Answer :

They are part of an organ system
They are the part of digestive system.  i.e organ-system

esophagus ----> from where food reaches the stomach
stomach -------> place where the food reacts with different chemicals to release
                        the energy
intestines ------>place where wastes are stored to get ejaculated out from
                       the  body . Large intestine also absorbs water and small intestines
                        absorb the nutrients