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Perhaps the answer to this question depends on who is in charge of the character. While many artists have been involved in Swamp Thing story arcs over the decades, I am most familiar with the Alan Moore arc, with art by Stephen Bissette and John Totleben.
In ‘The Anatomy Lesson,’ there is an autopsy performed on the body of Swamp Thing, who is presumed to be dead and whose body has been frozen for study. The autopsy reveals structures inside the chest cavity that resemble anatomically correct human organs. However, although they look like organs, they don’t function like them. The ‘lungs’ inside Swamp Thing don’t actually take oxygen into the body and expel carbon dioxide as they do in mammals, reptiles, and birds.
Bramlett, Frank, "Does Swamp Thing have a penis?" (2013). English Faculty Publications. 29.