The Respiratory System (Book Excerpt)

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


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

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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, New York. He was an FEI veterinarian and worked internationally with the United States Equestrian Team. He died in 2014.

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