One in five adults know they have high blood pressure, but there are many more who don't recognise the problem.

If your blood pressure stays high for a long time, it can greatly increase your risk of heart disease, strokes and kidney failure.

Blood pressure is created by the force of the heart pumping blood around your body. It's measured as two numbers. The higher number (known as "systolic") is the pressure in your blood vessels when your heart contracts. The lower number ("diastolic") is the pressure when your heart is relaxed between heartbeats.

Normal adult blood pressure is around 120/80 millimeters of mercury (mmHg) or lower, though it'll vary. Your blood pressure will be higher if you're stressed, or if you've been exercising a lot. It'll be higher in the afternoon than in the morning. It also increases as you get older and after consuming certain food and drink, like caffeine and salt.

How high is too high?

There's no magic number that means your blood pressure is dangerously high - only your doctor can tell you. But as your blood pressure rises you increase your risk of heart disease and other health conditions.

In general, a doctor will consider it elevated if it's consistently over 140/90. But elevated blood pressure, also called hypertension, doesn't necessarily mean you're at high risk of heart disease. And, conversely, your doctor might think you're at risk even if only one of the two blood pressure measurements is elevated.

In weighing up your risk, your doctor will consider other factors such as your age, gender, family history, weight and whether you are a smoker.

High blood pressure may be inherited. It can also be brought on by illness, pregnancy or certain medicines.