Red blood cells. Coloured scanning electron micrograph (SEM) of red blood cells, also known as erythrocytes. These disc-shaped cells transport oxygen to the body tissues and carbon dioxide back to the lungs during respiration. They contain large amounts of haemoglobin, a pigmented protein that binds oxygen. The erythrocyte is one of several different types of blood cell, all of which develop from parental blood stem cells present in bone marrow. Erythrocyte production, known as erythropoiesis, occurs constantly and takes about five days. The lifespan of an individual erythrocyte is around 120 days. Magnification unknown.