Anatomically the GI tract is a tube that from the exterior to the lumen is composed of connective tissue , smooth muscle layers with embedded nervous plexus, connective tissue, and an inner epithelial layer. The epithelial type, muscle thickness, gladular elements and nervous supply differ in the various functional regions, as does the diameter and shape of the tube. The features of the oral cavity participate in digestion by reducing the food to smaller pieces, moistening and lubricating the food and by the addtion of amylase for the breakdown of carbohydrates. The teeth are specialized for cutting and grinding the food to small pieces so that there is more surface area for digestive enzymes to work on and to ease swallowing. The tongue moves the food around within the mouth and has taste receptors for sweet, salt, sour, and bitter.
Salivary glands add saliva which contains water, ions, salivary amylase and mucous.
When the food bolus is of the proper consistency, the tongue moves it to the back of the mouth where swallowing is initiated. The epiglottis covers the trachea as the bolus is passed into the esophagus. The bolus stretches the esophagus initiating smooth muscle contractions that push the bolus along toward the stomach in peristaltic waves. The esophagus joins the stomach near the diaphragm which normally pinches the esophagus closed due to its muscular tone. Stomach movement also helps to keep the esophagus closed.