The many colours of blood

Much ado was made about the recent reports in the press of a man who was discovered to have dark-green blood. The patient was undergoing emergency surgery at St Paul’s Hospital, Vancouver when the surprised doctors discovered the 42-year-old’s migraine medication had caused a condition known as sulfhaemoglobinaemia, where sulfur is taken up into the oxygen carrying compound in red blood cells.

This comes as no surprise to biology students, who are aware that a fabulous palette exists for blood varieties, dependent upon what molecule is used to carry oxygen through the body. In human beings, iron atoms at the centre of each haemoglobin molecule are responsible for binding to oxygen, giving our blood a bright red colour when oxygenated (i.e. in the arteries) and deep red when deoxygenated (in the veins).

Now, were you to remove that iron and replace it with copper, you would have the second most widely-used oxygen carrier: haemocyanin. This molecule is colourless when deoxygenated, but becomes a delightful pale blue when bound with oxygen. It is perhaps most famously found in that living fossil, the Horseshoe Crab, although it is also present in many molluscs and arthropods. Although inferior to haemoglobin as an oxygen transporter, haemocyanin is highly sensitive to impurities, a property that has many uses in medical science and lends it a hefty price tag of around $15,000 per litre.

Another crab, the Common Shore Crab, displays a whole myriad of colours in its blood. Typically colourless, the blood turns pink (especially in males) just before a period of moulting. In females, the onset of maturity of the ovaries is signalled by the blood taking on a yellow colour.

Another type of oxygen carrier is chlorocruorin, found in polycheate worms. Solutions of this pigment are green when dilute but vivd red at higher concentrations. And then there are the bottom dwelling worms that use a moelcule called hemerythrin. This is colourless in the veins but when oxygenated becomes bright pink or violet. Finally, small creatures known as sea squirts use a pigment called vandium chromagen, which gives them green coloured blood - although it can also appear blue or orange in the presence of other chemicals in the blood.

So as you can see, blood comes in a true rainow of hues, although in humans, red is still very much de rigueur. For more detailed information on blood varieties, including a discussion of theoretical blood colours that could be found in aliens, see here.

3 comments June 9th, 2007