The red blood cell (RBC), or erythrocyte, is a flexible biconcave disc 8 �m in diameter. Its main function is to carry oxygen from the lungs to the tissues of the body and to perform this it contains a pigment, hemoglobin. During its 120-d lifespan, it travels about 300 miles around the arteriovenous circulation, repeatedly passing through the capillary bed. As the mean diameter of a capillary is about 3 �m, the red cell has to retain a high degree of flexibility which requires energy. Energy is generated as adenosine triphosphate (ATP) by the anaerobic, glycolytic pathway.
