The Equine Respiratory System
Oxygen powers exercise, and the equine respiratory system is responsible for bringing that oxygen into the body. Air enters through the nostrils, travels through the nasal passages, past the throat, down the trachea, and into the lungs, where it reaches the alveoli, or air sacs. There, oxygen diffuses into the pulmonary capillaries and is transported by the blood throughout the body to support energy production during exercise.
The body uses oxygen to make ATP, the molecule that provides energy to working muscles. As muscles generate energy, they also produce carbon dioxide as a waste product. Blood carries that carbon dioxide back to the lungs, where it diffuses into the alveoli and is exhaled. The diaphragm helps drive this process by expanding and contracting the lungs to move air in and out.
Moving Air at High Speed
The athletic horse has extraordinary respiratory demands. By the time a horse finishes a 5-furlong race, completes a Grand Prix jumping round, or covers one-sixth of a 3-star cross-country course, it will have moved roughly 1,800 liters of air through its lungs. That is about 475 gallons, or the equivalent of several bathtubs of air, moving in and out in less than a minute.
Of the air inhaled during intense exercise, only a portion of the oxygen is transferred into the bloodstream. That oxygen is essential because most of the energy required for performance still comes from aerobic metabolism, even in short, intense efforts. In a 5-furlong race, about 70% of the horse's energy demand is met aerobically. The remainder comes from anaerobic metabolism, which can generate energy quickly without oxygen but is less efficient and leads to lactic acid accumulation.
As speed increases, oxygen demand rises rapidly. The harder a horse works, the more air it must move through its respiratory system.
1,800
Liters of air moved in a 5-furlong race
70%
Energy from aerobic metabolism in a sprint
475
Gallons of air—several bathtubs full
Inhaling Oxygen
Air first passes through the upper respiratory tract, including the nostrils, nasal passages, larynx, and trachea, before moving into the lower airways and ultimately the alveoli deep within the lungs.
The trachea is relatively wide near the larynx, but as it branches within the lungs it divides again and again into progressively smaller airways. By the time air reaches the smallest passages, those airways are only fractions of a millimeter in diameter.
At the alveoli, oxygen must cross an extremely thin barrier (pulmonary capillary membrane) separating air from blood. This membrane is only a tiny fraction of the thickness of a human hair, yet it provides an enormous total surface area for gas exchange. In fact, the total area available for oxygen diffusion in the horse's lungs is immense.
Exercise-Induced Pulmonary Hemorrhage
Because these pulmonary capillary membranes are so delicate, they can rupture under the extreme stresses of exercise. When that happens, red blood cells leak into the airspaces of the lung, a condition known as Exercise-Induced Pulmonary Hemorrhage, or EIPH.
Transporting Oxygen to Working Muscle
Once oxygen enters the bloodstream, it binds to hemoglobin within red blood cells. The heart then pumps this oxygen-rich blood throughout the body to the muscles. At the muscle level, oxygen leaves the blood and diffuses into muscle cells, where it is ultimately used inside the mitochondria to produce energy.
Efficient oxygen transfer from the airways to the bloodstream, and from the bloodstream to the muscle, is critical to performance. Horses with exceptional cardiovascular capacity, including large hearts and a strong ability to use oxygen, still depend on efficient airflow to perform at their best.
Exhaling Carbon Dioxide
The respiratory system is just as important for removing carbon dioxide as it is for bringing in oxygen. Carbon dioxide produced in the muscles travels back to the lungs in the blood, diffuses into the alveoli, and is exhaled.
Clearing carbon dioxide efficiently matters. If carbon dioxide accumulates, it can contribute to fatigue and impair performance. A well-functioning respiratory system helps the horse both deliver oxygen and remove metabolic waste.
Other Essential Functions of the Lungs
The lungs do far more than exchange gases.
They also act as an important filter for the body. All circulating blood passes through the lungs, allowing them to trap small clots or gas bubbles before those can reach more sensitive organs such as the brain or heart.
The lungs also play a role in hormone regulation, helping activate or deactivate certain substances in circulation. In some respects, the lungs function as an endocrine organ.
Because the lungs are one of the body's main interfaces with the outside world, they also contain a highly specialized immune defense system. Along with the skin and gastrointestinal tract, they are constantly exposed to environmental challenges such as dust, bacteria, fungi, and viruses.
Regulating Body Temperature
A frequently overlooked role of the respiratory system is thermoregulation. Horses use breathing not only to support oxygen delivery, but also to help dissipate heat.
That is one reason horses continue to breathe heavily after exercise. While it may appear they are simply trying to get more oxygen, they are also working to cool themselves. In warm conditions, respiratory heat loss becomes an important component of recovery.
