Blood pressure, plainly.
Two numbers. A cuff. Five minutes a year. Why the most ordinary measurement in medicine is also one of the most consequential — and what it actually means when it climbs.
A blood pressure measurement is two numbers — say, 128 over 78. Most adults have had this measured dozens of times. Most have no idea what those numbers actually mean.
This essay is the explanation I give in clinic when a patient asks. It takes about ten minutes spoken; a little longer to read.
What the two numbers are
Your heart pumps in a cycle. It contracts, pushing blood out into your arteries — that surge is the systolic pressure, the top number. Then it relaxes and refills — and during that pause, the pressure in your arteries falls but does not drop to zero, because your arteries themselves are elastic and hold some pressure between beats. That resting pressure is the diastolic, the bottom number.
Both numbers are measured in millimetres of mercury, which is a historical unit from when blood pressure was measured by literally pushing a column of mercury up a glass tube. Modern cuffs do not contain mercury, but the unit stuck.
A reading of 120 over 80 means: when your heart contracts, the pressure in your arteries is enough to push a column of mercury up 120 millimetres; when it relaxes, that pressure falls to 80.
That is the entire physical meaning of the number.
What “high” actually means
The thresholds for “normal,” “elevated,” and “high” blood pressure are not natural categories — they are statistical ones. They were chosen because, in large populations, people with readings above certain numbers have measurably higher rates of stroke, heart attack, kidney damage, and dementia over the following decades.
The current American thresholds, used by most physicians worldwide:
| Reading | Category |
|---|---|
| Below 120 / 80 | Normal |
| 120–129 / under 80 | Elevated |
| 130–139 / 80–89 | Stage 1 hypertension |
| 140 / 90 or higher | Stage 2 hypertension |
| 180 / 120 or higher | Hypertensive crisis (urgent) |
These thresholds have shifted over the past twenty years, and they will shift again. The reason they keep getting lower is that better long-term studies keep showing meaningful risk increases at numbers we used to call normal. A reading of 135/85, which would have been called “borderline” in 1995, is now considered active disease worth treating.
This shift is real and based on evidence — but it also means that more people are now categorized as having hypertension than ever before, including people who feel completely fine.
Why it matters when it doesn’t hurt
Untreated high blood pressure does its damage slowly and silently. The mechanism is simple: your arteries were not built to carry that pressure year after year. Over time, the high pressure damages the inner lining of arteries everywhere in your body. Damaged arteries are more prone to atherosclerosis — the buildup of plaque that eventually narrows or blocks them.
Where the artery happens to be when it finally fails determines what disease you get:
- An artery in the brain narrows or bursts → stroke.
- An artery feeding the heart muscle blocks → heart attack.
- The fine arteries in the kidney slowly stiffen → chronic kidney disease.
- The fine arteries in the back of the eye get damaged → vision changes.
- The small arteries deep in the brain get progressively damaged → vascular dementia.
This is why hypertension is sometimes called “the silent killer.” It does not announce itself. By the time it announces itself, the damage has been accumulating for ten or twenty years.
What the data actually shows
The strongest evidence for treating high blood pressure comes from a series of large randomized trials over the past forty years. They consistently show that lowering blood pressure reduces stroke risk by roughly 30–40%, heart attack risk by roughly 20%, and overall mortality meaningfully — even in people who feel fine.
The benefit is biggest for people with higher starting blood pressures, but it persists down into ranges that used to be considered borderline. The SPRINT trial in 2015 was a turning point: it showed that aiming for a systolic blood pressure under 120, rather than the older target of 140, reduced cardiovascular events and deaths even in people without diabetes.
This is also why the thresholds keep moving lower.
What you can actually do
Most people with hypertension can lower their blood pressure substantially without medication, if they are willing to make changes that, in most adult lives, are difficult.
The interventions with the strongest evidence:
— Lose weight if you are carrying extra. Roughly 1 mmHg drop in blood pressure for every kilogram of weight lost, sustained.
— Reduce salt. The DASH diet — heavy on vegetables, fruits, whole grains, and lean protein, light on processed foods — drops systolic pressure by 5–10 mmHg in most people who follow it. Most of that effect is the salt reduction.
— Move regularly. At least 30 minutes of moderate aerobic activity most days, and resistance training twice a week if you can. Together they drop blood pressure 5–8 mmHg, plus all the other benefits.
— Drink less alcohol. More than two drinks a day for men, one for women, raises blood pressure measurably. Cutting back lowers it just as measurably.
— Sleep. Untreated sleep apnoea is a major and often-missed driver of hypertension. If you snore heavily or wake up unrefreshed, ask about a sleep study.
If lifestyle changes are not enough — and for many people they are not — the medications work. They are well-studied, generally well-tolerated, and the side effects are manageable. Refusing medication for hypertension because you “don’t want to start” is a bet against decades of evidence.
What to actually do with this
If you don’t know your blood pressure, get it measured. A pharmacy machine, a home cuff, or a routine doctor’s visit will all do. One reading is not enough — blood pressure varies hour to hour and day to day. Two or three readings on different days, all elevated, is the threshold for taking it seriously.
If you do know your blood pressure and it is fine, recheck once a year. If it is high, have a conversation with a doctor — not the internet — about the next step.
The whole point of hypertension treatment is that it is a wager about a future that is decades away. The disease prevented is the one that does not happen. Most patients who take their treatment seriously will never know what they avoided. That is the nature of preventive medicine, and it is, when it works, the quietest kind of success in clinical practice.
Coming soon: an essay on what diabetes actually is — the second of the chronic-disease pieces. Subscribe if you’d like it in your inbox.