Oxygen Uptake

Air is breathed in through the nostrils and mouth before entering the left and right bronchi of the lungs. Gas exchange in the lungs occurs in the 150 million alveoli humans have in each bronchi. The average adult male can inhale approximately 4 liters (close to 4 quarts) of air with each breath, from which the oxygen is diffused into the alveoli, passes into the blood through capillaries, and enters the hemoglobin in red blood cells. At the same time, carbon dioxide in the blood passes to the alveoli and is exhaled. The oxygen content of air is approximately 21 percent, and the oxygen content of expired air is approximately 15 percent, indicating that only a small proportion of the oxygen in air is captured by the lungs. The typical water content of air is .5 percent, while the water content of expired air is approximately 6 percent, demonstrating why rapid respiration is a major route of water loss in athletes.

The rate of cellular respiration increases with exercise intensity, with vigorous high-intensity exercise causing a 25-fold increase in the demand for oxygen in working muscles. This increase in oxygen requirement is satisfied by an increase in the rate and depth of breathing. However, it is the rising rate of carbon dioxide, rather than the higher demand for oxygen, that triggers the increased breathing rate. A higher carbon dioxide level causes the medulla to stimulate the motor nerves controlling the intercostal and diaphragm muscles to increase their activity.

Diseases that affect the lungs, such as pneumonia, asthma, emphysema, bronchitis, chronic obstructive pulmonary disease, and lung cancer, compromise an individual's ability to obtain sufficient oxygen and excrete sufficient carbon dioxide. An area of particular interest to athletes is exercise-induced asthma (EIA), which affects a significant proportion of the athlete population. The prevalence of EIA in athletes is not fully established, but there are published reports of a 55 percent prevalence in cross-country skiers and a lower (12 percent) prevalence in basketball players.^2,3

EIA is an airway obstruction that occurs as a result of exercise (either during or after); includes cough, wheezing, and a tightness in the chest; and may occur in people who do not suffer from chronic asthma.⁴ The recommended nonpharmacologic treatment for EIA includes the following:⁵

1. Become well conditioned for the activity that induces EIA. Well-conditioned athletes can exercise at lower breathing rates at any given work intensity and are therefore less likely to suffer from EIA.
2. Avoid exercising in cold and dry air. If the sport mandates exercising outside in these conditions (as in cross-country skiing), athletes should cover the mouth and nose with a scarf or ski mask to warm and humidify the breathed air.
3. A warm-up period is important, with the intensity of the warm-up dependent on how each athlete responds to EIA.
4. A cool-down period may diminish the severity of EIA by slowing airway changes.
5. Exercise on days when chronic asthma symptoms that may result from allergies or other conditions (if they exist) are controlled.

Oxygen Delivery and Cellular Utilization

The Oxygen-Nutrient Performance Relationship