Red blood cells. Colored scanning electron micrograph (SEM) of a group of red blood cells. Red blood cells, or erythrocytes, are small biconcave disc-shaped cells. Their red color comes from the pigment hemoglobin, which is made up of a protein and an iron-containing structure. The hemoglobin in red blood cells is responsible for transporting oxygen from the lungs to organs and tissues where it is needed for respiration. Red blood cells are different from most cells in that they do not possess a nucleus. Magnification: x2520 at 6x7cm size.