22.1:
The Respiratory System
The respiratory system consists of a series of passageways and ducts that bring air from the outside of the body to the lungs inside.
Air first enters through the nostrils and into the nasal cavity where particulate matter, potentially harmful to breathe like bacteria and dirt, is filtered out by hairs and mucus and cleaner air is warmed and humidified.
Air can also flow through the mouth, reaching the pharynx and larynx before being funneled into the main airway, the trachea.
As the trachea leads into the thoracic cavity, the ends bifurcate into two bronchi, corresponding to the left and right lungs, which are surrounded by pleural membranes and bound inferiorly by the diaphragm.
Within each lung a bronchus splits into increasingly smaller branches called bronchioles. At the smallest tips are alveolar ducts subdivided into tiny sacs of alveoli, thin layers of parenchymal cells.
Such organization provides a large surface area for gas exchange to occur with the circulatory system.
22.1:
The Respiratory System
The respiratory system is comprised of the organs that enable breathing. Air enters the nostrils and mouth, followed by the pharynx (throat) and larynx (voice box), which lead to the trachea (windpipe). In the thoracic cavity, the trachea splits into two bronchi that allow air to enter the lungs. The bronchi split into progressively smaller bronchioles and terminate in small groups of tiny sacs in the lungs called alveoli, where gas exchange occurs.
Removal of Debris
Air is cleansed in the nasal cavity, but anything that passes those defenses or enters through the mouth can be caught in the lungs. The lungs produce mucus that traps foreign particles, and the bronchi and bronchioles are lined with cilia that beat mucus and debris upward toward the throat for disposal (i.e., swallowing). Smoking damages the cilia, making removal of the excess mucus produced by smoking more difficult. This is one of the reasons smokers are more susceptible to respiratory infections.
Trachea
The trachea is a 10-12 cm long tube located in front of the esophagus that allows air to enter and exit the lungs. Its C-shaped hyaline cartilage keeps the trachea open. When the smooth muscle of the trachea contracts, the diameter of the trachea decreases and exhaled air is pushed out with great force (e.g., coughing). In cases of damage to the throat or mouth that blocks breathing, a tracheostomy, a surgically-created hole in the trachea, can allow air into and out of the lungs.
Alveoli
A single alveolar duct at the end of a bronchiole divides into approximately 100 alveolar sacs, which each resembling a bunch of grapes on a stem. Each alveolar sac has 20-30 alveoli, which look like little bubbles, or grapes. Alveoli are the sites of gas exchange and have direct contact with capillaries. This structure is crucial to the function of the lungs—providing the body with oxygen and removing carbon dioxide—because it maximizes the surface area of the lungs. The lungs contain 75 m2 of alveoli surface area, the size of a small apartment!
Suggested Reading
Hogan, Brigid L. M., Christina E. Barkauskas, Harold A. Chapman, Jonathan A. Epstein, Rajan Jain, Connie C. W. Hsia, Laura Niklason, et al. “Repair and Regeneration of the Respiratory System: Complexity, Plasticity, and Mechanisms of Lung Stem Cell Function.” Cell Stem Cell 15, no. 2 (August 7, 2014): 123–38. [Source]