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Some animals have blue blood. People only have red blood. It's a surprisingly common misconception that deoxygenated human blood is blue.
Why Blood Is Red
Human blood is red because it contains a large number of red blood cells, which contain hemoglobin.
Hemoglobin is a red-colored, iron-containing protein that functions in oxygen transport by reversibly binding to oxygen. Oxygenated hemoglobin and blood are bright red; deoxygenated hemoglobin and blood are dark red.
Human blood does not appear blue under any circumstances.
Vertebrate blood, in general, is red. An exception is skink blood (genus Prasinohaema), which contains hemoglobin yet appears green because it contains a large amount of the protein biliverdin.
Why You Can Appear Blue
While your blood never actually turns blue, your skin can take on a bluish cast as a result of certain diseases and disorders. This blue color is called cyanosis.
If the heme in hemoglobin becomes oxidized, it may become methemoglobin, which is brownish. Methemoglobin, can't transport oxygen, and its darker color may cause the skin to appear blue.
In sulfhemoglobinemia, the hemoglobin is only partly oxygenated, making it appear dark red with a bluish cast. In some cases, sulfhemoglobinemia makes blood appear green. Sulfhemoglobinemia is very rare.
There Is Blue Blood (And Other Colors)
While human blood is red, some animals do have blue blood.
Spiders, mollusks and certain other arthropods use hemocyanin in their hemolymph, which is analogous to our blood. This copper-based pigment is blue.
Although it changes color when it is oxygenated, hemolymph typically functions in nutrient transport rather than gas exchange.
Other animals use different molecules for respiration. Their oxygen transport molecules may produce blood-like fluids that are red or blue, or even green, yellow, violet, orange, or colorless.
Marine invertebrates that use hemerythrin as a respiratory pigment may have pink or violet fluid when it's oxygenated, which becomes colorless when deoxygenated.
Sea cucumbers have a yellow circulatory fluid because of the vanadium-based protein vanabin. It's unclear whether vanadins participate in oxygen transport.
See For Yourself
If you don't believe human blood is always red or that some animal blood is blue, you can prove this to yourself.
- You could prick your finger in a cup of vegetable oil. There is no oxygen in oil, so the red oxygenated blood would change to blue if the myth were true.
- A really interesting way to examine blood is to view the toes of a living frog under a magnifying glass or low-powered microscope. You can see that all of the blood is red.
- If you want to see blue blood, you can examine the hemolymph of a shrimp or crab. The oxygenated blood is blue-green. Deoxygenated hemolymph is more of a dull grayish color.
- Donate blood. You'll get to watch it leave your veins (oxygenated) and collect in a bag (where it becomes deoxygenated).
You can adapt the slime recipe to make blue blood for projects.
One of the reasons many people think deoxygenated blood is blue is because veins appear blue or green beneath the skin. Here's an explanation of how that works.