Alveoli are structural elements of the lung. During respiration, gas exchange beween blood and alveolar air takes place here. They are 0.2 mm big. Alveoli consist of small alveolar cells (pneumocytes type I) that sit on a basal membrane and big alveolar cells (pneumocytes type II) that produce surfactant to reduce surface tension and alveolar macrophages that phagocytes dust. The alveoli are closely connected to the capillary system of the lung. Both basal membranes are mostly merged.