Alveole. The alveoli are the structural unit in the lung in which, during the respiration, the gas exchange between blood and (alveolar) air happens. They measure ca. 0.2mm. Alveoli consist of small alveolar cells (pneumocytes type I) on a basement membrane, of big alveolar cells that produce surfactant to decrease the surface tension, and of alveolar macrophages that phagocytize dust particles. The alveoli are closely connected with the pulmonary capillary system. Both basal membranes are largely melted into each other.