Blood is red due to hemoglobin (the molecule that carries oxygen around) which is held in the red blood cells. In the hemoglobin are four atoms of iron which can be in an oxidized state, i.e with an atom of oxygen attached to it, making it red, or in a non-oxidized state, when it is dark red-blue (maroon). Blood is pumped through the lungs, where it is oxygenated. Then it travels around the body to where the oxygen is required. When the oxygen is used up, the hemoglobin is routed back via the veins, and then back through the lungs again.