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If blood is red, why do veins look blueish?

It’s commonly said that oxygen-rich blood is red, while oxygen-poor blood is blue. But is that really true?

In a word, no. Blood is always red. Every molecule of hemoglobin — a protein in red blood cells that carries oxygen — contains four atoms of iron, which reflect red light and so give our blood the red color. The shade of red changes depending on the level of oxygen in the blood. When the hemoglobin picks up oxygen in the lungs, the blood is a bright cherry red as it heads into the arteries and out to the tissues around your body. 

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