The respiratory system of the horse is the biological system by which a horse circulates air for the purpose of gaseous exchange.

Anatomy

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 Inhalation (or Inspiration) happens when air or other gases enter the lungs. Inhalation of air Inhalation of air, as part of the cycle of breathing, is a vital process for all human life. The process is autonomic (though there are exceptions ...

. 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 The larynx (), commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about ...

via the pharynx. The pharynx is about 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. Pferdeschädel

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: * Frontal sinuses: occupy the dorsal (top) part of the skull, between the eyes. There are two, one on each side, divided by a bony septum. These communicates with the inside of the conchae, forming the concho-frontal sinuses. Drainage into the nasal passages is via the caudal maxillary sinus. * Maxillary sinuses: within the maxilla, above the

tooth roots Dental anatomy is a field of anatomy dedicated to the study of human tooth structures. The development, appearance, and classification of teeth fall within its purview. (The function of teeth as they contact one another falls elsewhere, under den ...

. Each is divided into two components, the rostral maxillary sinus in front and the caudal maxillary sinus behind. They do not communicate. In addition, each of these is subdivided into a medial (inside) and lateral (outside) component, by an incomplete bone wall that carries the infraorbital canal containing nerves and blood vessels. The close proximity to the tooth roots mean that as the teeth erupt with age, the maxillary sinuses become larger. * Sphenopalatine sinuses: within the palatine and sphenoid bones, these connect to the caudal maxillary sinus. Kehlkopf Pferd

A flap of tissue called the soft palate blocks off the pharynx from the mouth (oral cavity) of the horse, except when

swallowing Swallowing, sometimes called deglutition in scientific contexts, is the process in the human or animal body that allows for a substance to pass from the mouth, to the pharynx, and into the esophagus, while shutting the epiglottis. Swallowing i ...

. 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

pant Pant may refer to: Clothing * Pants or trousers, an article of outer clothing worn on the lower half of the body * Underpants, an item of underwear Places * Pant, Denbighshire, Wales; a township of Llysfaen *Pant, Merthyr Tydfil, Wales *Pan ...

as a method of thermoregulation. The genus ''

Equus Equus may refer to: * ''Equus'' (genus), a genus of animals including horses, donkeys and zebras * ''Equus'' (play), a play by Peter Shaffer * ''Equus'' (film), a film adaptation of the Peter Shaffer play * Equus (comics), a comic book characte ...

'' 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. The larynx lies between the pharynx and the trachea, and is made up of 5 pieces of cartilage which serve to open the

glottis The glottis is the opening between the vocal folds (the rima glottidis). The glottis is crucial in producing vowels and voiced consonants. Etymology From Ancient Greek ''γλωττίς'' (glōttís), derived from ''γλῶττα'' (glôtta), va ...

. The larynx not only allows the horse to vocalize, but also prevents aspiration of food and helps to control the volume of air inhaled. The trachea is the tube which carries air from the oral cavity and into the lungs, and is about in length in the adult. It is held permanently open by 50–60 "C"-shaped rings of cartilage, in diameter. At the bifurcation of the trachea, there are two bronchi, the right of which is slightly larger in size. The bronchi then branch into smaller

bronchioles The bronchioles or bronchioli (pronounced ''bron-kee-oh-lee'') are the smaller branches of the bronchial airways in the lower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start o ...

, which in turn branch off into smaller bronchioles until they reach the

alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Uses in anatomy and zoology * Pulmonary alveolus, an air sac in the lungs ** Alveolar cell or pneumocyte ** Alveolar duct ** Alveolar macrophage * ...

(which absorb oxygen from the air and releases the carbon dioxide waste). The bronchi and bronchioles are all held within the

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

s of the horse, which is located in the animal's thoracic cavity. The lung is made up of a spongy, but very stretchy, material which has 2 lobes on the right and left side (a smaller, apical lobe and a large, caudal lobe) in addition to the accessory lobe. Blood is carried into the lungs via the pulmonary artery, where it is oxygenated at the alveoli and then returned to the heart by the pulmonary veins. The lungs are expanded with the help of the

diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...

, a muscular sheet of tissue which contracts away from the thoracic cavity, thereby decreasing the pressure and pulling air into the lungs. When fully expanded, the lungs can reach to the 16th rib of the horse.

Respiration rate of the horse

An adult horse has an average rate of respiration at rest of 12 to 24 breaths per minute. Young foals have higher resting respiratory rates than adult horses, usually 36 to 40 breaths per minute. Heat and humidity can raise the respiration rate considerably, especially if the horse has a dark coat and is in the sun. The respiration will often change if the horse becomes excited or distressed, and can therefore be useful in determining the health of the animal. At the gallop, the horse breathes in rhythm with every stride: as the abdominal muscles pull the hind legs forward in the "suspension phase" of the gallop, the organs within the abdominal cavity are pushed backward from the diaphragm, thereby bringing air into the lungs and causing the horse to inhale. As the neck is lowered during the extended phase of the gallop, the hind legs move backward and the gut contents shift forwards, pushing into the diaphragm and forcing air out of the lungs.

Ability to smell

The horse's olfactory receptors are located in the mucosa of the upper nasal cavity. Due to the length of the nasal cavity, there is a large area of these receptors, and the horse has a better ability to smell than a human. Additionally, the horse also has a vomeronasal organ, or Jacobson's Organ, which is in the hard palate, and is able to pick up pheromones and other scents when a horse exhibits the flehmen response. The flehmen response forces air through slits in the nasal cavity and into the vomeronasal organ. Unlike many other animals, the horse's Jacobson's Organ doesn't open into the oral cavity.

Respiratory diseases

Influenza Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms ...

* Left Recurrent Laryngeal Hemiplegia ("Roaring") * Recurrent airway obstruction ("Heaves"), akin to COPD or

emphysema Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the alve ...

in humans. *

Strangles Strangles (equine distemper) is a contagious upper respiratory tract infection of horses and other equines caused by a Gram-positive bacterium, ''Streptococcus equi''. As a result, the lymph nodes swell, compressing the pharynx, larynx, and tr ...

(''Streptococcus equi'' infection) * Rhinopneumonitis *

Exercise induced pulmonary hemorrhage Exercise-induced pulmonary hemorrhage (EIPH), also known as "bleeding" or a "bleeding attack", refers to the presence of blood in the airways of the lung in association with exercise. EIPH is common in horses undertaking intense exercise, but it h ...

* Equine nasal cysts * Ethmoid hematoma * Nasal polyps * Bacterial sinusitis ( Empyema) * Bacterial pleuropneumonia * Dorsal displacement of the soft palate *

Silicosis Silicosis is a form of occupational lung disease caused by inhalation of crystalline silica dust. It is marked by inflammation and scarring in the form of nodular lesions in the upper lobes of the lungs. It is a type of pneumoconiosis. Silicos ...

External links

The Horse: The Airways and the Lungs

Respiratory Problems

References

{{reflist Horse anatomy Respiratory system