What physiological factors cause us to feel hungry?

Stomach acids have an extremely low pH level, making them extremely acidic. We use them to digest food. When they are done digesting food, they simply sit in the stomach, this can cause the acid to eat through the lining of the stomach, and cause extreme medical problems. We get "hungry" when the acid begins to burn at our stomach lining, this is simply our body telling us give me more food so your stomach doesnt burn, that is one possibility, another possibility is when you are low on any type of chemical, vitamin, or nutreint your body needs, then your brain sends your stomach a message, and you get hunger pangs.