Introduction To The Respiratory System In Horses

The system of gas exchange is isolated from the rest of the body because of two things.  First air is dirty and filled with viruses, bacteria, mold spores, pollutants, dust and dirt with an occasional insect.  These need to be trapped or prevented from entering this gas filtering system.  The second part is a membrane system that allows gas molecules to pass from the outside air through a thin respiratory membrane, through a thin blood vessel (capillary) and attach to a blood cell for distribution throughout the body.  As with the other body systems, this air exchange system also eliminates waste products from the body (carbon dioxide, hormones, medications) to the outside air.

Billions of years ago the air on this planet was mostly methane and carbon dioxide.  With the development of plants, the oxygen content increased. The 1 cell organisms that were precursors to the animals we know today (horses and humans) needed to adapt to this change in gas composition. The development of lungs in mammals was part of this adaptation. 

This evolution included the pathways for this air to the lungs from our nostrils (air intakes).  It needed to be warmed when the air was cold (turbinates).  It needed to be filtered (hairs).  It needed to be moistened (secretions).  And on the way by, it might as well be used to make noises (vocal cords, larynx).  It also had to block food from getting in (the epiglottis).  It needed to expel foreign objects (cough) and fight off bad guys (inflammation, exudate). It needed to help equalize the pressures outside the ear drums (Eustachian tube).  To add some uniqueness, the horse developed the guttural pouches and a complex sinus system.

The air all of us are breathing today is 78% nitrogen and only about 20% oxygen and about 2% other gasses.  In fact oxygen above 20% becomes toxic.  When the air gained more oxygen billions of years ago, a bacteria moved into our cells (called mitochondria) which allowed for the conversion of oxygen into energy.  This was a good symbiotic relationship because most 1 cell organisms were dying due to the extra oxygen.  Today the multi cell organism (horse and human) take in the air gasses and deliver the oxygen to the mitochondria in the cell which keeps them alive.  In return the mitochondria make the energy the cells of the body need to remain alive.  Additionally, all proteins are made of 3 gasses: nitrogen, oxygen and hydrogen and all come in either through food or air.

What goes out in the expelled breath is very interesting.  Carbon is in every part of our body (sugar, fat and protein) and when a cell uses fuel to create energy, the waste product for all animals is carbon dioxide (CO2).  Interestingly, carbon is heavier than water (the 2 gasses hydrogen and oxygen or H2O) so most long term weight loss is actually the loss of carbon through our breath and not the water loss which is short term weight loss. Carbon dioxide then is the most abundant material that is breathed out.

Also eliminated in expelled breath are other molecules with water being the most common.  Other molecules include adrenaline, epinephrine and other hormones.  The control of the rate and depth of breathing is controlled by sensors in the kidney and brain that determine how much oxygen is needed for fuel metabolism and how much carbon dioxide is accumulating in the body.  Just hold your breath or exercise to see this happen in you.  Therefore exercise physiology is an important part of understanding how to train your horses.

Problems in the respiratory system include trauma (pneumothorax), blockage (sinus cyst, collapsed vocal cord), immune reaction (allergic bronchitis, COPD) and infection (viral, bacterial, fungal) of any part. 

Physics and math play an important part in this air flow.  The volume of air entering the lungs is directly proportional to the area of a circle.   The area of a circle equals π (pi) times radius squared (π r squared).  Any slight decrease in radius (inflammation, allergy) leads to a massive decrease in volume of air delivered to or from the body.  There is also laminar flow which describes the increased friction of fluid or gas passing next to a vessel wall.  Fluid or gas moving down the center of the pipe will move faster.  When turbulence is created from any blockage within an airway (inflammation, larynx problem), the flow of air or fluid normally in laminar flow is disrupted and volume is decreased by the decreased radius.  Think of a traffic accident disrupting the flow of cars on a highway. 

Any disruption of laminar flow or decreased radius will lead to decreased exchange of air needed to fuel the body and is usually seen as exercise intolerance at best and at worst, pneumonia and death.  Drowning for example is the total absence of gas exchange leading to an absence of oxygen for cell metabolism.  This will occur in a few minutes.  A 10% decrease in air delivery will lead to a decrease in the ability to metabolize fuel but will not cause death but rather exercise intolerance. 

One more thing.  The lungs have a microbiota consisting of trillions of bacteria that live in the lungs.  They help to defend the lining of the lungs and are essential to their health.  When a bad bacteria gets into the lungs this microbiota is disrupted and lung tissue becomes damaged.  Not much is said about these bacteria as most research is looking at the gut microbiota but bacteria live anywhere outside the body and this includes inside the lungs, sinuses and nostrils. 

I expect more will be learned about the protective benefits of these but deep breathing should be considered a part of this.  Why? Because you need to think of the lungs like a kitchen sponge filled with tiny air bubbles. When squeezed by your hand and then placed in a bowl of water, when you release, the sponge fills with water.  Take your sponge out of the water and squeeze it hard.  Almost all the water is removed.  The lungs work similarly in that when you fully express the lungs (exhale) they not only are devoid of air, the squeezing is also allowing in all of the blood allowed in the lungs.  When you fully inflate the lungs you are filling them with air AND pressing out all the blood.  This massive exchange does 2 things.  First it completely replaced the old air and blood with fresh air and blood.  Second it prevents sections of the lung from becoming solid from disuse (atelectasis).  When this happens, no exchange of air and blood will ever occur (permanent loss).  Taking deep breaths by exercising the horse hard for a brief period a few times a week will prevent atelectasis caused from disuse or adhesions from inflammation, promote replacement of the stagnant blood and keeps the good bacteria in the respiratory tract healthy as they are most likely air loving bacteria (aerobic).  Air hating bacteria (anaerobic) would love stagnated blood and solid lung tissue. 

So sprint with your horse once in a while and feel the difference.

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Here is a short video just to see the breath of a horse just to get you to start thinking about how critical breathing really is to all of us.

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