Snails, spiders and octopi have something in common- they all have blue blood! We're not talking in the sense of royalty, these creatures literally have blue blood. So why is their blood blue and ours red?
One of the purposes of blood is to carry oxygen around the body. That transport system is actually quite complex, not just a matter of oxygen dissolving in liquid blood.
In humans, the oxygen is carried around by a molecule known as hemoglobin which has an iron atom imbedded in its structure. Oxygen binds to this iron atom and is transported to cells where it is released. If there is insufficient iron in the system, a condition known as iron deficiency anemia results, which basically is a form of oxygen starvation. The oxyhemoglobin molecule absorbs all colours of light but reflects red, thereby accounting for the red colour of blood.
Unlike mammals, snails, spiders and octopi do not use hemoglobin to transport oxygen but rely on a related compound known as hemocyanin. This molecule, instead of having an atom of iron in its middle, has an atom of copper that binds oxygen. Hemocyanin absorbs all colours except blue which it reflects, making their blood appear blue.
So why do human veins appear blue from the outside? Since the white light that comes from the sun or out lightbulbs contains within it all colours of light, some colours penetrate our skin deeper than others. Blue light doesn't penetrate our skin as well as red light, so it bounces back to our eyes and makes our veins appear blue.
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