The respiratory system consists of the external nose, internal nose, and paranasal sinuses; the pharynx, which is the common passage for air and food; the larynx, where the voice is produced; and the trachea, bronchi, and lungs. Accessory structures necessary for the operation of the respiratory system are the pleurae, diaphragm, thoracic wall, and muscles that raise and lower the ribs in inspiration and expiration. The muscles of the anterolateral abdominal wall are also accessory to forceful expiration (their contraction forces the diaphragm upward by pressing the contents of the abdominal cavity against it from below) and are used in “abdominal” respiration. Certain muscles of the neck can elevate the ribs, thus enlarging the anteroposterior diameter of the thorax, and under some circumstances, the muscles attaching the arms to the thoracic wall can also help change the capacity of the thorax.
In Plates 1-1 through 1-16, the anatomy of the respiratory system and significant accessory structures is shown. It is important not only to visualize these structures in isolation but also to become familiar with their blood supply, nerve supply, and relationships with both adjacent structures and the surface of the body. One should keep in mind that these relationships are subject to the same degree of individual variation that affects all anatomic structures. The illustrations depict the most common situations encountered. No attempt is made to describe all of the many variations that occur.
An important and clinically valuable concept that is worth emphasizing at this point is the convention of subdividing each lung into lobes and segments on the basis of branching of the bronchial tree. From the standpoint of its embryologic development, as well as of its function as a fully established organ of respiration, the lung is indeed the ultimate branching of the main bronchus that leads into it. Knowledge of the subdivision of the lung on this basis is essential to anatomists, physiologists, pathologists, radiologists, surgeons, and chest physicians because without this three-dimensional key, there is no exact means of precisely localizing lesions within the respiratory system.
