Each compartment and accessory organ serves a specific function. At each stage, the food is transformed into a slightly different form that allows it to be passed along to the next compartment. The coordination of functions is done through the nervous and endocrine systems. Through the process of digestion, food is transformed from large complex particles into basic elements.
The process of digestion begins in the mouth. Within the mouth lie the teeth, tongue and jaws. Through a chewing motion, the food is mechanically broken down between the teeth and mixed with saliva, which aids in chemical digestion. Upon stimulation, saliva is produced in the salivary glands and brought into the mouth. It contains salivary amylase, an enzyme that digests starch. Once the digestion in the mouth is completed, the first phase of swallowing is initiated. This stage is voluntary and is characterized by contraction of the muscles of the floor of the mouth and tongue that propel the food bolus into the pharynx.
The role of the pharynx is to facilitate the passage of the food bolus into the esophagus. The pharynx is designed to direct the food bolus in this direction. It is here where the second phase of swallowing takes place. After the moistened food bolus is moved to the back of the mouth by the tongue, an involuntary swallowing reflex is triggered which prevents food from entering the respiratory tract. The tongue closes off the mouth, the soft palate blocks the nose, and the larynx rises such that the epiglottis closes off the trachea. Food then moves from the pharynx into the esophagus.
The esophagus is the first part of the digestive tract proper. It is also where the third phase of swallowing occurs. Approximately 10 inches in length, it consists of three tissue layers consistent with the rest of the gut. Once in the proximal portion of theesophagus, the muscles of peristalsis begin propelling the food bolus through the esophagus into the stomach.
No further digestion takes place in this compartment. The pharynx and esophagus serve only as conduits for digestion.
The stomach is a C-shaped pouch that receives the food bolus from the esophagus. It aids both in mechanical and chemical digestion. Acting like a churn, the stomach mixes the food with gastric acid and breaks down the food into a milky substance known as chyme. The acid reduces the pH of the stomach, in the process allowing activation of an enzyme called pepsin. This starts the chemical digestive process.