Quirks & Quarks

Do all creatures on Earth have red blood?

Our blood gets its red colour from the protein hemoglobin, but there are other proteins and blood colours among other species.

Our blood gets its red colour from the protein hemoglobin, but there are other colours in other animals

Canadian Blood Services announced its decision to drop its masking and physicial distancing requirements on Monday (Azami Adiputera/Shutterstock)

This week's question comes to us from Jim Bishop in Thunder Bay, Ont. He asks: 

Is the blood of all creatures on Earth red in colour? Have there been any changes in any blood colour over time?

Graham Scott, an associate professor in the department of biology at McMaster University, explains that our blood gets its red colour from large amounts of the protein hemoglobin. This protein transports oxygen from our lungs to tissues throughout the body. Our blood is bright red when it is carrying oxygen, but turns a dark burgundy colour after it releases oxygen to the tissues. 

Vertebrates, including mammals, birds, amphibians, reptiles and fish also have red blood because they too use hemoglobin as an oxygen transport protein. 

Many spiders, centipedes, crustaceans and molluscs use an oxygen transport system called hemocyanin, a protein that is blue when it carries oxygen. As a result, hemocyanin gives their blood a blue appearance.

Some marine worms have an oxygen transport protein that makes their blood look green. Insects have an entirely different oxygen transport system called trachea. The colour of the blood in these animals is determined by pigments from their diet. 

It is unlikely that the colour of blood, or the colour of proteins like hemoglobin or hemocyanin have changed over the course of evolution. But as animals have adapted to different environments over time, with different needs for oxygen transport, the way in which these proteins bind oxygen has changed.

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