The alveoles are the structural elements of the lungs where the respiratory gases are exchanged between blood and alveolar air. They measure ca. 0,2mm. Alveoles consist of small cells (pneumocytes type II) that produce surfactant which reduces the surface tension, and alveolar macrophages that phagocytize dust. The alveoles are tightly connected to the capillary system of the lungs. Both basal membranes are largely fused together.