What exactly happens in our stomachs when we eat?

 Even before we have taken the first bite of a meal, the brain sendsimpulses to the stomach. These impulses start the production of secretions inthe stomach and the upper part of the stomach, which acts as a reservoir,expands. The food passes into the stomach through the cardia at the stomachentrance. This is a muscle that acts like a valve, closing the top of the stomach.The top part of the stomach, the fundus, is where food and the air that we swallowwith every bite is collected. This is where the stomach volume starts to adapt.

  What exactly happens in our stomachs when we eat?

The fundus is an extremely adaptable structure. The more food goes into the stomach, the more it actively expands. In the middle part of the stomach, called the body, gastric juice is produced and mixed with the mass of chewed food. The main component of gastric juice is gastric acid. This eliminates bacteria in the food and also helps to prepare the food for the following stages of digestion. 

The muscular contractions of the stomach wall churn the food mass and mix it with gastric juice. Three contractions per minute move the food mass back and forth. 

After a certain time, the contents of the stomach are adequately broken down and mixed and are passed in small quantities into the intestine via the pylorus. As the stomach empties, it gradually returns to its previous size. Bayer - Science For A Better Life