The equine respiratory tract is so highly specialized for exercise that even the slightest deviation from normal can limit a horse's athletic career.

The respiratory tract of the horse, which moves extremely large volumes of air in and out of the lungs, is a highly specialized organ system that serves one primary function: to exchange oxygen for carbon dioxide. Compared to humans (whose respiratory tract is highly specialized for speech) and other mammals, a horse inspires and expires a staggering amount of air.


"At maximal exercise, a horse's upper airway is subjected to marked fluctuations in flow and pressure during inspiration and expiration," explains Jon Cheetham, VetMB, PhD, Dipl. ACVS, from the Department of Clinical Sciences at Cornell University's College of Veterinary Medicine.


According to Cheetham, tracheal pressures ranging from -4905 Pa (pascals, which are a measurement of force per unit area; one pascal is a Newton per square meter) to 2746.8 Pa have been reported, as well as airflow velocities of up to 80 liters per second in horses exercising on a treadmill. To compare, the airflow of an average hair dryer is 40 L/s.


"A horse's maximal oxygen uptake at maximal exertion is approximately 160 mL/kg/minute, which is about 40 times greater than their oxygen uptake at rest," says Cheetham. "This is far higher than an elite human athlete's maximal oxygen uptake, which is only about six to eight times higher at exercising compared to resting values."


With the exception of the lungs, the remainder of the equine respiratory tract is essentially a glorified tube--the other components of the respiratory system are, in some ways, considered ancillary and serve primarily as a conduit for the air to move between the environment and lungs. That is not to say that the other parts of the respiratory system are unimportant. In fact, respiratory system dysfunction is the second-leading cause of exercise intolerance and poor performance in athletic horses, following musculoskeletal disorders. Structural, functional, and infectious conditions can occur at any point along the respiratory tract.

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