blood pressure
Meaning of blood pressure: Pressure is never a fixed number, moving through the day with activity, sleep, posture and stress, which is why single readings mislead.
Definition of blood pressure
Blood pressure is the force circulating blood exerts against the walls of the arteries as the heart pumps. It is written as two numbers: the systolic pressure, reached as the heart contracts, over the diastolic pressure, the level it falls to between beats while the heart refills. Both are reported in millimeters of mercury.
What do the two numbers mean?
| Systolic, the upper number | Diastolic, the lower number | |
|---|---|---|
| When it occurs | As the heart contracts and ejects blood | Between beats, while the heart refills |
| What it reflects | The force of ejection and the stiffness of the large arteries | Resistance in the small arteries and the tone of their walls |
| Change with age | Tends to rise as arteries stiffen | Often levels off or falls in later life |
The gap between the two is the pulse pressure, and it widens as large arteries lose their elasticity. Pressure itself is the product of two things working together: how much blood the heart pumps out each minute, and how much resistance the blood vessels offer to that flow. Anything altering either alters the reading, and small arteries can narrow or widen within seconds, which is how the body adjusts pressure from moment to moment.
Why does it vary so much?
Blood pressure is a moving quantity rather than a fixed property. It falls during sleep and climbs before waking. It rises with physical effort, with pain, with cold, with a full bladder, with caffeine and nicotine, and with anxiety, including the anxiety of being measured. It shifts with posture, dropping briefly on standing until reflexes correct it. Recent exercise, a recent meal, talking during the measurement, an unsupported arm, a cuff of the wrong size or crossed legs will each move a reading measurably. For all these reasons an assessment rests on repeated measurements taken under consistent conditions, often including readings taken away from a clinical setting, rather than on any single result.
What happens when it stays high or low?
Persistently raised pressure, or high blood pressure, forces the heart to work against greater resistance and damages artery walls across years, which is why it is associated with heart attack, stroke, kidney disease and vision loss. It usually causes no symptoms whatever, and that silence is exactly why it is measured routinely rather than waited for. Pressure that runs too low matters in a different way, reducing the flow reaching the brain and other organs, which can cause lightheadedness or fainting. What counts as too low depends heavily on the individual and the circumstances, since a reading that is unremarkable for one person may represent a substantial fall for another.
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Used in a sentence
The nurse recorded the patient's blood pressure at each visit throughout the treatment course.
Good to know
A single blood pressure reading can be temporarily elevated by anxiety or recent activity, which is why repeated measurements over time provide a more reliable picture than one isolated reading.