Is blood blue? Are veins blue? The answer is no. Blood is always red (at least for humans). Veins may appear blue, but they aren’t actually pigmented.
Is Deoxygenated Blood Blue?
Blood contains the iron-based pigment hemoglobin, which brings oxygen to cells and carries away carbon dioxide. Hemoglobin appears bright red when oxygenated and a darker maroon color when deoxygenated. The reason for the color change is that the shape of hemoglobin changes, depending on whether or not it has oxygen, which alters the way it scatters and absorbs visible light.
Blood circulating in the body is never blue, whether it’s oxygenated or not. But, veins sometimes appear blue.
Is Blood Is Red, Why Do Veins Look Blue?
Arteries, veins, and lymphatic vessels are essentially colorless. Veins sometimes appear blue because they contain darker blood than arteries and have thinner walls than arteries, making blood more visible. The blue color comes from the way skin scatters and absorbs light.
Why blue? Blue light does not penetrate skin as well as red light. It’s more likely to be scattered by skin and returned to the eye than red light. Also, skin contains melanin, which overlays underlying tissue with brown pigment. Part of the blue color is perception, where the brain compares the darker color of a vein against the paler color of skin and interprets the color as blue. If skin is more pink, veins appear blue. Against golden skin, veins appear more green. Veins in neutral skin may appear blue, green, or even purple.
Arteries don’t appear blue mainly because they are deeper in skin than veins and aren’t visible. Similarly, deeper veins aren’t blue.
Animals With Blue Blood
While human blood is never blue, there are animals with blue blood. For example, spiders, crabs, and octopuses use the pigment hemocyanin instead of hemoglobin. The copper-based pigment is blue so their hemolymph looks blue rather than red. Deoxygenated hemocyanin appears as a dull blue-gray color.
More exotic colors occur in the animal kingdom, too. Sea cucumbers have a vanadium-based protein called vanabin in their circulatory fluid that appears yellow. Some other marine invertebrates use hemerythrin as a pigment that is pink or violet when oxygenated and colorless when deoxygenated.
Skinks are vertebrates, like humans, but their blood appears green. The reason is that skink blood contains the protein biliverdin in addition to hemoglobin.
Why Some People Think Blood Is Blue
If you’ve always known blood is red, you may wonder why anyone would think otherwise. There are a few reasons for the misconception that either blood in the body is blue or else blood in veins/deoxygenated blood is blue.
- Textbooks usually depict arteries as red and veins as blue so that it’s easier to distinguish between them.
- Depending on skin color, veins may appear blue.
- When a person holds their breath, they may turn “blue.” In cyanosis, even arterial blood becomes deoxygenated. The blood darkens, but remains red. It looks blue because of changes in blood vessels that appear blue when viewed through skin.
- A trustworthy person may tell a child that blood is blue until it hits air.
Prove It to Yourself
If you need convincing, prove to yourself that blood is always red:
- Prick your finger in a cup of vegetable oil. The oil keeps out air, so there isn’t oxygen to react with blood. If blood in the body is blue, it should show up blue in the oil. (Spoiler: It’s red.)
- Examine the toes of a living frog under a magnifying glass or a low-powered microscope. Frog skin is thin enough that you can actually see the red blood cells running through the blood vessels.
- Donate blood. Watch the blood flow directly from a vein through the tube and note the color. Here, the blood is deoxygenated, so it is a dark maroon color and not the bright red of arterial blood.
References
- Allen, Michael (2014). Misconceptions in Primary Science (2nd revised ed.). McGraw-Hill Education. ISBN 978-0335262663.
- Brusca, R.C.; Brusca, G.J. (2003). Invertebrates (2nd ed.). Sinauer Associates. ISBN 978-0-87893-097-5.
- Wyatt, G. R. (1961). “The Biochemistry of Insect Hemolymph”. Annual Review of Entomology. 6: 75–102. doi:10.1146/ANNUREV.EN.06.010161.000451