Blood Pressure Explained: What the Two Numbers Mean
Blood pressure is the force of blood against artery walls, written as systolic over diastolic in mmHg. Learn what each number measures and how readings are rated.
Blood pressure is the force that circulating blood exerts on the walls of the arteries. It is written as two numbers, such as 120/80 mmHg, measured in millimetres of mercury. The top number, systolic pressure, is the peak reached with each heartbeat. The bottom number, diastolic pressure, is the lowest level between beats.
What does each number measure?
The heart is a muscular pump with four chambers. When the ventricles contract, they push blood into the aorta, the body's main artery, and pressure in the arteries rises to its peak. That peak is the systolic reading. When the ventricles relax, the aortic valve shuts and pressure falls until the next contraction. The lowest point is the diastolic reading. The pumping sequence and the valves that open and close it are described in how the heart works.
Arteries are not rigid pipes. Their walls are elastic, so each surge of blood stretches them, and they recoil between beats. That recoil keeps blood moving while the heart refills. The gap between the two numbers, called pulse pressure, partly reflects how much blood each beat adds and how stiff the arteries are.
How does resistance change the reading?
Pressure depends on two things: how much blood the heart pumps each minute, and how hard it is for blood to pass through the smallest arteries, called arterioles. Arterioles work like narrow gates. Their walls are muscular, so when they tighten the gate narrows and pressure builds upstream. When they relax, pressure drops.
The body adjusts these settings all the time. Nerves and hormones change the heart rate and the tone of the arterioles within seconds to minutes. The kidneys adjust blood volume over hours to days, which depends partly on water balance. Keeping pressure in a working range is a clear case of homeostasis. Pressure is therefore not a fixed property of a person. It rises with effort, stress and standing up, and it normally falls during sleep.
How is blood pressure measured and classified?
Clinicians usually measure pressure with an inflatable cuff around the upper arm. The cuff is pumped above the systolic pressure, then released slowly while the examiner listens over the artery below it. The first tapping sound marks the systolic pressure, and the point where the sounds disappear marks the diastolic pressure. Cuff size, posture, rest and arm position all change the result. Readings taken in a clinic can also run higher than readings taken at home, a difference often called the white-coat effect.
A single reading varies from hour to hour, so guidelines rely on several readings taken on separate occasions. The table shows the adult categories in the 2017 guidelines of the American College of Cardiology and the American Heart Association. European guidelines use somewhat different cut-offs. When the two numbers fall into different categories, the higher category applies.
| Category | Systolic (mmHg) | Diastolic (mmHg) |
|---|---|---|
| Normal | Below 120 | Below 80 |
| Elevated | 120 to 129 | Below 80 |
| Stage 1 hypertension | 130 to 139 | 80 to 89 |
| Stage 2 hypertension | 140 or higher | 90 or higher |
| Hypertensive crisis | Above 180 | Above 120 |
A reading with systolic pressure above 180 and/or diastolic pressure above 120 is in the crisis range, and it needs prompt medical attention. Home monitors can be useful when they are validated and used with correct technique.
Why does high blood pressure matter?
High blood pressure often causes no symptoms, which is why it is sometimes called a silent condition. Over years, the constant load on arterial walls damages them, and the heart muscle thickens as it pushes against higher resistance. High pressure is one of the leading modifiable risk factors for stroke, heart attack, heart failure and kidney disease.
Low readings can also cause problems. A pressure that falls too far may cause dizziness or fainting, especially on standing up quickly. Diet, physical activity, alcohol, salt intake and body weight all influence readings over time. A single number is rarely a diagnosis on its own. Anyone with readings in the higher categories, or with symptoms, should talk to a clinician, who weighs several readings together with other risk factors.
Tags
cardiovascular circulation heart medicine physiology
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