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The Horse’s Respiratory System - Breath is Life
The respiratory system consists of the large and small airways and the lungs. The most important function of the respiratory system is to deliver oxygen to the blood and remove carbon dioxide.
The horse is an obligate nasal breather. The nasal passages in the horse are separated from the oral (mouth) cavity. They do not breathe through their mouths unless they have an injury or abnormality to the soft palate.
At canter and gallop normal horses take one breath perfectly in time with one stride. This is referred to as respiratory-locomotor coupling. The amount of time taken to inhale is the same as the time taken to exhale.
The amount of air moved in and out of the lungs increases in direct proportion to how fast the horse is running. If a horse runs twice as fast, it must move twice as much air in and out.
When horses inhale during exercise, around 90% of the resistance (obstruction) to air movement is in the airways in the head; the nostrils, the nasal passages, and the larynx. When horses exhale, most resistance to air movement (55%) lies within the lung airways.
An overtight girth will affect the horse’s performance–not because of constricting the chest and preventing the lungs from expanding, but because it decreases the effectiveness of the muscles around the front of the chest and shoulder that move the forelimbs.
Horses breathe by expanding and contracting their chests during rest, when breathing at walk and trot, and perhaps most noticeably when blowing hard after exercise. During the canter and gallop, all air movement comes from movement of the diaphragm.
Horses hold their breath over jumps, breathing out when landed.
The amount of air moved in and out by an unfit horse at a fixed speed will be the same six months later when that horse is fully fit.
The pressure in the pulmonary blood vessels during galloping is four to five times higher than resting pressure. This is one of the factors that puts stress on the very thin walls of the blood vessels and leads to some of them rupturing, causing Exercise Induced Pulmonary Hemorrhage.
If all the airways in the lung were opened out and laid flat on the ground, they would occupy a total area equivalent to 10 tennis courts.