Cardiovascular system

Published by Bupa's Health Information Team, February 2011.

This factsheet is for people who would like information on how the cardiovascular system works.

The cardiovascular system supplies oxygen from the lungs to the tissues around the body. It also transports carbon dioxide, a waste product, from the body to the lungs. Breathing out removes carbon dioxide from the body.

How the heart works

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About the cardiovascular system

Your cardiovascular system is your:

• heart
• blood vessels – arteries, veins and capillaries (small blood vessels)
• blood

How does your cardiovascular system work?

Oxygen makes up about a fifth of the atmosphere. You breathe air through your mouth and nose and it travels to your lungs. Oxygen from the air is absorbed into your bloodstream through your lungs. Your heart then pumps oxygen-rich ('oxygenated') blood through a network of blood vessels – the arteries – to tissues including your organs, muscles and nerves, all around your body.

When blood reaches the capillaries in your tissues it releases oxygen, which cells use to make energy. These cells release waste products, such as carbon dioxide and water, which your blood absorbs and carries away.

The used (or 'deoxygenated') blood then travels along your veins and back towards your heart. Your heart pumps the deoxygenated blood back to your lungs, where it absorbs fresh oxygen, and the cycle starts again.

The heart

Your heart is roughly the size of a clenched fist and weighs about 300g. It lies just to the left in your chest, surrounded by a protective membrane called the pericardium.

Your heart is a pump, divided into left and right sides. It has walls, made of muscle, which squeeze (contract) to pump blood into the blood vessels and around your body. You have around 8 pints of blood in your body, and in an average day your heart beats 100,000 times to keep the blood moving around your body.

Your veins deliver deoxygenated blood to the right side of your heart. Your heart pumps this blood back to your lungs, where it absorbs more oxygen. This oxygenated blood then returns to the left side of your heart, which pumps it out to the rest of your body through the arteries. The muscle on the left side of your heart is slightly larger because it has more work to do than the right: the right side only pumps blood to your lungs, the left side pumps blood around your body.

Each side of your heart is divided into an upper chamber called an atrium and a larger, lower chamber, called a ventricle. Blood flows from each atrium to the ventricle below, through a one-way valve.

The lungs

Your lungs are on either side of your heart in your chest (thorax) and consist of spongy tissue with a rich blood supply.

Your diaphragm is a sheet of muscle that separates your chest from your abdominal cavity and forms the floor of your thorax. Movement of your diaphragm as you breathe in makes your lungs inflate.

Air passes from your nose and mouth into your trachea (windpipe) and into each lung, through two airways called the bronchi. These divide into smaller airways, called bronchioles, which repeatedly divide and end in tiny sacs called alveoli. These are air sacs with walls just one cell thick. It's here that oxygen and carbon dioxide filter into and out of your blood. In this process, known as gaseous exchange, molecules of oxygen and carbon dioxide bind to the haemoglobin, a protein in your red blood cells.

There are about 300 million alveoli in each lung, which provide a vast surface area for gaseous exchange – around the size of a tennis court if it could be spread out.

In an average day, you breathe 10,000 litres of air in and out of your lungs.

Blood pressure

Blood carrying oxygen and nutrients is pumped around your body by your heart. The blood is under pressure as a result of the pumping action of your heart and the size and flexibility of your arteries. This blood pressure is an essential part of the way your body works.

When blood pressure is measured, the result is expressed as two numbers, such as 120/80mmHg (one hundred and twenty over eighty millimetres of mercury).

The first figure – the systolic blood pressure – is a measure of the pressure when your heart muscle is contracted and pumping blood. This is the maximum pressure in your blood vessels.

The second figure – the diastolic blood pressure – is the pressure between heart beats when your heart is resting and filling with blood. This is the minimum pressure in your blood vessels.

The lower your blood pressure, the better for your health, although very low blood pressure can make you feel dizzy or faint. Doctors recommend that blood pressure is kept below 140/85. If you have diabetes, kidney disease or cardiovascular disease, your blood pressure should be lower than this – ideally less than 130/80.

Your cardiovascular health

Your lifestyle plays an essential part in maintaining your long-term cardiovascular health. A healthy diet, moderate drinking, plenty of exercise, and not smoking can all help to maintain a healthy cardiovascular system.

For answers to frequently asked questions on this topic, see Common questions.

For sources and links to further information, see Resources.