Structure and Function of the Digestive System

Mouth, Pharynx, and Esophagus

Food is ingested and partially digested in the mouth before passing through the pharynx and esophagus during swallowing.

Food is ingested in the mouth (oral cavity), where it is mechanically broken down by teeth (one set of teeth is called a dentition) and the tongue while being mixed with saliva. Saliva is a fluid secretion that includes mucus, antimicrobial agents, the digestive enzyme amylase to break down starches, a lipase enzyme to break down the lipid triglyceride, minor amounts of other enzymes, and electrolytes. Saliva is produced by salivary glands and is released when various types of sensory receptors are stimulated by chewing, chemicals in the mouth, visual cues, smell, and even thoughts of food. The sensory nerves send information to the pons and medulla of the brain for processing. Functions of saliva include creating an acidic environment, dissolving molecules in food for taste, softening food, fighting microbes, and chemically digesting carbohydrates (specifically starches) and fats (triglycerides). Two types of salivary glands are intrinsic and extrinsic. Intrinsic salivary glands are scattered within the epithelial tissues of the mouth. The intrinsic salivary glands play a minor role in salivary production, with the extrinsic glands producing most of the saliva. Three extrinsic salivary glands are paired and include the parotid glands on the inside of each cheek, the sublingual glands under the tongue, and the submandibular glands on the bottom of the lower jaw.

The oral cavity is separated from the nasal cavity by the arch at the roof of the mouth, called the palate. The anterior portion of the palate is referred to as the hard palate and is made up of the maxilla and palatine bones. Behind it, the soft palate is made mostly of skeletal muscle. This part of the palate is flexible and mobile and aids in swallowing. At the end of the soft palate is the uvula, a small piece of tissue that hangs in the back of the throat that helps regulate the passage of food into the throat and produces a watery saliva. The remainder of the oral cavity is enclosed by the lips, cheeks, and tongue.

Teeth are embedded in the jaws by their roots, forming a gomphosis joint. The visible portion of the tooth is the crown, which has dentin, a bone-like material on the inside and enamel, a hard mineralized substance that bears the force of chewing, on the outside. The innermost part of the tooth is the pulp, which is a cavity containing nerves and blood vessels. The pulp cavity opens up internally through the root canal. In between the crown and the root of the tooth is the neck, which is surrounded by the gums. Teeth are classified as incisors, canines, premolars, and molars. By the time a person is two years old, they have 20 primary teeth that consist of the incisors, canines, and premolars. These will be replaced with permanent teeth as the child ages. A child is born with molars in their adult form, and as such, these teeth get larger in size, but are not lost like the primary teeth. As a person ages, the adult permanent teeth erupt through the gums, resulting in two pairs of incisors, one pair of canines, two pairs of premolars, and three pairs of molars on the top and bottom jaws for a total of 32 teeth. Incisors specialize in cutting and slicing foods, canines pierce foods, and premolars and molars grind and crush foods.

Structure of a Tooth

Above the gumline, the crown of the tooth is covered by hard enamel. The innermost part of the tooth is the pulp, which is a cavity containing nerves and blood vessels. The pulp cavity opens up internally through the root canal. The root anchors the tooth into the jaw. In between the crown and the root of the tooth is the neck, which is surrounded by the gums.
Mastication (or chewing) is the process of using the teeth to mechanically digest food and helping to create a food bolus. A bolus is a rounded mass of food formed in the mouth when food is mixed with saliva prior to swallowing. Although jaws move voluntarily, chewing is a conditioned reflex. When food enters the mouth, pressure receptors are stimulated to relax jaw muscles. When the lower jaw is relaxed, pressure receptors are no longer stimulated, and the jaw muscles tighten once again. Repeated up and down movements of the mandible (lower jaw) create a chewing motion. The walls of the oral cavity, the tongue, and teeth are all used to soften food, increasing exposure of the food to enzymes in saliva that begin to break it down. In the mouth, the enzyme salivary amylase begins the breakdown of starch and lingual lipase begins to break down triglycerides (especially in infants). When swallowing is initiated by the mouth, the food bolus is moved to the pharynx.

Behind the oral and nasal cavities is the pharynx, which is a passageway for both food and air. The food bolus moves quickly through the upper oropharynx and lower laryngopharynx during swallowing and enters the esophagus when the epiglottis closes the entrance to the larynx. Deglutition (swallowing) is the transport of the food bolus from the mouth to the stomach, which involves a series of muscle contractions. As the bolus begins to move down the pharynx, reflex mechanisms suppress breathing to prevent the aspiration of food into the respiratory tract. The upper esophageal sphincter relaxes to allow the bolus to enter the esophagus, and the sphincter closes once the bolus moves through. Stretch receptors in the esophagus trigger peristalsis, waves of muscular contraction propelling the bolus toward the stomach. When the bolus reaches the end of the esophagus, the lower esophageal sphincter relaxes briefly to allow the bolus to enter the stomach.

The esophagus is a long flexible muscular tube that transports food to the stomach by waves of muscle contraction. It runs the length of the thoracic cavity and does not serve to mechanically or chemically digest food. Esophageal glands in the esophagus secrete mucus to lubricate the esophagus and facilitate food transport. The mucus also acts to protect the lining of the esophagus. When the food bolus reaches the stomach, it enters through the gastroesophageal sphincter. This sphincter is necessary to keep acidic stomach contents from entering the esophagus.