It's absorbed to a much lesser extent along the larger bronchial tubes, too, but the bulk of the blood-gas-THC/lipophilic exchange occurs in the alveoli. That's really the only area of the lungs that allows for the microscopic exchange of any chemical, whether it's cannabis/THC, cigarette smoke/nicotine, or environmental toxins like benzenes. In the development of aerosolized THC-containing meds, they've also found that the bulk of the best aborption occurs in the alveoli. (Google "alveolar absorption of THC" and you can see some of the abstracts about the development/transmission of those meds.)

You have to read pretty far down in this link, but you'll see the alveolar details in here:
Marijuana Research: The Effects of Marijuana Smoke