What is the primary structure for gas exchange in the respiratory system, how is it deisgnedf or the job?

Answer 1

Alveoli.

http://oac.med.jhmi.edu/res_phys/Encyclopedia/Alveoli/Alveoli.HTML
http://en.wikipedia.org/wiki/Alveoli
http://lungdiseases.about.com/od/glossaryofterms/g/alveoli.htm
http://science.nhmccd.edu/biol/respiratory/alveoli.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/ptens2.html

I hope this helps. Thanks for asking.