Moderate to heavy physical exercise greatly increases the amount of oxygen skeletal muscles use. For example, a young man at rest will utilize about 250 milliliters of oxygen per minute, but he may require 3,600 milliliters per minute during maximal exercise. While oxygen utilization is increasing, the volume of carbon dioxide produced increases also. Since decreased blood oxygen and increased blood carbon dioxide concentration stimulate the respiratory center, it is not surprising that exercise is accompanied by an increased breathing rate. However, studies have revealed that blood oxygen and carbon dioxide concentrations usually remain nearly unchanged during exercise--a reflection of the respiratory system's effectiveness in obtaining oxygen and releasing carbon dioxide to the outside.

What other factors might increase breathing rate during exercise? The cerebral cortex and the proprioceptors associated with muscles and joints seem to be involved. Specifically, the cortex transmits stimulating impulses to the respiratory center whenever it signals the skeletal muscles to contract. At the same time, muscular movements stimulate the proprioceptors, triggering a joint reflex. In this reflex, sensory impulses are transmitted from the proprioceptors to the respiratory center, and breathing rate increases.

The increase in breathing rate during exercise requires an increase in blood flow to meet the needs of skeletal muscles. Thus, physical exercise increases demand on both the respiratory and the circulatory systems. If either of these systems fails to keep up with cellular demands, the person will begin to feel out of breath. This sensation however, is usually due to the inability of the heart and circulatory system to move enough blood between the lungs and the body cells, rather than to the inability of the respiratory system to provide enough air.

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