The food we eat needs to be broken down into small pieces which we chew up into even smaller ones before swallowing them. Once the food gets to the stomach the food is broken down further by the stomach's muscular walls. This is known as physical digestion.

Substances which our body needs cannot be absorbed into our blood until they have been broken down further and converted into small soluble chemicals. Enzymes are responsible for this process. This process is called chemical digestion.

Peristalsis is the movement of food through the digestive system by the contractions of two sets of muscles in the walls of the gut. The two sets of muscles produce wave-like contractions enabling food to move down the gut.

Within the stomach the following happens to the foods 

Carbohydrate is turned into glucose, which our bodies need to make energy.

Protein is turned into amino acids, required for cell growth and repair.

Fats and oils are turned into fatty acids and glycerol, needed to insulate our bodies and make cell membranes.

Vitamins and minerals do not have to be digested because they are already small enough to get into our blood.