The respiratory tract extends from the lips and nostrils to the alveoli of the lungs (Figure 23.1). It comprises a network of conducting and respiratory airways allowing air to pass into (inspiration) and out of (expiration) the lungs for gas exchange. The respiratory tract can be divided into upper and lower respiratory tracts for descriptive purposes.
Upper respiratory tract
Nasal cavities
These extend from the nostrils anteriorly to the posterior nasal apertures (choanae) posteriorly, and are divided into right and left halves by the nasal septum. Except for the vestibule of the nose, mucosa lines the nasal cavity. Air passing over the mucosa is warmed, filtered and moistened in preparation for efficient gas exchange, before passing further into the respiratory tract. The mucosa lining the majority of the respiratory tract is composed of pseudostratified ciliated columnar epithelium (respiratory epithelium). In certain areas within the nasal cavity the mucosa is modified for the specialised function of olfaction (olfactory epithelium). The nasal conchae, which increase the surface area, further divide each nasal cavity into four spaces: spheno-ethmoidal recess, superior nasal meatus, middle nasal meatus and inferior nasal meatus (Figure 23.1). The paranasal sinuses are air-filled cavities found within the frontal, maxilla, sphenoid and ethmoid bones (Figures 23.1 and 23.2). They communicate with the nasal cavities via small apertures.
Oral cavity
This extends from the lips anteriorly to the oropharynx posteriorly. It is divided into the oral vestibule and oral cavity proper. The oral vestibule is the slit-like space between the lips cheeks, teeth and gingivae. The oral cavity proper is the space posterior and medial to upper and lower dental arches; it contains the teeth and tongue.
Pharynx and larynx
This is the superior expansion of the alimentary system and is involved in the conduction of air to the larynx, trachea and lungs. It is a fibromuscular passage divided into three regions (Figure 23.1): nasopharynx (posterior to the nasal cavity), oropharynx (posterior to the oral cavity) and the laryngopharynx (posterior to the larynx). The larynx connects the oropharynx to the trachea. It has various important functions, including routing air into the lower respiratory tract, maintaining a patent respiratory tract and voice production.
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