Breathing and exchange of gases chapter will tell you that pCO_2 plays an important role in the transportation of oxygen. The low pCO_2 and high pO_2 favors the formation of oxyhaemoglobin take place at alveolus. And at the tissues, the high pCO_2 and low pO_2 favor the dissociation of oxygen from oxyhaemoglobin. So, the affinity of haemoglobin for oxygen is enhanced by the decrease of pCO_2 in blood. Therefore, oxygen is transported in the blood as oxyhaemoglobin and oxygen dissociate from it at the tissues.