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Horses performing intense exercise can breathe more than twice each second, and maximal oxygen uptake during exercise can be 40 times as great as when the horse is at rest. The horse’s respiratory system is designed to meet this enormous need for oxygen, but even during periods of inactivity, it must take in, warm, humidify, and filter a steady supply of air. These functions are handled by the various structures in the upper and lower portions of the horse’s respiratory tract.

Air inhaled through the horse’s nostrils is warmed and humidified by the turbinates, which are blood-rich scrolls of bone, before entering the trachea. Larger airborne particles are also trapped by the turbinates before they can proceed further into the respiratory tract. From the trachea, the air travels along larger airways (bronchi), which branch into small airways (bronchioles).

In the horse, the airways are lined by cilia, tiny hairlike projections that continuously sweep mucus and particles toward the nostrils. Mucus-producing cells are also found in the linings of the airways. Normally the mucus is thin and easily moved, but in disease states, such as with infection, the mucus can become thick and sticky. Groups of lymphoid cells are scattered throughout the airways, helping to maintain lung tissue’s immunity to many infectious agents.

The inward journey of air through the airways comes to an end at the alveolar sacs. It is within these sacs and across a very fine membrane, known as the alveolar membrane, that gas exchange occurs. Oxygen passes across the membrane into the red blood cells and carbon dioxide passes back across the other way. Oxygen is necessary for the survival of the tissues and carbon dioxide is a by-product of energy use. Stale air moves back out of the lungs as the horse exhales.

A final defense barrier exists in the alveoli. Tiny inhaled particles that get passed through the turbinates and airways land in the air sacs and are cleaned up by cells called macrophages. These cells engulf material ranging from tiny particles of dust to bacteria. However, they can be overloaded. For example, heavy burdens of dust can decrease the ability of those cells to fight infectious agents such as bacteria. A horse in a dusty environment will therefore be more prone to infection than a horse in a cleaner environment. Providing ventilation in the barn, using bedding material that is dust-free, and dampening hay before feeding are measures that help to minimize inhalation of dust and mold particles.

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