries example: 160 / 80 mmHg Pulse pressure = 160 − 80 = 80 mmHg (wider gap)
How it is measured
Classic blood pressure measurement uses a cuff around the upper arm. The cuff inflates until it stops the flow in the artery, then slowly releases. The clinician listens (or a machine senses) for the moment blood first spurts through — that pressure is the systolic — and the moment the sound disappears as flow becomes smooth again — that pressure is the diastolic.