ALT Infographic on hunger, digestion, and feeling full. Ghrelin, produced by an empty stomach, causes feelings of hunger. Stomach rumbling is caused by the migrating motor complex, contractions that sweep indigestible substances from the stomach and through the small intestine. Smelling food signals the brain's primary salivary centers in the medulla oblongata. Neurotransmitters then signal glands to produce saliva.
Mechanical and chemical digestion break down the food we eat. Different enzymes break up different biological molecules. The salivary glands produce amylase, a carbohydrase that breaks carbohydrates into simple sugars.