Blood pressure is the force your blood exerts against the walls of your arteries as your heart pumps it through your body. It’s measured with two numbers: the top number (systolic) captures the peak pressure when your heart contracts, and the bottom number (diastolic) captures the lowest pressure between beats. A normal reading is below 120/80 mmHg.
How Your Body Creates Blood Pressure
Every time your heart beats, it squeezes blood out of the left ventricle into the aorta, your body’s largest artery. That surge of blood pushes outward against the artery walls, creating systolic pressure. Your arteries are elastic, so they stretch slightly to absorb each pulse and then recoil between beats, maintaining steady flow to smaller blood vessels even while the heart refills. The pressure during that resting phase is your diastolic reading.
Two factors determine your overall blood pressure: how much blood your heart pumps per minute (cardiac output) and how much resistance your smaller blood vessels put up against that flow. If your arteries stiffen, which happens naturally with age or from plaque buildup, the heart has to push harder to move the same volume of blood. That’s one reason systolic pressure tends to climb as people get older.
What the Numbers Mean
Blood pressure is recorded in millimeters of mercury (mmHg), a unit inherited from the original mercury-column devices used to measure it. The 2025 guidelines from the American Heart Association and American College of Cardiology define the categories like this:
- Normal: below 120 systolic and below 80 diastolic
- Elevated: 120 to 129 systolic with diastolic still below 80
- Stage 1 hypertension: 130 to 139 systolic or 80 to 89 diastolic
- Stage 2 hypertension: 140 or higher systolic or 90 or higher diastolic
If your systolic and diastolic numbers fall into two different categories, you’re classified by the higher one. So a reading of 135/75 counts as stage 1 hypertension because of the systolic number, even though the diastolic is normal.
Why High Blood Pressure Is Dangerous
Chronically elevated pressure damages the inner lining of your blood vessels. Over time this reduces the availability of nitric oxide, a molecule that keeps arteries flexible and relaxed. The result is increased inflammation, scarring, and progressive stiffening of the vessel walls. That stiffening, in turn, raises pressure further, creating a cycle that accelerates damage to the heart, kidneys, brain, and eyes.
The danger is compounded by the fact that high blood pressure rarely produces symptoms until something goes seriously wrong. Globally, about 1.4 billion adults aged 30 to 79 have hypertension. Of those, an estimated 600 million (44%) don’t know they have it. Only about 23% of people with hypertension worldwide have it under control.
Why a Single Reading Can Be Misleading
Your blood pressure is not a fixed number. It shifts throughout the day based on body position, mood, physical activity, food intake, and even the time of day. During sleep, pressure naturally drops by 10 to 20 mmHg. Simply moving your arm from a horizontal to a vertical position can change the reading by 5 to 6 mmHg. In some people, the stress of having a cuff inflated can spike pressure by up to 40 mmHg.
This variability explains a phenomenon called white-coat hypertension, where readings taken in a clinic fall in the hypertensive range but home readings are normal. Somewhere between 15% and 30% of people with elevated office readings actually have white-coat hypertension rather than sustained high blood pressure. That’s why guidelines recommend sitting quietly for 5 to 10 minutes before a reading, and why home monitoring over several days often gives a more reliable picture than a single office visit.
How Blood Pressure Is Measured
Traditional measurement uses a cuff wrapped around the upper arm. A clinician inflates the cuff until it temporarily stops blood flow, then slowly releases the pressure while listening through a stethoscope for the sounds of blood resuming flow. The point where sound first appears marks the systolic pressure; the point where it disappears marks the diastolic.
Most home monitors and many clinic devices now use an oscillometric method instead. These detect tiny vibrations in the cuff caused by blood pulsing through the artery. The device identifies the point of maximum oscillation (which corresponds to the average pressure in the artery) and then uses an algorithm to estimate systolic and diastolic values. For most people, an upper-arm oscillometric monitor is accurate enough for routine tracking.
Primary vs. Secondary Hypertension
About 90% to 95% of hypertension cases are classified as primary, meaning there’s no single identifiable cause. It develops gradually from a combination of genetics, aging, diet, weight, and activity level. Secondary hypertension, on the other hand, is caused by a specific underlying condition. The most common culprits include obstructive sleep apnea, narrowing of the arteries that supply the kidneys, overproduction of the hormone aldosterone, and certain medications or substances including alcohol.
Your doctor may suspect secondary hypertension if your blood pressure was previously stable and suddenly spikes, if you develop high blood pressure before age 30, or if three or more medications fail to bring it under control. Treating the underlying cause can sometimes resolve the blood pressure problem entirely.
When Blood Pressure Becomes an Emergency
A reading of 180/120 mmHg or higher is considered a hypertensive crisis. Symptoms can include severe headache, blurred vision, chest pain, shortness of breath, confusion, nausea, or seizures. If you get a reading that high and experience any of these symptoms, especially chest pain, difficulty breathing, or signs of stroke (sudden numbness, trouble speaking, vision changes), call emergency services immediately.
How Lifestyle Changes Affect Your Numbers
For people with elevated or stage 1 readings, lifestyle changes alone can make a meaningful difference. Reducing salt intake by about 4.4 grams per day (roughly a teaspoon less of table salt) lowers systolic pressure by an average of 5 mmHg in people with hypertension and about 2 mmHg in those with normal pressure. That effect holds across ethnicities and in both men and women. While 5 mmHg may sound small, at a population level it translates into significantly fewer heart attacks and strokes.
Other changes with well-documented effects include losing excess weight, increasing aerobic activity, eating more fruits and vegetables while reducing saturated fat (an approach known as the DASH diet), limiting alcohol, and managing stress. These interventions tend to be additive, meaning combining several of them produces a larger drop than any one alone. For stage 2 hypertension or people at higher cardiovascular risk, medication is typically needed alongside these changes.