Science
What is blood and what does it do?
Quick answer
Blood is the red liquid pumped around your body by your heart. It carries oxygen and food to every cell, takes away waste and helps fight germs and heal wounds.
In one sentence
Blood is the fluid that flows through your body, delivering oxygen and nutrients and carrying away waste.
Explained
Your heart pumps blood round your body day and night through tubes called blood vessels. An adult has about five litres of blood. Blood has four main parts.
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- Plasma is the pale yellow liquid, mostly water, that makes up a little over half of your blood. It carries cells, nutrients and waste.
- Red blood cells carry oxygen from your lungs to the rest of your body. They contain a red substance called haemoglobin, which gives blood its colour. They are shaped like tiny, squashed discs, and there are billions of them in each teaspoon of blood.
- White blood cells protect you from germs. They are part of the immune system.
- Platelets are tiny fragments that stick together when you cut yourself. They help blood clot into a scab, which seals the wound.
There are three kinds of blood vessel. Arteries carry blood away from the heart, veins carry it back, and tiny capillaries link the two, passing oxygen and food into your body's cells. You can feel your heart pushing blood through an artery as a pulse on your wrist or neck.
The blood goes on a loop. It picks up oxygen in the lungs, gets pumped by the heart to the body, gives up the oxygen, picks up carbon dioxide, and returns to the lungs to exchange it. Nutrients from digested food also travel in the blood.
Everyone has a blood group, such as A, B, AB or O. Some people give blood at donation centres, which can help patients in hospital. Doctors check that donated blood matches the patient's type. A healthy diet with iron, found in beans, meat and green vegetables, helps your body make red blood cells.
Examples
| Part | Job |
|---|---|
| Red blood cells | carry oxygen |
| White blood cells | fight germs |
| Platelets | clot blood |
| Plasma | carries everything |
Common mistakes
- "Blood is blue inside veins." It is always red, darker when it has less oxygen.
- "Veins carry blood away from the heart." Arteries do that.
- "All blood is the same." Blood groups differ.
Quick quiz
Blood
Check what you know with 4 quick questions.
Question 1 of 4
Which blood cells carry oxygen?
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Related topics
The heart
The heart is a muscle about the size of your fist that pumps blood around your body, carrying oxygen and food to every cell and taking waste away.
Lungs
The lungs are two spongy organs in your chest that take oxygen from the air into your blood and remove carbon dioxide when you breathe out.
The human body
The human body is made of trillions of cells that form organs, such as the heart, lungs and brain. Organs work together in systems that keep you moving, breathing, eating and thinking.
The brain
The brain is the control centre of your body. It sends and receives messages through your nerves so you can think, feel, move, remember and breathe.
Digestive system
The digestive system is the long tube and organs that break food down into tiny pieces your body can absorb for energy and growth, then get rid of what is left as waste.
Healthy eating
Healthy eating means eating a variety of foods from all the main food groups, with plenty of fruit and vegetables and water to drink, so your body gets everything it needs to grow.