Only some of a humans body functions could ever need to be simulated when creating the most ultra realistic humanoid robot.
Here we will talk about some areas like the respiratory system, and digestive system.
The Gemmy toy Halloween/Christmas company produced in the past a 5 foot tall $200 singing Edwardian Butler who would stand in one spot but would move its head and eyes/mouth and sing songs.
But it also had an interesting feature where the voice would start heavy breathing and the chest would actually be made to move back and forth to replicate the look of a creature or human breathing.
So if this can be done in a $200 robot its certainly simple enough to add some pneumatics to the robot build so that either the chest can move back and forth continually or now and again and perhaps some air could actually come out of the mouth.
Robot building can be just like Hollwyood especially if one is bringing a robot onto a televions show like the Jimmy Fallon show to impress an audience.
So you could have a mirror placed in front of the robot to show that it is breathing if you want to go for the spectacle of the Hollywood look.
Its your robot…so you can decide how far you want to go with it.