Red blood cells contain hemoglobin and are covered with a membrane composed of proteins and lipids. Hemoglobin—an iron-rich protein that gives blood its red color—enables red blood cells to transport oxygen and carbon dioxide. Red blood cells do not have nuclei, allowing for more room for hemoglobin.
The shape of red blood cells is a unique biconcave shape (round with a flat, indented center). Their lack of nuclei makes them so flexible that they can pass through extremely small blood vessels.