Systolic and diastolic blood pressure

Lesley Dobson / 13 March 2017

Knowing your blood pressure numbers is important, but what do the two numbers in a blood pressure reading mean?



What do the blood pressure numbers mean?

When you have your blood pressure checked at your doctor’s surgery, health clinic, or hospital, the doctor or nurse taking your blood pressure is looking for two readings. These are the systolic blood pressure number, and the diastolic blood pressure number.

The numbers tell your doctor or nurse – and you, if you’re taking your blood pressure at home – the force at which your blood is travelling through your body. These measurements show whether your blood pressure is low, ‘normal’ – in other words, at a healthy level - or too high.

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What does the Systolic Blood Pressure number mean?

The first figure in blood pressure reading is the systolic pressure. This tells you the pressure recorded when your heart beats and pushes the blood out of its chambers.

So a blood pressure reading of 120/80 mmHg means that your systolic pressure has been recorded as 120, and your diastolic pressure, is 80. Your doctor or nurse may talk about your blood pressure as 120 over 80, and you may see this written as 120/80. These figures are just examples of what your reading might be, but they are regarded generally, as being at the upper end of the normal, or ideal blood pressure.

What is high blood pressure?

What does the Diastolic Blood Pressure number mean?

The second reading - 80 in this example - is the diastolic pressure. This shows the pressure recorded when your heart is between beats, in a resting state. Because your heart isn’t working hard to push your blood into your arteries and veins, and around your body, this resting figure should always be lower than the systolic reading.

How is blood pressure measured?

There are two devices that are used to measure blood pressure. They work in similar ways, and should produce the same blood readings.

The equipment used for reading blood pressure is known as a sphygmomanometer. You will almost certainly have seen one of these in doctor’s surgeries in the past, and have had your blood pressure readings taken with one of these.

The doctor (or nurse) taking your blood pressure will ask you to sit quietly and try to relax for a short while before they take your blood pressure. When they are ready they will wrap a cuff around your upper arm. (Usually they only measure the pressure in one arm, but they may do both arms if they feel this is necessary.)

Once the cuff is in place, they will pump air into the cuff so that it inflates. It will feel as if your arm is being squeezed.

The old-fashioned models are pumped up manually, using a bulb on the end of a rubber hose. This pumps air into the cuff around your arm, and allows the monitor to take a reading of your blood pressure. The modern models are plugged into an electrical socket, and pumped up electronically.

The doctor will then check the reading on the dial or screen, to see what your blood pressure is. In some cases, if your blood pressure is high, for instance, they might ask you to wait for a short while and then take your blood pressure measurement again.

Your blood pressure doesn’t stay the same all the time. It can go up and down during the course of the day – if you’ve been doing exercise, or are feeling worried. Just going to your GP’s can make it rise, as can any stressful event or environment.

What causes high blood pressure?

How often should I get my blood pressure checked?

As we get older our blood pressure tends to go up, so it’s a good idea to have it checked regularly.

If your blood pressure readings are consistently higher than they should be, your GP or nurse will want to check your blood pressure more often than they would for someone with a lower blood pressure.

However, we don’t all have the same blood pressure. What is normal for some people may be a low blood pressure for others. Your medical team is likely to take this and other factors – such as age and weight – into account when deciding whether you need medical treatment.

Your GP may ask you to go to their surgery a couple of times a week to have your blood pressure taken. This will help to show whether your blood pressure is consistently high, and whether you should be taking medication to bring it down. Your GP or nurse may also suggest home checks.

Read more on how to monitor your blood pressure

Can I check my blood pressure at home?

Doing blood pressure checks at home can be a good way of finding out what your blood pressure is in different situations. It may be high if you’re worried about something, lower when you’re watching television, and lower still if you’re doing something you find relaxing, like gardening or listening to music.

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Some GP surgeries have electronic blood pressure monitors that they let patients take home for 24 hours (or longer), so that you can check your blood pressure at different times throughout the day.

You can buy easy to use blood pressure monitors, if you’d like to check your blood pressure at home. These can be useful for finding out what activities and situations affect your blood pressure. So listening to soothing music, or doing a jigsaw puzzle might bring your blood pressure down, while watching the news, or checking interest rates on your savings might push your blood pressure up.

If you do decide to buy a home blood pressure monitor, make sure that you choose one that has been ‘clinically validated’ by the British Hypertension Society. This means that it has been tested for use at home, and has been found to give reliable results.

Digital blood pressure monitors are probably the easiest to use, as they are automatic, so there are no complicated instructions to follow. Make sure that you choose a monitor that has a cuff that will fit around your upper arm, with a little space to spare, otherwise you may not get an accurate blood pressure reading. The British Heart Foundation suggest enough room for two fingertips to slide underneath.

The British Hypertension Society has a table of suitable blood pressure monitors designed to be used at home, which may help you choose a home monitor. You could also ask your doctor or nurse for their advice at your next appointment. Visit the British Hypertension Society table for more information.

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