Red blood cells. Light micrograph of red blood cells (erythrocytes) in a venule (small vein). Red blood cells are responsible for carrying oxygen around the body, and for carrying waste carbon dioxide to the lungs for exhalation. Their red colour comes from molecules of the pigment haemoglobin. Each haemoglobin molecule can carry four molecules of oxygen. The unique biconcave shape of red blood cells maximizes the absorption and release of oxygen. Magnification: x540 when printed 10cm wide.