Contrary to popular belief, the blood inside your body isn't blue.
You may have heard the popular urban myth that your blood is blue until it mixes with the Earth's atmosphere when you cut yourself.
While there are some creatures that have blue blood, humans definitely are not one of them.
So if our blood is always red, then why are the veins that carry our blood blue?
Human blood is always red. Even if it flows out from a wound, human blood doesn't change color.
Our veins only appear to be blue because of the way that our skin reflects different light waves. Human skin scatters blue light waves more than red ones.
The veins appear blue because they are under our skin.
This effect is amplified by an optical illusion. The skin around our veins looks more red than the vein itself, so the contrast enhances the veins' bluish hue.
Human blood is red because it is full of iron. When iron mixes with the oxygen in our lungs, it gives our blood that deep red color.
Oxygen-rich blood is bright red when it leaves the heart, but when it returns to the heart without much oxygen, it appears as a deeper, darker red.
However, Crustacean and spider blood is full of copper, so when it comes in contact with oxygen, it turns blue!
Learn more in the video below!
[h/t Goodfulness]