Every universe is a lung.
A left lung, more precisely, because it shares space with the heart of that universe, and all life needs some TLC.
The right lung is the parallel universe to that in the left lung. It is bigger but harsher, because of the distance to the heart. Every being existing in the left lung has a mirror version in the right - only robotic. Robots are calculating, cruel and deterministic - they lack a heart.
The alveoli are where the galaxies and planets are; little cavities at the end of the air supply, supporting life (in both lungs, although the robots doesn't technically breathe).
The only way to reach the parallel universe is to travel through a worm hole junction called the Primary Bronchi. But beware, steer clear of the trachea or you may travel to far and eventually leave the body. If so, aim for the nearest mouth, keep going left and pray to your gods that the next universe is a kind one.