- The subject begins by breathing pure air (21% oxygen, 0.03% carbon dioxide, and about 79% inert gases by volume), first from the room and then from the tank. This control reveals what, if any, changes in response can be expected just by breathing from the tank (because of an unpleasant taste or increased air resistance, for example). The two graphs show that no appreciable change does occur when breathing air.
- When 100% oxygen is used instead, no marked change in rate ("breaths/minute") or depth ("vital capacity") of breathing occurs either, although there is a tendency for depth of breathing to decrease slightly.
- When the subject inhales a gas mixture consisting of 92% oxygen and 8% carbon dioxide, however, a most dramatic increase in the rate and depth of inspiration takes place. Note that there is no question of tissues lacking oxygen. The gas mixture contains four times as much oxygen as air.
- Note that after a period of breath-holding, the rate and depth of inspiration are markedly greater than before the breath-holding began (1). This can be explained by the build-up of CO2 during the breath-holding period.
- Vigorous, forced hyperventilation reduces the CO2 content of the alveolar air and blood to below its normal value, leading to a period of shallow breathing before its concentration builds back up to normal (2).
- The length of time that one can hold his or her breath to the breaking point can be substantially increased by hyperventilating just prior to the period of breath-holding (3).
It may seem curious that the rate at which one breathes and thus supplies oxygen to the body is controlled by carbon dioxide rather than oxygen. But cellular respiration produces CO2 as fast as oxygen is consumed, so the CO2 given off by active muscles triggers increased ventilation of the lungs and thus automatically supplies additional oxygen. While CO2 is the major stimulus for controlling breathing, the carotid body in the carotid arteries does have receptors that respond to a drop in oxygen. Their activation is important in situations (e.g., at high altitude in the unpressurized cabin of an aircraft) where oxygen supply is inadequate, but there has been no increase in the production of CO2. People who live at high altitudes, e.g., in the Andes, have enlarged carotid bodies.