Let me find my post regarding this question...
..ah, here we go:
https://www.rollitup.org/toke-n-talk/66308-facr-myth-holding-smoke-longer.html
Full text of response:
the psychotropic ingredients in weed (THC etc) is absorbed in the first few seconds of inhalation. You will get some more by holding in for a while, but I think the 'buzz' you get from holding it in is more a case of the carbon monoxide and lack of oxygen than anything. Holding it in will also allow more tars to settle in your lungs as well, which may not be a good thing. When I pull a cone, I usually hold it for about five seconds, all you really ned I think (in my experience at least...32 years worth)
I then clarified the post with this:
Let me clarify the statement I made. The alveoli in your lungs can only hold so much air/smoke etc. Once the alveoli have been filled, you can't absorb anymore of whatever is in the passageways to your alveoli. The reason a shotgun works is because there is still THC in all the myriad passageways that go to the alveoli. When you exhale into the lungs of your shotgun recipient, that's what they get, the leftover THC that your body can't absorb.
Remember, there are a lot of passageways to the alveoli, including your trachea and the bronchi (the passages that lead to your alveoli).
If you hold your breath long enough, all you'll do is reabsorb the carbon dioxides and monoxides that your lungs are designed to expell when you exhale, because they (the carbon dioxides etc) replace the THC and oxygen that your body absorbed from the alveoli originally. Plus, the tars will settle in your alveoli and start to block them, leading the way to emphysema and other nasty lung diseases/conditions.
I hope this little biology lesson helps clarify the shotgun thing.