Digestive System PDF - Human Anatomy and Physiology

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23 The Digestive System Prostock-studio/Shutterstock.com Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Digestive System Chapter 23 963   Chapter Introduction What was the last thing you ate? How long ago was that? While behaviorally we don’t eat all of the time, the digestive system is working relatively constantly throughout the day. You may be taking a walk or studying or sleeping, having for- gotten all about your last meal or snack, but your stomach and intestines are busy digesting it and absorbing its vitamins and other nutrients. Because humans have evolved over hundreds of thousands of years of food scarcity, our digestive systems are masterful at stripping away all that it can from our food before waste material is excreted. This chapter exam- ines the structure and functions of these organs, and explores the mechanics and chemistry of the digestive processes. In this chapter we will learn… …about the structure and function …and how food is broken of the digestive system… down into monomers. 23.1 Overview of the Digestive System Learning Objectives: By the end of this section, you will be able to: 23.1.1 Describe the major functions of the digestive 23.1.3 List and identify the organs that compose system. the gastrointestinal (GI) tract. 23.1.2 Explain the differences between the 23.1.4 Trace the pathway of ingested substances gastrointestinal (GI) tract (alimentary canal) through the gastrointestinal (GI) tract. and the accessory digestive organs. The functions of the digestive system are to break down the foods you eat, release LO 23.1.1 their nutrients, and absorb those nutrients into the body as well as to generate, store, and excrete some of our wastes. When you take a bite of food, the food enters a long and complicated tract of connected organs. Each of the organs in the LO 23.1.2 The Human Anatomy and Physiology Society includes more than 1,700 educators who work together to promote excellence in the teaching of this subject area. The HAPS A&P Learning Outcomes measure student mastery of the content typically covered in a two-semester Human A&P curriculum at the undergraduate level. The full Learning Outcomes are available at https://www.hapsweb.org. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 964 Unit 5 Energy, Maintenance, and Environmental Exchange gastrointestinal (GI) tract has its own function and contributes to the breakdown gastrointestinal (GI) tract (alimentary of your food and absorption of nutrients. There are additional organs outside of canal) the tract that function in digestion, but food does not pass through them. These accessory digestive organs are vital to the functioning of the digestive system even though they are not part of the GI tract. The digestive organs are represented in Figure 23.1. As is the case with all body systems, the digestive system does not work in iso- lation; it functions cooperatively with the other systems of the body. Consider, for example, the interrelationship between the digestive and cardiovascular systems. Figure 23.1 Organs of the Digestive System The digestive system is composed of the GI tract and the accessory digestive organs (liver, pancreas, gallbladder, salivary glands). Pharynx Salivary glands: Parotid gland Submandibular gland Mouth Sublingual gland Esophagus Liver Stomach Gallbladder Pancreas Small intestine: Large intestine: Duodenum Transverse colon Jejunum Ascending colon Ileum Descending colon Cecum Sigmoid colon Appendix Rectum Anus Anal canal Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Digestive System Chapter 23 965   Table 23.1 Connections Among Body Systems Body Functional Connections Cardiovascular Blood supplies digestive organs with oxygenated blood and removes deoxygenated blood. Blood supply is used to carry absorbed nutrients to the liver for filtering and all over the body for distribution, use, or storage. Endocrine Hormones help regulate secretion in digestive glands and accessory organs. Lymphatic Mucosa-associated lymphoid tissue and other lymphatic tissue defend against entry of pathogens and maintain immune tolerance to the foods we eat; lymphatic vessels absorb and transport lipids. Muscular Skeletal and smooth muscle (depending on location) provide motility within the GI tract. Nervous Sensory and motor neurons help regulate secretions and muscle contractions in the digestive tract, provide information to the CNS about “fullness” and “hunger.” Skin Manufactures Vitamin D in response to sunlight, which enables calcium absorption. Urinary Kidneys convert vitamin D into its active form, allowing calcium absorption in the small intestine. Arteries supply the digestive organs with oxygen and processed nutrients, and veins drain the digestive tract. These intestinal veins, constituting the hepatic portal system, are unique; they do not return blood directly to the heart. Rather, this blood travels to the liver for filtering before the blood completes its circuit back to the heart. The inter- relationship of the digestive and endocrine systems is also critical. Hormones secreted by several endocrine glands, as well as endocrine cells of the pancreas, the stomach, and the small intestine, contribute to the control of digestion and nutrient metabolism. Of course, the digestive system is responsible for absorbing the nutrients that fuel all the cells of the body. In turn, the digestive system provides the nutrients to fuel endo- crine function. Table 23.1 gives a quick glimpse at the connections between the diges- tive system and other body systems. Organs of the Gastrointestinal Tract The GI tract is a one-way tube about 25 feet in length during life and closer to 35 feet in length when measured after death (the differ- ence has to do with muscle tone, the tract is longer after the muscles relax). Within the organs of the GI tract, our food is broken down into the smallest components possible. Nutrients are absorbed across the wall of the GI tract organs and into the bloodstream or lymph vessels. Any material that cannot be broken down into pieces small enough to absorb, or material that is unusable to us, is compacted into feces and eliminated from the end of the GI tract. Thus, the tube begins at the mouth and terminates at the anus. Between those two points, the food moves through the pharynx, the esophagus, the stomach, the small intestine, and the colon (Figure 23.2A). It is progressively broken down LO 23.1.3 all along its route. At both ends (the mouth and anus), the tube is open to the external environment; thus, food and wastes within the GI tract are technically considered to LO 23.1.4 be outside the body. Only through the process of absorption, which occurs across the cells of the wall of the tract, do the nutrients in food enter into and nourish the body’s “inside.” For this reason, the secretions that are added to the food to help break it down are considered exocrine secretions (secretions to the outside of the body). Accessory Structures Each accessory digestive organ aids in the breakdown of food. Within the mouth, the teeth and tongue begin the mechanical breakdown of the food through chewing. The rest of the accessory digestive organs all are secretory in nature, Student Study Tip adding water and enzymes to the food. Each contribution is produced in a gland, and contributes to specific components or stages of the breakdown process. These glands Secretory accessory digestive organs are not a part of the main outfit (GI tract), but are the salivary glands, the gallbladder, the liver, and the pancreas; each is connected to they ADD to it. the gut by ducts. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 966 Unit 5 Energy, Maintenance, and Environmental Exchange Figure 23.2 The GI Tract and Accessory Organs of the Digestive System The digestive system includes both (A) a tract of connected organs (the gastrointestinal or GI tract) and (B) organs that contribute to digestion but are found outside the tract (accessory organs). Salivary glands: Oral cavity Parotid gland Submandibular gland Tongue Pharynx Sublingual gland Esophagus Liver Stomach Gall bladder Pancreas Small intestine: Duodenum Large intestine: Jejunum Transverse colon Ileum Ascending colon Descending colon Cecum Appendix Rectum Anus A B Learning Check 1. Proteolytic enzymes are secreted into the digestive tract by the pancreas. These enzymes break down protein in the small intestine. Are these secretions considered endocrine (released into the body fluids) or exocrine (released outside the body) secretions? a. Endocrine b. Exocrine 2. When are nutrients considered to be inside your body? a. When they are ingested into the mouth b. As soon as they reach the stomach c. When they have been absorbed by the cells lining the wall of the GI tract d. When they reach the colon Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Digestive System Chapter 23 967   23.2 General Gross and Microscopic Anatomy of the Gastrointestinal (GI) Tract Learning Objectives: By the end of this section, you will be able to: 23.2.1 Identify and describe the gross anatomic 23.2.5 Identify and describe the location, structure, and microscopic structure and function of and function of the visceral and parietal each of the gastrointestinal (GI) tract tunics peritoneum, serous fluid, and the peritoneal (layers): mucosa, submucosa, muscularis cavity. (muscularis externa), and serosa or adventitia. 23.2.6 Compare and contrast the locations of the mesenteries (e.g., mesentery proper, 23.2.2 Compare and contrast mechanical digestion mesocolon, lesser omentum, greater and chemical digestion, including where omentum). they occur in the digestive system. 23.2.7 Define mesentery and explain its function. 23.2.3 Define peristalsis. 23.2.8 Explain the difference between an 23.2.4 Describe the enteric nervous system (ENS) intraperitoneal and a retroperitoneal organ. and explain its role in controlling digestive system function. 23.2.9 Identify which digestive system organs are intraperitoneal or retroperitoneal. 23.2a Microscopic Structure of the GI Tract Throughout its length, the GI tract is composed of the same four tissue layers; similar to the tunics of the blood vessels, the composition and structure of each of these layers varies with the function of each organ. Remember that the inside of a hollow body structure (such as the stomach) is called its lumen. The four layers of the wall of the GI LO 23.2.1 tract, starting with the innermost layer, are the mucosa, the submucosa, the muscularis, and the serosa (Figure 23.3). The mucosa is a layer of mucous membrane, meaning an epithelium enriched with mucus, similar to the epithelial linings of the nose or respiratory system. The epithelium is in direct contact with ingested food, and, like all epithelia, is anchored by a basement membrane. The basement membrane in this case is termed a lamina propria because it is more substantial than a typical basement membrane. It is a layer of connective tissue analogous to the dermis of the skin. In addition, the mucosa has a thin smooth muscle layer called the muscularis mucosa (not to be confused with the muscularis layer, described in the following text). Epithelium. In the mouth, pharynx, esophagus, and anal canal, the epithelium is primarily a non-keratinized, stratified squamous epithelium. In the stomach and intestines, it is a simple columnar epithelium. Notice that the epithelium is in direct contact with the lumen and the food contents within. The difference between the organs that have a stratified squamous epithelium versus those lined with columnar epithelium is a matter of internal friction. In the mouth, for example, food is sometimes rough as it is not far along in the digestion process, and so the epithelium is stratified to protect the layers beneath from friction. By the time the digested food reaches the stomach and intestines, it is more of a smooth paste and therefore friction is minimal. Moreover, in the small intestine in particular, nutrients must cross the mucosa to be absorbed, so a thicker epi- thelium is not desirable. Interspersed among its epithelial cells are goblet cells, which secrete mucus and fluid onto the epithelial surface. Enteroendocrine cells Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 968 Unit 5 Energy, Maintenance, and Environmental Exchange Figure 23.3 Layers of the wall of the GI Tract Every organ in the GI tract has four layers of different tissues that make up its walls. The hollow center of the organ is the lumen. From the lumen outward, the layers are the mucosa, the submucosa, the muscularis, and the serosa. While all organs have these four layers, their composition and structure vary. Nerve Lymphatic vessel Vein Artery Epithelium Serosa Areolar connective tissue Myenteric plexus Submucosal plexus Submucosal glands Lymphatic tissue Longitudinal muscle Muscularis Circular muscle Lumen Submucosa Epithelium Mucosa Lamina propia Muscularis mucosae are features of the epithelium of the intestines and function to secrete hormones that regulate some digestive processes. Epithelial cells have a very brief lifespan, averaging only a few days. This process of rapid renewal helps preserve the health of the GI tract and replace cells that are lost to friction. Lamina propria. The lamina propria contains numerous blood and lymphatic ves- sels that transport nutrients absorbed across the wall of the GI tract organs to other parts of the body. The lamina propria also contains clusters of lymphocytes, making up the mucosa-associated lymphoid tissue (MALT). These lymphocyte clusters are particularly substantial in the small intestine. These patches of MALT serve a dual purpose. These patches of MALT serve a dual purpose: on the one hand, they filter out potentially harmful pathogens from our foods (despite our best efforts, the food we eat is hardly sterile), which contain bacteria, viruses, and fungi; we rarely suffer from foodborne illness because the pathogens stay “outside” the body within the lumen and cannot cross the wall, and because the lymphocytes of the MALT defend us. On the other hand, however, the MALT are also believed to play a role in tolerance—learning about the foods we eat and quelling possible immune reac- tions to them. Food allergies are a breakdown of this tolerance process. Muscularis mucosa. This thin layer of smooth muscle is in a constant state of tension, pulling the mucosa of the stomach and small intestine into undulating folds. These folds dramatically increase the surface area available for digestion and absorption. As its name implies, the submucosa lies immediately beneath the mucosa. The submucosa is a layer of dense connective tissue that connects the mucosa to the mus- cularis. Because the mucosal epithelium is avascular, the submucosa is rich with blood Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Digestive System Chapter 23 969   and lymphatic vessels and also contains some submucosal glands that release digestive secretions. Additionally, it serves as a conduit for a dense branching network of nerves, the submucosal plexus, which functions as described in the following text. The third layer of the GI tract is the muscularis. The muscularis in most organs Student Study Tip is made up of a double layer of smooth muscle: an inner circular layer and an outer The inner layer is called Circular, starting longitudinal layer. The contractions of these layers propel food along the tract and also with a “C” just like the word center. function to mix and churn the digested food. The muscularis is not uniform in each organ, rather, its composition varies from organ-to-organ according to function. In the stomach, there are three layers to the muscularis for extra grinding power. At the ends of the GI tract, including the mouth, pharynx, anterior part of the esophagus, and anus, the muscularis is made up of skeletal muscle instead of smooth muscle, which gives voluntary control over swallowing and defecation. The breakdown of food requires the extensive use of enzymes to break the bonds LO 23.2.2 among the food molecules. Once broken, the monomers—the smallest units of the pro- teins and sugars and fats we eat—can be transported across the membranes of the cells that line the GI tract. This is called chemical digestion. However, enzymes cannot act alone. We take food into the body in rather large pieces, and the enzymes can only act on the exposed outer surfaces of these pieces. Mechanical digestion, which is initiated by the teeth and continued by the muscularis layer throughout the GI tract, physically grinds LO 23.2.3 the food into smaller and smaller chunks upon which the digestive enzymes can act. Another mechanical act performed by the muscularis is peristalsis. Peristalsis Student Study Tip consists of sequential, alternating waves of contraction and relaxation of two muscula- Peristalsis in the GI tract performs the ris layers, and functions to propel food along the tract (Figure 23.4). These waves also same function that your fingers do when play a role in mixing food with digestive juices. Peristalsis is so powerful that foods and they squeeze a toothpaste tube; the liquids you swallow enter your stomach even if you are upside down. muscular contractions force the contents along, and eventually out of, the tube. Figure 23.4 Peristalsis Peristalsis is the rhythmic contraction of muscle within the muscularis that propels food along the GI tract. Direction of food propulsion Muscle contraction Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 970 Unit 5 Energy, Maintenance, and Environmental Exchange The serosa is the portion of the tract which is the furthest from the lumen. It consists of a layer of connective tissue that helps to hold bigger arteries, veins, and nerves to the GI tract wall. All organs of the GI tract have a layer of connective tissue superficial to their muscularis; however, this layer is referred to as the serosa only within the abdominal cavity. The mouth, pharynx, and esophagus have a similar dense sheath of collagen fibers; for these organs, however, this layer is called the adventitia. The adventitia serves to hold these organs of the GI tract in place near the vertebral column. 23.2b Nerve Supply As soon as food enters the mouth, or even when it is just smelled or seen, an orches- trated symphony of events begins throughout the digestive system. The salivary glands and pancreas begin to generate their secretions. Throughout the GI system, coordi- nated muscle contractions begin to increase motility. All of this activity is coordinated enteric nervous system (intrinsic by the nervous system. The sight, smell, feel, or taste of food is detected by receptors nervous system) that send impulses along the sensory neurons of cranial nerves. With this input, the central nervous system can begin to initiate the process of digestion. LO 23.2.4 The central nervous system is not solely or even mainly responsible for GI innervation. There is intrinsic innervation of the GI tract provided by the enteric nervous system, a network of nerves that connects the GI tract organs. The enteric nervous system runs from the esophagus to the anus, and contains approximately myenteric plexus (plexus of Auerbach) 100 million neurons—motor and sensory neurons, as well as interneurons. These enteric neurons are grouped into two plexuses. The myenteric plexus lies in the submucosal plexus (plexus of muscularis layer and is responsible for motility. The submucosal plexus lies in the Meissner) submucosal layer and is responsible for regulating digestive secretions and reacting to the presence of food. Extrinsic innervation of the GI tract is provided by the autonomic nervous system, which includes both sympathetic and parasympathetic nerves. In general, sympathetic activation (the fight-or-flight response) decreases GI secretion and motility. In contrast, parasympathetic activation (the rest-and-digest response) increases GI secretion and motility. 23.2c Blood Supply The blood vessels serving the digestive system have two functions. As it does for all organs of the body, the blood brings oxygen and nutrients to the digestive organs so that the cells can perform cellular respiration and get rid of wastes. Specifically, within the head, neck, and thorax, arteries branching off the aortic arch and thoracic aorta feed the digestive organs. Below the diaphragm the digestive organs are supplied with blood by arteries branching from the abdominal aorta. Whereas the celiac trunk ser- vices the liver, stomach, and duodenum, the superior and inferior mesenteric arteries supply blood to the remaining small and large intestines. The second function of the blood vessels in the digestive system is the transporta- tion of protein and carbohydrate nutrients absorbed across the wall of the GI tract. Lip- ids are not immediately carried in the blood but are absorbed via lacteals, tiny vessels of the lymphatic system. The veins that collect this nutrient-rich blood empty into the hepatic portal sys- tem. This venous network takes the blood into the liver, where the nutrients are either processed or stored for later use. Only then does the blood drained from the GI tract circulate back to the heart. To appreciate just how demanding the digestive process is on the cardiovascular system, consider that while you are “resting and digesting,” about one-fourth of the blood in the body is circulating within the intestines. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Digestive System Chapter 23 971   23.2d The Peritoneum The abdominal cavity is a fairly large space that holds many organs. These organs are not free to move within the cavity, but rather are held in place by the peritoneum, a LO 23.2.5 broad serous membranous sac. The peritoneum is made up of squamous epithelial tis- sue surrounded by connective tissue. Like the pleura of the lungs and the pericardium of the heart, the peritoneum is composed of two different layers: the parietal perito- neum, which lines the abdominal wall, and the visceral peritoneum, which envelops the abdominal organs (Figure 23.5). The peritoneal cavity is the space between the visceral and parietal peritoneal surfaces. Within this space, the membranes secrete a watery fluid that acts as a lubricant to minimize friction. lesser omentum (small omentum, The visceral peritoneum adheres tightly to the surface of the organs and includes gastrohepatic omentum) multiple large folds that surround various abdominal organs, hugging them to the walls of the abdominal cavity. Within the peritoneum are blood vessels, lymphatic vessels, greater omentum (great omentum, and nerves. There are four major peritoneal folds. The lesser omentum is a vertical omentum majus, gastrocolic omentum, sheet that hangs down from the inferior edge of the liver, tying it to the superior edge epiploon) of the stomach (Figure 23.6). The greater omentum hangs down from the inferior sur- face of the stomach like a curtain, covering the intestines. The transverse mesocolon LO 23.2.6 anchors the transverse colon of the large intestine to the posterior wall of the abdomi- nal cavity, and the mesenteries perform the same function for the small intestine, LO 23.2.7 suspending each fold from the posterior abdominal wall. Notably, the posterior parietal peritoneum provides the posterior wall of the abdominal cavity, where the mesenteries attach. The posterior parietal peritoneum is directly adjacent to the vertebral column, though there is a small space behind the peritoneum where the kidneys reside; this space is described as retroperitoneal. Some digestive organs, such as the stomach, the LO 23.2.8 Figure 23.5 The Peritoneum The peritoneum is the double-layered membrane that contains most of the organs in the abdominal and pelvic cavities. Organs behind the peritoneum are termed retroperitoneal. Parietal Peritoneal cavity peritoneum Visceral peritoneum Stomach Gallbladder Small intestine Large Large intestine intestine Pancreas Liver Small intestine Spleen Kidney Kidney Vertebra Spinal cord Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 972 Unit 5 Energy, Maintenance, and Environmental Exchange Figure 23.6 The Mesenteries (A) Transverse view. The lesser omentum connects the stomach to the liver; the greater omentum hangs down like a curtain over the intestines. The mesentery anchors the small intestine to the posterior body wall. (B) In the cadaver, the omentum hangs in place over the intestines. (C) Once the omentum is removed, the intestines can be seen. Liver Lesser Omental bursa omentum Pancreas Stomach Transverse Duodenum mesocolon Transverse Mesentery colon Greater omentum Small intestine A © Nayak SB, George BM, Mishra S, Ashwini LS, Marpalli S. Omento-cystic peritoneal fold and rudimentary quadrate lobe: A case report. OA Case Reports 2013 Jun 21;2(5):46 Liver Gallbladder Transverse colon Ralph T. Hutchings/Science Source Small intestine B C LO 23.2.9 jejunum, and the ileum of the small intestine, and portions of the large intestine, lie completely within the abdominal cavity; they are described as intraperitoneal. Other digestive organs, such as the ascending colon, descending colon, and portions of the duodenum, rectum, and pancreas, are retroperitoneal. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Digestive System Chapter 23 973   Learning Check 1. Which of the following layers of the wall of the GI tract is the most superficial? a. Mucosa c. Muscularis b. Submucosa d. Serosa 2. Which type of epithelium would you expect to find in the small intestine? a. Simple squamous c. Simple columnar b. Stratified squamous d. Simple cuboidal 3. Which of the following groups of nerves are responsible for motility of the food through the GI tract? Please select all that apply. a. Enteric nervous system c. Submucosal plexus b. Myenteric plexus d. Brachial plexus 4. Which of the following hangs down vertically over the small intestines and covers their anterior surface? a. Lesser omentum c. Transverse mesocolon b. Greater omentum d. Mesenteries 23.3 The Mouth, Pharynx, and Esophagus Learning Objectives: By the end of this section, you will be able to: The Mouth and Pharynx 23.3.11 Describe the general functions of the esophagus. 23.3.1 Identify and describe the boundaries of the oral cavity. 23.3.12 Describe the anatomic specializations of the esophageal tunics (e.g., composition of the 23.3.2 Define mastication. mucosa and muscularis [muscularis externa]) 23.3.3 Compare and contrast the composition and compared to the tunics of the rest of the GI functions of the hard palate, soft palate, and tract. uvula. 23.3.13 Relate the anatomic specializations of the 23.3.4 Identify and describe the structures (e.g., esophagus to the organ’s functions. taste buds, papillae) and the functions of the tongue. The Stomach 23.3.5 Describe the structure and function of the 23.3.14 Describe the general functions of the stomach. salivary glands. 23.3.15 Identify and describe the gross anatomy of 23.3.6 Describe the composition and functions of the stomach, including its location relative to saliva. other body structures. 23.3.7 Describe the structure and function of teeth. 23.3.16 Describe the compositions, locations, and functions of the inferior esophageal (cardiac, 23.3.8 Identify and describe the different regions of lower esophageal) sphincter and the pyloric the pharynx with respect to the passage of sphincter. air and/or food. 23.3.17 Identify gastric folds (rugae) and discuss their 23.3.9 List the structures involved in deglutition and functional significance. explain the process of deglutition, including the changes in position of the glottis and 23.3.18 Describe the anatomic specializations of the larynx that prevent aspiration. stomach tunics compared to the tunics of the rest of the GI tract. The Esophagus 23.3.19 Relate the anatomic specializations of the 23.3.10 Identify and describe the gross anatomy of stomach tunics (e.g., number of layers of the esophagus, including its location relative muscle in the muscularis [muscularis externa]) to other body structures. to the organ’s functions. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 974 Unit 5 Energy, Maintenance, and Environmental Exchange 23.3.20 Identify and describe the gastric glands, [plicae circulares], villi, microvilli) compared to including their cells (e.g., parietal cells, chief the tunics of the rest of the GI tract. cells). 23.3.31 Relate the anatomic specializations of the 23.3.21 Describe the functions, production, and small intestine tunics (e.g., circular folds regulation of secretion of hydrochloric acid [plicae circulares], villi, microvilli) to the (HCl). organ’s functions. 23.3.22 Explain the effects of the cephalic phase, 23.3.32 Identify and describe the function of the gastric phase, and intestinal phase following small intestine structures: duodenal of digestion on various parts of the glands (Brunner glands), intestinal glands gastrointestinal (GI) tract. (crypts of Lieberkuhn), and Peyer patches (lymphoid [lymphatic] nodules). 23.3.23 Explain how volume, chemical composition, and osmolarity of chyme affect motility in the 23.3.33 Compare and contrast the following: stomach and in the duodenum. peristalsis, mixing waves, segmentation, and mass movement. The Small Intestine The Large Intestine, Rectum, and Anal Canal 23.3.24 Describe the general functions of the small intestine. 23.3.34 Describe the general functions of the large intestine, rectum, and anal canal. 23.3.25 Identify the specific segments of the small intestine (i.e., duodenum, jejunum, ileum), 23.3.35 Identify and describe the gross anatomy of including their relative length. the large intestine, rectum and anal canal, including their location relative to other body 23.3.26 Identify and describe the gross anatomy of structures. the small intestine, including its location relative to other body structures. 23.3.36 Identify the specific segments and related flexures of the large intestine. 23.3.27 Describe the major functions of the biliary apparatus. 23.3.37 Compare and contrast the location, composition, and innervation (i.e., somatic 23.3.28 Identify and describe the biliary apparatus versus autonomic) of the internal and external components (i.e., left and right hepatic anal sphincters. ducts, common hepatic duct, cystic duct, common bile duct, main pancreatic duct, 23.3.38 Describe the specializations of the large hepatopancreatic ampulla [ampulla of Vater], intestine tunics (e.g., composition of the hepatopancreatic sphincter [sphincter of muscularis [muscularis externa]) compared to Oddi], major duodenal papilla). the tunics of the rest of the GI tract. 23.3.29 Trace the path of bile and pancreatic juice 23.3.39 Relate the specializations of the large through the biliary apparatus. intestine tunics (e.g., composition of the muscularis [muscularis externa]) to the organ’s 23.3.30 Describe the anatomic specializations of functions. the small intestine tunics (e.g., circular folds 23.3a The Mouth oral cavity (buccal cavity) The cheeks, tongue, and palate frame the mouth, which is also called the oral cavity (or buccal cavity). The structures of the mouth are illustrated in Figure 23.7. LO 23.3.1 At the entrance to the mouth are the lips, or labia (singular = labium). On their outer surface the labia are composed of skin—keratinized stratified squamous epi- labia (lips) thelium. On their inner surface the labia are made of a mucous membrane—mucus- covered non-keratinized stratified squamous epithelium. The lips are highly vascu- lar, and their keratin layer is very thin, so the dense vascularization lends a reddish hue to their outer surface. The lips have a huge representative area in the primary Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Digestive System Chapter 23 975   Figure 23.7 The Mouth The mouth is the entrance to the GI tract. Its structures enable mechanical digestion of ingested food. Superior labial Upper lip frenulum Gingiva (gum) Teeth Hard palate Glossopalatine arch Soft palate Palatine arch Uvula Palatine tonsil Tongue Inferior labial frenulum Lower lip somatosensory cortex, which means that the brain receives a tremendous amount of sensory information from them. This fact probably explains the human fascination with kissing, as well as why babies explore their environments by putting objects in labial frenulum (lip frenulum) their mouths. The labial frenulum is a fold of mucous membrane that attaches the inner surface of each lip to the gum found at the midline of the mouth. The cheeks make up the oral cavity’s sidewalls. Like the lips, their outer surface is skin and their inner covering is mucous membrane. Between the skin and mucous membranes mastication (chewing) of the cheeks and lips are connective tissue and skeletal muscles. These muscles, along with the tongue, enable the movements of chewing, otherwise known as mastication. LO 23.3.2 The oral cavity is bordered by the teeth, gums, and cheeks on the lateral sides. The inferior border is the tongue, and the posterior border contains the opening between the posterior of the oral cavity and throat (oropharynx), called the fauces. The supe- rior border, or roof, is called the palate. The anterior region of the palate serves as both a barrier between the oral and nasal cavities and a rigid shelf against which the tongue can push food. It is created by the maxillary and palatine bones of the skull and, given soft palate (velum, palatal velum, its bony structure, is known as the hard palate. If you run your tongue along the roof of muscular palate) your mouth, you’ll notice that the hard palate ends in the posterior oral cavity, and roof becomes softer. This part of the palate, known as the soft palate, is composed mainly LO 23.3.3 of skeletal muscle. You can therefore manipulate, subconsciously, the soft palate—for instance, to yawn, swallow, or sing (see Figure 23.7). If you examine your posterior soft palate in the mirror, you will notice a flap of soft tissue called the uvula that hangs down like an icicle from the center. While it may look uvula (palatine uvula) strange, it serves an important purpose. When you swallow, the soft palate and uvula move upward, helping to keep foods and liquid from entering the nasal cavity. Unfor- tunately, it can also contribute to the sound produced by snoring. Two muscular folds extend downward from the soft palate, on either side of the uvula. Toward the front, Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 976 Unit 5 Energy, Maintenance, and Environmental Exchange Student Study Tip the palatoglossal arch lies next to the base of the tongue; behind it, the palatopha ryngeal arch forms the superior and lateral margins of the fauces. Between these two The Greek suffix -glossal means “tongue.” arches are the palatine tonsils, clusters of lymphoid tissue that protect the pharynx. The This is also seen in the “hypoglossal” cranial nerve. lingual tonsils are located at the base of the tongue. 23.3b The Tongue LO 23.3.4 The tongue is one of the strongest muscles in the body. It is a workhorse, facilitating ingestion, mechanical digestion, sensation (of taste, texture, and temperature of food), palatoglossal arch (glossopalatine swallowing, and language. arch, anterior pillar of fauces, anterior arch) The tongue is attached to the mandible, the styloid processes of the temporal bones, and the hyoid bone. The hyoid is unique in that it does not articulate with other bones but rather serves as an anchor to the tongue. palatopharyngeal arch (posterior arch) The top and sides of the tongue are studded with papillae, small raised bumps of stratified squamous epithelium (Figure 23.8) in which taste buds are housed. For more reading on the sense of taste, please see Chapter 15. A fold of mucous membrane on the underside of the tongue, the lingual frenulum, tethers the tongue to the floor of the mouth. 23.3c The Salivary Glands LO 23.3.5 Many small salivary glands are housed within the mucous membranes of the mouth and tongue. These minor exocrine glands are constantly secreting saliva, a watery sub- stance enriched with several enzymes, either directly into the oral cavity or indirectly through ducts, even while you sleep. In addition to these small contributing glands, there are three major salivary glands. Together, the salivary glands of most adults pro- duce an average of 1 to 1.5 liters of saliva each day. Secretion increases when you see, Figure 23.8 The Tongue The tongue is a small but incredibly powerful muscular organ. Its surface is dotted with papillae, which contain taste buds. Epiglottis Palatopharyngeal arch Lingual tonsil Palatine tonsil Palatoglossal arch Papilla Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Digestive System Chapter 23 977   smell, or taste food, because saliva is essential to moisten food and initiate chemical digestion. The three pairs of major salivary glands, which secrete the majority of saliva into ducts that open into the mouth, are: The submandibular glands: these are in the floor of the mouth and secrete saliva into the mouth through the submandibular ducts. The sublingual glands: these lie below the tongue and use the lesser sublingual ducts to secrete saliva into the oral cavity. The parotid glands: these lie between the skin and the masseter muscle, near the ears. They secrete saliva into the mouth through the parotid duct, which is located on the posterior upper cheek (Figure 23.9). Saliva Saliva is essentially (over 95 percent) water. Dissolved within the water are ions, LO 23.3.6 enzymes, signaling molecules, antibacterial compounds, and waste products. One important ingredient in saliva is the enzyme salivary amylase, an enzyme that initiates the breakdown of carbohydrates. While adult salivary glands produce a carbohydrate- digesting enzyme, infant salivary glands actually produce salivary lipase, an enzyme that breaks down the fats presents in breast milk. When we are around 2 years of age, the production of enzymes changes to amylase. Many of our favorite foods, includ- ing soda, coffee, tea, yogurt, pickles, and many others, are acidic. An overly low pH environment in the mouth can compromise enzymatic function and the integrity of tooth enamel, so bicarbonate and phosphate ions are added to the saliva as buffers, Figure 23.9 The Salivary Glands There are three salivary glands on each side of the mouth. The parotid gland is under the skin in the posterior cheek. The submandibular gland sits below the mandible. The sublingual salivary gland is found under the tongue. Parotid duct Parotid salivary gland Sublingual ducts Submandibular salivary gland Sublingual Submandibular duct salivary gland Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 978 Unit 5 Energy, Maintenance, and Environmental Exchange preventing the pH of the mouth from becoming too low. Salivary mucus helps lubricate food; this facilitates movement and swallowing, and enables small taste molecules to reach the taste buds within the papillae. Saliva contains immunoglobulin A—one type of antibody that prevents microbes from penetrating the epithelium—and lysozyme, an antibacterial compound that kills off invading bacteria. Saliva also contains histos- tatin, which helps to heal wounds quickly. The presence of histostatin, antibodies, and lysozyme in saliva is likely why many animals lick their wounds. Each of the major salivary glands secretes a unique formulation of saliva. For example, the parotid glands secrete a watery solution that contains salivary amy- lase. The submandibular glands have cells similar to those of the parotid glands, as well as mucus-secreting cells. The sublingual glands contain mostly mucous cells, and they secrete the thickest saliva with the least amount of salivary amylase. The saliva in our mouths is the end product of the mixing of the individual glands’ contributions. Regulation of Salivation The autonomic nervous system regulates the secretion of saliva. In the absence of food, parasympathetic stimulation keeps saliva flowing at a comfortable level for speaking and breathing. During times of stress, such as before speaking in public, sympathetic stimulation takes over, reducing salivation and produc- ing the symptom of dry mouth often associated with anxiety. When you are dehy- drated, salivation is reduced, causing the mouth to feel dry and prompting you to take action to quench your thirst. Production and secretion of saliva can be stimulated by the sight, smell, and taste of food. Taste receptors on the tongue communicate both with the taste-processing insula and nuclei in the brainstem that regulate the parasympathetic impulses that stimulate salivation. Most saliva is swallowed along with food and is reabsorbed, so that fluid is not lost. dentes (teeth) 23.3d The Teeth LO 23.3.7 The teeth, or dentes (singular = dens), are organs similar to bones that you use to tear, grind, and otherwise mechanically break down food. deciduous teeth (baby teeth, milk Types of Teeth Teeth begin to erupt from the gums at about 6 months of age. These teeth, primary teeth) first 20 deciduous teeth, or baby teeth, will be replaced between approximately DIGGING DEEPER: Mumps Paramyxovirus is a virus that spreads through saliva—often spread through coughing, sneezing, sharing utensils, or kissing. Paramyxovirus can infect the nasal passages, the pharynx, and any salivary gland; however, the parotid glands are the usual site of infection. Enlargement and inflammation of the parotid glands is typical, causing a characteristic swelling between the ears and the jaw. Other symptoms include fever and throat pain, which can be severe when swallowing acidic substances such as orange juice. Mumps was once well-controlled through vaccination, and there were years in the United States in which no or only a few cases of mumps were reported. However, in the past few decades, some individuals and communities have become hesitant to vaccinate their children, and as vaccination rates began to decline, cases of mumps in the United States rose. While vaccination is sufficient to prevent infection in most cases, if the challenge is significant enough, meaning that if the person transmitting the virus to you has a high viral load, then so-called breakthrough infections—infections in a vaccinated person—can occur. Outbreaks of mumps involving a mix of vaccinated and unvaccinated individuals have become frequent on college campuses. Vaccination reduces the chance that you will contract mumps by 88 percent, and breakthrough infections are much less severe. In unvaccinated individuals, however, mumps can be serious or even deadly. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.