The |alveoli| are the tiny sacs at the ends (or "leaves") on the bronchial tree. Each small |bronchiole| divides into half a dozen or so alveolar ducts, which are the narrow inlets into alveolar sacs. Each alveolar duct subdivides, leading into three or more alveolar sacs. Each large alveolar sac is like a grape cluster which contains ten or more |alveoli|. Because the membrane separating the |alveolus| and the |capillary network| which carries ~blood~ over them is very thin and semipermeable, oxygen can transfer from the ~air~ into the |blood cells| within the |capillaries|. Likewise, carbon dioxide and other waste gases can transfer out of the ~blood~ and into the ~air~ to be exhaled from the |lungs|. The |alveoli| are particularly susceptible to ~infection~, as they provide bacteria and viruses a perfect place to grow. This accounts for the tendency for a chest cold or other |lung| problem to advance into pneumonia and pneumonitis, both potentially dangerous conditions in which the innermost parts of the |lungs| become infected and inflamed, diminishing ~air~ flow and oxygen transport.
