The Circulatory System
Your circulatory system (also called the cardiovascular system) is the network of blood, blood vessels and the heart that delivers oxygen, food and other essential things to every cell in your body, and carries away waste products. At its centre is your heart: a fist-sized muscular pump that beats around 100,000 times a day and pushes blood through over 100,000 km of blood vessels. Without the circulatory system, no cell more than a few millimetres from a blood vessel could survive.
- Heartbeats per dayapprox. 100,000approx. 3 billion in a lifetime
- Heart weightapprox. 300 gSize of a fist
- Blood vessel lengthapprox. 100,000 km2.5 times around the Earth
- Blood volumeapprox. 5 litresIn an average adult
- Red blood cellsapprox. 25 trillionIn an adult body
- Time blood takes to circulateapprox. 1 minuteFrom heart, around body, back to heart
The heart: a four-chambered pump
Your heart is about the size of your fist and sits roughly in the middle of your chest, slightly to the left. It is made of cardiac muscle (a special muscle found nowhere else in your body) and works as two pumps in one:
- The right side pumps blood to your lungs to pick up oxygen.
- The left side pumps the oxygen-rich blood out to the rest of your body.
Each side has two chambers: an atrium on top (collects the incoming blood) and a ventricle below (pumps it out). One-way valves between and out of the chambers stop blood flowing the wrong way. The classic "lub-dub" sound of the heartbeat is the valves snapping shut after each beat.
The two circulations
The circulatory system actually does two separate loops every time the heart beats.
- Pulmonary circulation: blood from the right heart goes to the lungs, picks up oxygen, drops off carbon dioxide, and returns to the left heart.
- Systemic circulation: blood from the left heart goes to the rest of the body, delivers oxygen and nutrients, picks up waste, and returns to the right heart.
The two loops happen together with every heartbeat. Each side of the heart fills and empties at the same time, so the blood is always moving.
Blood vessels: the body's plumbing
Your blood travels through three main kinds of blood vessel.
- Arteries: carry blood AWAY from the heart. They have thick muscular walls that can stand high pressure from the heart's pumping. The biggest is the aorta, as thick as a garden hose.
- Veins: carry blood BACK to the heart. They have thinner walls and one-way valves to stop the blood flowing backwards.
- Capillaries: the tiny vessels (just one cell thick) where blood actually exchanges oxygen, food and waste with nearby cells. Capillaries are so fine that red blood cells have to squeeze through one at a time.
If you laid out all your blood vessels end to end, they would stretch about 100,000 km: more than twice the way around the Earth. Almost all of that length is capillaries, which form a vast network reaching every corner of your body.
What is in blood?
Blood is not just a red liquid; it is made of several different parts.
- Plasma: the yellow watery liquid (approx. 55% of blood), carrying dissolved substances around.
- Red blood cells (approx. 45%): carry oxygen using a protein called haemoglobin, which gives blood its red colour. You have about 25 trillion of them.
- White blood cells (approx. 1%): fight infection. They are part of your immune system.
- Platelets: tiny cell fragments that stick together to form clots and stop bleeding when you cut yourself.
What blood does for you
Blood is doing many jobs at once.
- Carries oxygen from your lungs to every cell in your body.
- Carries carbon dioxide back from your cells to your lungs to be breathed out.
- Delivers food (sugars, amino acids, fats) from your gut to your cells.
- Removes waste (especially urea) and takes it to your kidneys to be filtered out.
- Carries hormones from your glands to wherever they need to act.
- Fights infections via white blood cells.
- Heals wounds via platelets and clotting.
- Regulates temperature: blood flow to your skin can be increased (to lose heat) or reduced (to keep heat in).
Deeper dive: blood types and why they matter
Not all blood is the same. Human blood comes in four main types: A, B, AB and O. The differences come from special molecules called antigens on the surface of your red blood cells. There are also Rhesus positive (Rh+) and Rhesus negative (Rh-) types based on another antigen, giving 8 main combined types (A+, A-, B+, B-, AB+, AB-, O+, O-).
The system matters because if you receive blood of the wrong type in a transfusion, your immune system will recognise the foreign antigens and attack the donated blood cells. This can cause a deadly reaction. Blood transfusions only became safe after Austrian doctor Karl Landsteiner discovered the blood types in 1901 (for which he later won the Nobel Prize).
Different blood types can give and receive blood in different combinations:
- Type O negative is the "universal donor": their blood has no antigens, so almost anyone can receive it. Vital for emergencies when there is no time to test.
- Type AB positive is the "universal recipient": they can receive blood from anyone, because they have all the antigens themselves.
Your blood type is inherited from your parents and stays the same for life. Different blood types are common in different parts of the world: type B is common in Asia; type O is the commonest in Latin America; type AB is rare everywhere (around 4% of people). Blood is in constant short supply at hospitals, which is why blood donor services everywhere always need more volunteer donors.
For the lungs the heart pumps blood to, see the respiratory system. For the cells that defend you, see the immune system.