Red blood cells. Coloured scanning electron micrograph (SEM) of two red blood cells (RBCs). RBCs, or erythrocytes, make up 99% of cells in the blood. They carry red-coloured haemoglobin molecules, which in turn carry oxygen around the body. Haemoglobin is required because oxygen is not very soluble in water and not enough oxygen would be carried by the bloodstream if it was simply dissolved in blood. Each haemoglobin molecule can carry four molecules of oxygen and the unique surface shape (biconcave) of RBCs maximises the absorbtion and release of oxygen. Magnification: x 13000.