Upper Respiratory Tract Anatomy

• The upper respiratory tract (URT) includes cavities in the head and upper neck, that connects the nostrils to the larynx and trachea.
• The URT serves as a series of complex airways.

Nasal Cavity

• Nostrils, also known as nares, are openings to the outside air that lead to the nasal cavity.
• The nasal cavity occupies the upper third of the head, up to the level of the back of the eyes.
• Conchae, also called turbinates, are structures that occupy part of the nasal cavity.
• The false nostril, or nasal diverticulum, is a blind pouch leading from each nasal opening.
• Nasogastric tubes could mistakenly pass into the false nostril.
• If the tube is stopped from moving, back it out and try again in a different direction, without forcing it, to avoid penetrating the wall of the diverticulum.

Nasopharynx

• The nasal cavity has an opening in the floor that leads into the nasopharynx between the space between the eyes.
• The oropharynx and laryngopharynx connect with the nasopharynx, but are used for swallowing food.
• The nasopharynx connects with the cavity within the larynx, which continues to the trachea.
• The junction of the larynx and trachea separates the upper and lower parts of the respiratory tract (URT and LRT).

Larynx

• The larynx is the 'gate keeper' between the URT and LRT.
• The larynx is a hollow valve made of cartilage and muscle.
• It opens during breathing and vocalization, and closes during swallowing.
• It is located at the back of the jaw in the midline between the mandibles.

Mouth, Palate

• The oral cavity is separated from the nasal cavity by the hard and soft parts of the palate
• In horses, the oral cavity route is closed off to air by a valve involving the palate and the epiglottis, causing the horse to breathe through its nose ('obligate nasal breather').

Sinuses and Guttural Pouch

• The frontal sinus connects with the nasal cavity.
• Sinuses are interconnected air-filled cavities within skull bones lined with mucous membrane.
• Sinuses are prone to infections from airborne bacteria and viruses that enter through the nasal cavity.
• Guttural pouches are cavities behind the nasopharynx.
• The guttural pouches are large outpouchings of the auditory tube (Eustachian tube), which runs from the middle ear to the pharynx.
• There is one guttural pouch on each side of the head.
• The pouches are deep, near the midline of the head, behind and below the eyes.
• Like the sinuses, guttural pouches are prone to infections, primarily from fungi.

Passage of Air Through the URT

• Air enters and leaves the nasal cavity through the nostrils during breathing.
• On breathing in, air leaves the nasal cavity through openings (choanae) into the nasopharyngeal cavity and re-enters the same way when breathing out.
• From the nasopharynx, air enters the larynx, to go into the trachea and down into the lungs.
• Air in the nasal cavity also exchanges freely with that in the frontal sinus.
• Air exchange also occurs between the nasopharynx and guttural pouches but may not occur continuously.
• The opening to each pouch from the nasopharynx may only open occasionally (e.g., during swallowing).