Why is blood red? A simple explanation for kids

Why is blood red? - Asher, six years 11 months, New South Wales.

What a great question about something in our body, Asher.

Blood is inside our body, but we see it on the outside when we bleed, like when we get a cut or a nose bleed.

Haemoglobin function

 Blood is red because of something called haemoglobin. Haemoglobin is red, and this makes our blood red.

But what is haemoglobin for? Well, we need haemoglobin to carry oxygen in our blood.

This might sound a bit complicated, so let’s look more closely at why we need oxygen and haemoglobin to carry oxygen in our blood.

  Everyone needs oxygen to stay alive. Our body is made up of millions and millions of tiny cells, and all the cells in it need oxygen from the air we breathe and nutrients from the food we eat.

Cells use oxygen and nutrients to make energy to do their job. For example, cells in our muscles need energy to move us, and our brain cells need energy so we can learn.

Every time you breathe in, you breathe oxygen into your lungs. Our heart pumps blood to the lungs to pick up this oxygen.

The heart then pumps the blood to all our body’s cells with this oxygen.

After the blood drops off oxygen for the cells to use, it travels back to the heart and lungs to pick up more oxygen again.

Our cells need oxygen all the time because they are always working.

Oxygen transport

  So where does haemoglobin come in? Oxygen travels in blood to our cells in tubes called blood vessels. But oxygen doesn’t dissolve very well in blood. If we just had oxygen on its own in our blood, we could get air bubbles. These bubbles would stick to the sides of the blood vessels. This means the oxygen could get stuck and not travel to our cells.

Luckily, haemoglobin carries oxygen in our blood, so it doesn’t form air bubbles and get stuck.

We can think of this as trying to move a stone down a river. A stone can’t float down a river because it will sink. But if we put the stone in a container that floats, the container can float down the river and carry the stone with it.

Haemoglobin is like a red-coloured container of oxygen. It’s our oxygen carrier.

So, because we have red haemoglobin in our blood to carry oxygen, our blood is red.

Blood appearance

 Blood can be bright red or dark red. When haemoglobin carries more oxygen, it is a brighter shade of red.

This means blood travelling from the heart and lungs to the cells with lots of oxygen is bright red.

After haemoglobin loses oxygen to the cells, it turns dull and dark red. This means darker red blood travels back to the heart and lungs with less oxygen.

 We can see some of our veins (the blood vessels that carry blood back to the heart) just under our skin, on the back of our hands, for example.

The blood in these veins can appear green-blue. That’s because we look at this blood through our skin, which changes the colour we see.

But we know our blood is not blue or green when we bleed – it’s red.

 Not all animals have red blood. An octopus can have blue blood; some lizards even have green blood.

This blue and green blood is also because of the colour of their oxygen carrier. These animals don’t have red haemoglobin like we do. They have a blue or green oxygen carrier. This makes their blood blue or green.

If you could choose blood of any colour, what colour would you choose? 

Theresa Larkin, University of Wollongong (Australia)

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