Blood pressure is the force your blood exerts against the walls of your arteries as it moves through your body. It’s measured with two numbers, like 120/80, representing the pressure during two distinct moments in each heartbeat. Understanding what those numbers mean, what’s normal, and what affects your reading can help you make sense of one of the most common measurements in medicine.
What the Two Numbers Mean
Every blood pressure reading has a top number and a bottom number. The top number, called systolic pressure, measures the force in your arteries when your heart contracts and pushes blood out. The bottom number, called diastolic pressure, measures the force when your heart relaxes between beats. A reading of 120/80 means your arteries experience 120 units of pressure during a heartbeat and 80 units during the pause.
Those units are millimeters of mercury, written as mmHg, a holdover from the original devices that measured pressure using a column of liquid mercury. Modern devices are digital, but the unit stuck.
Blood Pressure Categories
The American Heart Association breaks blood pressure into five categories:
- Normal: below 120/80 mmHg
- Elevated: systolic 120 to 129, with diastolic still below 80
- High blood pressure, Stage 1: systolic 130 to 139, or diastolic 80 to 89
- High blood pressure, Stage 2: systolic 140 to 179, or diastolic 90 to 119
- Severe hypertension: systolic 180 or higher, or diastolic 120 or higher
Notice that the “elevated” category only involves the top number. You can have a perfectly normal diastolic reading and still be in the elevated range if your systolic pressure sits between 120 and 129. This category is essentially a warning zone, signaling that full hypertension may develop without lifestyle changes.
How Your Body Controls Blood Pressure
Your blood pressure isn’t a fixed number. It shifts constantly based on signals from your kidneys, blood vessels, brain, and adrenal glands, all working together to keep blood flowing at the right force.
The kidneys play a central role. When blood pressure drops or sodium levels fall, your kidneys release an enzyme that triggers a chain reaction, ultimately producing a hormone that tightens blood vessels and tells the kidneys to hold on to more sodium and water. More fluid in the system means more pressure. When you eat a lot of salt and pressure rises, this system dials back, letting the kidneys release more sodium and water to bring things down.
Your nervous system handles more immediate adjustments. When you stand up suddenly, exercise, or feel stressed, your brain signals blood vessels to constrict and your heart to beat faster. This is why your blood pressure can spike during a stressful conversation and settle back down minutes later.
Average Readings by Age and Sex
Blood pressure tends to rise with age, and the pattern differs slightly between men and women. Average readings for adults:
- Ages 18 to 39: 110/68 for women, 119/70 for men
- Ages 40 to 59: 122/74 for women, 124/77 for men
- Age 60 and older: 139/68 for women, 133/69 for men
In children and teens, normal readings are much lower. A newborn’s systolic pressure can be as low as 60 mmHg, gradually climbing through childhood. By adolescence, typical systolic readings range from 112 to 128. Pediatric blood pressure is evaluated differently than adults, using percentiles based on age, sex, and height rather than fixed cutoffs.
Why Readings Can Be Wrong
Blood pressure is surprisingly easy to measure incorrectly. Small errors in technique can shift your reading by 10 to 20 points, enough to push a normal reading into the hypertension range or mask a genuinely high one. Data from the American College of Cardiology shows how much common mistakes can inflate systolic readings:
- Using a cuff that’s too small: can add 5 to 20 mmHg
- Not resting for a few minutes before the reading: adds 10 to 20 mmHg
- Talking or texting during measurement: adds 10 to 15 mmHg
- Having a full bladder: adds 10 to 15 mmHg
- Measuring over clothing: can shift the reading by 10 to 50 mmHg in either direction
- Crossing your legs or sitting on an exam table with legs dangling: adds 5 to 8 mmHg
- Drinking coffee beforehand: adds 5 to 8 mmHg
For the most accurate reading at home, sit in a chair with your feet flat on the floor, rest quietly for five minutes, use the right size cuff on bare skin, and keep your arm supported at heart level. Don’t talk during the measurement.
White Coat and Masked Hypertension
Some people consistently read high at the doctor’s office but normal at home. This is called white coat hypertension, and it affects roughly 14% of people. The stress of a medical visit temporarily raises their numbers. On its own, it’s generally considered less dangerous than true sustained hypertension, but it’s worth monitoring.
The opposite pattern is more concerning. About 10% of people show normal readings at the clinic but have elevated pressure the rest of the time. This is masked hypertension, and because it goes undetected in routine checkups, it can silently damage organs for years. Home blood pressure monitoring is the main way to catch it.
What Chronic High Blood Pressure Does
High blood pressure rarely causes obvious symptoms day to day, which is why it’s often called a silent condition. But over months and years, the extra force damages blood vessels and the organs they supply.
In the heart, sustained high pressure narrows and stiffens the arteries that feed heart muscle, a process called coronary artery disease. This can lead to chest pain, irregular heart rhythms, or a heart attack. The heart itself also changes shape over time. Because it has to pump harder against higher resistance, the left side of the heart thickens and eventually weakens. This is one of the most common pathways to heart failure.
In the brain, damaged blood vessels increase the risk of strokes and mini-strokes, where blood supply to part of the brain is briefly interrupted. Even without a full stroke, long-term high blood pressure is linked to cognitive decline as small vessels in the brain slowly deteriorate.
The kidneys are especially vulnerable. They filter blood through millions of tiny vessels, and sustained high pressure damages those filters over time, gradually reducing kidney function. This creates a vicious cycle: damaged kidneys are worse at regulating blood pressure, which accelerates the damage further.
How Blood Pressure Is Measured
Traditional blood pressure measurement uses an inflatable cuff around the upper arm. In the manual method, a clinician inflates the cuff until it temporarily stops blood flow, then slowly releases the pressure while listening through a stethoscope for the sound of blood resuming flow. The point where the sound begins is the systolic pressure; the point where it disappears is the diastolic pressure. This technique remains the gold standard.
Most home monitors and many clinic devices now use an automated approach. Instead of listening for sounds, these devices detect small oscillations in cuff pressure caused by the pulsing artery underneath. A microchip in the device calculates systolic and diastolic values from the pattern of those oscillations. They eliminate the human error that comes with manual readings, though they need to be validated for accuracy and used with the correct cuff size to produce reliable numbers.