What blood pressure is
Blood pressure is the force that circulating blood exerts against the walls of the arteries. The heart pumps blood out into the arteries, and the pressure inside them is what keeps blood moving to the brain, muscles, kidneys and other organs. Some pressure is essential. The question is whether it is within a range the body can tolerate comfortably over many years.
Two main things set the level: how much blood the heart pushes out and how much resistance the blood vessels offer. When small arteries are narrowed or stiff, resistance rises and so does pressure. The kidneys, nerves and hormones all help adjust it from moment to moment.
The two numbers
A reading is written as one number over another, for example 120/80, and spoken as “120 over 80.” The unit is millimeters of mercury, abbreviated mmHg, a historical unit based on the mercury columns used in older devices.
The top number: systolic pressure. This is the pressure in the arteries when the heart contracts and pushes blood out. It is the peak of the cycle.
The bottom number: diastolic pressure. This is the pressure in the arteries when the heart relaxes between beats and refills with blood. It is the lowest point of the cycle.
Because the heart goes through this cycle with every heartbeat, pressure is not constant. It rises and falls with each beat, and the two numbers capture the high and low points. The gap between them is sometimes called the pulse pressure.
The numbers in the example above are used only to show how a reading is written. They are not advice about what any individual’s reading should be.
How it is measured
Most measurements use an inflatable cuff around the upper arm. The cuff is inflated until it briefly squeezes blood flow shut, then slowly released. In the traditional method, a clinician listens with a stethoscope for the first sounds of blood flowing again, which marks the systolic value, and for the point where the sounds disappear, which marks diastolic. Automatic devices detect small pressure oscillations in the cuff instead and calculate the numbers.
Measurements can be affected by many things:
- cuff size and position, since a cuff that is too small or too big can give inaccurate numbers
- body position, arm support and whether the person has been sitting quietly
- recent caffeine, nicotine, exercise or a full bladder
- stress or nervousness, including the well-known effect where readings are higher in a clinic than at home, sometimes called white-coat effect
For this reason, clinicians often take more than one reading, and sometimes ask people to measure at home or wear a monitor that records over 24 hours.
Why readings vary
Blood pressure is naturally dynamic. It tends to be lower during sleep, rise on waking, and change with activity, emotion, pain, temperature and meals. A single reading is a snapshot. A high number on one occasion does not mean a person has high blood pressure, and a normal one does not rule it out. Patterns over time matter more than any single measurement.
High and low blood pressure
When pressure stays high over a long time, a condition known as hypertension, it puts extra strain on the heart and blood vessels. It is associated with increased risk of heart disease, stroke, kidney disease and other problems. It often causes no symptoms, so it is sometimes called a silent condition, and that is why checking is routine in many health visits.
Pressure that is lower than usual can cause dizziness or fainting in some people, though some people naturally run low and feel fine. Context matters: a sudden drop with symptoms is different from a stable low reading without them.
Health organizations publish categories and thresholds to classify readings, such as normal, elevated and different stages of high blood pressure. These cutoffs differ somewhat between countries and guidelines, and they can change over time, so this article does not give exact boundaries. A doctor will interpret readings together with age, other health conditions and overall cardiovascular risk, including factors like cholesterol levels and diabetes.
What influences blood pressure over the long term
Many factors contribute, and some cannot be changed:
- age, since arteries tend to stiffen over time
- family history and genetics
- body weight
- salt intake and overall diet
- physical activity levels
- alcohol and tobacco use
- sleep problems, and chronic stress effects
- certain medical conditions and medicines
General health habits, such as regular exercise and a balanced diet, are widely discussed in connection with blood pressure, but their effect differs from person to person. When lifestyle measures are not enough, doctors may prescribe medicines, and the choice depends on the individual.
Keeping the numbers in context
Blood pressure readings are tools for estimating risk, not diagnoses on their own. Two people with the same numbers may have different needs depending on the rest of their health. Anyone who is unsure what their readings mean, or who has symptoms such as severe headache, chest pain or sudden weakness, should contact a health professional or emergency services as appropriate. This article is general background, not medical advice.