The largest part of the solid components of the blood is made up by red blood cells, also known as erythrocytes. These tiny, disk shaped cells do not have a nucleus and contain large amounts of an iron-containing blood pigment, haemoglobin, to which oxygen can bind for transport around the body. A proportion of carbon dioxide is also transported in the erythrocytes, the rest is simply dissolved in the blood plasma. Erythrocytes have a diameter of 7 micrometres (= 7 µm). One millilitre of blood contains between 6.5 and 10.5 thousand million erythrocytes.