When fully mature, erythrocytes typically have a rounded, biconcave shape that gives them the appearance of a dumbbell when viewed from the side. Their design facilitates oxygen exchange at a continuous rate over the greatest possible area. Though most average humans have a small fraction of oval-shaped red blood cells as well, an unusually high number of such cells or other alternatively shaped erythrocytes is a characteristic of certain diseases. For instance, large numbers of oval red blood cells is a sign of pernicious anemia and crescent-shaped erythrocytes are a distinguishing feature of sickle cell anemia.