At the carina' this divides in two main bronchi, one to the right lung and one to the left. These main bronchi' divide serially into two or three branches through approximately 25-27 divisions until they end in alveoli. As they divide into smaller and smaller tubes they lose the cartilage in their walls and cross sectional area becomes bigger allowing air flow to slow and become more laminar. Until about the 19th or 20th division the bronchi serve simply as conducting airways. From this point onwards however, they start to develop alveoli where gas exchange can occur and they are known as respiratory bronchioles'.
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As the bronchi divide and lose their cartilage rings, becoming bronchi, they become smaller and smaller in diameter. Eventually they terminate in a group of alveoli. These are thin walled sacs which have a very elaborate capillary network on their surface facilitating gas exchange. The barrier between blood and air at this point is extremely thin comprising only the specialised respiratory cell epithelium, a basement membrane, a small interstitial space, a basement membrane for the capillary, and the endothelium of the capillary. Oxygen and carbon dioxide readily diffuse across this membrane under normal conditions.
Functionally, the lungs comprise two parts: the airways or gas conducting/conditioning part, and the alveoli or gas exchange part. These two functions are closely related and are obviously both important but these two functions can be measured separately and may be affected differently by different diseases.
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The other components of the respiratory tract, respiratory muscles, nervous system, ribs, etc, while not primarily involved in respiration or gas exchange are important in supporting and controlling the functions of the respiratory tissues. Respiratory muscles and the diaphragm provide the mechanical means to move air in and out of the lungs, the pleura provide a lubricated surface allowing the chest wall to move over the ribs easily, and nerves are important in controlling respiratory movements as well as other functions such as mucous secretion and airway calibre.
