Exercise is an energy intensive process. Working muscles have high demands for energy; these high energy particles are broken down to form carbon dioxide and water. While the carbon dioxide levels in the veins are increased, healthy people normally have normal carbon dioxide levels in their arteries. The increased carbon dioxide produced during exercise has a number of important effects in the body that help increase oxygen delivery to muscle.
How Carbon Dioxide is Formed
The body requires energy for a number of chemical reactions needed for creating proteins, sending nerve impulses and pumping the heart. Nearly every chemical reaction requires energy, and this comes from breaking down the chemical bonds that are found in sugars and fatty acids. This process, known as cellular respiration, converts sugars to carbon dioxide and water. Oxygen is a necessary part of this reaction.
Changes in Carbon Dioxide During Exercise
During exercise, the body's energy requirements increase. Active muscles require large amounts of calories to contract. As with any other cellular reaction, the source of this energy is sugars and fatty acids. Because more energy is required, additional carbon dioxide is formed. This extra carbon dioxide leaves the muscle, enters the veins and travels to the lungs, where it is expired. The venous concentration of carbon dioxide is raised. However, because the lungs blow off this gas, the arterial carbon dioxide, which is normally measured in medicine, stays the same.
Carbon Dioxide and Breathing
The rise in venous carbon dioxide plays two important roles during exercise. The respiratory center, the part of the brain that controls the rate of breathing, is very sensitive to the effects of carbon dioxide. The raised levels of carbon dioxide in the brain stimulate the respiratory center, which causes an increase in the frequency of breathing. This extra breathing rate helps to get rid of the carbon dioxide and also helps to ensure that the blood is adequately oxygenated.
Carbon Dioxide and Hemoglobin
The carbon dioxide released by active muscles also serves a second function. Oxygen in the blood vessels is temporarily carried by a molecule called hemoglobin. Each molecule of hemoglobin can carry up to four molecules of oxygen. When the fully-loaded hemoglobin enters a tissue, the oxygen can unbind from it and go into cells. Carbon dioxide can facilitate this unbinding process. Extra carbon dioxide released from working muscles can bind to hemoglobin, hastening its unloading and providing additional oxygen supplies for muscles during times of stress.
References
- "Textbook of Medical Physiology"; John Hall; 2010
- "Physiology"; Linda Constanzo; 2009
