Structure of the Respiratory System

The respiratory system is represented by the following structures, shown in Figure 1:

  • The nose consists of the visible external nose and the internal nasal cavity. The nasal septum divides the nasal cavity into right and left sides. Air enters two openings, the external nares (nostrils; singular, naris), and passes into the vestibule and through passages called meatuses. The bony walls of the meatuses, called concha, are formed by facial bones (the inferior nasal concha and the ethmoid bone). From the meatuses, air then funnels into two (left and right) internal nares. Hair, mucus, blood capillaries, and cilia that line the nasal cavity filter, moisten, warm, and eliminate debris from the passing air.

  • The pharynx (throat) consists of the following three regions, listed in order through which incoming air passes:

    • The nasopharynx receives the incoming air from the two internal nares. The two auditory tubes that equalize air pressure in the middle ear also enter here. The pharyngeal tonsil (adenoid) lies at the back of the nasopharynx.

    • The oropharyrnx receives air from the nasopharynx and food from the oral cavity. The palatine and lingual tonsils are located here.

    • The laryngopharynx passes food to the esophagus and air to the larynx.

  • The larynx receives air from the laryngopharynx. It consists of several pieces of cartilage that are joined by membranes and ligaments, shown in Figure 2:

    • The epiglottis, the first piece of cartilage of the larynx, is a flexible flap that covers the glottis, the upper region of the larynx, during swallowing to prevent the entrance of food.

    • The thyroid cartilage protects the front of the larynx. A forward projection of this cartilage appears as the Adam's apple (anatomically known as the laryngeal prominence).

    • The paired arytenoid cartilages in the rear are horizontally attached to the thyroid cartilage in the front by folds of mucous membranes. The upper vestibular folds (false vocal cords) contain muscle fibers that bring the folds together and allow the breath to be held during periods of muscular pressure on the thoracic cavity (straining while defecating or lifting a heavy object, for example). The lower vocal folds (true vocal cords) contain elastic ligaments that vibrate when skeletal muscles move them into the path of outgoing air. Various sounds, including speech, are produced in this manner.

    • The cricoid cartilage, the paired cuneiform cartilages, and the paired corniculate cartilages are the remaining cartilages supporting the larynx.

  • The trachea (windpipe) is a flexible tube, 10 to 12 cm (4 inches) long and 2.5 cm (1 inch) in diameter (Figure 2).

  • The mucosa is the inner layer of the trachea. It contains mucus‐producing goblet cells and pseudostratified ciliated epithelium. The movement of the cilia sweeps debris away from the lungs toward the pharynx.

  • The submucosa is a layer of areolar connective tissue that surrounds the mucosa.

  • Hyaline cartilage forms 16 to 20 C‐shaped rings that wrap around the submucosa. The rigid rings prevent the trachea from collapsing during inspiration.

  • The adventitia is the outermost layer of the trachea. It consists of areolar connective tissue.

  • The primary bronchi are two tubes that branch from the trachea to the left and right lungs.

    • Inside the lungs, each primary bronchus divides repeatedly into branches of smaller diameters, forming secondary (lobar) bronchi, tertiary (segmental) bronchi, and numerous orders of bronchioles (1 mm or less in diameter), including terminal bronchioles (0.5 mm in diameter) and microscopic respiratory bronchioles. The wall of the primary bronchi is constructed like the trachea, but as the branches of the tree get smaller, the cartilaginous rings and the mucosa are replaced by smooth muscle.

    • Alveolar ducts are the final branches of the bronchial tree. Each alveolar duct has enlarged, bubblelike swellings along its length. Each swelling is called an alveolus. Some adjacent alveoli are connected by alveolar pores.

  • The respiratory membrane consists of the alveolar and capillary walls. Gas exchange occurs across this membrane. Characteristics of this membrane follow:

    • Type I cells are thin, squamous epithelial cells that constitute the primary cell type of the alveolar wall. Oxygen diffusion occurs across these cells.

    • Type II cells are cuboidal epithelial cells that are interspersed among the type I cells. Type II cells secrete pulmonary surfactant (a phospholipid bound to a protein) that reduces the surface tension of the moisture that covers the alveolar walls. A reduction in surface tension permits oxygen to diffuse more easily into the moisture. A lower surface tension also prevents the moisture on opposite walls of an alveolus or alveolar duct from cohering and causing the minute airway to collapse.

    • Alveolar macrophage cells (dust cells) wander among the other cells of the alveolar wall, removing debris and microorganisms.

    • A thin epithelial basement membrane forms the outer layer of the alveolar wall.

    • A dense network of capillaries surrounds each alveolus. The capillary walls consist of endothelial cells surrounded by a thin basement membrane. The basement membranes of the alveolus and the capillary are often so close that they fuse.

Figure 1.A view of the entire respiratory system and the upper respiratory tract.

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Figure 2.Anterior and sagittal sections of the larynx and the trachea.


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