External respiration is the process that occurs in the lungs by which gasses, especially O2 and CO2, are exchanged between the blood and the air. The specific structures for this exchange are the minute, surface-area-ridden alveoli. The process by which the gasses are exchanged is simple diffusion. The higher concentration of carbon dioxide in the blood causes it to diffuse into the air whereas the higher concentration of oxygen in the air causes it to diffuse into the blood. Since most of the CO2 in the blood is kept in the form of the bicarbonate ion, this ion must be decomposed before CO2 gas can be expelled. The enzyme carbonic anhydrase catalyzes this reaction. De-oxy-haemoglobin (Canadian Spelling) combines with oxygen that has diffuses into the blood to form oxy-haemoglobin. Also, those molecules of haemoglobin that had been carrying CO2 molecules, in the complex carbamino-haemoglobin, liberate those CO2 molecules at this point, allowing the CO2 to diffuse across the membrane and a greater amount of O2 to be absorbed.