THE NUCLEUS Issue 2 Spring 2024

WHAT IS BLOOD?

BY ALEXANDER HORNSEY

Many cultures around the world have an interesting relationship with blood - blood rituals, menstrual blood, even vampires. Let’s take a tour of different types of blood and why they evolved.First off, what is the purpose of blood? Well, human blood has four components, as your GCSE biology syllabus says: red blood cells, white blood cells, plasma and platelets. Red blood cells distribute oxygen around the body, white blood cells fight off pathogens, platelets clot, and plasma have a myriad of functions, including homeostasis (a very complex process, which does not change much across the animal world, however, animals and birds, as warm-blooded animals, have specific proteins that allow them to keep their high body temperature relative to their environment),a relatively simple job because there is not much selective pressure acting on them.

So . . . Why is blood red? We have haemoglobin in our blood, a protein which contains iron (the ion of which is . . . red!), so that it will bind to oxygen. There are other available blood colours however! For example, octopuses, and other molluscs, have hemocyanin instead, which is a blue/blue-green colour and uses copper instead of iron. One of the reasons for this is because seawater has more dissolved copper than iron in it. The other main evolutionary benefit is that hemocyanin does not denature at lower temperatures; and so is better for cold environments than haemoglobin. Some marine worms and leeches have green blood – this contains chlorocruorin, which can carry more oxygen than haemoglobin, and has nitrogen in its formula. There is not known to be a specific evolutionary reason for this, beyond it being more efficient at its job. Some other marine worms have hemerythrin or vanabin, which cause a purple hue, and can function in very-low oxygen environments, like the deep sea. Now the most interesting groups. Some fish have transparent blood – they have no pigmented protein to carry oxygen with! They are small enough, and have a sophisticated enough respiratory system, where they can rely on plasma to transport oxygen for them. Finally, some insects and annelids have haemolymph, which relies on dissolved oxygen – this is more efficient than haemoglobin, but only works for very small creatures (think surface area to volume ratios!).

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