13 RESPIRATORY SYSTEM
NASAL CAVITIES AND PARANASAL SINUSES
Each nasal cavity, separated from the other by the septum, consists of the vestibule, the respiratory portion, and the olfactory area (Figure 13-1).
The respiratory portion is lined by a pseudostratified ciliated epithelium with goblet cells supported by the lamina propria, which consists of connective tissue with seromucous glands. The lamina propria has a rich superficial venous plexus, known as cavernous or erectile tissue. The lamina propria is continuous with the periosteum or perichondrium of bone or cartilage, respectively, forming the wall of the nasal cavities.
Nasopharynx
The auditory tubes (eustachian tubes), extending from the middle ear, open into the lateral walls of the nasopharynx.
Olfactory epithelium
The olfactory epithelium contains three major types of cells (Figures 13-2 and 13-3): (1) basal cells, (2) olfactory cells (bipolar neurons), and (3) supporting or sustentacular cells.
The olfactory cell is highly polarized (see Figure 13-3). The apical region, facing the surface of the mucosa, forms a knoblike ending (called olfactory vesicle or olfactory knob) with 10 to 20 modified cilia. The basal region gives rise to an axon. Several axons, projecting from the olfactory cells, form small unmyelinated bundles (called olfactory fila; from Latin filum, thread) surounded by glial-like cells. Nerve bundles cross the cribriform plate of the ethmoid bone and contact in the glomerulus dendrites of mitral cells, neurons of the olfactory bulb, to establish appropriate synaptic connections (see Box 13-A).
Box 13-A Olfactory epithelium
LARYNX
The wall of the larynx is made up of the thyroid and cricoid hyaline cartilage and the elastic cartilage core of the epiglottis extending over the lumen (Figure 13-4).
The larynx can be subdivided into three regions:
During forced inspiration, vocal cords are abducted, and the space between the vocal cords widens.
During phonation, the vocal cords are adducted and the space between the vocal cords changes into a linear slit. The vibration of the free edges of the cords (a cover consisting of both the stratified squamous epithelial covering and the superficial layer of the lamina propria, known as Reinke’s space) during passage of air between them produces sound. The contraction of the intrinsic muscles of the larynx, forming the body of the cords, increases tension on the vocal cords, changing the pitch of the produced sound (see Box 13-B).
Box 13-B True vocal cords or folds
Laryngeal seromucous glands are found throughout the lamina propria, except at the level of the true vocal cords. The lamina propria of the true vocal cords consists of three layers (see Figure 13-4): (1) a superficial layer containing extracellular matrix and few elastic fibers. This layer is known as Reinke’s space; (2) an intermediate layer with an increased content of elastic fibers; and (3) a deep layer with abundant elastic and collagen fibers.
TRACHEA
The right lung has three lobes, whereas the left lung has two lobes.
The trachea and main bronchi are lined by pseudostratified columnar ciliated epithelium resting on a distinct basal lamina. Several types of cells can be identified (Figure 13-5):
The lamina propria contains elastic fibers. The submucosa displays mucous and serous glands that, together with goblet cells, produce components of the airways mucus (see Box 13-C).
Box 13-C Airway mucus
The framework of the trachea and extrapulmonary bronchi consists of a stack of C-shaped hyaline cartilages, each surrounded by a fibroelastic layer blending with the perichondrium. In the trachea and primary bronchi, the open ends of the cartilage rings point posteriorly to the esophagus. The lowest tracheal cartilage is the carinal cartilage. Transverse fibers of the trachealis muscle attach to the inner ends of the cartilage. In branching bronchi, cartilage rings (see Figure 13-5) are replaced by irregularly shaped cartilage plates (Figure 13-6), surrounded by smooth muscle bundles in a spiral arrangement.