How does a fart travel to the anus?

One may wonder why fart gas travels downward toward the anus when gas has a lower density than liquids and solids, and should therefore travel upwards.
The intestine squeezes its contents toward the anus in a series of contractions, a process called peristalsis. The process is stimulated by eating, which is why we often need to poop and fart right after a meal. Peristalsis creates a zone of high pressure, forcing all intestinal contents, gas included, to move towards a region of lower pressure, which is toward the anus. Gas is more mobile than other components, and small bubbles coalesce to from larger bubbles en route to the exit. When peristalsis is not active, gas bubbles may begin to percolate upwards again, but they won't get very far due to the complicated and convoluted shape of the intestine. Furthermore, the anus is neither up nor down when a person is lying down.

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