When you exercise, the muscles you use require more energy. When exercise can be sustained, this demand is met primarily by aerobic means. Aerobic energy production in muscles results in increased gas exchange at the lungs, because more oxygen is taken in and more carbon dioxide is released. Your blood transports these metabolic gases to and from your tissues.
Gas Exchange at Rest
Oxygen and carbon dioxide are both present in the atmosphere. Oxygen comprises 20.9 percent of the air you breathe, while carbon dioxide makes up 0.03 percent. When you breathe, you transfer these metabolic gases between the atmosphere and your blood in your lungs. Oxygen moves into your blood, which transports it to tissues where it liberates energy from foods you have eaten. This results in production of carbon dioxide, which must be removed. At rest, you consume 3.5 ml of oxygen per kg of body weight each minute to satisfy energy needs. Your muscle fiber type, glycogen content, dietary fat intake, training and blood metabolites all influence how much carbon dioxide you produce in association with this oxygen consumption. The least amount is 0.7 L for every liter of oxygen if fat is burned exclusively.
Gas Exchange During Exercise
During exercise, your body needs more energy, which means your tissues consume more oxygen than they do at rest. Consuming more oxygen means you will also produce more carbon dioxide because your metabolic rate is elevated. The ratio of carbon dioxide produced per oxygen consumed also increases during exercise because a shift from fat to carbohydrate utilization takes place. At the most challenging work rates, you burn carbohydrates exclusively and produce 1.0 L of carbon dioxide for every liter of oxygen you consume.
Non-Metabolic Carbon Dioxide
At rest and during moderate exercise, lactic acid will not increase in your muscles because all that is produced is also used. Once you reach more challenging work rates, production exceeds use and the acid enters your blood. To maintain a healthy pH, sodium bicarbonate in your blood buffers most lactic acid by breaking it down to water and carbon dioxide. This results in additional carbon dioxide that must be released by your blood.
Example
When you jog at 6 mph, you require 10.2 times the resting oxygen requirement, or 35.7 ml per kg each minute. If this is a very challenging pace. Your muscles will burn carbohydrates exclusively and produce 35.7 ml of carbon dioxide per kg each minute. This equates to 3 L of carbon dioxide each minute for a 185-lb. person sustaining that pace. The challenging exercise would also result in lactic acid accumulation and subsequent buffering to carbon dioxide. It is not unusual to produce an additional 0.1 L of carbon dioxide for every liter of oxygen consumed for this reason. This means total carbon dioxide production by a 185-lb. person jogging 6 mph could be 3.3 L per minute.
References
- "American Journal of Physiology Endocrinology and Metabolism"; Determinants of the Variability in Respiratory Exchange Ratio at Rest and During Exercise in Trained Athletes; Julia Goedecke, et al.; 2000
- "Exercise Physiology: Human Bioenergetics and Its Application"; G. Brooks, et al.; 2005.
- "ACSM's Guidelines for Exercise Testing and Prescription, 6th Edition"; American College of Sports Medicine; 2000.
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