Why is it so, that food does not pass through the digestive system

Why is it so, that food does not pass through the digestive system by ‘gravity’, but still we are able to digest food even when we are lying down ? Explain how food passes through our digestive system ?

The lining of the oesophagus, stomach and intestine is muscular, so that food can be pushed through it easily by its own by the action caused peristalsis. These movements of muscles help the food to pass through the digestive system.