Upper Digestive Tract

 

The Upper digestive trat is comprised of the mouth, throat and esophogus. These structures are responsible for preparing food for digestion within the lower tract: the stomach and intestine. A small amout of digestion of carbohydrates occur in upper digestive tract.


Mouth

The mouth is adapted to receive food and to prepare it for digestion by mechanically reducing the size of solid particles and mixing them with saliva. The mouth is surrounded by the lips, cheeks, tongue, and palate, and includes a chamber between the palate and tongue, called the oral cavity, as well as a narrow space between the teeth, cheeks, and lips, called the vestibule.

Cheeks and Lips

The cheeks consist of outer layers of skin, pads of subcutaneous fat, muscles associated with expression and chewing, and inner linings of moist stratified squamous epithelium. The lips are highly mobile structures. They contain skeletal muscles an a variety of sensory receptors, which are useful in judging the temperature and texture of foods. Their normal reddish color is due to an abundance of blood vessels near their surfaces.

Tongue

The tongue nearly fills the oral cavity when the mouth is closed. It is covered by muous membrane and is connected in the midline to the floor of the mouth by membranous fold called the frenlum.

The body of the tongue is composed largely of skeletal muscle. These muscles aid in mixing food particles with saliva during chewing and in moving food toward the pharynx during swallowing. Rough projections, called papillae, on the surface of the tongue provide friction, which is useful in handling food. These papillae also contain taste buds.

The posterior region, or root, of the tongue is anchored to the hyoid bone and is covered with rounded masses of lymphatic tissue, called lingual tonsils.

Palate

The palate forms the roof of the oral cavity and consists of a hard anterior part (hard palate) and a soft posterior part (soft palate). The soft palate forms a muscular arch, which extends posteriorly and downward as a cone-shaped projection, called the uvula.

During swallowing, muscles draw the soft palate and uvula upward. This action closes the opening between the nasal cavity and pharynx, preventing food from entering the nasal cavity.

In the back of the mouth, on either side of the tongue and closely associated with the plate, are masses of lymphatic tissue, called palatine tonsils. These structures lie beneath the epithelial lining of the mouth and, like other lymphatic tissues, help protect the body against infections.

Still another mass of lymphatic tissue, called the pharyngeal tonsil, or adentoids, occurson the posterior wall of the border of the soft plate.

Teeth

Teeth are unique in that two different sets form during devolpment. The members of the first set, the primary teeth (deciduous teeth) usaully erupt through the gums at regular intervals between the ages of six months and two and one-half years.There are twenty decidous teeth- ten in each jaw.

The primary teeth are usually shed in the same order they appear. Before this happens, their roots are resorbed. The teeth are then pushed out of their sockets by pressure from the developing secondary teeth (permanent teeth). This secondary set consists of thirity-two teeth - sixteen in each jaw.

The seccondary teeth usaully begin to appear at about age six years,but the set may not be completed until the third molars appear between seventeen and twenty-five years of age.

The teeth break pieces of food into smaller pieces. This action increases the surface area of food particles and thus makes it possible for digestive enzymes to reactmore effectively with the food molecules.

Different teeth are aapted to handle food in different ways. The incicors (front teeth) are chisel-shaped, and their sharp edges are used to bite off reletively large pieces of food. The cuspids (canine teeth) are cone shaped, and they are useful in grasping or tearing food . The bicuspids (premolars) and molars have somewhat flattened surfaces and are specialized for grinding food particles.

Each tooth consists of two main portions, called the crown, which projects beyond the gum (gingiva), and the root, which is anchored to the aveolar procreaces of the maxilla and mandible. The region where these portions meet is called the neck of the tooth.

The crown is covered by glossy, white enamel, which consists mainly of calcium salts and is the hardest substance in the body. Unfortunately, if damaged by abrasive action or injury, the enamel is not replaced.

The bulk of a tooth beneath the enamel is composed of dentin, a substance much like bone but somewhat harder. Dentin, in turn, surrounds the tooth's central cavity (pulp cavity), which contains blood vessels, nerves, and connective tissue (pulp). Blood vessels and nerves reach this cavity through tubular root canals, which extend into the root.

The root is enclosed by a thin layer of bone like material called cementum, which is surounded by a periodontal ligament. This ligament contains bundles of thick collagenous fibers, which pass between the cementum and the bone of the alveolar process, thus firmly attaching the tooth to the jaw. It also contains blood vessels and nerves.

Salivary Glands

The salivary glands secrete saliva. This fluid moistens food particles, helps bind them together, and begins the digestion of carbohydrates. Saliva also acts as a solvent by dissolving various food chemicals- a process that is necessary before they can be tasted- and by helping to cleanse the mouth and teeth.

Salivary Secretions

Within a salivary gland are two types of secretory cells, serous cells and mucous cells. These cells occur in varying proportions within different glands. The serous cells produce a watery fluid that contains a digestive enzyme, called amylase. This enzyme splits starch and glycogen molecules into disaccharide's- the first step in the digestion of carbohydrates. The mucous cells secrete the thick, stringy liquid called mucus, which binds food particles together and acts as a lubricant during swallowing.

When a person sees, smells, tastes, or even thinks about pleasant food, parasympathetic nerve impulses elicit the secretion of a large volume of watery saliva. Conversely, if food looks, smells, tastes unpleasant, parasympathetic activity is inhibited so that less saliva is produced, and swallowing may become difficult.

Major Salivary Glands

There are three pairs of major salivary glands- the parotid, submandibular, and sublingual glands- and minor ones, which are associated with the mucous membrane of the tongue, palate, and cheeks.

The parotid glands are the largest of the major salivary glands. One gland lies in front of and somewhat below each ear, between the skin of the cheek and the masseter muscle. The parotid glands secrete a clear, watery fluid that is rich in amylase.

The submandibular glands are located in the floor of the mouth on the inside surface of the lower jaw. The secretory cells of these glands are predominantly serous, but some mucous cells are present. Consequently, the . glands secrete a more viscous fluid than the parotid glands.

The sublingual glands are the smallest of the major salivary glands. They are on the floor of the mouth under the tongue. Because their cells are primary the mucous type, their secretions tend to be thick and stringy.


Pharynx and Esophogus

The pharynx is a cavity behind the mouth from which the tubular esophagus leads to the stomach. Although neither of these organs contributes to the digestive process, both are important passageways, and their muscular walls function in swallowing.

Structure of the Pharynx

The pharynx connects the nasal and oral cavities with the larynx and esophagus. It can be divided into the nasopharynx, oropharynx, and laryngopharynx.

The nasopharynx communicates with the nasal cavity and provides a passageway for air during breathing.

The oropharynx opens behind the soft palate into the nasopharynx. It functions as a passageway for food moving downward from the mouth and for air moving to and from the nasal cavity.

The laryngopharynx is located just below the oropharynx. It opens into the larynx and esophagus.

Swallowing Mechanism

The act of swallowing involves a set of complex reflexes and can be divided into three stages. In the first stage, which is initiated voluntarily, food is chewed and mixed with saliva. Then this mixture is rolled into a mass (bolus) and forced into the pharynx by the tongue.

The second stage begins as the food reaches the pharynx and stimulates sensory receptors located around the pharyngeal opening. This stimulation triggers the swallowing reflex, which includes the following actions:

1. The soft palate is raised, preventing food from entering the nasal cavity.
2. The hyoid bone and the larynx are elevated , so that food is less likely to enter the trachea; a flap like structure attached to the larynx, called the epiglottis, is pressed downward by the base of the tongue, and this action also helps to prevent food from entering the trachea.
3. Muscles in the lower portion of the pharynx relax, opening the esophagus.
4. A peristaltic wave begins into the pharyngeal muscles, and this wave forces the food into the esophagus.

As the swallowing reflex occurs, breathing is momentarily inhibited. Then, during the third stage of swallowing, the food is transported by the esophagus to the stomach peristalsis.

Esophogus

The esophagus is a straight, collapsible tube about twenty-five centimeters long, which provides a passageway for substances between the pharynx and stomach. It begins at the base of the pharynx and descends behind the trachea, passing through the mediastinum. The esophagus penetrates the diaphragm and is continuous with the stomach on the abdominal side of the diaphragm.

Just above the point where the esophagus joins the stomach, some circular smooth muscle fibers in its wall are thickened. These fibers usually are contracted and function to close the entrance to the stomach. In this way, they help prevent regurgitation of the stomach contents into the esophagus. When peristaltic waves reach the stomach, these muscle fibers relax and allow food to enter.

Mucous glands are scattered throughout the mucosa of the esophagus, and their secretions keep the inner lining of the tube moist and lubricated.