What do blood pressure numbers actually mean?

Blood pressure is reported as two numbers in millimeters of mercury. Systolic, the top number, is the pressure in your arteries when the heart contracts. Diastolic, the bottom, is the pressure between beats while the heart refills. Current American Heart Association categories put normal below 120 over 80, elevated at 120 to 129 systolic with diastolic below 80, stage 1 hypertension at 130 to 139 or 80 to 89, and stage 2 at 140 or above or 90 or above. A reading above 180 over 120 with symptoms is a hypertensive crisis requiring emergency care.

The reason it matters is mechanical. Sustained high pressure damages arterial walls, accelerating atherosclerosis and straining the heart, and the organs that suffer most are the heart, brain, kidneys, and eyes, producing heart attack, stroke, kidney failure, and retinal damage. Hypertension is called the silent killer because it produces no reliable symptoms until damage is advanced, which means the only way to know is measurement. Roughly half of American adults meet the criteria and a substantial share are unaware.

Measurement technique changes readings more than people expect. Sit with back supported and feet flat, arm supported at heart level, no caffeine, exercise, or smoking for thirty minutes beforehand, empty bladder, and rest quietly for five minutes first, then take two readings a minute apart and average them. A single elevated reading does not establish a diagnosis, and white coat hypertension, elevated readings only in clinical settings, is common enough that home monitoring or ambulatory monitoring is often used to confirm. Interventions with strong evidence include reducing sodium, the DASH dietary pattern, regular aerobic activity, limiting alcohol, weight loss where relevant, and adequate sleep, with medication added when those are insufficient.