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What Blood Pressure Is and How We Measure It

Two numbers, one cuff, and a surprising amount of meaning. Learn what systolic and diastolic pressure actually represent, how a reading is taken properly, and why a single high number is not yet a diagnosis.

Two numbers, two moments of the heartbeat

Blood pressure is simply the push of blood against the walls of your arteries. Because the heart pumps in beats, that push rises and falls, so we report it as two numbers. The top number, systolic pressure, is the peak as the heart squeezes (systole). The bottom number, diastolic pressure, is the resting low between beats as the heart refills (diastole). A reading of “120 over 80” means a peak of 120 and a trough of 80, in units of millimetres of mercury (mmHg).

Two derived numbers add nuance. Pulse pressure is the gap between the two (120 − 80 = 40), and it widens when the large arteries stiffen with age. Mean arterial pressure is the average pressure driving blood into the tissues, and it is what your organs actually feel across the whole heartbeat.

Reading: 120 / 80 mmHg

Pulse pressure  = systolic − diastolic
                = 120 − 80 = 40 mmHg

Mean arterial pressure (MAP)
  ≈ diastolic + (1/3 × pulse pressure)
  = 80 + (1/3 × 40)
  = 80 + 13.3
  ≈ 93 mmHg

(The 1/3 weighting reflects that the heart spends
 more of each cycle in diastole than in systole.)
Working pulse pressure and mean arterial pressure out from one reading.

Taking a reading that you can trust

Blood pressure measurement looks effortless, but small details swing the result by 10 mmHg or more. The cuff size, your posture, a full bladder, recent coffee, talking, or simply rushing in from outside can all push the number up. A trustworthy reading is taken after a few quiet minutes of rest.

  1. Sit quietly for 5 minutes, back supported, feet flat on the floor, legs uncrossed.
  2. Rest the arm on a table so the cuff sits at heart level; use a cuff that fits the arm.
  3. Stay silent during the reading — talking can raise the number by several mmHg.
  4. Take two or three readings a minute apart and average them, rather than trusting one.

Why one high number is not a diagnosis

Blood pressure is meant to vary. It is lowest in sleep, climbs on waking, jumps with exercise, stress, pain, or cold, and settles again at rest. A single elevated reading after climbing the stairs tells you almost nothing. Real high blood pressure is a pattern, not a moment — confirmed by repeated readings on separate occasions, or by a 24-hour recording.