Respiratory System

The primary function of the respiratory system is gas exchange. Air, containing 21% oxygen (O²) is inhaled from the nostrils through the nasal passages, pharynx, larynx and trachea to the lungs. It then diffuses from the alveoli of the lung into the pulmonary capillary circulation where it is picked up by the hemoglobin and transported via the circulatory system to the muscles. The muscles use oxygen to burn fuels from the horse’s diet (carbohydrates and fats) to produce the energy necessary for muscle contraction. Carbon dioxide CO², which is by-product, is exhaled. The horse must breathe at a respiratory rate sufficient to satisfy the body’s oxygen needs. If the needs are not met, the horse accumulates an oxygen deficit. Horses have an oxygen deficit, for example, at the end of a steeplechase or cross- country course. The amount of oxygen required to repay the deficit is directly correlated to the intensity and duration of the exercise (speed and distance). The amount of air breathed over a period of time is a product of the number of breaths (respiratory rate) and the volume of each breath (tidal volume). The respiratory rate for the horse at a gallop is linked to stride frequency at a one to one ratio (i.e. the horse takes a breath with every stride) with the horse exhaling when the front limbs are on the ground. For a horse running at maximum speed, the respiratory rate can reach 150 breaths per minute (2.5 breaths per second) and the tidal volume can exceed 12 liters per breath. The total amount of air entering the horse’s lungs while racing is approximately 1800 liters per minute (about 475 gallons of air).

Anatomically and physiologically, the horse’s respiratory tract is divided into the upper respiratory tract and the lower respiratory tract. The upper respiratory tract includes the nostrils, nasal passages, pharynx. Larynx and trachea. Its function is to warm, humidify, and filter the air on its way to the lungs. The lower respiratory tract includes the structures, which lie within the chest (thorax), primarily the bronchi and aveoli in the lungs. 

The upper respiratory tract has little margin for error to accommodate the high flow rates through the relatively narrow opening of the larynx. For the running horse with his head down and nose extended, the air follows a fairly straight pathway to the lungs if all structures function normally. Horses must breathe through the nose, unlike other domesticated species that have the option of breathing either through the nose or mouth. This physiological characteristic is termed an obligate nasal breather.

Horses are subject to many of the same lower respiratory diseases as people, such as allergic or infectios bronchitis, pnemonia, pleuritis, and pleuropneumonia. These diseases are treated similarly to the same conditions in humans. Pneumonia, pleuritis and pleuropneumonia are particularly serious illnesses of the horse. Historically they were most often associated with long distance shipments of horses hence the term “shipping fever”.