Most likely you won't even recognize the lungs of Mamenchisaurus as lungs, they're so different from yours. And the lungs were just one part of a complex, highly efficient breathing system like the one found in birds today. The efficiency of this system was key to the gigantic size of sauropods.
The breathing system of any animal does two things: its gets air into and out of the body and its delivers oxygen to the blood while removing carbon dioxide from it. We mammals do both jobs with one organ--the bellows lung. Sauropods used two structures to accomplish the job--as do modern birds and living crocodiles. The two-structure system delivers more oxygen with less effort.