Blood cells. Coloured scanning electron micrograph (SEM) of a normal human blood cells, showing red blood cells (erythrocytes, red), a white blood cell (leucocyte, lower right) and platelets (thrombocytes, upper left). White blood cells are involved in defending the body against both infectious disease and foreign materials. The main function of red blood cells is to distribute oxygen to body tissues and to carry waste carbon dioxide back to the lungs. Platelets are fragments of white blood cells in involved in the clotting process.