Alveolar gas exchange
The alveoles are the structural elements of the lung in which, during respiration, the gas exchange between blood and alveolar air takes place. They measure ca. 0,2mm. Alveoles consist of small alveolar cells (type I pneumocytes) which sit on a basement membrane, big alveolar cells (type II pneumocytes) which produce surfactant to reduce the surface tension, and alveolar macrophages that phagocytize dust. The alveoles are tightly connected to the capillary system of the lung. Both basement membranes are largely coalesced.