This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Which is responsible for respiratory drive:September 2007 
Select and explain. 
A. O2, B. CO, C. CO2, D. Bicarbonate ions
C: CO2
Answer: C: CO2 
Explanation: Ans. C: CO2Although the body requires oxygen for metabolism, low oxygen levels do not stimulate breathing. Rather, respiratory centre is directly stimulated by higher carbon dioxide levels or excess hydrogen ions in the blood. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (such as pure nitrogen) can lead to loss of consciousness without ever experiencing air hunger.The respiratory centers try to maintain an aerial CO2 pressure of 40 mm Hg. With intentional hyperventilation, the CO2 content of aerial blood may be lowered to 10-20 mm Hg (the oxygen content of the blood is little affected), and the respiratory drive is diminished.