Why are veins blue if our blood is red

We all know that blood is red. But the veins that show through our skin sometimes look frighteningly blue. Why?

Even when the blood dries up, it does not turn blue. What’s the catch? 

Many people believe that oxygen-rich blood is red, while oxygen-poor blood is blue. But it is not. Let’s figure it out!

Blood is always red 

Each molecule of hemoglobin, the oxygen-carrying protein in red blood cells, contains four iron atoms, which reflect red light and therefore give our blood its red color. Hemoglobin absorbs blue and green light waves, so blood cannot be these colors.

However, the shade of red can change depending on the level of oxygen in the blood. When hemoglobin absorbs oxygen from the lungs, it turns the blood bright cherry. In this color, it enters the arteries and tissues throughout the body.

But on the way back to the lungs, after the blood cells have delivered oxygen to tissues throughout the body, the deoxygenated blood (that is, the blood that flows through the veins) becomes darker, turning purple.

What about vein color? 

Your blood cannot be blue or blue. And although it seems to us that the veins have a blue tint, the blood in them still flows red.

Scientists explain that this color of the vessel is an illusion caused by the fact that the vein is under a small but significant layer of skin. The colors we see are based on the wavelength our retina perceives. And the layers of skin cause the wavelengths to scatter.

For example, the darker a person’s skin, the greener a vein will appear. With fair skin, the veins appear blue. This is because green and blue wavelengths of light are shorter than red ones. Red waves are absorbed by our skin, green and blue waves are reflected and scattered back.

Other blood vessels, such as tiny capillaries closer to the surface, are not as affected by this illusion. The fingertips appear pink precisely because the vessels are located much closer to the surface.

However, blue blood exists in nature! Crabs, lobsters, octopuses and spiders have it. The fact is that the blood of these creatures contains copper instead of iron, which gives it a blue color.