Updated 08 February 2018

What do the numbers mean?

When your doctor mumbles 120 over 80, or 160 over 100, what does it mean?

Blood pressure is created by your heart pushing the blood through your arteries and results in resistance created by the walls of your blood vessels. The play between the pressure generated by your heart and the resistance of the blood vessels determines your blood pressure level.

Your heart is an enormously powerful pump, which can force blood into openings too small to see.

Your blood pressure increases and decreases according to the needs placed on it. This enables you to play a vigorous game of tennis and then to get a good night’s sleep.

The terminology
If your blood pressure is recorded as 120/80, the number on top is the systolic pressure, and the bottom number the diastolic. It is measured in millimeters of mercury. 120/80mmHg also happens to be the optimal blood pressure

Systolic pressure is the pressure generated by each heartbeat. This occurs during the contraction of the heart muscle, which is called a systole.
Diastolic blood pressure is the pressure between the heartbeats when the heart is resting. Systolic pressure is obviously always higher than diastolic blood pressure.
Pulse pressure is the difference between the two readings.

If any of these are significantly elevated, it increases the risk for heart disease, stroke or kidney damage.

If your systolic pressure is 130 and your diastolic pressure is 90, it will be written as 130/90 or referred to as “one-thirty over ninety.”

Blood pressure works in the same way as putting your thumb over a hosepipe: the narrower the aperture, the higher the pressure. But pressure also decreases the further away from the heart it’s measured. So how do you get a uniform reading?

Blood pressure is measured using a sphygmomanometer. It consists of a cuff that’s wrapped around your arm, then inflated to put pressure on the arm and on the brachial artery that runs the length of the limb. This is linked to a column of mercury with precisely calibrated numbers.

Combining the pressure and listening to your heart rate gives your healthcare professional a clear idea of your blood pressure. Listening to the heartbeat while deflating the pumped up cuff, he or she watches the mercury fall. When the heartbeat becomes audible, it means the pressure is just not high enough to cut off the blood flow in your brachial artery. The point at which this happens is measured and is counted as your systolic pressure.

Releasing the pressure even more, your healthcare professional listens for when the heartbeat disappears again, which provides a reading of your diastolic pressure.

- (Mari Hudson, Health24, updated February 2012)



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Hypertension expert

Dr Jacomien de Villiers qualified as a specialist physician at the University of Pretoria in 1995. She worked at various clinics at the Department of Internal Medicine, Steve Biko Hospital, these include General Internal Medicine, Hypertension, Diabetes and Cardiology. She has run a private practice since 2001, as well as a consultant post at the Endocrine Clinic of Steve Biko Hospital.

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