Bronchi: At the lower end of the trachea, there is a division into the right main stem bronchus and left main stem bronchus (primary bronchi) at the carina. The walls of the bronchi are made of cartilage with a ciliated mucous lining. The extrapulmonary portion of each bronchus enters the lung on its own side and immediately divides into branches called lobar (secondary) bronchi. The secondary bronchi continue to branch, forming bronchioles which become smaller and smaller, forming microscopic branches which divide into alveolar ducts. At the terminal ducts are grape-like formations known as alveolar sacs which provide surfaces where the exchange of air and carbon dioxide takes place. The intrapulmonary (lobar) bronchi are lined with ciliated columnar, mucin-producing epithelium. The walls are composed of a submucosa, smooth muscle, fibrous tissue and cartilage.