The digestive system includes the organs of the alimentary canal and accessory structures. The alimentary canal forms a continuous tube that is open to the outside environment at both ends. The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive structures include the teeth, tongue, salivary glands, liver, pancreas, and gallbladder.
After you've chewed food, you push food to the back of your mouth with your tongue. That's a conscious act using voluntary muscles. The other processes are automatic. The epiglottis closes off your trachea - the tube to your lungs. This stops food from going down the trachea. Instead, it goes down the esophagus towards the stomach.
Like the esophagus, your stomach is wrapped in layers of muscles. They also contract in a peristaltic rhythm - contract in waves. This mixes the food with liquids that your stomach produces. These are gastric juices secreted by microscopic glands that line the inner wall of your stomach. Stomach glands secrete two digestive substances: enzyme pepsinogen and hydrochloric acid. The acid converts pepsinogen into pepsin, an active enzyme pepsin breaks down complex protein molecules. Break down of sugars and fats happen in your small intestine.
This steady contraction and relaxation is called peristalsis. It occurs throughout your digestive system.