Blood pressure measurement

Written by: 
Dr Roger Henderson

This page offers you the essentials for understanding what blood pressure is, and how and why it is measured.

Blood pressure is the pressure (force) that circulates bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. from the heart, around the body and back to the heart.

The heart creates this pressure when it contracts, when it squeezes into a smaller volume to push bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. out - either to the lungs to collect fresh oxygen or out to the arteries that carry oxygen-rich bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. to the rest of the body.

When the heart relaxes after a heartbeat, it fills with bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. again, either back from the lungs or from the rest of the body.

This cycle of contraction and relaxation is repeated, usually between 60 and 80 times a minute. This figure is known as the pulse rate, or heart rate.

When the heart squeezes, the blood pressure in the body's arteries is greatest. This is called the 'systolic' pressure. The lower blood pressure when the heart relaxes is called the 'diastolic' pressure. The difference between these two pressures is called the 'pulse pressure'.

How blood pressure is measured

The traditional way of taking a bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid.-pressure measurement is for a health professional to:

  • Ask you to be seated, with your arm resting on a desk or other support
  • Wrap a rubber cuff around your upper arm
  • Pump up this cuff until it feels tight; this is painless but may be uncomfortable and can occasionally make the arm feel warm or 'tingly'
  • Using a stethoscope, listen to the pulse at the elbow, and when a pulse can no longer be heard, gradually release the cuff pressure. When the pulse can be heard again, the reading on a measuring scale attached to the cuff is noted - this is the systolic bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. pressure
  • Continue to release the cuff pressure until again no pulse can be heard - this reading is the diastolic bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. pressure.

In many places, electronic blood pressure monitors are now used instead of the traditional type. Rather than needing a stethoscope to listen to the sounds, the blood pressure cuff is attached to a device that automatically measures the pressures and displays them on a digital scale.

Electronic monitors may give a more accurate blood pressure reading and can also be used at home if people wish to monitor their own bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. pressure.

The reading

Blood pressures are measured in millimetres of mercury, symbolised by 'mmHg'. (This traditional measure of pressure refers to the height of a column of mercury that balances the pressure, as seen in a traditional mercury barometer; 'Hg' is the chemical symbol for mercury.)

Systolic and diastolic bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. pressures are described as, for example, '120 over 80'. A normal systolic blood pressure is typically 130 or under, and a normal diastolic blood pressure is 80 or under.

Persistently raised readings may indicate high bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. pressure, also called hypertensionHigh blood pressure.. However, blood pressure varies normally through the day and under the influence of other factors such as exercise, stressRelating to injury or concern., alcohol, caffeine and medication.