Why is blood red?

Blood is red because it is made up of cells that are red, which are called red blood cells. But, to understand why these cells are red you have to study them on a molecular level. Within the red blood cells there is a protein called hemoglobin. Each hemoglobin protein is made up sub units called hemes, which are what give blood its red color. More specifically, the hemes can bind iron molecules, and these iron molecules bind oxygen.