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The cardiovascular system and how it 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 then removes carbon dioxide from the body.

About the cardiovascular system 

The cardiovascular system is your:

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

Cardiovascular diagram

How does your cardiovascular system work? 

When you breathe air through your mouth and nose it travels to your lungs. Oxygen from the air is absorbed into your bloodstream through your lungs. Your heart then pumps oxygenated (oxygen-rich) 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 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 a pump, 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.

The heart has muscular walls which squeeze (contract) to pump blood into the blood vessels and around your body. You have just under four litres of blood in your body, and in an average day your heart beats 100,000 times to keep the blood moving around your body.

The heart is divided into left and right sides. Your veins deliver deoxygenated blood to the right side, and 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. In an average day, you breathe 10,000 litres of air in and out of your lungs.

Your chest is separated from your abdominal cavity by a sheet of muscle called the diaphragm, which forms the floor of your thorax. When you breathe in, movement of your diaphragm 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 200 million alveoli in each lung, which provide a vast surface area for gaseous exchange. If it could be spread out, it’d be around the size of a tennis court.

Blood pressure

Blood pressure is the measure of the force of blood against the walls of the arteries as it’s pumped around your body. Two major factors determine blood pressure: the force and volume of the blood and the size and flexibility of the arteries.

When blood pressure is measured, the result is expressed as two figures, one over the other. The top or first figure is the systolic pressure, the bottom or the second is the diastolic:

  • Systolic blood pressure. This is a measure of the pressure in your arteries when your heart muscle is contracted and pumping blood
  • Diastolic blood pressure. This is the pressure between heart beats when your heart is resting between beats and filling with blood.

‘Normal’ blood pressure can vary from person to person and fluctuates throughout the day according to many factors. Some of these include whether you’re resting or active, your age, mood, general health and what kinds of medication you’re taking. And what is in the high-normal range for one person may be considered high in a person with diabetes, for example.

However, The National Heart Foundation offers a general guide:

Normal blood pressure Generally less than 120/80 mmHg
Normal-to-high blood pressure between 120/80 and 140/90 mmHg
High blood pressure 140/90 mmHg or higher
Very high blood pressure 180/110 mmHg or higher

Australian blood pressure guidelines recommend that blood pressure be kept below 140/90. If you have diabetes, kidney disease or cardiovascular disease, your blood pressure should be lower than this – ideally less than 130/80.

High blood pressure can’t be diagnosed from a single blood pressure reading. However, if readings are persistently higher than what’s considered a safe level, then your GP may diagnose you as having high blood pressure, known medically as hypertension. This puts you at greater risk of heart attack, stroke, heart failure, certain types of aneurysm and kidney failure. The higher your blood pressure, the higher your risk of other health conditions.

Your cardiovascular health 

All areas of your lifestyle in some way contribute to your health and wellbeing, and this includes your long-term cardiovascular health. Poor habits increase your risk of developing many chronic health problems including heart disease, stroke and type 2 diabetes.

The more risk factors you have, the greater your risk. It’s true you can’t change some risk factors like your age, background and family history of health problems. But you can modify many others with positive lifestyle changes.

These modifiable risk factors in your control include high blood cholesterol level, high blood pressure, being overweight, tobacco smoking, physical inactivity and poor eating and drinking habits.

Planning and carrying out important lifestyle changes can help you maintain a healthy cardiovascular system. These changes include:

  • not smoking
  • eating a healthy well-balanced diet
  • maintaining a healthy weight
  • regular checks of health vitals such as your blood pressure and cholesterol levels
  • regular physical activity
  • making sensible drinking decisions.

More information about heart health.

Further information 

National Heart Foundation of Australia

Baker IDI Heart and Diabetes Institute


Australian Better Health Initiative. Measure Up. [online] Canberra, ACT: Commonwealth of Australia. c2007 [updated Oct 2010, accessed 30 Jun 2011]

Australian Government Department of Health and Ageing. Physical activity guidelines. [online] Canberra, ACT: Commonwealth of Australia. 2005 [Accessed 30 Jun 2011] Available from:

The Australian Lung Foundation. The lungs – an overview of how they work. [online] Bowen Hills, QLD: Australian Lung Foundation. [Accessed 30 Jun 2011] Available from:

Clinical Knowledge Summaries. Diabetes type 2. [online] London: National Institutes for Health and Clinical Excellence. 2008 [last updated Nov 2010, accessed 6 Jul 2011] Available from:

Diabetes Australia. Living with diabetes. [Online] Canberra, ACT: Diabetes Australia. c2011 [Accessed 5 July 2011] Available from:

National Heart Foundation of Australia. How your heart works [online] National Heart Foundation of Australia. [ accessed 30 Jun 2011] Available from:

National Heart Foundation of Australia. Your blood pressure. [online] National Heart Foundation of Australia. 2008 [Accessed 30 Jun 2011] Available from:

National Heart Foundation of Australia. Absolute risk. [online] National Heart Foundation of Australia. [Accessed 30 Jun 2011] Available from:

National Heart Foundation of Australia. Guide to management of hypertension 2008. [online] National Heart Foundation of Australia. 2008 [Last updated Dec 2010, accessed 30 Jun 2011] Available from:

Ochs M Nyengaard JR Jung A et al. The number of alveoli in the human lung. American Journal of Respiratory and Critical Care Medicine. 2004; 169: 120-124. Available from:

Simon C, Everitt H, van Dorp F. Oxford handbook of general practice. 3rd ed. Oxford: Oxford University Press, 2010

Last published: 30 July 2011

This information has been developed and reviewed for Bupa by health professionals. To the best of their knowledge it is current and based on reputable sources of medical research. It should be used as a guide only and should not be relied upon as a substitute for professional medical or other health professional advice.

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