At the end of each alveolar duct there are a number of sac-like structures called alveoli, it is within these structures that surfactant is produced. The alveoli are grouped together like a lot of interlinked caves, rather than existing as separate individual sacs.
Gas exchange of oxygen and carbon dioxide takes place in the alveoli. Oxygen from the inhaled air diffuses through the walls of the alveoli and adjacent capillaries into the red blood cells. The oxygen is then carried by the blood to the body tissues. Carbon dioxide produced by the body’s metabolism returns to the lung via the blood. It then diffuses across the capillary and alveolar walls into the air to be removed from the body with expiration. The alveoli have a structure specialised for efficient gaseous exchange: