^^^ Well, they do have artificial gravity. It would stand to reason that they have structural integrity fields and inertial dampeners as well. The BSG showrunners have always made a habit of not focusing on such technical minutiae for the sake of the story, unless the story itself demanded some kind of specific technical explanation - Hera's blood and the comparative blood types between human and cylon come to mind for NuBSG. As far as the ships go, aside from using Tylium for fuel, we know very little how any of them really work.