In human blood, erythrocytes (red blood cells) contain a red-coloured, iron-containing complex protein called haemoglobin . About 97 percent of oxygen is transported by red blood cells in the blood . Oxygen binds reversibly to haemoglobin to form oxyhaemoglobin, with each haemoglobin molecule capable of carrying a maximum of four molecules of oxygen . Therefore, haemoglobin acts as the main oxygen carrier in the human body.