There’s a detailed look at the slightly more complex reasons why veins appear blue, despite being red, in this paper examining the question. However, this appearance is in fact caused by the interaction of light with both the blood and the skin and tissue covering the veins. It’s a commonly believed myth that deoxygenated blood is blue – after all, if you look through your skin at any of your veins, which carry deoxygenated blood away from your body’s cells, they have a definite blue-grey hue.

The presence of the iron atom modifies this absorption slightly, and as such haemoglobin is a red colour when oxygenated, and a slightly darker red when deoxygenated. The individual haems are conjugated molecules – they have lots of alternating double and single bonds between carbon atoms in their structure – and this conjugation causes them to absorb light wavelengths in the visible portion of the spectrum, leading to a coloured appearance. The iron atoms are also responsible for haemoglobin’s colour. This can ‘bind’ to oxygen, giving red blood cells their oxygen transporting ability.

The large protein consists of four smaller units which themselves contain small sections called haems, each of which contains an iron atom. Haemoglobin is what’s known as a respiratory pigment, and it plays a vital role in the body, ferrying oxygen around the body to your cells and helping carbon dioxide back to the lungs where it can be exhaled. Most people will have learnt that human blood, as well as that of most other vertebrates, is red as a result of haemoglobin, a large protein found in red blood cells which contains iron atoms within its structure. So, let’s start with what we already know. However, red is not the only blood colour available – it also comes in blue, green, violet, and even colourless varieties – and this is a result of the specific chemicals that make up blood in different organisms. You probably already have a pretty good idea of the reasons behind the red colouration of human blood that fake blood mimics.

Halloween’s almost here, which, for a large number of costumes, will require a liberal dousing of fake blood to complete the look.

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