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Why is our blood red? Why does it look blue through white skin?

Donna Coutts, February 11, 2019 6:45PM Kids News

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You can see the cherry red blood of humans in this test tube. Picture: iStock media_cameraYou can see the cherry red blood of humans in this test tube. Picture: iStock

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Reading level: green

The average adult human has about 5 litres of red blood pumping through their body. It’s always red: sometimes bright, sometimes slightly darker, but always red. (Except when it looks blue through pale skin, but we’ll come to that later.)

It’s red because of an important ingredient: iron. Iron is a pure and very common metal. It forms much of the Earth’s inner core* and it’s used for everything from making saucepans to metal roofs. In animals including humans, iron has the very special job of being able to join to — and then deliver — oxygen around to every part of the body.

The iron is carried in the blood in a substance called haemoglobin.

Red blood cells. Picture: supplied media_cameraRed blood cells. Picture: supplied

We breathe air into our lungs, we take the oxygen out of that air and it is absorbed into our bloodstream for the iron to carry it off to get to work.

When blood is full of oxygen and on its way around your body, it is bright red. Once the blood has delivered the oxygen, it picks up a substance you don’t need, carbon dioxide, and takes that back to your lungs for you to breathe out. On this return trip the blood isn’t quite so bright in colour: more a dark red than a light, bright cherry red.

WHY DO VEINS LOOK BLUE?
If you look at the wrist of someone with pale skin, you’ll see blood moving through the vessels* of the body’s circulation system (veins, arteries and capillaries). It looks blue. And the paler the person’s skin, the bluer it looks.

An illustration of the veins and arteries in the human head. Source: iStock/Getty Images media_cameraAn illustration of the veins and arteries in the human head. Source: iStock/Getty Images

No matter how blue it looks, however, the blood pumping beneath the skin is still red.

The reason is an optical* illusion*.

Light we see is made up of a range of colours, even though we see it mostly as white.

When light, such as sunlight, hits the skin, blue light can’t go as far through the skin as red light. More of the red part of light is absorbed and doesn’t reflect back out into your eyes for you to see. More of the blue part of light is reflected back to your eyes. The deeper the blood vessel below your skin, the bluer it looks.

Contrast* is also important. The whiter the skin, the bluer it looks, too.

In historic times, rich and powerful white people, such as royalty in England, were nicknamed “blue bloods”. Because they didn’t work out in the fields as peasants* did, their skin was pale and the blood beneath their skin looked bluer. They probably had a bath more often to keep their skin whiter than the poor workers!

An ethereal octopus, which has blue blood. Picture: iStock media_cameraAn ethereal octopus, which has blue blood. Picture: iStock

THE REAL BLUE BLOODS
Squid, octopus, cuttlefish, lobsters and horseshoe crabs really do have blue blood. Their bodies use a different pure metal, called copper, instead of iron to carry oxygen around.

Other animals have purple blood, some have clear blood. And some lizards even have green blood. In New Guinea, there’s a green-blooded skink, which is a type of scincid lizard.

WHAT ELSE IS IN BLOOD?
Blood is the juice that carries all the ingredients you need around your body to keep it working smoothly. Then, when it’s delivered its cargo, it picks up the waste and takes it to be processed and disposed of.

Blood carries lots of water, nutrients to fuel you, white blood cells to help you fight infection*, platelets to help your blood clot if you are injured and hormones, which are a big range of chemical messengers that communicate to your body when to do particular things, such as grow.

VIDEO: Healthy Australians donate blood each year to help patients in our hospitals.


HOW MUCH BLOOD DO YOU HAVE?
A child has 70-75ml of blood for each kilogram of body weight.

Multiply the number of kilograms you weigh by the number 70. Write that number down.

Then multiply the number of kilograms you weigh by 75. Write that number down too.

The two numbers give you the likely range of the volume of blood you have in your body, in millilitres.

If you’d like to know how many litres that is, divide that number by 1000.

GLOSSARY

  • core: centre
  • vessels: arteries, veins and capillaries which carry blood
  • optical: how we see things
  • illusion: something we see that is not real
  • contrast: very different
  • peasants: poor farm hands
  • infection: illness through disease, virus or poison

EXTRA READING

What is at the centre of the Earth?

Aussie medical invention will save lives

Vaccinations eliminate rubella from Australia

QUICK QUIZ

  1. What substance in the blood is the iron carried in?
  2. Name three vessels of the circulation system?
  3. Where is the green-blooded skink from?
  4. Does blood have water in it?
  5. How much blood does a child have for each kilogram of body weight?


LISTEN TO THIS STORY

CLASSROOM ACTIVITIES
1. How Much?
In the story, you can read about how you can work out how much blood a person has.

How many millilitres of blood would you have if you weighed:

  • 15 kilograms
  • 30 kilograms
  • 45 kilograms
  • 55 kilograms

Do you know how much you weigh? Work out how much blood you have!

Time: allow 25 minutes to complete this activity
Curriculum Links: Mathematics

2. Extension
Create a storyboard for a short animation. Your animation will be used in schools to help kids understand the most important facts about blood. Don’t forget to include words for a voiceover, music and anything else that will make kids enjoy and learn from your animation.

Time: allow 45 minutes to complete this activity
Curriculum Links: English, Science, The Arts — Media Arts


VCOP ACTIVITY
After reading the article, with a partner, highlight as many connectives as you can find in pink. Discuss if these are being used as conjunctions, or to join ideas and create flow.

HAVE YOUR SAY: What did you learn about blood in this story?
No one-word answers. Use full sentences to explain your thinking.

Extra Reading in explainers