Cellular Structure of the Lower Respiratory Tract

Respiration is accomplished via the function of a multitude of molecules, cells, glands, and muscles. The epithelial lining in the respiratory tract provides protection by secreting mucus, removing irritants, and activating the immune system. Air is approximately 79% nitrogen and 21% oxygen.5 Air is conducted through the respiratory system into the structures involved in gas exchange, which occurs primarily in the bronchioles, alveolar ducts, alveolar sacs, and the alveoli. Three layers make up the walls of the trachea and bronchi: respiratory mucosa, basement membrane and submucosa. The inner surface of the respiratory mucosa is lined with ciliated columnar epithelial cells containing goblet cells, which produce mucus. The basement membrane provides support for the mucosa. The submucosa contains cells that include muscle, mucous and serous glands, mast cells, lymphocytes, fibroblasts and blood vessels.6 Normally the muscle fibers are relaxed, but bronchoconstriction may occur as a protection mechanism or as part of an overreaction in hypersensitive airways. For example, cold air may produce bronchoconstriction for some people. Epithelial cells line the airways but can become inflamed and hyper-reactive. Goblet cells produce mucus, but excess mucus can narrow the airway and cause asthma symptoms. Mast cells may mediate inflammation, but may also increase swelling in asthma. Muscle tissue normally regulates the open lumen, but may contract and limit airflow.

The exchange of oxygen into the bloodstream and carbon dioxide out of the bloodstream occurs via diffusion, which is simply the movement of gases from an area of high concentration to an area of low concentration. Specifically, there is a high concentration of oxygen in the alveoli, so oxygen moves from the alveoli into the blood, which has a lower concentration of oxygen. The opposite is true for carbon dioxide, which moves out of the blood and into the alveoli.