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When doctors take your blood pressure, they're measuring how hard your blood pushes against the walls of your arteries and veins as it moves around your body. Your blood pressure may rise and fall slightly throughout the day and night. But when it stays up, it's called high blood pressure.

If you have high blood pressure you probably won't feel ill or have any symptoms. But high blood pressure increases your chance of having a heart attack or a stroke. It can also lead to kidney disease and heart failure.

The good news is that there are many things you can do to lower your blood pressure. You can change your lifestyle, for example, or take medications.

Your doctor may call high blood pressure hypertension.

• High blood pressure is common, but it usually doesn't cause any symptoms so many people don't know they have it.
• The only way you can find out if your blood pressure is high is to have it checked.
• High blood pressure that is not properly treated after many years can lead to a heart attack, heart failure, a stroke and kidney disease.
• Taking medication or making changes to your lifestyle, such as eating less salt, exercising and losing weight, can help to keep your blood pressure down.

To understand why you have high blood pressure and how treatments work, it helps to know a bit about how blood flows through your body and what controls the pressure.

• Your blood is pumped around your body by your heart.
• It travels through a system of blood vessels called arteries and veins.
• Blood leaves your heart through arteries. This blood carries oxygen and food to all the cells in your body.
• Blood is then carried back to your heart through your veins. When blood is returned to your heart, it makes a detour through your lungs to pick up oxygen.

You need a certain amount of pressure to keep blood moving through your blood vessels. When your heart beats it pushes blood out and around your body. This causes the pressure to rise. When the heart relaxes and fills with blood, the pressure drops.

There are two parts to the blood pressure reading.

• The first number is the systolic pressure. It measures the pressure of the blood when your heart pumps.
• The second number is the diastolic pressure. This is the pressure measured when your heart relaxes and fills up with blood.

There are three main things that help control blood pressure:¹

• How fast and forcefully your heart pumps blood around your body
• How open and flexible your arteries are
• How much blood you have going around your body.

These three things are controlled by:¹

• Nerves going to the heart and arteries
• Muscles around the blood vessels
• Chemicals in the blood itself.

The amount of pressure your blood needs to be under to keep flowing varies slightly from one minute to the next. It depends on how much oxygen and food the various organs in your body need. When you exercise, your muscles need lots of oxygen and your heart has to pump faster, so your blood pressure may rise.

If your body senses that your blood pressure is too low to meet its demands, the brain sends messages through nerves to the heart to tell it to pump faster and harder.

This pushes more blood out and around the body with each beat, and the blood surges through the arteries under higher pressure.

If your body senses that your blood pressure is too high, the brain sends messages through another nerve to slow down the heart. Your body also releases chemicals to open up (dilate) the arteries so blood can flow through easily without putting extra pressure on the walls of your blood vessels. Your blood pressure then drops.

The system works a bit like a garden hose. If you turn on the tap full, the water shoots out through the narrow hose under high pressure. As you turn off the tap the pressure drops.

Your kidneys control how much fluid is in your blood vessels, so they have a role in controlling blood pressure. To learn more, see How your kidneys help control your blood pressure.

It's normal for your blood pressure to rise and fall throughout the day. But if it stays high for a long time (usually for at least four months), then it's called high blood pressure.

Usually your doctor will say you have high blood pressure if your blood pressure reading has been at least 140 (top number) or over 90 (bottom number) on at least two occasions.² This is written as 140/90.

To learn more, see What the numbers tell you.

No one knows exactly what goes on in your body to cause high blood pressure. But researchers think that you get it when the balance of certain chemicals in the blood is upset. These chemicals control how fast your heart beats, how open the arteries are and how much blood there is in your blood vessels. If the balance of chemicals is upset:

• Your heart may beat too fast
• Your arteries and veins may become thicker and narrower
• The amount of blood in your arteries and veins may go up.

All these things can make your blood pressure rise.¹

Your doctor probably won't be able to tell you why you have high blood pressure. More than 9 in 10 people with high blood pressure never know the exact cause.

But researchers do know that some people are more likely to get high blood pressure than others. Doctors call the things that increase your chance of getting a condition risk factors. The most well-known risk factors for getting high blood pressure are:

• Getting older
• Having a relative with high blood pressure
• Being pregnant if you are a woman
• Belonging to certain ethnic groups
• Being overweight
• Not exercising
• Smoking
• Eating and drinking the wrong things
• Stress.

To find out more, see Risk factors explained.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.