If you inhale to your maximum capacity and hold your breath for an extended period of time, it begins to feel uncomfortable; slowly releasing your breath at this point seems to provide relief from this discomfort.
Is there a biological reason why this is so, or is this something psychological?
The exact causes of the discomfort relief after the breath-hold breakpoint is unknown yet.
It has been stated that breath "pacemaker" continue to work independently of voluntary breath holding. Thus, while breath holding, the breath center is trying to activate the diaphragm and the afferent feedback from the diaphragm normally causes the feeling of discomfort. Thus, upon starting breathing again, i.e. actual diaphragm movements, the discomfort will diminish and the subject will feel relief.
Breath-holding and its breakpoint. Experimental Physiology Volume 91, Issue 1, January 2006.
This occurs because the feelings of needing more air to breath result from changes in the concentration of carbon dioxide in the bloodstream. This happens because the carbon dioxide gets dissolved in blood, turning into carbonic acid (H2CO3). This, since it is an acid, dissociates into ions, and your body can detect that change in acidity. These changed PH levels are not desirable because they reduce the efficiency of proteins and can cause negative effects. Therefore, the body detects these changes and creates the discomfort from the desire to breath out. This also holds true for too little carbon dioxide, because it will result in the reverse effect on the acidity in the blood stream (it will raise the PH/make it more basic). Passing out occurs when the acid levels are too high and nothing is making them return to normal (breathing out) so the brain says "this idiot cannot breath correctly" and shuts off the control center to do it itself (unconsciousness).
