The Colors of Blood

Blood color is not always Red. As you all might be knowing, Haemoglobin gives blood its red colour. But here’s an interesting fact – Not all blood is red. Blood comes in a variety of colours like green, blue and even purple! How does that happen?

Colour of Blood in the Human Body

Red Blood (Human Blood)
So, let’s start with what we already know. Which is – human blood, as well as the blood of most other vertebrates, is red. This is, of course, due to the presence of haemoglobin, a large protein found in red blood cells which contains iron atoms within its structure. Haemoglobin, a respiratory pigment, plays a vital role in the body – that of carrying oxygen around the body to our cells and carrying carbon dioxide back to the lungs where it can be exhaled. The large protein consists of four smaller units which themselves contain small sections called haems, each of which contains an iron atom. These can ‘bind’ to oxygen (a process called oxygenation), giving red blood cells their oxygen transporting ability.

You also probably know that the colour of an object comes from its ability to absorb light wavelengths from a specific spectrum. Which is why blood is red in colour – Oxygenated blood is bright red in colour, whereas deoxygenated blood is a darker red.

Colour of Blood in Different Animals

Blue Blood
Do you know why our veins appear bluish green though? It’s because of interaction of light with our skin and the tissues covering the blood. Interesting, huh? That puts to rest the myth that “human blood is blue, and turns red when exposed to air”- All human blood is red, whether inside the body or out.

Some creatures, though, do have blue blood. Creatures that we come across every day, like spiders, octopuses, squids (you’ve gotta be underwater for these) as well as some molluscs have blue blood. And it all comes down to having a different respiratory pigment, called haemocyanin. Haemocyanin has a different atomic structure that contains copper atoms instead of iron atoms. Because of this, the blood is colourless when deoxygenated, and blue, when oxygenated.

Green Blood
There’s green blood too, in some species of worms and leeches. Their blood has a protein called chlorocruorin, which is responsible for the green coloration. In appearance, this protein is very much similar to haemoglobin, with the only difference being the presence of an aldehyde group in the place of a vinyl group in the chemical structure.

Violet Blood
Finally, violet blood is also possible, as is seen in some marine worms. These worms have in their blood yet another different respiratory pigment, called haemorythrin. Haemorythrin contains individual units which themselves contain iron atoms; when deoxygenated, the blood is colourless, but when oxygenated it is a bright violet-pink.

The most interesting thing of course, is the fact that humans are blessed with haemoglobin, which is the most powerful in terms of its oxygen carrying capacity. It is, however, funny to think that if our respiration pigments consisted of copper instead of iron, we might all be slapping on a different colour fake blood for Halloween instead of red!

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