Blood is pretty important stuff; we've even got banks to make deposits and withdrawals, and anything stored in a bank must be very valuable. Banks aside, we all know why it's so important: lose too much of it, and you die.
Image by Mate Marschalko - source: Pixabay
In the evolution of life on earth, the appearance of blood and circulatory systems was one of the more revolutionary developments. While simple organisms could just absorb oxygen directly by diffusion through the skin, more complex organisms needed a way to deliver oxygen, nutrients and dispose waste materials to and from many different organs in the body. In our own circulatory system blood is pumped through arteries by the heart, through the lungs where oxygen is added and subsequently transported to tissues and organs, and finally returns to the heart through our body's veins.
Blood's main function is the delivery of oxygen; this is needed for all living organisms to burn fat or carbohydrates in order to generate energy. The color of blood is determined by the chemicals used to bind the oxygen to the blood.
Red
Our blood, and that of most animals is red. And it's always red, contrary to the popular belief that blood is blue when it runs through our arteries and veins. Sure, it looks blue when you're white skinned, but that's just because skin reflects more blue, and absorbs more red light, when it reaches a certain depth under the skin. Our blood is red because we use hemoglobin. A globin molecule has a feature that binds to an iron-atom; it's this iron that gives our blood it's rich red color. Blood in arteries is a blighter red because it still has the oxygen in it, where blood from our veins is a darker red; our bodies have smart circulatory design, as arteries are mostly buried deep inside, and veins are often right under our skin, limiting the risk of accidental cuts and other surface wounds.
Blue
There are however animals that actually have blue blood, such as spiders and crustaceans. Their blood is blue, because they use a different protein to transport oxygen, called hemocyanin, that bind to a copper-atom instead of iron. When that copper comes into tough with oxygen, it turns blue. Hemocyanin and hemoglobin are the most common oxygen carrying blood-proteins found in animals today.
Underside of a Cancer productus crab. If you every doubt that arthropods have pigment in their blood which have an open circulatory systems (blood runs free in the body without vessels), turn over a crab and if you are a little lucky, you will see the purple stain seen here left from the pigment. They do not have hemoglobin, but sure do have hemocyanin. - source: Wikimedia Commons
Purple / violet
Then there's some animals that use yet another oxygen carrying protein, called hemerythrin. This uses iron too, but has a much simpler structure; it turns a bright violet when oxygenated.
Green
Blood can be green to; some lizards for example have a green pigment in their blood called biliverdin, which is produced when hemoglobin is broken down.
White
Lastly there's the occasional animal that have gotten rid of all blood-proteins, which has turned their blood white. The icefish lives in oxygen-rich waters and has a slow metabolism to make up for this.
There's a lot more to say about this, and about the evolution of life's circulatory systems. If you'd like to learn more, the below video is good place to start.
How Blood Evolved (Many Times)
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