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 in millimeters of mercury (mmHg) and expressed as two numbers: a normal reading falls below 120/80 mmHg. Those two numbers tell you different things about how your cardiovascular system is performing, and understanding them is the first step toward knowing whether yours is healthy.
What the Two Numbers Mean
The top number, called systolic pressure, is the maximum force against your artery walls when your heart contracts and pushes blood out. The bottom number, diastolic pressure, is the minimum pressure between beats, just before the next contraction. A reading of 120/80 means your blood hits the artery walls with 120 mmHg of force during a heartbeat and drops to 80 mmHg while your heart refills.
The gap between these two numbers also matters. If your systolic pressure is 130 and your diastolic is 70, the difference (60 mmHg) is called your pulse pressure. A wider gap signals stiffer arteries, which often happens with age. Even a 10 mmHg increase in pulse pressure raises cardiovascular risk by roughly 20%. People with a pulse pressure above 61 mmHg develop irregular heart rhythms (atrial fibrillation) at more than four times the rate of those with a pulse pressure of 40 or below.
What Determines Your Blood Pressure
Two factors set your blood pressure at any given moment: how much blood your heart pumps per minute (cardiac output) and how much resistance your blood vessels create. Multiply those together and you get your blood pressure. Anything that increases either one, whether it’s a faster heart rate, more fluid in your bloodstream, or narrower arteries, pushes the number up.
Vessel size has an outsized effect. The resistance to blood flow is inversely proportional to the fourth power of the vessel’s radius, meaning even a small narrowing of an artery dramatically increases pressure. This is why conditions that stiffen or constrict blood vessels, like plaque buildup or chronic inflammation, have such a powerful impact. On the flip side, when you lose a large amount of blood, cardiac output drops. Your body compensates by tightening blood vessels to try to keep pressure stable, but if the loss is severe, pressure falls dangerously low.
Blood Pressure Categories
The American Heart Association classifies blood pressure into five ranges:
- Normal: below 120/80 mmHg
- Elevated: systolic 120 to 129, with diastolic still below 80
- Stage 1 hypertension: systolic 130 to 139 or diastolic 80 to 89
- Stage 2 hypertension: systolic 140 to 179 or diastolic 90 to 119
- Severe hypertension: systolic 180 or higher and/or diastolic 120 or higher
Notice that “elevated” only applies to the top number. You can also qualify for a hypertension stage based on either number being high, not necessarily both. Many people are surprised to learn that the threshold for Stage 1 hypertension starts at 130, not 140, which was the older cutoff used before guidelines were updated.
Why High Blood Pressure Is Dangerous
Chronically high pressure damages your body silently, often for years before symptoms appear. The harm happens through several overlapping mechanisms, and it reaches beyond the heart.
In the heart itself, high pressure forces the muscle to work harder with every beat. Over time the walls of the heart’s main pumping chamber thicken, a change called left ventricular hypertrophy. This thickened muscle becomes stiffer and less efficient, eventually leading to heart failure. High pressure also accelerates plaque buildup in the coronary arteries, raising the risk of heart attacks and sudden cardiac death.
In the brain, sustained pressure stresses artery walls and promotes atherosclerosis, which can cause both hemorrhagic and ischemic strokes. It also reduces overall blood flow to the brain and contributes to small vessel disease, the cumulative effect of many tiny areas of damage. Over decades, this is a major contributor to vascular dementia and cognitive decline.
The kidneys are especially vulnerable because they filter blood through millions of tiny structures that rely on precise pressure regulation. Chronically elevated pressure increases the pressure inside these filters, accelerates the loss of functional units, and disrupts the kidneys’ own ability to help regulate blood pressure, creating a damaging feedback loop. This can progress from protein leaking into urine to full chronic kidney disease.
Low Blood Pressure
Blood pressure below 90/60 mmHg is generally considered low. Unlike hypertension, low blood pressure isn’t always a problem. Some people run low without any symptoms and face no health risks from it. It becomes a concern when it causes noticeable symptoms: dizziness, lightheadedness, blurred vision, fatigue, trouble concentrating, or fainting. A drop of just 20 mmHg from your usual level can be enough to trigger these symptoms.
Extremely low blood pressure can lead to shock, a medical emergency where organs don’t get enough blood. Signs include cold and clammy skin, rapid shallow breathing, a weak pulse, confusion, and pallor.
Hypertensive Crisis
A reading of 180/120 mmHg or higher is a hypertensive crisis. Symptoms can include severe headache, chest pain, shortness of breath, blurred vision, confusion, nausea, anxiety, and in serious cases seizures or unresponsiveness. This level of pressure can cause immediate damage to the brain, heart, kidneys, and major blood vessels, making it a medical emergency.
Getting an Accurate Reading
Blood pressure is surprisingly sensitive to how and when you measure it. A few details make a real difference in accuracy. Sit in a comfortable chair with your back supported for at least five minutes before taking a reading. Both feet should be flat on the ground, legs uncrossed. Rest the arm wearing the cuff on a table at chest height. The cuff should sit snug against bare skin, not over a sleeve, and should fit properly for your arm size.
Skipping the rest period, crossing your legs, or letting your arm hang at your side can each inflate the reading by several points, enough to push a normal result into the elevated range.
White Coat and Masked Hypertension
Your blood pressure in a clinic may not reflect your blood pressure at home. White coat hypertension means your readings run high at the doctor’s office but normal everywhere else. In one study of over 1,200 patients with borderline or mildly elevated readings, nearly 22% had white coat hypertension. Among those specifically classified as Stage 1 hypertensives in the clinic, almost half (48.9%) turned out to have normal pressure at home.
Masked hypertension is the opposite and more worrying pattern: normal readings in the office but high readings in daily life. About one in five people with prehypertensive office readings had masked hypertension, and that figure climbed to nearly 28% among those with systolic readings between 130 and 140 in the office. Because these individuals look fine during checkups, the condition often goes undetected. Home monitoring is the simplest way to catch either pattern, which is one reason many clinicians now recommend tracking your numbers outside the office over several days rather than relying on a single visit.