We feel "out of breath" when the body detects that CO2 levels in the blood have risen, indicating that more Oxygen will be needed. An above post suggests that the receptors for this are in the brain, but they are in fact in the blood vessels close to the heart, the carotid arteries and aorta to be precise. Chemoreceptors in the walls of these blood vessels monitor the pH of the blood. When CO2 levels rise, the levels of carbonic acid also go up, as CO2 dissolves, forming this weak acid. The pH of the blood therefore falls, and a negative feedback mechanism sends a message to the brain that there are higher levels of CO2 in the blood. The brain then sends signals down the accelerator nerve to the sino-atrial node in the heart, which stimulates a faster heart rate (for information on the sino-atrial nerve and heart beat, see heart).

Similarly, when carbonic acid levels fall, the pH of the blood rise and so the Chemoreceptors relay this information to the brain, which sends more signals, this time down the vagus nerve, which tell the sino-atrial node to stop beating so fast.

Normally if someone is forced to breathe the same air over and over again (eg through a balloon), they will begin to feel out of breath, as respiration uses up Oxygen and produces CO2, activating the Chemoreceptors and making us breathe faster. However, if a chemical such as Potassium Hydroxide is put into the air being breathed, the CO2 will continuously be removed from the air. The subject will therefore not feel out of breath, and so can quite happily sit there breathing air with virtually no Oxygen in it and not notice.. that is until they collapse and fall unconscious. Some more sadistic teachers conduct this experiment on their students, asking them to ride exercise bikes and monitoring breathing rates when forced to breathe the same air, which has the CO2 removed with each breath. The teacher will ask the subject to stop at some point, hopefully before they have fainted!