Blood Colour

The chemistry behind blood’s colour is something fundamental as most secondary school students are aware that blood contains a type of red pigment molecules named haemoglobin. It is a quaternary structured conjugate protein found in the red blood cells (erythrocytes). It transports respiratory gases (oxygen and carbon dioxide).

The red colouration of blood is due to the sub-units (2 alpha and 2 beta chains) of the haemoglobin protein. Each of the alpha or beta chain consists of a tertiary structured protein chain which is bound to a haem group (a non-protein prosthetic group). The haem groups, which contain iron atoms (Fe), that give rise of blood’s dark red colour.

Why is it red?

The chemical structure of haem group consists of alternating double and single bonds absorbs light of wavelengths but reflects red light, causing us to see it as red.


A common textbook misconception about our blood is that deoxygenated blood is blue. It is understandable that we believe that fact as most biology textbooks depicted oxygenated blood with red colouration while deoxygenated blood with blue. (Although veins do appear blue underneath our skin, we will discuss that in another time.)

The real fact is oxygenated haemoglobin is a bright red while deoxygenated haemoglobin is a darker red colour.

If you’ve ever had a wound bleed, you’ll have probably noticed that any blood you stem with a tissue or cloth turns a dark brown colour as it dries. This is due to the oxidation of the iron atoms in the haemoglobin subunits, from iron (II) to iron (III), producing methaemoglobin which is a dark brown colour.


How to make fake blood?



If you’ve ever bitten your tongue or tasted your own blood, you’ll have noticed that blood has a metallic taste (similar to the taste if you lick your house key).

This is partly due to the presence of the iron in haemoglobin (not the only reason). The iron in Haemoglobin also react with fat molecules to produce a range of compounds that help to produce a metallic flavour.

The compounds made include oct-1-en-3-one, which is described as having a mushroom-like, metallic odour. This is also the compound behind the metallic smell you can detect on your skin after touching metal objects – so in these cases it’s not the metal you’re smelling, but chemical breakdown products of molecules in your own skin.










Blood itself smells metallic all on its own. Researchers found that a compound in blood that contributes this faint metallic odour named trans-4,5-epoxy-(E)-2-decenal. An important compound detected by predators.


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