I think a coupling of established scientific research and one's own perception and experience is the best way to come to an acceptable conclusion. What birdgirl said makes perfect sense, esp. the fact that the essential gases in a breath of standard air are absorbed instantly. But are the lungs just as efficient with a breath of thicker, heavier gas (i.e. potsmoke)? How would the lungs even recognize which particles they were supposed to absorb and which they are supposed to expel when given exposure to a big cloud of smoke?

Could it be that the pure THC content is absorbed instantly and other active cannabinoids are discarded through exhalation? If not, how would one explain the efficacy of hotboxing a car? Being surrounded by a captured cloud of second-hand potsmoke (in many cases, the smoke is all second-hand, without any fresh runoff smoke from the bowl), seems to add to the feeling of being stoned. And I would discount placebo and oxygen-deprivation, since I've done it so many times, and people don't complain about getting light-headed in a closed car under normal conditions.

Also, if you're running low on herbage, use a balloon to blow your hit into. Breathe fresh air, then re-inhale the contents of the balloon. No oxygen deprivation, and heightened stoney effects. The body seems to adapt quickly to this method and a plateau is reached, which is why I recommend doing it sparingly, during times of famine.

It's obvious that without free, unrestricted research on mj, it's hard to come to a consensus, and we will all be content with our own notions.