Coronary heart disease (CHD) is usually caused by a build-up of fatty deposits on the walls of the arteries around the heart (coronary arteries).

The fatty deposits, called atheroma, are made up of cholesterol and other waste substances.

The build-up of atheroma on the walls of the coronary arteries makes the arteries narrower, restricting the flow of blood to the heart muscle. This process is called atherosclerosis. Your risk of developing atherosclerosis is significantly increased if you:

• smoke
• have high blood pressure (hypertension)
• have a high blood cholesterol level
• don't take regular exercise
• have diabetes

Other risk factors for developing atherosclerosis include being obese or overweight and having a family history of CHD (the risk is increased if you have a male relative under the age of 55, or a female relative under 65, with CHD). 

Cholesterol

Cholesterol is a fat made by the liver from the saturated fat in your diet. Cholesterol is essential for healthy cells, but too much in the blood can lead to CHD.

Cholesterol is carried in the bloodstream by molecules called lipoproteins. There are several different types of lipoproteins, but two of the main ones are low-density lipoproteins (LDL) and high-density lipoproteins (HDL).

LDL, often referred to as "bad cholesterol", takes cholesterol from the liver and delivers it to cells. LDL cholesterol tends to build up on the walls of the coronary arteries, increasing your risk of heart disease.

HDL, often referred to as "good cholesterol", carries cholesterol away from the cells and back to the liver, where it's broken down or passed from the body as a waste product.

In the UK, the government recommends that total cholesterol levels should be:

• 5mmol/L or less for healthy adults
• 4mmol/L or less for those at high risk

Levels of low-density lipoprotein (LDL) should be:

• 3mmol/L or less for healthy adults
• 2mmol/L or less for those at high risk

An ideal level of high-density lipoprotein (HDL) is above 1mmol/L. A lower level of HDL can increase your risk of heart disease.

Your ratio of total cholesterol to HDL may also be calculated. This is your total cholesterol level divided by your HDL level. Generally, this ratio should be below four because a higher ratio increases your risk of heart disease.

Read more about high cholesterol. 

High blood pressure

High blood pressure (hypertension) puts a strain on your heart and can lead to CHD.

Blood pressure is measured at two points during the blood circulation cycle. Your:

• systolic pressure - is a measure of your blood pressure as the heart contracts and pumps blood out
• diastolic pressure - is a measure of your blood pressure when your heart is relaxed and filling up with blood

Blood pressure is measured in terms of millimetres of mercury (mmHg).

When you have your blood pressure measured, the systolic pressure is the first, higher number to be recorded. The diastolic pressure is the second, lower number to be recorded.

High blood pressure is defined as a systolic pressure of 140mmHg or more, or a diastolic pressure of 90mmHg or more.

Read more about high blood pressure.

Blood Pressure UK have also produced a useful guide explaining high, low and normal blood pressure readings.   

Smoking

Smoking is a major risk factor for coronary heart disease. Both nicotine and carbon monoxide (from the smoke) put a strain on the heart by making it work faster. They also increase your risk of blood clots.

Other chemicals in cigarette smoke can damage the lining of your coronary arteries, leading to furring of the arteries. If you smoke, you increase your risk of developing heart disease by 24%.

Read more about how to stop smoking and stop smoking treatments.

Diabetes

A high blood sugar level doesn't directly increase your risk of developing CHD, but it may lead to diabetes, which can more than double your risk of developing CHD.

Diabetes can lead to CHD because it may cause the lining of blood vessels to become thicker, which can restrict blood flow.

Thrombosis

A thrombosis is a blood clot in a vein or artery. If a thrombosis occurs in a coronary artery (coronary thrombosis), it will cause the artery to narrow, preventing the blood supply from reaching the heart muscle. This increases your chance of having a heart attack.

A coronary thrombosis usually happens at the same place the atherosclerosis is forming.