That's it, just certain Aristolochia species. This is a list I borrowed from another site on the net. Just because you haven't seen any (in the wild, I assume?), that doesn't mean they aren't around somewhere. Many are hard to spot unless they are blooming...and may not be common in your area.
Aristolochia macrophylla - big leaf pipevine
Aristolochia tomentosa - woolly pipevine
Aristolochia serpentaria - Virginia snakeroot
Aristolochia californica - California pipevine
Aristolochia fimbriata - white-veined Duchman's pipe
Aristolochia trilobata - Dutchman's Pipe
Aristolochia clematitis - Birthwort
BTW, that list includes some that aren't native to the U.S.