Diagram of an Alveolus Capillary Alveolus (air sac) Oxygen in From pulmonary artery Oxygenated blood Oxygen Carbon dioxide out To pulmonary vein Deoxygenated blood Carbon dioxide Short diffusion distance One cell thick Diagram of an Alveolus Capillary Alveolus (air sac) Oxygen in From pulmonary artery Oxygenated blood Oxygen Carbon dioxide out To pulmonary vein Deoxygenated blood Carbon dioxide Short diffusion distance One cell thick