The Upper Respiratory Tract

The upper respiratory tract consists of the nasal cavities, the nasopharynx, the larynx and the trachea. The nasal cavities are located in the bridge of the nose and in the forehead and are open to the outside world via the nostrils, which open from the nose. The nostrils have a cartilaginous structure and are mostly open to allow air to enter.

- The nasal cavities and nasopharynx. The nasal cavities include turbinate bones and nasal sinuses and are separated by a median bony partition. The turbinate bones spiral in on themselves to form a cone shape. For this reason, the mucous membrane that covers them has an extensive surface area, richly furnished with blood vessels, which enhances its role of warming the air and saturating it with water vapor.The nasal glands contained within the nasal cavities secrete mucus, which traps noxious particles from the air (dust, microbes, etc.). Another portion of the mucous membrane has an olfactory function, allowing the dog to smell.After leaving the nasal cavities, the air passes through the posterior nares into the nasopharynx in the back of the mouth. At this point, it is nearly at body temperature and impurities have been removed.- The larynx. The air continues its passage to the lungs through the larynx and trachea. The larynx is composed of four cartilage structures (the cricoid cartilage, thyroid cartilage, arytenoid cartilage and epiglottis) and is attached to the bones of the skull by the hyoid bone. A number of muscles are responsible for moving the cartilage structures in relation to each other so that the larynx is open during respiration but closes when the dog swallows, thus avoiding entry of food into the trachea if it is swallowed the wrong way. The larynx controls the flow of air by opening or closing. It also contains the vocal cords, which vibrate when air passes through them, allowing the dog to make sounds (for example, growling and barking).- The trachea is a long tube formed of about 40 cartilaginous rings closed by tracheal muscle. It carries the air from the larynx (in the throat) to the bronchi (in the thoracic cavity). Tracheal muscle contracts the trachea, reducing its diameter and so regulating the flow of air: It also prevents excessive dilation, e.g., when the dog coughs.

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