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Respiratory: overview


Upper respiratory tract

External nares
  • Much of the resistance that the nares cause to airflow at rest is eliminated by dilation during exercise.
  • A C-shaped alar cartilage forms the rigid medial border.
  • Nasolabialis muscles of lateral margin of nares have motor supply via dorsal buccal branches of facial nerve.
  • Any damage to the muscles that dilate the nares, or their nervous supply, severely affects the performance of athletic pursuits.
  • Alar folds form the dorsal margin and attach to the ventral conchus.
  • Blind pocket of false nostril lies dorsally.

Paranasal sinuses

  • 5 pairs of paranasal sinuses: frontal (divided into conchofrontal and frontal portions); caudal maxillary; rostral maxillary (has a bony lateral section and a turbinate section medially); ethmoidal and sphenopalatine.
  • Air filled spaces lined with mucoperiosteum.
  • Dynamic drainage is required to remove secretions as drainage ostia are not at lowest points, and are very narrow.
  • Impairment of this mechanism or obstruction of ostia results in sinusitis.
  • Frontal sinus drains through frontomaxillary foramen into caudal maxillary sinus (CMS).
  • Ethmoidal and sphenopalatine also drain via CMS into middle nasal meatus.
  • Rostral maxillary sinus (RMS) has a separate drainage ostium into the middle meatus.
  • The two parts of the RMS are separated by the infra-orbital canal and a sheet of bone joining it ventrally to the roots of the cheek teeth.
  • In youngsters the lateral compartment is completely filled by tooth roots.
  • Roots of 3rd cheek teeth form rostral wall of RMS.
  • Roots of 4th, 5th and 6th teeth are in maxillary sinuses.
  • These recede with age.

Borders of paranasal sinuses

Frontal sinus

  • Caudal border: zygomatic process of frontal bone.
  • Rostral border: a line halfway between the medial corner of eye and the rostral point of the facial crest.
  • Lateral border: base of the zygomatic process of frontal bone.

Maxillary sinus

  • Caudal border: middle of orbit.
  • Rostral border: a line joining the rostral point of the facial crest with the infra-orbital foramen.
  • Ventral border: facial crest.
  • Dorsal border: a line from the infra-orbital foramen to the medial corner of eye.
  • Septum dividing the maxillary sinus is approximately 50 mm from the point of the facial crest.
  • In foals the sinuses do not extend so far, they expand over the first 5 years.

Nasal meatii

  • Usually the ventral meatus is used for intubation or endoscopy.
  • To examine the drainage ostia of sinuses, the middle meatus must be used.


  • A structure purely of soft tissue that joins the mouth to the esophagus and the nasal cavity to the larynx.
  • Oropharynx extends from palatoglottal arch to the epiglottis.
  • Lined with oral mucosa.
  • Borders: dorsally caudal border of soft palate; laterally the fauces of the pharynx and ventrally by base of tongue.
  • Nasopharynx is lined with respiratory epithelium and lies dorsally to the soft palate.
  • Laryngopharynx extends from the epiglottis caudally to the esophagus.

Guttural pouches

  • The openings of the guttural pouches are in the dorsolateral wall of the nasopharynx, one on either side. The ostia are slits formed by a fold of mucosa strengthened with cartilage.
  • Divided into a small lateral and a larger medial compartment.
  • Volume approximately 300 ml.
  • Natural drainage occurs when horse lowers head to eat from ground.
  • Top of guttural pouch is in direct contact with skull from occipital condyle to level of temporomandibular joint.
  • Cranial nerves 9, 10 and 11 and the internal carotid artery are in direct contact with the top of guttural pouch.
  • Cranial nerve 7 lies alongside the tympanic bulla in the middle of this region.
  • Retropharyngeal lymph nodes lie ventrally.
  • Maxillary artery and vein are also adjacent to guttural pouch.
  • These vessels are susceptible to erosion by mycotic infections of guttural pouches   Guttural pouch: mycosis  , causing potentially fatal epistaxis.
  • Externally the guttural pouch is within the area of Viborg's triangle (formed by angle of the mandible, sternocephalicus muscle and linguofacial vein).
  • The guttural pouch may have evolved as a mechanism for cooling arterial blood before it reaches the brain. Blood temperature equilibrates with that of the air in the guttural pouch during passage through the internal carotid artery, protecting the brain from hyperthermic damage during strenuous exercise.


  • Joins trachea to pharynx, situated mainly caudally to the jaw.
  • Formed by 4 cartilages: epiglottic, thyroid, cricoid and arytenoid (paired).
  • The laryngeal recurrent nerve lies under the rostral horn of the thyroid cartilage.
  • The thyroid is formed by two lateral lamina that join rostroventrally.
  • Ventrally between the lamina is a broad, deep gap covered by a ligament. The inner larynx can be reached surgically via this route.
  • Entrance to larynx is formed rostrally by the epiglottis; laterally by the aryepiglottic folds and caudally by the corniculate processes of the arytenoid.
  • Vocal chords are situated in the middle portion of larynx.
  • Just cranially to the vocal chords, on either side, are the laryngeal ventriculi (mucosal pockets).
  • Equines are obligatory nose breathers.
  • The laryngeal cartilages are locked into the caudal wall of the nasopharynx by the 4th branchial arch (air-tight seal).
  • The epiglottis is dorsal to the free border of the soft palate.
  • The anatomical and functional integrity of the arch is essential to maintain the conducting airways.
  • Momentary disengagement occurs for deglutition.
  • Any condition that disrupts this anatomy or function results in either a respiratory noise or lack of performance.

Conducting airways

  • In horses the main form of defense is retreat (flight).
  • This requires aerobic muscular function and an efficient respiratory system.
  • Olfaction is relatively unimportant in comparison to other domestic species, and the anatomy of the nasal chonchae is simple and streamlined to decrease airflow resistance. This lack of complication in the turbinate pattern means that filtering of inspired air is decreased and higher levels of particles such as pollen/spores may be given access to the small airways with possible consequences for clinical disease.
  • At rest the upper respiratory tract (nostrils and larynx) causes 80% of resistance to airflow.
  • This is reduced to below 50% by active dilation of soft tissues of nares and larynx together with straightening of the airway during exercise.
  • Airflow at rest is 4 l/s increasing to 70 l/s at a gallop.
  • Failure of muscular dilation of soft tissue causes a marked resistance to airflow, dramatically affecting performance.

Lower respiratory tract

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Respiration and locomotion

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Further Reading


Refereed papers
  • Recent references fromPubMedandVetMedResource.
  • Kock C, Straub R, Ramseyer A, Widmer A, Robinson N E & Gerber V (2007)Endoscopic scoring of the tracheal septum in horses and its clinical relevance for the evaluation of lower airway health in horses. Equine Vet J39(2), 107-112PubMed.
  • Seedorf J, Schroder M, Kohler L & Hartung J (2007)Suitability of biocompost as a bedding material for stabled horses: respiratory hygiene and management practicalities. Equine Vet J39(2), 129-135PubMed.
  • Baptiste K E et al(2000)A function for the guttural pouch in the horse. Nature403(6768), 382-383VetMedResource.

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