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Human blood is red because of the iron in the haemoglobin. This gives red blood cells their red colour. Veinous blood is much darker than arterial blood, and your teacher is right that oxygenated (arterial) blood is 'redder' than deoxygenated (veinous) blood. However, human blood is not blue. Some animals do have blue blood though.
Iron in Your Body. The majority of your bodyâs iron is found in hemoglobin, the part of red blood cells that carries oxygen for delivery to your cells. Hemoglobin is a highly complex molecule that has one atom of iron at its center. That one atom is what gives blood its red coloring.