The Respiratory System (Book Excerpt)

Editor's Note: This is an excerpt from Understanding Basic Horse Care by Michael A. Ball, DVM. This book is available from

The respiratory system's main goal is to transfer oxygen from the air we breathe to the red blood cells where the oxygen will be transported throughout the body and be available for all organs and tissues.  In addition, carbon dioxide, a waste product of metabolism, is eliminated from the body via the lungs.  Average room air (at sea level) contains about 21% oxygen, some 70% nitrogen, and the remainder a variety of gases including pollution.  So, in fact, the air we typically breathe is not very rich in oxygen.  As the altitude gets higher the percentage of oxygen in the air becomes less (the air becomes "thinner"). 

Red blood cells contain an iron-rich protein called hemoglobin.  Oxygen molecules bind to hemoglobin when the red blood cells are in the lungs and are released from the hemoglobin in the tissues.  Several factors can affect the red blood cells' ability to bind or release oxygen.  For example, iron deficiency can lead to anemia and a reduced oxygen-carrying capacity of the blood.  Red maple leaf poisoning in horses causes a change in the hemoglobin that decreases its binding of oxygen, and carbon monoxide toxicity prevents the release of oxygen from hemoglobin.  There are numerous other toxins that can have an effect on the binding affinity of oxygen for hemoglobin.

Ways to evaluate the respiratory system include respiratory rate and character of breathing, bearing in mind that there are other things that can have an effect on these parameters.  For example, pain can increase the respiratory rate and cause shallow breathing.  A good physical examination is required to assess the respiratory system adequately.  Is there a nasal discharge and if so what is the content of it -- clear thin fluid, thick green snot, thick yellow snot, blood content, foul smelling, one nostril or both? Is there normal airflow out of both nostrils? Is there any asymmetry to the head? Has the horse been coughing and if so what is the character of the cough (dry/wet)? Can a cough be induced by squeezing the windpipe (trachea)? Is the horse extending its head and neck to breathe? Are there any painful swellings around the head? Does the horse make a noise while breathing? (at rest? at work? on inspiration? on expiration?) Does the horse have extra abdominal movement while breathing? Is there a fever? Are there any other horses on the property with similar signs? All of these things must be assessed and can give clues to the specific problem.  Other important factors include: travel history, horse's age, vaccination status, time of year, and prior illness.

Respiratory problems are typically classified as upper or lower respiratory.  The anatomy of the upper airway is complex, but requires a good understanding to aid in diagnosis and recognition of disease. 

Let's start with the nostrils as these are the entry point for air and that ever so precious oxygen.  The numerous muscles of the nostrils generate the flaring (dilating) action that allows for air movement with the least resistance at this point. 

Occasionally some horses will have small nostrils that don't open well while exercising, affecting airflow and thus performance.  There are minor surgical procedures and other interventions to help correct this problem. 

Going up the nostrils, there are three passages stacked on top of each other that lead toward the throat.  It is possible for these delicate passageways to become inflamed and lead to nasal passage obstruction.  Occasionally foreign bodies (plant material, wood, etc.) can become lodged in a nasal passage and cause inflammation/obstruction.  It also has been reported that foreign objects such as gauze sponges have purposefully been inserted into these passages.  The horse's head has extremely large sinus cavities that drain into the nasal passages.  Horses have a long nasal septum so that the sinuses drain into the nasal passages on the same side -- a nasal discharge only coming from one nostril by sinus drainage. 

The next stop on the way toward the back of the throat are the openings to the guttural pouch.  The guttural pouch is a structure unique to horses, rhinos, and the Tapir.  This structure is an outpouching of the eustachian tube.  Whereas our eustachian tube drains from the inner ear to the back of our throat, the horse has a pouch that is fairly large (it holds several hundred milliliters) with the opening draining just behind the nasal septum on each side.  A discharge from the guttural pouch can drip down the outer wall of the throat and out the nostril on the same side as the affected pouch, or, if the discharge is great and flowing rapidly, it can come from both nostrils. 

The guttural pouch is a unique but somewhat scary structure for several reasons.  The interior of the guttural pouch contains important structures that can be affected by guttural pouch disease.  There are four major nerves coming directly out of the brain (cranial nerves) that travel on the inside of the pouch as well as two major arteries (the internal carotid artery on the way to the brain and the maxillary artery supplying much of the blood to the head).  Guttural pouch disease will be discussed further in this chapter. 

As we continue on, we come to the voice box or "larynx."  Several common disease processes affect airflow in the laryngeal area.  In the normal horse, the larynx coordinates swallowing and protects the trachea and thus the lungs from inhalation of feed material.  It should be noted at this point that the normal horse cannot breathe through its mouth.  Horses are obligate nasal breathers, so anything that occludes the upper airway will cause an airway obstruction and respiratory distress.  The laryngeal area is where the soft palate ends (the soft palate separates the oral cavity from the nasal cavity) and the larynx itself is the junction between the nasal cavity and the trachea (windpipe).  A triangle-shaped structure called the epiglottis jets out with the point facing forward and sits on top of the soft palate. 

As we go further back, just before entering the trachea, there is a V-shaped structure with small bulges on each side called the arytenoids (these are adjacent to the vocal cords and often are called "the flappers").  The arytenoids are pulled open wide during inspiration to allow for maximum unobstructed airflow.  There are several problems that occur in this area that we will examine later.  Just after going past the arytenoids, we are in the trachea and on the way down to the lungs.  Before leaving the head, I want to mention that there are many lymph nodes in the head area that can cause significant problems if they become enlarged. 

The windpipe of an adult horse is about three inches in diameter and travels down the center of the neck into the thoracic cavity where it splits into the two main branches just above the heart.  The branches of the trachea, now called bronchi, divide many times until all the little (microscopic) branches connect to what is called an "air sac."  The air sac is where it all happens.  The structure at this level becomes so fine that there is nothing but a single thin membrane separating a single red blood cell from the oxygen in the air sac.  It is at this level that the oxygen diffuses across the membrane into the red blood cell and the carbon dioxide diffuses out of the red blood cell and into the air sac to be eliminated when a horse exhales.

The chest or thoracic cavity also can be called the pleural cavity.  The "pleura" is the thin, cellular membrane lining the chest cavity (inside of the ribs) and covering the surface of the lungs.  Within this cavity the lungs are fully expanded and in close contact with the chest cavity.  There is only a very small amount of fluid present within the chest cavity when everything is normal.  

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at or by calling 800/582-5604.

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