system
Given the following post, respond as if you're commenting on Reddit:
human
How are humans able to fart gas out downwards if gasses naturally rise in the body?

Inside of a human's rectum, shouldn't solids sink to the bottom gasses rise to the top? How can we fart gas followed by solids? Doesn't physics say it should be the other way around?
gpt
The human digestive system is blocked off at several points by several sphincters. This prevents too-early passage of food as well as back-draft from gases. Since most of the gases build up in the final stretch, called the large intestine (which way shorter than the small intestine, but a bit wider), it simply becomes a matter of pressure build-up within a closed container. Once the pressure inside is bigger than the air pressure outside, any opening of the outside sphincter causes the gases to be expelled until pressure equalizes.