blood pressure

Blood pressure is simply the push of blood against the walls of your arteries—the same idea as the pressure inside a water pipe. It rises and falls with each heartbeat, which is why it is reported as two numbers: the higher one when the heart squeezes and the lower one when it relaxes.

It is written as systolic over diastolic, in millimeters of mercury (mmHg), for example 120/80. The systolic number is the peak pressure as the left ventricle contracts; the diastolic number is the lowest pressure between beats while the heart fills. Blood pressure is, at its core, the product of how much blood the heart pumps per minute (cardiac output) and how much resistance the arteries offer to that flow (systemic vascular resistance), with arterial stiffness shaping the waveform.

Healthy adults are generally healthiest in a low-normal range, and persistently high values (hypertension) quietly damage arteries, the heart, kidneys, brain and eyes over years—which is why it is called a ‘silent’ condition. A single reading is only a snapshot, though: pressure swings with activity, emotion, posture, pain and even the stress of being measured, so diagnosis relies on repeated, properly taken readings rather than one number.