Blood and What It Does

The river inside you

If you could shrink down and travel inside your body, you would find a busy red river flowing through tubes everywhere — into your fingers, your toes, your brain and back again. That river is your blood. It never stops moving, day and night, carrying important things to every corner of you.

A grown-up has about 5 litres of blood. That sounds like a lot, but your blood is always working hard. It is pushed around your body by your heart, which you can read about in the circulatory system: heart and blood. Let's open up a drop of blood and see what is inside.

What is blood made of?

Blood may look like a plain red liquid, but it is really four parts mixed together.

1. Red blood cells. These tiny discs are the most common cells in your blood. They carry oxygen from your lungs to the rest of your body. They are red because they hold a substance called haemoglobin, which grabs onto oxygen. This is what makes all your blood look red.

1. White blood cells. These are your body's tiny soldiers. They hunt down germs — bacteria and viruses — and destroy them before they can make you ill. You have far fewer white cells than red ones, but they are a key part of how the immune system works.

1. Platelets. These are tiny pieces that rush to any cut and clump together to stop the bleeding. They make a plug, then a hard scab forms over it like a natural sticking plaster.

1. Plasma. This is the pale yellow liquid that the cells float in. Plasma is mostly water, and it carries the cells, plus food, vitamins and warmth, all around your body.

All the jobs blood does

Blood is one of the hardest workers in your body. Here are its main jobs:

• Carrying oxygen. Every cell needs oxygen to make energy. Blood delivers it from your lungs.
• Carrying food. After you digest a meal, the goodness goes into your blood and is delivered to your cells.
• Taking away rubbish. Blood collects waste, like carbon dioxide, and carries it to your lungs and kidneys to be removed.
• Fighting germs. White blood cells defend you from illness.
• Healing cuts. Platelets seal wounds so you do not lose too much blood.
• Keeping you warm. Warm blood spreads heat evenly around your body.

Why blood matters so much

Without blood, your cells could not get oxygen or food, and they would quickly stop working. That is why losing a lot of blood is dangerous, and why doctors sometimes give people extra blood, called a blood transfusion, kindly donated by other healthy people.

Your blood also tells doctors a lot about your health. A small drop tested in a lab can show whether you have enough iron, whether you are fighting an infection, and much more. That is why a nurse sometimes takes a tiny sample of blood when you are unwell.

Try it: make pretend blood

This safe, edible activity shows you the parts of blood — no real blood needed!

You will need a clear cup, water, a spoonful of red sweets or red beads, a few white beads or mini marshmallows, and some glitter or tiny coloured sprinkles.

1. Half-fill the cup with water. This is your plasma, the liquid part.
2. Stir in a big spoonful of the red sweets or beads. These are your red blood cells — there should be lots of them.
3. Add just a few white beads or marshmallows. These are your white blood cells. Notice how there are far fewer of them.
4. Sprinkle in a pinch of glitter or sprinkles. These are your platelets, the tiny pieces that fix cuts.
5. Gently swirl the cup and watch everything float and move in the plasma.

Why it works: Real blood is exactly like this — a liquid (plasma) with lots of red cells, a few white cells and tiny platelets all floating along together. By using the right amounts, you can see why blood looks red (so many red cells) and how each part has its own special job, just like the real river flowing inside you right now.