An Overview of the Equine Respiratory System and Its Functions

a bay horse trotting with a diagram of the equine respiratory system drawn over it

Anatomy of the Equine Respiratory System

The equine respiratory system (we’ll say ERS for short) is made up of an upper and lower respiratory tract. To understand the function of the ERS, we must first examine its anatomy.

diagram of the equine respiratory system

FUN FACT: Over the last several decades, researchers have found growing evidence that the ERS may be the main limiting factor in a horse’s maximal performance. In a future post, I will go into detail on the horse’s respiratory response to exercise and training as well as the limitations of equine respiration.

Upper Respiratory Tract

Nares

The nares (nostrils) are the two openings to the nasal cavity that bring air into the lungs. When a horse takes a breath, the respiration process starts here. They can flare when more oxygen intake is necessary.

Nasal Cavity

Next is the nasal cavity, a large, air-filled cavity just inside the nostrils where heat and water exchange occurs. Air flow through this part of the ERS may be restricted by nasal septum defects or even tight nosebands.

Pharynx

After passing through the nasal cavity, the air comes through the pharynx. The pharynx is involved in both breathing and digestion. We’ll just focus on its respiratory function for now.

The nasopharynx is the upper part of the pharynx that connects with the nasal cavity above the soft palate. The soft palate is a musculomucosal sheet that separates the pharynx into the oral (oropharynx) and nasal (nasopharynx) compartments.

Larynx

The larynx is simply the structure that connects the nasal passage to the trachea after the pharynx.

Epiglottis

The epiglottis is a flap of mucous covered elastic cartilage located between the pharynx and the larynx. It forms a boundary between the two and prevents food particles from entering the trachea. 

When a horse eats, the epiglottis closes the trachea to allow food to pass to the esophagus. The epiglottis conversely closes down on the soft palate when the horse is breathing, thus allowing clear passage to the trachea.

Guttural Pouch

The guttural pouch is a pair of sacs of air that expand from the Eustachian tube (structure that connects the ears to the nose and mouth and helps to regulate air pressure). These pouches are only found in the equine species with one on each side of the horse’s head positioned beneath the ear.

According to Nature.com, “their anatomical association with the upper respiratory tract suggests that the horse’s guttural pouches might function during selective brain-cooling”, especially during exercise.

diagram of the upper equine respiratory system

Lower Respiratory Tract

Trachea

After passing through the larynx, oxygen travels through the trachea. The trachea is an approximately 27 to 31 inch (70 to 80 cm) flexible tube that is involved in bringing gases to and from the lungs.

Anatomically, the trachea consists of c-shaped, cartilaginous rings that support it as well as allow for movement and flexing. Think like a vacuum hose. The trachea eventually splits into two sections to travel to the left and right bronchi.

Bronchi

The trachea then branches repeatedly to form bronchi in the lungs that eventually end up in the bronchioles, the smallest airways in the ERS.

Alveoli

The final part of the lower respiratory tract are the alveoli. Alveoli act as blind sacs (having only one entry and exit point) involved in gas exchanges. Interestingly, horses have more alveoli than most other mammals.

Diaphragm

The diaphragm is a muscle that contracts and relaxes to increase the size of the chest cavity to allow air into the lungs. With each inhale, the diaphragm contracts allowing more space. At each exhale, the diaphragm relaxes and closes that space.

Overview

Horses are obligate nasal breathers meaning they are only able to breathe through the nose and not the mouth. This is due to the epiglottic cartilage blocking feedstuff from entering the trachea.

Evolutionarily speaking, this anatomical set up may be advantageous to the horse. Since they are able to smell and breathe while they eat, they are able to stay alert for predators at all times. Not to mention, since horses spend 12 to 14 hours a day grazing, it makes sense for them to be able to breathe while they eat.

However, there is also a small disadvantage to the ERS being that the anatomy creates a bottleneck which can lead to less oxygen uptake and exchange. This is one of the main reasons that the ERS may be the limiting factor for maximum performance.

Functions of the Equine Respiratory System

When it comes to the ERS, we look at seven main functions:

1. Gas exchange

  • the alveoli and capillaries that encompass the lungs exchange oxygen and carbon dioxide with every inhale and exhale

2. Warming and humidification

  • as the horse inhales and air passes through the nasal cavities, it is warmed and humidified. This allows the air that reaches the lungs to be warm and moist.

3. Filter air

  • defends the horse against antigens and other large foreign particles

4. Thermoregulation

  • when horses begin to overheat, the respiration rate will increase; since heat is lost during exhalation, allowing the horse to cool down

5. Acid/base regulation

  • the gas exchange mentioned above regulates the carbonic levels in the blood. When horses exercise, CO2 concentration in the blood increases. CO2 is then transported to the lungs through the bloodstream to be exhaled.

6. Sense of smell

  • allows horses to smell their surroundings and assess dangers

7. Vocalization

  • without the respiratory system, horses would not be able to communicate vocally

Thank you so much for reading about the equine respiratory system. Let me know if any of the information in this post surprised you! I’d love to hear from you.

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References and bonus resources