Red blood cells are erythrocytes The function of the erythrocytes is to carry oxygen from lungs to tissues and carbon dioxide from tissues to lungs. The percentage of blood which is composed of red cells is termed the haematocrit; this may be lowered in some forms of anaemia. Red cells are biconcave discs. This shape provides a large surface area for gaseous exchange and also gives the cell flexibility. Some capillaries are so small the red cells have to squeeze through by deforming their shape. Red cells are about 7 micrometres in diameter (i.e. 7 one thousandths of a millimetre). Every cubic millimetre (mm3) of blood contains about 5 million red blood cells.