- carbon dioxide is carried: 7-8% dissolved in plasma, 22% coined withe the globin part of haemoglobin (forms carbaminohaemoglobin), 70% carried in plasma in bicarbonate ions - blood in the capillaries: most carbon dioxide reacts with water to form carbonic acid (H2CO3) - carbonic acid then dissolves into hydrogen ions and bicarbonate ions - CO2 + H20 <-> H2CO3 <-> H + HCO3