The main goal of the respiratory system is to transfer oxygen from the air we breathe to the red blood cells, where the oxygen will be transported throughout the body and be available for all organs and tissues. In addition, carbon dioxide, a waste product of metabolism, is eliminated from the body via the lungs. While this seems a simple task, it is one that, if compromised in any way, will severely affect the athletic ability of a horse, if not his very life.

A horse with a sinus infection with an indwelling drain for flushing.

Average room air (at sea level) contains about 21% oxygen, 70% (or thereabouts) nitrogen, and the remainder is made up of a variety of gases, including pollution. So, in fact, the air we typically breathe is not very rich in oxygen. As the altitude gets higher, the percentage of oxygen in the air becomes less (the air becomes "thinner"). The body can do a few things to compensate for the reduced concentration of oxygen in the air of higher altitudes, but it takes several weeks for the adjustment. That is the reason careful training and conditioning are required when athletes (human and horse alike) work at higher-than-normal altitudes