CO2 and blood acidity levels are the brain's primary trigger to breathe. A low O2 level in the bloodstream is a second and less strong trigger. Some smokers who have developed COPD (chronic obstructive pulmonary disease) have lost their CO2 trigger and rely on the O2 trigger to breath. If you were to put a COPD patient on O2, it is possible for them to stop breathing all together due to the high levels of O2 in their system.
One other bit of trivia. Freedivers have to be careful of overstaying their bottom time at deep depths. As a freediver decends the ppo2 of their lung volume increases just as a diver on scuba. It has been shown that gasses will difuse and mix from a higher concetration to that of a lower. As the freediver decends the higher concentration of O2 in his/her lungs difuses to their blood as one would expect. However, as time passes and the O2 is metabolized by the tissues, CO2 levels begin to increase. Freedivers train to ignore this CO2 buildup and resist the urge to breathe more easily than most. Thus, the lack of O2 in their system becomes the driving force to trigger them to breath. While the freediver is at the bottom the increased ppo2 of his/her lung and blood gas fools their brain into thinking there is more O2 than there actually is. The freediver at the bottom may not feel like they need to breath to badly. As he/she ascends and the ppo2 decreases, the amount of O2 may become less in the lungs than it is in the bloodsteam because the molecules are becoming farther apart with expansion. Reverse difusion could begin and O2 could pulled from the blood stream to equalize the lower O2 content in the lungs. The last 20 feet or so is the danger area where most blackouts of freedivers occurs. Sometimes, with well trained freedivers, blackouts occur from hypoxia rather than hypercapnia (CO2 buildup).
just if you were wondering....
