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!