Lower (Distal)
-Bronchi split (bifurcate) repeatedly into two smaller passages (17 times,2 = 131,072) called bronchioles.
-Diameters decrease accordingly.
-Bronchioles end in microscopic sacs called alveoli (site of gas exchange).
-Alveolar membrane is one cell thick (pneumocytes), surrounded by capillaries.
-Passive diffusion.