The digestive system of the body starts with the mouth and ends with the anus. Commonly termed “the digestive tract,” it is about 9 m (29.5 ft) long in humans. Obviously, it is shorter in some animals and longer in others. Its primary purpose is the same in all species: convert complex food products into simple nutrients or monomers, absorb those nutrients, and expel the undigested or unabsorbed residual. This entire process from the mouth to the anus is commonly referred to as digestion.