Anatomy of the Digestive System
The organs of the digestive system are located within a tube called the alimentary canal, or gastrointestinal tract. The tube begins with the mouth and ends with the anus (Fig. 15.1). Although the term digestion, strictly speaking, means the breakdown of food by enzymatic action, we will expand the term to include both the physical and chemical processes that reduce food to small, soluble molecules.
The functions of the digestive system are to:
1. ingest the food;
2. break food down into small molecules that can cross plasma membranes;
3. absorb these nutrient molecules;
4. eliminate nondigestible wastes.
The mouth, which receives food, is bounded externally by the lips and cheeks. The space between the lips and cheeks and the teeth is the vestibule. The tongue is composed of skeletal muscle whose contraction changes the shape of the tongue. Muscles exterior to the tongue cause it to move about. Rough projections on the tongue, called papillae, help it handle food and also contain the sensory receptors called taste buds. A fold of mucous membrane, called a frenulum, on the underside of the tongue attaches it to the floor of the mouth. If the frenulum is too short, the individual cannot speak clearly and is said to be tongue-tied. Posteriorly, the tongue is anchored to the hyoid bone.
The mouth has a roof that separates it from the nasal cavities. The roof has two parts: an anterior (toward the front) hard palate and a posterior (toward the back) soft palate (see Fig. 15.2). The hard palate contains several bones, while the soft palate is muscular only. The soft palate ends in a finger-shaped projection called the uvula.
Three pairs of salivary glands send juices (saliva) by way of ducts to the mouth. The parotid glands lie anterior and somewhat inferior to the ears between the cheek and the masseter muscle. They have ducts that open on the inner surface of the cheek at the location of the second upper molar. The parotid glands swell when a person has the mumps, a disease caused by a viral infection. The sublingual glands are located beneath the tongue, and the submandibular glands are in the floor of the mouth on the inside surface of the lower jaw. The ducts from the sublingual and submandibular glands open under the tongue. You can locate the openings for the salivary glands if you use your tongue to feel for small flaps on the inside of your cheek and under your tongue. Saliva contains bicarbonate and an enzyme called salivary amylase that begins the process of digesting starch.
Figure 15.1 Digestive system. Trace the path of food from the mouth to the anus. The large intestine consists of the cecum, the colon (composed of the ascending, transverse, descending, and sigmoid colon), the rectum, and the anal canal. Note also the location of the accessory organs of digestion: the pancreas, the liver, and the gallbladder.
Table 15.1 traces the path of food. From the mouth, food passes through the pharynx and esophagus to the stomach, small intestine, and large intestine. The food passage and the air passage cross in the pharynx because the trachea is anterior to the esophagus, a long muscular tube that takes food to the stomach (Fig. 15.2). The tonsils are embedded in the mucous membrane of the pharynx. The palatine tonsils are on either side of the tongue close to the soft palate, and the pharyngeal tonsils, or adenoids, are in the nasopharynx. The tonsils help protect the body against infection. When the tonsils become inflamed, the person has tonsillitis. If the tonsillitis keeps recurring, the tonsils may be surgically removed (called a tonsillectomy). The pharynx has three parts: (1) The nasopharynx, posterior to the nasal cavity, serves as a passageway for air; (2) the oropharynx, posterior to the soft palate, is a passageway for both air and food; and (3) the laryngopharynx, just inferior to the esophagus, is a passageway for food entering the esophagus.
Figure 15.2 Swallowing. When food is swallowed, the soft palate closes off the nasopharynx, and the epiglottis covers the glottis, forcing the bolus to pass down the esophagus. Therefore, a person does not breathe while swallowing.
During swallowing, food normally enters the esophagus because other possible avenues are blocked. Swallowing is a reflex action performed automatically (without our willing it). When we swallow, the soft palate moves back to close off the nasopharynx, and the trachea moves up under the epiglottis so that food is less likely to enter it. (We do not breathe when we swallow.) The tongue presses against the soft palate, sealing off the oral cavity, and the esophagus opens to receive a food bolus (Fig. 15.2). Unfortunately, we have all had the unpleasant experience of having food “go the wrong way.” The wrong way may be either into the nasal cavities or into the trachea. If it is the latter, coughing will most likely force the food up out of the trachea and into the pharynx again.
The esophagus is a muscular tube that passes from the pharynx through the thoracic cavity and diaphragm into the abdominal cavity, where it joins the stomach. The esophagus is ordinarily collapsed, but it opens and receives the bolus when swallowing occurs. A rhythmic contraction called peristalsis pushes the food along the alimentary canal. Peristalsis begins in the esophagus and continues in all the organs of the alimentary canal. Occasionally, peristalsis begins even though there is no food in the esophagus. This produces the sensation of a lump in the throat. The esophagus plays no role in the chemical digestion of food. Its sole purpose is to conduct the food bolus from the mouth to the stomach. Sphincters are muscles that encircle tubes and act as valves; tubes close when sphincters contract, and they open when sphincters relax. The entrance of the esophagus to the stomach is marked by a constriction, often called the esophageal sphincter, although the muscle is not as developed as in a true sphincter. Relaxation of the sphincter allows the bolus to pass into the stomach, while contraction prevents the acidic contents of the stomach from backing up into the esophagus. Heartburn, which feels like a burning pain rising up into the throat, occurs during reflux when some of the stomach contents escape into the esophagus. When vomiting occurs, a contraction of the abdominal muscles and diaphragm propels the contents of the stomach upward through the esophagus.
Figure 15.3 Wall of the alimentary canal. a. Several different types of tissues are found in the wall of the alimentary canal. Note the placement of circular muscle inside longitudinal muscle. b. Micrograph of the wall of the esophagus.
The Wall of the Digestive Tract
The wall of the esophagus in the abdominal cavity is comparable to that of the alimentary canal, which has these layers (Fig. 15.3):
Mucosa (mucous membrane layer) A layer of epithelium supported by connective tissue and smooth muscle lines the lumen (central cavity). This layer contains glandular epithelial cells that secrete digestive enzymes and goblet cells that secrete mucus.
Submucosa (submucosal layer) A broad band of loose connective tissue that contains blood vessels lies beneath the mucosa. Lymph nodules, called Peyer patches, are in the submucosa. Like the tonsils, they help protect us from disease.
Muscularis (smooth muscle layer) Two layers of smooth muscle make up this section. The inner, circular layer encircles the gut; the outer, longitudinal layer lies in the same direction as the gut. (The stomach also has oblique muscles.)
Serosa (serous membrane layer) Most of the alimentary canal has a serosa, a very thin, outermost layer of squamous epithelium supported by connective tissue. The serosa secretes a serous fluid that keeps the outer surface of the intestines moist so that the organs of the abdominal cavity slide against one another. The esophagus has an outer layer composed only of loose connective tissue called the adventitia.