The element oxygen is essential to all human life. It is the most abundant element in the Earth's crust, and it is the second most abundant element in the air, constituting approximately 21% of the Earth's atmosphere. The human body ingests oxygen primarily through the respiratory system, with the cardiovascular system the vehicle by which oxygen is then transported for uses throughout the body.

Oxygen is also a constituent part of such vital energy sources as the carbohydrates that are converted to glucose, which in molecular form is composed of carbon, hydrogen, and oxygen atoms aligned as C6H12O6. Oxygen is also essential to the structure of the dietary fats absorbed and stored in the body. Triglycerides represent the storage form of such fats; when released from the adipose storage cells, these molecules break down into fatty acids and glycerol, which is a substance with a carbon/hydrogen/oxygen structure similar to glucose. Oxygen is an agent in virtually all metabolic processes. Oxygen also is a part of most organic molecules in the body, the building blocks for all living things.

The importance of oxygen to an athlete is most obvious in the healthy function of the respiratory system, without which competitive athletics would be impossible. Air is inhaled through the mouth or nose and ultimately passes through the bronchial tubes into the lungs. Within the lung cavity, there are tiny, thin-walled alveoli, air sacs that are connected to the wall of the lung and which are each a part of a dense network of capillaries, the blood vessels that act as exchange mechanisms. Inhaled oxygen passes through the sac wall to directly enter the cardiovascular system, and waste carbon dioxide passes out into the lung.

Once the oxygen enters the bloodstream, it is transported by a component of the red blood cells, hemoglobin. The oxygen is taken to the cellular sites within the body where energy is being generated. While the body can produce energy without the presence of oxygen, through its two anaerobic systems, for limited periods of time, ultimately the aerobic system will be required. It is the presence of oxygen that permits the release of energy for long-term physical needs from the body's ultimate physical fuel source known as adenosine triphosphate (ATP). ATP is the product of a complex process that involves the breakdown of glucose or fatty acids.

Sports performance is impossible without the supply of oxygen in an uninterrupted fashion. When supplies of oxygen are restricted, the body must make adaptations to correct the shortage. The most common environmental change that limits the body's oxygen supply is as a result of the body performing at an increased altitude. As altitude increases, the atmosphere is described as becoming "thinner." The molecules in the air are less dense as the effect of gravity is less pronounced, and the amount of oxygen present in the elevated atmosphere is reduced. From the sea level percentage of approximately 21% oxygen, at Denver, Colorado's famous Mile High Stadium, located at 5,500 ft (2,400 m), the oxygen percentage is reduced to approximately 18%, and at the soccer stadium in Quito, Ecuador, with an elevation of 9,300 ft (2,850 m), oxygen is only 15% of the available air.

There are only two mechanisms available to counter the reduced amount of oxygen entering the body at those altitudes. The first is the process known as acclimatization, where the athlete spends a period of time in the oxygen-reduced environment prior to the anticipated event. Research confirms that the adaptations required by the body will be complete within two to three weeks of living and training in the thinner air.

Acclimatization spurs the production of a hormone in the kidneys known as erythropoietin (EPO), which is the chemical signal to the body to produce a greater number of erythrocytes (red blood cells) to counter the effect of less oxygen; if there is less oxygen available to be transported, the greater number of red blood cells can more efficiently acquire what available oxygen there is to be consumed.

In a related way, the goal of enhanced oxygen transport has been the reason for the development of synthetic versions of EPO, used as the primary blood-doping agent.

It is common in sporting events to see oxygen being administered on the sidelines to athletes. It is evident that there is a belief that such practices will aid athletes in their recovery and permit them to return to play more quickly. In events contested at sea level, such oxygen aids are not anything more than a placebo to the athlete; the lungs cannot absorb oxygen any more readily than if supplied by the air. At elevation, where the athlete is not acclimatized to the thinner atmosphere, the portable oxygen supply may be a modest assist to recovery.

SEE ALSO Blood doping; Cardiorespiratory function; EPO; Glycogen depletion.