Red blood cells are called erythrocytes and they are the most common type of blood cell in human blood. They give blood its red color. In terms of structure, they are oval shaped with dimples in the center. They are very simple cells with no nucleus or major organelles as found in most other types of cells. This is so that they can hold as much hemoglobin as possible. The hemoglobin is what allows the red blood cell to perform its primary function, to transport oxygen from the lungs to various parts of the body. Hemoglobin is an organic molecule with an iron atom in the center. The iron atom effectively binds to O2 molecules to transport blood in the bloodstream. The hemoglobin also carries some of the carbon dioxide waste from the body back to the lungs to be expelled.