[curtis]

The primary thing you need to know about holding your breath is that there's more oxygen capacity in your blood than there is in your lungs.

[alexfarran]

That may or may not be true, but it's not why this trick works. The need to take a breath is triggered by the level of CO2 in your blood. Hyperventilation exercises like Blaine's flush out the CO2 so it takes longer to build up to the threshold level.

[curtis]

Wikipedia agrees with you. According to http://en.wikipedia.org/wiki/Apnea: "Blood leaving the lungs is normally fully saturated with oxygen, so hyperventilation of normal air cannot increase the amount of oxygen available."

[sketerpot]

To elaborate a little: you don't sense the oxygen level in your blood. You sense the CO2 level, using acidity sensors in your aorta. CO2 dissolved in your blood reaches an equilibrium with carbonic acid, which means that the acidity of your blood is proportional to the amount of CO2. It's stuff like this that makes physiology fun to study.