When you think of blood, chances are you think of the color red. But blood actually comes in a variety of colors, including red, blue, green, and purple. This rainbow of colors can be traced to the protein molecules that carry oxygen in the blood. Different proteins produce different colors.
Humans, along with most other animals, birds, reptiles, and fish, have red blood. We all use an oxygen-carrying blood protein, known as hemoglobin, that contains iron. It’s the iron that gives blood its dark red color in the body. When blood comes into contact with air, it turns the classic scarlet red. Some people appear to have blue blood in their veins. That’s just an optical illusion caused by the way the skin filters light.
The term blue blood often refers to people related to kings and queens. But organisms with actual blue-colored blood are far from royal. They include snails, spiders, slugs, octopuses, and squid. The protein that carries oxygen in these creatures is called hemocyanin. Instead of iron, this protein contains copper. The blood appears clear when it’s not carrying oxygen. It turns blue when it picks up oxygen.
Science fiction aliens aren’t the only ones with green blood. Earthbound creatures with green blood include fantastically shaped sea worms, some leeches, and earthworms. These animals have a blood protein called chlorocruorin. It’s similar to hemoglobin but doesn’t hold oxygen as tightly. Also, it floats free in the bloodstream instead of being inside a blood cell.
Peanut worms, duck leeches, and bristle worms, all of which live in the ocean, use the protein hemerythrin to carry oxygen in the blood. Without oxygen, their blood is clear in color. When it carries oxygen, it turns purple.