describe what peristalsis is and how it allows you to drink water while hanging upside down.

Peristalsis is a series of involuntary, wave-like muscle contractions that occur throughout the digestive tract, especially prominent in the esophagus. These contractions push food and fluids forward through the digestive system, independent of gravity. When swallowing, the muscles in the throat and esophagus contract in a coordinated manner to propel the swallowed contents (bolus) down into the stomach. This muscular squeezing action allows a person to drink water even while hanging upside down because peristalsis works against gravity. The waves of contraction move the water up through the esophagus and into the stomach regardless of body position. The muscles actively push the liquid downward in the digestive tract, making gravity irrelevant in this process. This is why astronauts can eat and drink in zero gravity and why humans can swallow liquids upside down without the fluid flowing backward or out through the nose or ears.