Light microscopy of red blood cells (erythrocytes) in a blood vessel. Erythrocytes are biconcave in shape, about 7 micrometers in diameter and being deformable enables them to squeeze along the narrowest capillaries that may be only 4 micrometers wide. Filled with haemoglobin for transport of oxygen to tissues, they are formed in bone marrow and survive up to 4 months when most are removed by the spleen. Magnification x600 when printed at 10cm.