Erythrocytes, also known as red blood cells, are a key part of our blood system. Their main task is to transport oxygen from the lungs to all parts of the body and return carbon dioxide back to the lungs for exhalation. Erythrocytes are somewhat special cells because, unlike most other cells in the body, they do not contain a nucleus. They also have a special, biconcave shape, which allows them to adapt and travel through narrow capillaries. Their red color comes from hemoglobin, a protein that binds oxygen. A lack of erythrocytes or hemoglobin leads to anemia or blood deficiency.