The respiratory system is functionally divided into two parts: a conducting system, which conditions and moves air, and an exchange system, which removes carbon dioxide from the blood and replaces it with oxygen. The conduction system consists of the oral and nasal cavities, pharynx, larynx, trachea, bronchi, and proximal bronchioles. The exchange system is composed of respiratory bronchioles, alveoli, alveolar capillaries, and the thin respiratory membrane that separates the alveolar spaces from the capillary lumens. The respiratory system is powered by the skeletal muscle of the diaphragm, which is innervated by the phrenic nerve. A group of nuclei in the brainstem (including the nucleus tractus solitarius and nucleus ambiguous) drive the contraction and relaxation of the diaphragm, as well as coordinating muscles of the tongue, pharynx, and larynx.

The nasal cavity (see Figure 7.1) extends from the nares to the cribriform plate of the ethmoid bone and is bordered dorsally by the maxilla and palatine process of the incisive bone and ventrally by the hard palate. A septum, which is caudally continuous with the ethmoid bone and more rostrally composed of hyaline cartilage, separates the left and right portions of the nasal cavity. Much of the cavity is filled by delicate scrolls of turbinate bones, arranged into rostral (dorsal, middle, and ventral) and caudal (ethmoidal) conchae. The conchae define three passageways for air flow through the nasal cavities: the dorsal, middle, and ventral meatuses. These bones are covered by respiratory epithelium and aid in warming, filtering and humidifying inspired air and directing inhaled pheromones and odorants to the vomeronasal organ and olfactory epithelium.

The paranasal sinuses are air-filled spaces within the skull bones that are lined by respiratory epithelium. The number and location of paranasal sinuses vary by species (Table 7.1 and Figure 7.1; Alsafy et al. 2022). All the sinuses communicate with the nasal cavity except the lacrimal and palatine sinuses, which open into the maxillary sinus.

The auditory (or Eustachian) tubes connect the middle ear to the nasopharynx. In horses, the guttural pouch is a large diverticulum of the auditory tube which is believed to function in cooling the arterial blood flowing to the brain. The auditory tube should be examined, especially in horses with respiratory signs or nasal discharge. Figure 7.2 shows the location and contents of the guttural pouch.

The pharynx is divided by the soft palate into the oropharynx and nasopharynx. The larynx (Figure 7.3) connects the pharynx to the trachea and consists of three unpaired (epiglottic, thyroid, and cricoid) and one paired (arytenoid) cartilages. The epiglottic cartilage is composed of a stalk, which is attached to the base of the tongue, the basihyoid bones, and the body of the thyroid cartilage, and a flexible blade, which curves toward the soft palate at rest, but tilts caudodorsally during swallowing to partially cover the entrance to the larynx. In horses, rodents, and rabbits, the epiglottis overlaps the caudal edge of the soft palate, so that air entering the mouth cannot flow to the lungs. As a result, these animals are obligate nose breathers. The thyroid cartilage, the largest cartilage in the larynx, is “V” shaped and forms the bulk of the laryngeal floor. The cricoid cartilage is ring shaped and is the most caudal part of the larynx. The paired arytenoid cartilages are triangular and have vocal processes which project into the laryngeal lumen and serve as the attachment site for the vocal folds. The intrinsic muscles of the larynx are the cricothyroideus, cricoarytenoideus dorsalis and lateralis, thyroarytenoideus, and arytenoideus transversus.

The hyoid apparatus supports the larynx and the base of the tongue (Figure 7.3). This structure consists of four paired bones and one unpaired bone: the thyrohyoids, which articulate with the thyroid cartilage; the ceratohyoids; the epihyoids; the stylohyoids, which articulate with the base of the skull via the tympanohyoid cartilages; and the unpaired basihyoid, which connects the left and right sides of the hyoid apparatus.

The trachea and bronchi conduct air between the larynx and the lung parenchyma. The rigidity of the trachea is provided by “C”-shaped strips of cartilage which are joined along the dorsal aspect by the trachealis muscle, or dorsal tracheal ligament, to form tracheal rings (Figure 7.4

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