When you exercise, whether running, playing tennis or lifting weights, your breathing rate increases dramatically to supply oxygen to your muscles, but also to get rid of carbon dioxide. Muscles require energy, stored in the form of a compound called ATP, which they replenish using several metabolic pathways. Aerobic metabolism uses oxygen, while anaerobic metabolism does not. Which pathway your muscles prefer, and how much carbon dioxide is produced, depends on the intensity of the exercise.
Low-Intensity Exercise
During low-intensity exercise, such as walking, aerobic metabolism predominates. Muscles use oxygen to metabolize fats and carbohydrates, although at low levels of effort, fat is their preferred fuel source. They produce carbon dioxide as a result, so how much carbon dioxide you breathe out is directly related to how much oxygen you take in. As you step up the pace, both increase at similar rates.
Higher-Intensity Exercise
As exercise intensity increases, muscles shift their fuel mixture to include more carbohydrates along with fats. Because of the chemical structure of fats and carbohydrates, they produce more carbon dioxide for the amount of oxygen used when carbohydrates are the fuel. This ratio of carbon dioxide to oxygen can be measured, and is called the respiratory quotient. When you burn fat the RQ is around 0.7, and when the fuel source is carbohydrate the RQ is 1.0.
Very High-Intensity Exercise
The more exercise intensity increases, such as during circuit training or sprinting, aerobic metabolism alone is not able to replenish energy quickly enough. Muscles have to call upon anaerobic metabolism as well, which produces lactic acid as a byproduct. To protect tissues from becoming too acidic, your body buffers lactic acid using a compound called bicarbonate. Carbon dioxide is produced as a result of that reaction -- more than could be produced by aerobic metabolism alone.
The Ventilatory Threshold
Although scientists don't completely understand the mechanism, during very intense exercise, as lactic acid begins to accumulate in the bloodstream, there is a sharp rise in non-metabolic carbon dioxide production. This is called the ventilatory threshold because your breathing rate increases sharply at that point to expel the carbon dioxide. Athletes who can exercise at higher intensities before reaching the ventilatory threshold often have a competitive advantage. Interval exercise, such as circuit training, can help increase your ventilatory threshold.
References
- "Exercise Physiology for Health, Fitness and Performance"; Sharon A. Plowman and Denise L. Smith; 2011
- "Exercise Physiology: Energy, Nutrition, and Human Performance"; William D. McArdle, Frank I. Katch and Victor L. Katch; 2007
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