In order to understand why is blood red, we have to look at the molecules that it is made up of. The cells that form blood are known as red blood cells. Inside the red blood cells you find hemoglobin which is a type of protein. Hemoglobin proteins contain hemes. Hemes make blood red in color. They do this by binding iron molecules and oxygen. The iron molecules interact with the oxygen and this produces the red color. Iron reflects red light and there is a lot of iron in our blood.
Blood is made up of several components including red blood cells, white blood cells and platelets. They are contained within a clear fluid called plasma. Red blood cells make up the majority of cells thus dominating with an overall red color.
Blood needs to be able to carry oxygen around the body. As blood travels through the lungs, it gathers up oxygen. This oxygen is moved throughout the body. Once oxygen has been used up the blood will pick up more oxygen from the lungs for circulation. When there is a lot of oxygen in your blood, is is much brighter in color. When there is little oxygen, it will appear a lot darker.
Blue blood myth
Since our blood is red, people often wonder why our veins are blue. Some people may even think that our blood is blue inside and only turns red when it is exposed to air. But this is not true. Blood is always red. On people with a pale skin color, the veins look blue because of the way light is passing through the skin. Skin causes light to reflect differently on blood deeper in our veins and our eyes are picking this up as a blue or green color.
Interestingly, some animals do have blue blood. For example, a lot of crustaceans, including lobsters and crabs, have blue blood. But this is not the case for humans and all other animals with backbones.