i saw in the big post a mention to cannabis expanding alveoli... i was going to expand on that as i just learned about that in my A&P II class...

cannabis is a bronchiol dialator, which is why it can be beneficial as treatment for asthma (a little known fact). chemically it causes vasoconstriction (constriction of blood vessels). this is why people with hypertension (high blood pressure) should be careful with smoking as it can increase the likelyhood of a heart attack or stroke.

as the blood vessels contrict, it causes the airways and alveoli to be pulled apart and open more because of the tension in the walls surrounding the lumen (open space). this leads to an ease in breathing, as long as the lung tissue does not become irritated and swollen due to the heat and pollution of the smoke (hardly an issue if one vaporizes).

just FYI