The Conversation – 24th July 2017
Just as a village can’t grow into a city without some form of transport (road, rail or river) that provides necessary interconnections for it to flourish, living things are limited in the size they can reach unless they have some form of circulatory system to transport nutrients and remove waste.
Single celled organisms such as bacteria and fungi, and some multicellular creatures such as sponges, corals and flatworms, simply absorb the nutrients they need and get rid of their waste using a passive process known as diffusion (which is much like soaking in and draining out).
More complex animals have developed some kind of circulatory system. A variety of different systems and pumps (hearts) have developed, but they all have a few things in common. These include something to carry oxygen around their bodies, a fluid of some sort, and some “plumbing” – in humans (and a number of other species) the fluid is called blood and the plumbing is our arteries, veins and capillaries. The oxygen carrier is haemoglobin.
Depending on the organism and where it has adapted to live, its oxygen carrier can come in different forms, often giving its “blood” different colours. Spiders, crustaceans, octopuses and squid use haemocyanin, which is based on copper and gives them blue blood. This carrier works well in low oxygen environments and in the cold.
Segmented worms and some leeches use an iron based carrier called chlorocruorin, which can appear either green or red, depending on its chemical environment. Vertebrates, including humans, use haemoglobin, which makes their blood red.
A truly special case is the Antarctic icefish, which lost its haemoglobin long ago as a result of a presumably random mutation. It has adapted though, and now survives by transporting oxygen that is simply dissolved in its blood. This is possible thanks to the cold conditions it lives in.