What is considered the main trunk of the respiratory system?
Since this is not my area of expertise, I searched Google under the key words "'respiratory system' main trunk" to get these possible sources:
http://en.wikipedia.org/wiki/Respiratory_system
http://www.answers.com/topic/respiratory-system
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search.
I hope this helps. Thanks for asking.