Why Isn't All Blood Red? By Emily Jones

If you were asked to picture the colour of blood, chances are you would think of red, right? Wrong. Well, kind of. Humans and other vertebrates do exhibit red blood, but across the animal kingdom it can also be blue, green, violet, yellow or colourless. We rely on blood to transport vital substances around the body, and in most cases its colour is due to the type of molecule the organism uses to carry oxygen, known as a respiratory pigment. This infographic by chemistry teacher Andy Brunning on his blog, Compound Interest, explains why different animals exhibit different blood colours.


Humans and other vertebrates use a respiratory pigment called haemoglobin. It is found in red blood cells and transports oxygen away from the lungs and around the body. Our blood is red due to haemoglobin’s structure. It is made of small structures called ‘haems’ that absorb wavelengths of visible light, making it appear coloured. Each haem contains atoms of iron, which are what binds to oxygen. The blood is red whether it is carrying oxygen or not, although it is slightly darker when deoxygenated. If people are lacking in iron, a condition called anaemia can develop, resulting in fatigue and breathlessness. 


Alternatively, organisms such as spiders, crustaceans, molluscs and squid have blue blood. They transport oxygen using a respiratory pigment called haemocyanin. The pigment contains copper atoms in place of iron, and this, along with the differing structure of the pigment, gives it its blue colour when oxygenated, and little to no colour when not. It is a common (and false) myth that deoxygenated human blood is blue, due to the blue tinge of our veins. However, this occurs because of an optical effect when you look at your veins through layers of skin.

Despite the belief that blue blood is reserved for royalty, it is certainly valuable to humans, with a litre of blue horseshoe crab blood being worth around $15,000. Each year, the biomedical industry harvests over 400,000 of these critters because a compound in their blood can support the production of LAL, or Limulus amoebocyte lysate. This can detect human pathogens in patients, drugs and intravenous devices, which has allowed the safe delivery of vaccines and injectable medicines. Until recently, no other test had the same accuracy. 

There has been debate over the ethics of the procedure, as the crabs are harvested from the ocean, drained alive for a portion of their blood, and returned to the same spot within a day – but thousands die in the process. Luckily for the crabs, a synthetic alternative called recombinant Factor C (rFC) has been developed, and it is beginning to rise in popularity in the biomedical industry.

[Blue blood being obtained from horseshoe crabs]


Green blood can be found in segmented worms, leeches and marine worms. Their respiratory pigment is chlorocruorin, which has a similar structure to haemoglobin, although it has an aldehyde group instead of a vinyl group in its chemical structure. The blood is green when oxygenated, and, strangely, in higher concentrations it sometimes turns a light red.

However, green blood is not limited to worms. Some lizards from New Guinea are known to bleed in green, despite their blood containing haemoglobin like other vertebrates. Its colour comes from a build-up of a green pigment called biliverdin, which is produced as a by-product, along with bilirubin, when haemoglobin is broken down after delivering oxygen to the tissues. In other vertebrates, a build-up of either is toxic, and the liver acts to filter them out of the blood before the pigments cause jaundice or DNA damage. The lizard thrives under these levels of biliverdin, which is 40 times higher than the lethal concentration in humans, and scientists are not too sure why. One theory is that the build up occurred as an attempt to avoid malaria, as lizards can get infected by hundreds of malarial species. However, the lizards still get infected by malaria, and there is one strain that infects only the green-blooded species. This could be explained by the malaria continuing to evolve countermeasures to the increased levels of toxin - which continued to increase because of the presence of malaria. This process, where two organisms evolve to try and outcompete each other, is known as an evolutionary arms race.

[The green–blooded lizard from New Guinea]


Violet blood can also be found in a number of marine worms, brachiopods, and the unfortunately named penis worms. In this instance, the respiratory pigment is called haemorythrin, and it contains small units which each contain iron atoms. When the blood is deoxygenated it is colourless, and oxygenation causes it to be a bright violet-pink. Compared to haemoglobin, other respiratory pigments have a reduced capacity for carrying oxygen. In some cases of haemorythrin, it may only be a third that of haemoglobin’s.

[Thanks to its translucent skin, the purple blood of peanut worms can be seen]


Sea cucumbers, sea squirts and some beetles have yellow blood, although unlike the examples listed above it is not due to their respiratory pigment (which is haemocyanin). Their blood is yellow because of high levels of a pigment called vanabin - the role of which is unclear to scientists. One theory is that the vibrant colour will deter predators, but, ironically, sea cucumbers are consumed across many cultures for their supposed health benefits - some of which can actually be attributed to vanadium, which makes up the base of the yellow pigment.

[The sea cucumber has yellow blood]

Should we re-think our use of the term “blood-red”? Blood, despite being in different colours, exists for the same reason – transporting substances around the body. It is yet another example of how evolution has apparently come up with multiple solutions that each allow animals to overcome barriers to their survival.






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