The function of the respiratory system is to take in oxygen and expel carbon dioxide. Oxygen is essential for life and is involved in all bodily functions. Carbon dioxide is given off as waste. A healthy respiratory tract will exhibit itself by relaxed, rhythmic, and quiet breaths. At rest, a horse typically takes ten to fourteen breaths per minute. Each of these breaths will be followed by a pause. Exercise will cause a sharp rise in respiration, but a healthy horse should return to his resting rate within ten minutes. You can easily check your horse's respiration at rest by counting the rise and fall of his flanks for one minute. The horse's lungs are large and take up a good portion of the chest cavity. To give you an idea of size, the lungs of a horse can hold up to thirty quarts of air, equivalent to seven and a half gallons. Structures involved in the passage of air into the lungs are the nostrils, pharynx, larynx, esophagus, trachea, and diaphragm.
A well-known disease of the larynx, known commonly as “roaring,” is caused by the degeneration of the nerves that support the muscles that open and close the larynx. The sound a horse makes upon inhalation when afflicted with this disease sounds like a roar. This can be nothing to worry about. If, however, the noise is accompanied by any exercise intolerance, your veterinarian should examine the horse.