Respiratory system of the horse

In biology, the respiratory system of the horse is the means by which a horse circulates air around its internal organs.

The respiratory system begins with the nares, commonly known as the nostrils, which can expand greatly during intense exercise. The nostrils have an outer ring made of cartilage (the alar cartilage), which serves to hold them open during inhalation. Additionally, a small pocket within them, called the nasal diverticulum, filters debris with the help of the hairs lining the inner nostril. The nasal cavity contains the nasolacrimal duct, which drains tears from the eyes and out the nose.

The nasal passages contain two conchae on either side, which help to increase the surface area to which the air is exposed. Additionally, the sinuses within the skull are able to drain through the nasal passage. The nasal passage join to the larynx via the pharynx. The pharynx is about 15 cm long in an adult, and includes the nasopharynx, which protect the entrance to the auditory tubes, the oropharynx, which contains tonsillar tissue, and the laryngopharynx.

In parallel to the main nasal passages, the horse has a complex system of paranasal sinuses - air filled spaces within the head which communicate with the respiratory tract, and serve to reduce the weight of the head. These consist of:

A flap of tissue called the soft palate blocks off the pharynx from the mouth (oral cavity) of the horse, except when swallowing. This helps to prevent the horse from inhaling food, but does not allow use of the mouth to breathe when in respiratory distress, a horse can only breathe through its nostrils. For this same reason, horses also cannot as a method of thermoregulation.

The genus Equus has a unique part of the respiratory system called the guttural pouch, which is thought to equalize air pressure on the tympanic membrane. These (left and right, separated by a narrow septum) is located in "Vyborg's triangle", between the mandibles but below the occiput. With a capacity of 300 to 500 ml, it fills with air when the horse swallows or exhales.

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