The horse’s respiratory system provides him with the oxygen that’s essential for all cell functions throughout the horse’s body, including the removal of carbon dioxide.
Also, the respiratory system helps to protect the horse from allergens and disease, provides him with a sense of smell, and enables the horse to cope with the rigors of intense activity and exercise.
In this article, we take a closer look at your horse’s respiratory system and explain its importance for the competition horse.
Structure of the horse’s respiratory system
There are two distinct areas of the horse’s respiratory system; the upper and lower respiratory tract. Note that horses are “obligate nasal breathers.” That means that, unlike humans and many other mammals, horses can only breathe through their nose and not through their mouths.
The upper respiratory tract
- The horse’s nostrils (nares) are the first part of the respiratory tract. On both sides of the nostrils are structures that are called alar folds or false nostrils. The alar folds help to expand the nostrils when the horse breathes in, as well as acting as filters, removing large pieces of debris and dust from the air.
- The horse’s nostrils are separated by a nasal septum that extends right back to the larynx.
- Inside the nostrils are hairs that remove smaller items of debris. Also, each nostril contains coils of bone (turbinates) that are encased in a mucous membrane. The turbinates humidify and warm the air that the horse inhales, filtering out any remaining debris in the air.
- At the rear of the nasal passage is an area that is rich in olfactory receptors that give the horse his sense of smell. These nerve endings extend through the skull and into the brain.
- A structure called the vomeronasal organ is found in the horse’s hard palate. When the horse lifts his top lip (flehmen response), he is forcing odors over the vomeronasal organ so that he can smell them.
- Within the horse’s skull is a complex sinus system that reduces the weight of the skull while keeping it strong.
- At the bottom of the nasal palate is the hard palate. Further back, the hard palate enters the pharynx, where it becomes the soft palate. The hard and soft palate separate the nasal and oral cavities. At the top of the soft palate is the epiglottis, which divides the nasal and oral cavities.
- The esophagus lies next to the trachea. When the horse swallows, the food moves into the oropharynx, and the epiglottis rises, briefly covering the trachea so that the horse doesn’t inhale the food.
- Either side of the pharynx are the guttural pouches, which are an enlargement of the eustachian tube, which connects the back of the nose and the middle ear. The guttural pouches contain essential blood vessels and nerves.
- The horse’s pharynx leads to the larynx. The larynx consists of the arytenoid cartilages that work to open the larynx when the horse inhales and closes it during swallowing. The horse’s vocal cords are also situated in the larynx.
Lower respiratory tract
- The windpipe (trachea) takes air from the nasal cavities and associated structures to the lungs. The trachea is surrounded by rings of cartilage to keep it open, and it is lined with tiny hairs (cilia) that move debris, mucous, and fluid from the lungs and up to the pharynx where the horse can snort them out or swallow them.
- Inside the lungs, the trachea branches into two bronchi, which in turn, branch repeatedly, becoming bronchioles. At the end of the bronchioles are tiny sacks called alveoli. These structures allow the exchange of oxygen into the blood and move carbon dioxide from the blood to the air.
- The act of breathing is assisted by the horse’s diaphragm. The diaphragm is a sheet of muscular tissue that contracts away from the chest cavity, causing a decrease in pressure and dragging air into the lungs.
How does the horse’s respiratory cycle work?
When blood arrives in the horse’s lungs, it is low in oxygen because the body has used it. The blood carries carbon dioxide for removal from the body. The carbon dioxide is taken from the horse’s blood and passed into the air in the lungs, which is then exhaled out of the body.
When the horse inhales, the air is pulled into the lungs. The oxygen the air contains is transferred into the blood, which then flows to the heart to be pumped around the horse’s body, delivering oxygen to the cells of the body tissues. The blood then removes the carbon dioxide from the body’s tissues and returns it to the lungs, where the cycle starts again.
Respiratory conditions in horses
The horse’s respiratory system is complicated, and problems can occur at virtually any point along it.
Issues commonly include allergies or infections, but can also involve neurological or physical abnormalities.
In this part of our guide to the horse’s respiratory system, we outline some of the most common illnesses and conditions that could affect your horse.
Infection
Respiratory tract infections can be bacterial, although many are viral, for example, influenza.
DDSP
DDSP or dorsal displacement of the soft palate is a common condition.
DDSP occurs when the soft palate is displaced above the epiglottis rather than resting below it. The condition causes noisy breathing and poor performance due to obstruction of the respiratory tract.
Roaring
Roaring, or laryngeal hemiplegia, is caused by paralysis or weakness of the vocal fold and left arytenoid cartilage. The affected structures move into the trachea, vibrating when the horse inhales and causing the roaring noise and reduction in performance.
COPD or RAO
COPD is now known as recurrent airway obstruction or RAO.
The condition is triggered by an allergy, usually to dust and mold spores. RAO causes a narrowing of the horse’s airways, inflammation, and increased mucous production. Sometimes, an adverse reaction to pollens and grasses during the summer months causes a similar reaction, which is known as SPARAO, or summer pasture associated recurrent airway obstruction.
Tracheal or nasopharyngeal collapse
This is a common condition where the cartilage rings that support the trachea collapse.
The condition can be caused by infection, a tumor, and trauma, or it may be hereditary.
Exercise-induced pulmonary hemorrhage
Commonly referred to in horseracing circles as a burst blood vessel or bleeder, this condition usually manifests as a nosebleed.
The problem occurs during high-intensity exercise, which causes tiny blood vessels to burst.
Pneumonia
Pneumonia is a lung infection that causes the tissues to become inflamed and the alveoli congested with fluid.
Pneumonia can be caused by fungi, viruses, and bacteria.
Pleuropneumonia
Pleuropneumonia is a lung infection that affects the area between the ribcage and the lungs. Also known as shipping fever because it commonly appears in horses that have been traveling for long periods, the condition is often a secondary infection.
How to maintain good lung health
Recovery from respiratory tract conditions can be lengthy and problematic, so it’s in your horse’s best interests to take steps to keep his lungs healthy.
Vaccinate!
You should have your horse vaccinated against equine influenza to reduce his chances of catching the disease and reducing the condition’s severity if he did succumb.
Dampen hay and haylage
Even good quality forage can be dusty. Reduce the amount of dust that your horse breathes in while eating by wetting or steaming hay and haylage right before feeding it to your horse.
Also, feed forage from the floor, rather than asking your horse to reach up to a hayrack or net. Eating with his head down will help to prevent the inhalation of dust and hayseeds that could irritate the lungs.
Use dust-free bedding
If your horse has a lung condition, try changing his bedding to something that’s as dust-free as possible, such as dust-extracted shavings, paper, or plain rubber matting.
Maximize time outside
Perhaps the most effective way of reducing the effects of dust on your horse’s lung health is to maximize the amount of time that your horse spends turned out in the paddock or field.
Also, when mucking out, always remove your horse from the stable and take down any dust-harboring cobwebs that you may find lurking in dark corners.
Damping down
If you have an indoor or outdoor riding surface that gets dusty, water it or damp it down before working your horse to prevent the risk of dust inhalation.
Quarantine new horses
You should have an isolation stable available on your yard that’s sited well away from other horses.
Any new horse arriving on the yard should spend at least ten days in isolation. That will give you a chance to observe the horse for signs and symptoms of illness before introducing him to the rest of your horses.
Related Read: How to Quarantine Your Horse
Performance horses
The horse’s respiratory tract is the third largest organ in his body, after the skin and the gastrointestinal tract, and is probably the most important when it comes to performance.
No matter what discipline in which the horse is taking part, be it a short sprint, jumping a line of fences, competing in a Grand Prix dressage test, or galloping around the cross-country phase of a three-day event he will need oxygen to generate energy. However, the higher the intensity of the exercise the horse is undertaking, the more important the respiratory system becomes.
How does aerobic metabolism work?
So, if the horse’s respiratory system is compromised in any way, the animal’s muscles and organs would not receive the amount of oxygen required, and his performance would suffer.
Much of the horse’s energy is generated from aerobic (oxygen) metabolism. For example, in a cross-country round, as much as 90% of the horse’s energy comes from oxygen.
Aerobic metabolism occurs in three phases:
Phase I
Phase one involves the horse getting as much air in and out of the lungs as fast as possible.
Phase II
The second phase involves moving the oxygen from the horse’s airways into his bloodstream so that it can be carried in the red blood cells and pumped from the heart to the muscles and organs.
Phase III
Finally, the oxygen leaves the red blood cells, moves into the horse’s muscle cells, and thence to the mitochondria, which use the oxygen to metabolize the energy that’s stored in fats and sugars.
Tidal airflow
The horse’s usual respiratory rate is eight to 12 breaths per minute.
Each time he inhales, the horse draws in around 1.32 gallons of tidal air – tidal, because the air is drawn in and then exhaled. So, in total, your horse moves 15.8 gallons of air in and out of his lungs every minute.
To overcome the resistance that’s caused by the air movement around the respiratory tract, the horse has to expend an amount of energy. The amount of air that’s moved in and out increases in proportion to how hard the horse is working. The harder the horse works, the more air he must move, as more oxygen is used.
When making a simple transition from walk to trot, the horse may breathe a little faster to increase the in/out flow of air. However, when moving from trot to canter and from canter to gallop, the horse may start breathing deeper, while the actual rate of breathing remains the same. That’s because breathing and stride are linked 1:1 at canter and gallop.
To increase speed in the canter and gallop, the horse takes longer strides. A longer stride means that more time is available to fill the lungs, and thus the size of the breaths increases. At a gallop, a horse may be taking around 120 breaths per minute. That’s two breaths in and out per second, shifting a remarkable four gallons of air in and out with each breath!
In conclusion
The good health of your horse’s respiratory tract is essential if he is to perform to his optimum.
Be sure to have your horse vaccinated against equine flu and keep his environment as dust-free as possible to guard against allergies and dust-related lung conditions.
Do you have any top tips for keeping your horse’s lungs in good health? Share your thoughts with us in the comments box below.
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