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, written as one over the other (like 120/80), and a normal reading falls below 120/80 mm Hg. Understanding what those numbers mean, what pushes them too high or too low, and why it matters can help you make sense of one of the most common measurements in medicine.
What the Two Numbers Mean
Every time your heart beats, it pushes blood into arteries that carry it to your organs and tissues. That pumping action creates pressure in two phases, and each one gets its own number in your reading.
The top number is systolic pressure. It measures the force inside your arteries at the moment your heart contracts and pushes blood out. The bottom number is diastolic pressure. It measures the force when your heart relaxes between beats, refilling with blood before the next contraction. Systolic is always the higher number because the contraction phase generates more force.
Both numbers are reported in millimeters of mercury (mm Hg), a unit left over from the original mercury-column devices used to measure pressure. When someone says your blood pressure is “120 over 80,” they mean your systolic pressure is 120 mm Hg and your diastolic is 80 mm Hg.
Blood Pressure Categories
The 2025 guidelines from the American Heart Association and American College of Cardiology divide adult blood pressure into four categories:
- 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, the higher category is the one that applies. So a reading of 135/75 would count as stage 1 hypertension because of the systolic number, even though the diastolic number is normal.
On the other end, a reading below 90/60 mm Hg is considered low blood pressure, or hypotension. Not everyone with low readings has symptoms, but when pressure drops enough to reduce blood flow, it can cause dizziness, blurred vision, fainting, nausea, fatigue, and confusion.
Why High Blood Pressure Is Called the Silent Killer
High blood pressure rarely causes noticeable symptoms. You can carry dangerously elevated readings for years without feeling anything unusual. The damage it causes to your internal organs builds quietly, and by the time symptoms appear, serious harm may already be done. That’s why routine checks matter so much: the only reliable way to know your blood pressure is to measure it.
Between 85% and 95% of high blood pressure cases are classified as primary (or essential) hypertension, meaning there’s no single identifiable cause. It develops gradually from a combination of genetics, aging, diet, weight, and activity level. The remaining cases are secondary hypertension, caused by an underlying condition like kidney artery narrowing or adrenal gland problems. Secondary hypertension sometimes comes on more suddenly and runs higher than primary.
Damage High Blood Pressure Can Cause
Persistently high pressure forces your heart to work harder than it should. Over time, this extra workload can thicken the heart muscle, making it stiff and less efficient. Eventually the heart may weaken to the point where it can’t pump effectively, a condition known as heart failure. High pressure also damages coronary arteries, narrowing them and reducing blood flow to the heart itself, which can lead to chest pain or a heart attack.
Your kidneys filter waste from your blood through a dense network of tiny blood vessels. High pressure damages those vessels, gradually reducing the kidneys’ filtering ability. Kidney failure from long-term hypertension is common enough that it remains one of the leading reasons people need dialysis or a transplant.
The brain is vulnerable too. Hardened or narrowed arteries can block blood supply, causing a stroke or a transient ischemic attack (a temporary blockage sometimes called a ministroke that serves as a warning sign). Chronic high pressure has also been linked to mild cognitive impairment, affecting memory and thinking over time. The tiny blood vessels in your eyes face similar risks: damage to the retina can cause bleeding, blurred vision, and in severe cases, complete vision loss.
How to Get an Accurate Reading
Blood pressure fluctuates throughout the day based on activity, stress, caffeine, and even posture, so how you measure matters. Small mistakes can throw off results significantly. Resting in a quiet room for at least five minutes before measuring makes a real difference. Sit in a chair with your back supported and your feet flat on the floor.
Arm position is surprisingly important. Letting your arm hang at your side or rest in your lap, below heart level, can inflate the reading by anywhere from 4 to 23 mm Hg. Instead, support your arm on a table or armrest so the cuff sits at the same height as your heart. Don’t talk during the measurement, and avoid caffeine or exercise for 30 minutes beforehand. Taking two or three readings a minute apart and averaging them gives a more reliable picture than a single measurement.
What Affects Your Numbers
Some factors that influence blood pressure are outside your control. Age raises it: arteries gradually stiffen over the decades, increasing resistance to blood flow. Family history plays a role, and certain ethnic groups face higher average rates. Men tend to develop high blood pressure at younger ages than women, though the gap narrows after menopause.
The factors you can change tend to have a meaningful impact. Excess sodium causes your body to retain water, increasing blood volume and pressure. The World Health Organization recommends adults consume less than 2,000 mg of sodium per day, roughly the amount in just under a teaspoon of table salt. Most people in Western diets consume well above that, largely from processed and restaurant foods rather than the salt shaker.
Regular physical activity, maintaining a healthy weight, limiting alcohol, managing stress, and eating a diet rich in fruits, vegetables, and whole grains all help keep pressure in a healthier range. For people already in the elevated or stage 1 category, these lifestyle changes are often the first line of action before medication enters the picture. In stage 2 hypertension, lifestyle changes typically accompany medication rather than replace it.
Low Blood Pressure: When It’s a Problem
While most of the public health focus is on hypertension, pressure that’s too low causes its own set of problems. A reading below 90/60 mm Hg is the standard threshold. Some people naturally run low without any issues, but when blood pressure drops enough to starve your brain and organs of adequate blood flow, symptoms show up fast: lightheadedness, fainting, shallow breathing, fatigue, and difficulty concentrating.
Common causes include dehydration, blood loss, certain medications, prolonged bed rest, and sudden position changes (standing up too quickly). Severe infections and allergic reactions can also cause dangerous drops in pressure. Unlike hypertension, low blood pressure almost always announces itself through symptoms, making it less likely to go unnoticed.