Basically, the respiratory system for equines is to exchange oxygen and carbon dioxide. Oxygen enters the lungs during inspiration, and carbon dioxide exits with expiration. Oxygen is the fuel to be burned, and carbon dioxide is the exhaust to be removed. Air enters through the nostrils, then passes through the nasal cavity. A horse differs from many mammals in that it does not normally breathe through its mouth.

There is another distinct difference between the horse, and some other mammals the nasal cavity of the equine is quite long. This is of particular advantage in cold weather because it allows the inspired air to be warmed before it reaches the lungs. After passing through the nasal cavity (B), air continues its pathway to the lungs (K) by passing over the larynx (L) and pharynx (G), then entering the trachea (H).

Normally, there is a smooth passage of air from the nostrils into the lungs.

(A) Buccal cavity (B) Nasal Cavity (open to pharynx) (C) Inferior maxillary sinus (D) Superior maxillary sinus (E) Frontal sinuses (F) Guttural pouch (G) Pharynx (H) Trachea (I) Bronchus (J) Alveolus (K) Lungs (L) Larynx

The Throat

RIGHT: Normal LEFT :Paralised
(1). Trachea (2). Cartilage (3). Vocal cord (4). Epiglottis

When the horse swallows, for example, the pharynx and soft palate position themselves in such a way that food is directed into the esophagus rather than the trachea. The larynx(L), which houses the horse's vocal cords, also serves as a barrier that prevents food from moving into the trachea(H). The 50-60 cartilaginous rings of the horse's trachea form incomplete hoops opening dorsally. Smooth muscle fibers of the tracheal muscle join the inner surface of the free ends of the cartilaginous rings. Extra cartilaginous plates at the foot of the trachea(H) fill in the gaps between the free ends of the main rings. Irregular plates support the left and right principal bronchi(I) going to the lungs. At the end of the trachea is the bronchial tree(I). The conducting airways of the (I) divide into smaller and smaller bronchi. When cartilaginous plates no longer are present in the walls of the smallest bronchi, the airway is termed a bronchiole. The bronchioles, in turn, join with the alveolar(J) ducts that terminate in the functional units of the lungs where gas exchange actually occurs the alveoli(J). The alveoli(J)are small outpouchings along the walls of the alveolar sacs and alveolar ducts. It is through the walls of these tiny pouches that gas exchange actually takes place. The exchange occurs between air within alveoli and blood within capillaries in the alveolar walls. The alveoli(J) have very thin walls between oxygen laden air in the lungs and the blood vessels that contain red blood cells carrying oxygen throughout the body. Because of the thin structure of the walls, the carbon dioxide is able to move out of the blood, permeate the thin lining, and be expired. The alveolar structure allows the inspired oxygen to cross over and join with the red blood cells, where it is bound to hemoglobin for transport to the tissues. When the blood leaves the lungs, it is saturated with oxygen. The oxygen is transported through the arterial system to tissues where it is "burned" in cell metabolism. The use of oxygen produces "exhaust" carbon dioxide. Now the venous blood supply takes over. It picks up the carbon dioxide and returns it to the lungs, where it is expired.

How much oxygen is taken in and how much carbon dioxide is expired depend on how much the horse is exercising. When the horse is at rest, its respiration rate is very low, with the animal sometimes taking 10 to 14, or even fewer, breaths per minute. However, when strenuous exercise is involved, the respiration rate increases dramatically as the lungs move into high gear to satisfy the demand for oxygen throughout the body. When the horse breathes while at rest, little effort is involved. That changes when it breathes rapidly in an effort to increase the oxygen supply. With that demand, the intercostal muscles and diaphragm are called on to expand the chest, which in turn allows for an expansion of the lungs. The lung expansion allows a greater quantity of air to flow in.

Nature has both helped and hindered the horse in its effort to increase the flow of oxygen and remove carbon dioxide. The horse is constructed in such a way that respiration is assisted by limb motion. This is especially true when the animal is traveling at a gallop. At this pace, the respiration rate and stride rate are normally one to one. In other words, at every stride, the horse takes a huge breath. Inspiration occurs when the forefeet are being extended, and expiration occurs when the forefeet strike the ground. This unique feature is of benefit because it reduces the workload of the respiratory muscles, thus reducing the potential for fatigue.

The bad news is that the number of strides taken in a given distance also limits the intake of oxygen. The horse will reach a point at which it simply is running at its maximum stride frequency and, thus, has reached its limit to inspire oxygen.

The normal horse can handle the oxygen demand when the workload produces a heart rate up to 180 beats per minute. When the horse is stressed to the point that the heart rates hits 200 and beyond, the respiratory system might be unable to deliver a sufficient quantity of oxygen for fuel.

At this point, the blood is no longer saturated with oxygen when it leaves the lungs. This lack of oxygen saturation is called arterial hypoxemia.