What do the two numbers in a blood pressure reading mean?

The top number is the highest pressure in your arteries during a heartbeat, and the bottom number is the lowest pressure between beats. They are called systolic and diastolic pressure. A reading of 120/80 means 120 millimetres of mercury at the peak and 80 at the low point. Blood pressure is the force of circulating blood against the walls of your blood vessels, and most of it comes from the heart pumping.

Why two numbers? Because the heart does not push blood steadily. Each time the ventricles squeeze, pressure in the arteries surges to its maximum. Between beats, while the heart refills, it falls to a minimum. A single number would hide that rhythm, so doctors record both. The unit is millimetres of mercury because the traditional instrument, the sphygmomanometer, measured pressure by the height of a column of mercury. When people say blood pressure without more detail, they mean the pressure in the brachial artery of the upper arm, where it is most commonly measured.

What is normal? The American Heart Association and American College of Cardiology describe a normal resting adult reading as under 120/80, and guidelines put the lower bound around 90/60, though organisations draw the lines slightly differently, and low pressure is usually not harmful unless it causes symptoms. Pressure changes through the day with activity and emotion. Over years, though, pressure that is consistently too high is called hypertension, and it is a risk factor for stroke, heart disease and kidney failure.

Measuring it is a surprisingly short history. The first published measurement was made in 1733 by the English clergyman Stephen Hales. The arm cuff that turned high blood pressure into a recognised clinical condition came in 1896, and in 1905 Nikolai Korotkoff worked out how to listen with a stethoscope to read both numbers.