when you exercise intensely, your rate of gas exchange rises because
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 liter of carbon dioxide for every liter of oxygen you consume.
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.