human-body quick facts
- Laid end to end, your blood vessels would stretch more than 60,000 miles — more than twice around the Earth.
- A capillary is about 5 micrometres wide, so red blood cells must pass through single file.
- You have about 5 litres of blood, but more than 7,500 litres pass through your heart each day, because the same blood keeps coming back.
- Nearly 75 per cent of your blood is sitting in your veins at any one moment.
- William Harvey proved blood circulates in 1628 using maths, not a microscope. Capillaries were not seen until 1661.
The circulatory system at a glance
| Your circulatory system | |
|---|---|
| What it is | The heart, the blood vessels and the blood |
| Heart size | About the same as your clenched fist |
| Chambers | Four: two atria on top, two ventricles below |
| Valves | Four, all one-way |
| Beats | Around 100,000 times a day |
| Blood you have | About 5 litres (8 pints) in an adult |
| Vessel length | More than 60,000 miles (about 97,000 km) |
The Year 6 national curriculum for England asks pupils to name the main parts of the human circulatory system and describe what the heart, the blood vessels and the blood actually do. This guide covers all three.
What the circulatory system is
It is a delivery network. Three parts work as one: the heart, the blood vessels and the blood itself.
Every cell in you needs oxygen and food, and every cell makes waste. The circulatory system brings one and takes away the other, without stopping, for your whole life.
The heart is two pumps, not one
This is the idea that makes the rest make sense. A wall of muscle called the septum splits the heart down the middle.
The right side sends blood to the lungs. The left side sends blood to the rest of the body. They sit side by side and beat together, but they are doing two different jobs.
The heart is about the size of your own clenched fist, and an adult one weighs roughly 280 g (about 10 oz).
The four chambers
Each side of the heart has two chambers. The two on top are the atria. The two underneath are the ventricles.
The right atrium collects blood that has already been round the body and is low in oxygen. The right ventricle pushes that blood out to the lungs.
The left atrium takes in blood coming back from the lungs, full of fresh oxygen. The left ventricle then pushes it out to the whole body through a large artery called the aorta.
The left ventricle is the strongest chamber of the four. It has the longest journey to power.
The four valves
The British Heart Foundation describes the valves as doors that open and shut with every heartbeat.
There are four of them: the tricuspid, the pulmonary, the mitral and the aortic valve. The tricuspid sits between the right atrium and right ventricle. The mitral sits between the left atrium and left ventricle. The other two guard the exits where blood leaves the ventricles.
Their job is simple and vital. They stop blood sliding backwards, so it only ever travels one way.
The two journeys blood makes
Blood does not do one lap. It does two, one after the other, for ever.
The short loop is called pulmonary circulation: heart to lungs, pick up oxygen, straight back to the heart. The long loop is called systemic circulation: out from the heart to every part of the body, and back again.
Every drop goes round both. Short loop, long loop, short loop, long loop.
How much work the heart does
Your heart beats around 100,000 times every day.
Two numbers get muddled here, so it is worth separating them. The British Heart Foundation says your heart moves about 5 litres of blood around your body. That is how much blood you have.
The Cleveland Clinic says the heart pumps more than 7,500 litres a day. That is how much passes through, because the same 5 litres keeps coming back round.
So you do not make 7,500 litres of blood. You have about 5 litres, and it goes round and round all day.
Arteries, veins and capillaries
There are three kinds of blood vessel, and each is built for its job.
| Vessel | Direction | Walls |
|---|---|---|
| Artery | Carries blood away from the heart | Strong and muscular, for high pressure |
| Vein | Carries blood back towards the heart | Thinner, with one-way valves inside |
| Capillary | Joins arteries to veins | About 1 micrometre thick |
You have about 160 named arteries and about 200 named veins. At any moment, nearly 75 per cent of your blood is sitting in your veins.
The valves in your leg veins matter more than you would think. They help push blood back up to the heart against gravity, all day, while you stand.
Laid end to end, all your blood vessels would stretch more than 60,000 miles. That is more than twice around the Earth.
Inside a capillary
Capillaries are where the real work happens, and they are astonishingly small.
A capillary is about 5 micrometres across. A micrometre is one thousandth of a millimetre.
Red blood cells cannot swim through side by side. They go single file, lined up one behind the other.
The walls are only about 1 micrometre thick, which is thin enough for oxygen and food to pass straight through into your cells, and for waste like carbon dioxide to pass back.
What blood is made of
Blood is not one thing. It has four parts.
Red blood cells carry oxygen. White blood cells fight infection. Platelets are tiny fragments that make blood clot and seal a cut. Plasma is the straw-coloured liquid that carries them all, and it makes up a bit over half of your blood.
The parts do not last the same length of time. A red blood cell lives up to about 120 days. A platelet lasts only about 9 or 10 days before the spleen clears it away.
Why blood is red, and why veins look blue
Red blood cells carry oxygen using a protein called haemoglobin. Haemoglobin contains iron, and the iron is what makes blood red.
Blood is red all the time. Bright red when it is carrying plenty of oxygen, darker red when it is not.
So why do the veins in your wrist look blue? They are not. It is an illusion caused by the way your skin filters and scatters light on its way back to your eye.
Your pulse, and what is normal for a child
Every heartbeat sends a pressure wave along your arteries. You can feel that wave as your pulse.
To take one, press gently at your wrist or beside your windpipe on the neck, count the beats for 30 seconds and double it.
For an adult, 60 to 100 beats a minute at rest is normal. Children are faster.
A study of nearly 23,000 UK children aged 9 to 11, by Great Ormond Street Hospital and UCL, found girls averaged 82.2 beats a minute at rest and boys 78.7.
Exercise, fever, infection, worry and stress all push your rate up for a while. That is normal and expected.
Blood groups
There are four main blood groups: A, B, AB and O. Each one can be RhD positive or RhD negative, which makes eight groups in total.
The NHS says around 48 per cent of people in the UK have group O, making it the commonest. O negative is known as the emergency blood, because it can be given to almost anybody.
Giving blood
One donation is 470 ml, just under a pint, and the bag takes only 5 to 10 minutes to fill.
NHS Blood and Transplant needs about 5,000 donations every day for patients in England, and about 143,000 new donors each year to keep the mix of blood groups right.
Two-thirds of donated blood treats medical conditions such as anaemia, cancer and blood disorders. Nearly a third is used in surgery and emergencies, including childbirth.
The man who worked it out
For a long time nobody knew blood went round in a circle. An English doctor called William Harvey, who lived from 1578 to 1657, proved that it does.
He proved it with maths rather than a microscope. He worked out that in half an hour the heart pumps more blood than the whole body contains. So the blood could not be made fresh each time. It had to be going round again.
He published the idea in 1628, in a book usually shortened to De Motu Cordis.
One piece was still missing. Harvey guessed that tiny vessels must join arteries to veins, but he never saw one. Marcello Malpighi spotted capillaries under a microscope in 1661, four years after Harvey died.
When to get help
Never take medical advice from an article, including this one. A doctor or nurse is the right person to ask about your own body.
A pulse reading cannot tell you what is wrong with you. If your heart rate is always fast, slow or irregular, or you feel faint or dizzy, tell an adult and see a GP.
Some things are emergencies. The NHS says chest pain that feels crushing or squeezing, or pain spreading to the arm, neck or jaw, means calling 999 straight away. If someone is unresponsive and not breathing normally, call 999 and start CPR.
You can read more about the organ at the centre of all this in our guide to the human heart. Two other systems that never stop working are covered in our guides to the brain and the nervous system.
Three things people get wrong
“Blood is blue until it touches air.” It never is. Blood is red all the time, bright red with oxygen and darker red without it. Veins only look blue because of the way skin scatters light.
“The heart is on the left side of your chest.” It sits in the middle, tipped a little to the left. Put your hand flat on the centre of your chest and you are over it.
“A heart attack and a cardiac arrest are the same thing.” They are not. A heart attack is blood flow to the heart being blocked. A cardiac arrest is the heart stopping suddenly, and the person collapses. Both need 999.
Did you know? In a study of nearly 23,000 UK children aged 9 to 11, girls averaged 82.2 heartbeats a minute at rest and boys 78.7.
Try it
Find your pulse at your wrist. Count the beats for 30 seconds and double it. Now run on the spot for one minute and count again.
Knowledge check
Why is the heart described as two pumps?
Show answer
A wall of muscle called the septum splits it. The right side pumps blood to the lungs, and the left side pumps it to the rest of the body.
Why do your veins look blue if blood is red?
Show answer
They only look blue. It is an illusion caused by the way skin filters and scatters light on its way back to your eye.
Frequently asked questions
What are the main parts of the circulatory system?
The heart, the blood vessels and the blood. The heart pumps, the vessels carry, and the blood does the delivering. That is the wording the Year 6 national curriculum for England asks pupils to know.
How many chambers does the heart have?
Four. Two atria on top collect blood coming in, and two ventricles underneath push blood out. Four one-way valves make sure blood can never slide backwards.
What is the difference between arteries and veins?
Arteries carry blood away from the heart and have strong muscular walls for high pressure. Veins carry blood back to the heart, have thinner walls, and contain one-way valves.
How much blood is in your body?
An adult has about 5 litres, which is around 8 pints. That same blood keeps circulating, so more than 7,500 litres passes through the heart over a whole day.
What is blood made of?
Red blood cells that carry oxygen, white blood cells that fight infection, platelets that clot and seal cuts, and plasma, the straw-coloured liquid that carries all of them.
Who discovered that blood circulates?
William Harvey, an English doctor, who published the idea in 1628. He proved it with arithmetic, showing the heart pumps more blood in half an hour than the body contains.
Sources
- British Heart Foundation: how a healthy heart works — the chambers, the four valves, 100,000 beats a day, 5 litres of blood and where the heart sits
- Cleveland Clinic: circulatory system — pulmonary and systemic circulation, 7,500 litres a day and the 60,000 miles of vessels
- Cleveland Clinic: blood vessels — how arteries, veins and capillaries differ, and the counts of named vessels
- NIH National Institute of General Medical Sciences: roses are red and so is blood — haemoglobin, iron, and why veins only look blue
- NHS Blood and Transplant: how blood is used — what blood is made of and what donated blood treats; its donation-process page was also used
- Great Ormond Street Hospital and UCL: resting pulse rates of UK pre-teens — the measured resting heart rates of UK children aged 9 to 11
