When eating, a bolus of food plops into the stomach, but you don't feel plop. The hollow organ known as the stomach is made up of four different layers:

1. Serous coat: Covers most of the stomach.
2. Muscular coat: Forms the hollow and churns food.
3. Submucosal coat: Connects the muscular and mucosal layers. Mucose coat: Protects the inner lining of the stomach.

The muscular coat has three layers of muscle fibers, each of which has different directions. The three different types of muscle fibers allow the stomach to contract as necessary to churn the food one have digested.

The lining of the stomach ridges called rugae, which provide more surface area. As the stomach fills, the rugae smooth out, allowing the stomach to expand. Just like the esophagus and the mouth, a mucosa membrane covers the inside of the stomach. Actually, the mucous membrane runs continuously from the mouth all the way through the entire digestive system. One function of the mucous membrane is to protect the digestive organs from being eaten away by the strong acids secreted in the digestive system, such as hydrochloric acid.

Gastric juice is secreted from the millions of tiny gastric glands that are part of the mucosal lining of the digestive system. It contains hydrochloric acid and pepsinogen, which starts breaking down proteins into peptides. Hydrochloric acid is one of the most acidic substances found anywhere. It converts pepsinogen into pepsin, and it helps to break down the connective tissue in meats, etc.

The stomach's action of contracting to churn up its contents is part of physical digestion, like chewing, swallowing and peristalsis. But it's the stomach's contribution to chemical digestion that really helps break down the food one eats.

After the stomach churns and gastric juice mixes with the bolus of partially digested food, the food breaks down even more completely and turns into an oatmeal-like paste called chyme. 

The chyme then squirts into the small intestine through the pyloric sphincter, a muscular ring between the lower part of the stomach-called the pylorus-and the top of the small intestine-called the duodenum