Nobody in this thread has linked to any scientific studies showing that nothing is absorbed after a certain point.
Some people have simply said "scientists say X", while others haven't.
That is not something that really convinces me, ya know? Some random person on the internet claiming to quote numbers from a real scientist.
Instead, I trust my own experiments, though they are not double-blind.
I have undertaken two types of experiments.
In the first, I shotgunned my girlfriend , just as described by the OP, and she got plenty high on 2 or 3 hits that I first held in for around 5 seconds. I, also, got high.
In the second, I tried smoking a much, much smaller amount that I normally would, but breaking it down into tiny amounts at a time. For example, I would normally take 2 hits to get high, but I took a pinch of weed - a little less than 1 hit's worth - and broke it up into 4 portions. Each was very, very tiny. I took one at a time, ghosting each hit. I did this because I was smoking in a public place, and couldn't exhale any smoke at all. I was successful, but surprisingly, I was too high to continue after the fourth hit. This is despite the fact that I smoked a bit less than half the amount I would normally smoke.
I have experienced the increase in effects when I hold my hits in for a long time.
Finally, the lungs are larger at the bottom, and have more absorbent capacity per area where they are larger, so it does not make sense to force the air towards the top.
Learned this in yoga class
(I suspect that anyone who has practiced yogic breathing will know the truth of this. )
