Alveoli

When the immune response is compromised, Pseudomonas aeruginosa biofilms are able to colonize the alveoli, and to form biofilms

"This portion of the bronchi is called the respiratory portion. The epithelium in this section is much thinner then in the conducting portion. This section is branches from the terminal bronchioles to form the respiratory bronchioles. These branches divide to form smaller respiratory bronchioles, the smallest branches become alveolar ducts which are lined with simple squamous epithelium. At the distal end of this alveolar ducts terminate into alveolar sacs that are dilated. Along the previous branches (respiratory bronchioles and alveolar ducts) are out pocket sacs called alveoli (different from alveolar sacs). The thin wall covered with these alveolus sacs used to promote diffusion of gases between alveolus and blood in the pulmonary capillaries. There are about 300 to 400 million alveoli in the lungs. There are two types of cells in the alveolar walls. The first being alveolar type I cells (aka squamous alveolar cells) which promote rapid gas diffusion across the alveolar wall and are the more prominent cell. The second type of cell is the alveolar type II cell (aka septal cells), which secretes pulmonary surfactant which reduces the tension on the wall to prevent collapsing of the alveoli." http://anatowiki.wetpaint.com/page/Respiratory+system

sources: http://en.wikipedia.org/wiki/Pulmonary_alveolus