General Knowledge
Why does swallowed food still reach your stomach upside down?
- Muscles squeeze it along in waves
- Gravity does the work
- You cannot swallow upside down
The answer is Muscles squeeze it along in waves. Peristalsis - the same wave that moves food along a worm.
Because gravity is not doing the work. Rings of muscle in the oesophagus contract in a wave behind the mouthful and push it along - peristalsis - and the wave does not care which way is up. Astronauts eat perfectly well in free fall for the same reason.
The same wave runs the whole length of the gut, and it is largely automatic: the digestive tract has its own nervous system of several hundred million neurons, sometimes called the second brain, which can coordinate all of this with the connection to the actual brain cut.
It is also how an earthworm moves, and how a snail glides, and it is copied in industry as the peristaltic pump - a tube squeezed by rollers, so the fluid touches nothing but the inside of the tube. That makes it the standard choice for dialysis machines, heart-lung bypass and anything else where contamination is unacceptable. Swallowing itself is a surprisingly complex act involving over two dozen muscles, which is why losing the ability to do it safely is such a serious clinical problem.