The respiratory and digestive systems share structures in the back of the oral cavity that connect with both the trachea (of the respiratory system) and the esophagus (of the digestive system). What prevents swallowed food from “going down the wrong pipe”? With each swallow a structure called the epiglottis closes over respiratory structures. The swallowed bolus stays on course and is directed toward the esophagus. One peristaltic wave can be enough to move a bolus down the esophagus and into the stomach.
The walls of the alimentary canal include layers of smooth muscle controlled by the autonomic nervous system. Alternating contraction and relaxation of these muscles is called peristalsis. Peristaltic waves push the swallowed bolus down the esophagus. In the stomach, peristalsis churns swallowed food, mixing it with gastric juices. These mechanical and chemical actions further break down food into a substance called chyme. It takes about a minute to chew a piece of food into a bolus and a few seconds to swallow it. Once in the stomach foodstuffs take a few hours to become chyme.