Mythconception: Venous blood is blue

Why are our veins blue and do any creatures have different coloured blood?
23 October 2018


Computer generated image of Red blood cells travelling in a blood vessel


This week  - a myth that’s been making Tamsin Bell’s blood boil…

Tamsin - I remember at school a friend told me that blood is blue until it is exposed to air when it turns red. Looking down at my wrists and seeing those blue lines I had no reason to doubt them. But like almost everything you hear in the playground, it was complete nonsense!

Human blood is the same colour whether it’s inside or outside your body - and that’s red! The colour is brighter when blood is full of oxygen and darker red when it is oxygen-poor. But it’s definitely not blue!

So how did this myth start? It may come from the fact that the veins we can see on our body are used to carry oxygen-poor blood and these look blue. And to add to the confusion of children everywhere, the typical blood flow diagram of the body taught as school uses red lines to represent oxygen-rich blood, whilst blue lines correspond to oxygen-poor blood. To understand why this myth is false, let’s consider the science behind this red substance...

Blood is our internal delivery system. It takes nutrients and oxygen all around the body by a super-speedy highway known as arteries. And then takes waste products like carbon dioxide away through the veins. Our blood is made up of cells floating in a liquid called plasma. Blood plasma is predominantly composed of water, but also contains proteins, sugars, irons, hormones and carbon dioxide.

The cells we have in our blood include the little doughnut-shaped red blood cells which carry oxygen, white blood cells which fight infection, and platelets which help the blood clot, stopping it from endlessly pouring out of our bodies after a cut. Interestingly, our blood also contains small amounts of some metals, even expensive ones like gold.

The colour of blood is determined by the protein in the blood that carries oxygen around. In vertebrates like us, the colour comes from a protein called haemoglobin. This protein contains iron, which makes blood bright red when it’s stuck to oxygen and dark red without oxygen. Different proteins lead to different blood colours; for example, cut horseshoe crabs and they bleed blue, whilst sea squirts disgustingly have a mustard yellow coloured blood. Other weird and wonderful sea creatures have fabulously vibrant blood colours such as green, purple and pink.

So why does our red human blood look blue in veins close to the surface of our skin? Well, the size of the blood channel, the oxygen content of the blood, and the thickness of the skin above the channel affects how visible light travels through skin, which is known as absorption. And these aspects also affect how much light can escape or be scattered, changing the colour we see.

The oxygen-poor blood in our veins is best at absorbing red light, but blue light cannot reach deep into the skin. So a shallow vein appears blue because this colour is reflected back at the eye. We cannot see our arteries because they tend to be smaller and have thicker walls than our veins.

Hopefully, this misconception was not in vain, and you’ll remember that, unless you’re a sea squirt, our blood is always red.


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