- oxygen is carried: 3% dissolved in blood plasma, 96% as oxyghaemoglobin - Hb + O2 <-> HbO2: oxygen combines with haemoglobin where oxygen concentration is high - oxygenated blood is blood with a high proportion of oxyhaemoglobin - oxyhaemoglobin breaks down into oxygen and haemoglobin where oxygen concentrations are low - oxygenhaemoglobin is bright red in colour, so the blood in the arteries is bright red - haemoglobin is dark red or purplin in colour, the deoxygenated blood in veins is therefore dark red