Digestive System
1. Identify the components of the digestive tract and the accessory organs.
2. List six functions of the digestive system.
3. Describe the general histology of the four layers in the digestive tract wall.
4. Describe the features and function of the oral cavity, teeth, pharynx, and esophagus.
5. List and describe the location of the three salivary glands.
6. Explain the function of saliva.
7. Describe the structure and features of the stomach and its role in digestion.
8. Describe the structure and features of the small intestine and its role in digestion and absorption.
9. Describe the structure, features, and function of the large intestine.
10. Describe the structure and function of the liver, gallbladder, and pancreas.
11. Explain how substances are absorbed into the body through the small intestine.
12. Describe ways in which the aging of an individual affects the digestive system.
Introduction to the Digestive System
The digestive system includes the digestive tract and its accessory organs (Figure 14-1). The function of the digestive system is to process food into molecules that can be absorbed and used by the cells of the body. Food is broken down, bit by bit, until the molecules are small enough to be absorbed and the waste products are eliminated. The digestive tract (also called the alimentary canal or gastrointestinal [GI] tract) consists of a long, continuous tube that extends from the mouth to the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The tongue and teeth are accessory structures located in the mouth. The salivary glands, liver, gallbladder, and pancreas are not part of the digestive tract but are major accessory organs that have a role in digestion. These secrete fluids into the digestive tract.

Functions of the Digestive System
Food undergoes three types of processes in the body:
Digestion and absorption occur in the digestive tract. After the nutrients are absorbed, they are available to all cells in the body and are used by the cells in metabolism.
The digestive system prepares nutrients for use by the body’s cells through the following six activities:
• Ingestion—The first activity of the digestive system is to take in food. This process is called ingestion. Ingestion has to take place before anything else can happen.
• Mechanical digestion—The large pieces of food that are ingested have to be broken into smaller particles that can be acted on by various enzymes. This is called mechanical digestion. Mechanical digestion begins in the mouth with chewing, or mastication (mas-tih-KAY-shun), and continues with churning and mixing actions in the stomach.
• Chemical digestion—The complex molecules of carbohydrates, proteins, and fats are transformed by chemical digestion into smaller molecules that can be absorbed and used by the cells. Chemical digestion uses water to break down the complex molecules. This process is known as hydrolysis. Digestive enzymes speed up the hydrolysis process, which is otherwise slow.
• Movements—After ingestion and mastication, the food particles move from the mouth into the pharynx, and then into the esophagus. This movement is called deglutition (dee-gloo-TISH-un), or swallowing. Mixing movements occur in the stomach as a result of smooth muscle contraction. These repetitive contractions mix the food particles with enzymes and other fluids. The movements that propel the food particles through the digestive tract are called peristalsis. These are rhythmic waves of contractions that move the food particles through the various regions in which mechanical and chemical digestion takes place.
• Absorption—The simple molecules that are produced from chemical digestion pass through the lining of the small intestine into the blood. This process is called absorption.
• Elimination—The food molecules that cannot be digested need to be eliminated from the body. The removal of indigestible wastes through the anus, in the form of feces, is defecation (def-eh-KAY-shun).
General Structure of the Digestive Tract
The digestive tract is a long, continuous tube that is about 9 m (30 ft) in length. It opens to the outside at both ends, through the mouth at one end and through the anus at the other. Although there are variations in each region, the basic structure of the wall is the same throughout the entire length of the tube.
The wall of the digestive tract has four layers or tunics (Figure 14-2):

Mucosa
The mucosa, or mucous membrane layer, is the innermost tunic of the wall. It lines the lumen of the digestive tract. The mucosa consists of epithelium, an underlying loose connective tissue layer, and a thin layer of smooth muscle. In certain regions the mucosa develops folds that increase the surface area. Certain cells in the mucosa secrete mucus, digestive enzymes, and hormones. Ducts from other glands pass through the mucosa to the lumen of the digestive tract.
Submucosa
The submucosa is a thick layer of loose connective tissue that surrounds the mucosa. This layer also contains blood and lymphatic vessels, nerves, and some glands. Abundant blood vessels supply necessary nourishment to the surrounding tissues. Blood and lymph carry away absorbed nutrients that are the end products of digestion. The nerves in the submucosa form a network called the submucosal plexus that provides autonomic nerve impulses to the muscle layers of the digestive tract.
Muscular Layer
The muscular layer (labeled muscularis in Figure 14-2) consists of two layers of smooth muscle. The inner circular layer has fibers arranged in a circular manner around the circumference of the tube. When these muscles contract, the diameter of the tube is decreased. In the outer longitudinal layer the fibers run lengthwise along the long axis of the tube. When these fibers contract, their length decreases and the tube shortens. A network of autonomic nerve fibers, called the myenteric (mye-en-TAIR-ik) plexus, exists between the circular and longitudinal muscle layers. The myenteric plexus, along with the submucosal plexus, is important for controlling the movements and secretions of the digestive tract. In general, parasympathetic impulses stimulate movement and secretion in the GI tract and sympathetic impulses inhibit these activities.
Serosa or Adventitia
The fourth and outermost layer in the wall of the digestive tract is called the adventitia if it is above the diaphragm and the serosa if it is below the diaphragm. The adventitia is composed of connective tissue. The serosa, which is below the diaphragm, has a layer of epithelium covering the connective tissue. It is actually the visceral peritoneum and secretes serous fluid for lubrication. The serous fluid allows the abdominal organs to move smoothly against one another without friction.
Components of the Digestive Tract
Mouth
The mouth, or oral cavity, is the first part of the digestive tract. It is adapted to perform the following: receive food by ingestion, break it into small particles by mastication, and mix it with saliva. The lips, cheeks, and palate form the boundaries of the mouth. The oral cavity contains the teeth and tongue and receives the secretions from the salivary glands.
Lips and Cheeks
The lips and cheeks help hold food in the mouth and keep it in place for chewing. They are also used in the formation of words for speech. The lips contain numerous sensory receptors that are useful for judging the temperature and texture of foods. The cheeks form the lateral boundaries of the oral cavity. The main components of the cheeks are the buccinator muscle and other muscles of facial expression. On the outside, the muscles are covered by skin and subcutaneous tissue.
Palate
The palate is the roof of the oral cavity. It separates the oral cavity from the nasal cavity. The anterior portion, the hard palate, is supported by bone. The posterior portion, the soft palate, is skeletal muscle and connective tissue. The soft palate ends in a projection called the uvula. During swallowing, the soft palate and uvula move upward to direct food away from the nasal cavity and into the oropharynx.
Tongue
The largest and most movable organ in the oral cavity is the tongue. Most of the tongue consists of skeletal muscle. The major attachment for the tongue is the posterior region, or root, which is anchored to the hyoid bone. The anterior portion is relatively free but is connected to the floor of the mouth, in the midline, by a membranous fold of tissue called the lingual frenulum. The dorsal surface of the tongue is covered by tiny projections called papillae. The papillae provide friction for manipulating food in the mouth, and they also contain the taste buds (see Chapter 10). The lingual tonsils are embedded in the posterior surface of the tongue. The lingual tonsils provide defense against bacteria that enter the mouth.
Teeth
Two different sets of teeth develop in the mouth. The first set begins to appear at approximately 6 months of age and continues to develop until about years of age. This set is known as the primary or deciduous teeth. The primary teeth contain 10 teeth in each jaw for a total of 20 teeth. Figure 14-3, A illustrates the types of primary teeth. Starting at 6 years of age, the primary teeth begin to fall out and are replaced by the secondary or permanent teeth. This set contains 16 teeth in each jaw for a total of 32 teeth. These teeth are illustrated in Figure 14-3, B.

Different teeth are shaped to handle food in different ways. The incisors are chisel-shaped and have sharp edges for biting food. Cuspids (canines) are cone-shaped and have points for grasping and tearing food. Bicuspids (premolars) and molars have flat surfaces with rounded projections for crushing and grinding. Note the location of each type of tooth in Figure 14-3.
Although the different types of teeth have different shapes, each tooth has three parts:
The crown is the visible portion of the tooth, covered by enamel. The root is the portion that is embedded in the sockets (alveolar processes) of the mandible and maxilla. The neck is a small region in which the crown and root meet and is adjacent to the gingiva, or gum.
The central core of a tooth is the pulp cavity. It contains the pulp, which consists of connective tissue, blood vessels, and nerves. In the root, the pulp cavity is called the root canal. Nerves and blood vessels enter the root through an apical foramen. The pulp cavity is surrounded by dentin, which forms the bulk of the tooth. Dentin is a living cellular substance similar to bone. In the root, the dentin is surrounded by a thin layer of calcified connective tissue called cementum, which attaches the root to the periodontal ligaments. The ligaments have fibers that firmly anchor the root in the alveolar process. Enamel surrounds the dentin in the crown of the tooth. Enamel is the hardest substance in the body. Figure 14-4 shows a longitudinal section of a tooth and illustrates the major features.


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