Download 26. Digestive System

O U T L I N E 26.1 General Structure and Functions of the Digestive System 780 26.1a Digestive System Functions 26.2 Oral Cavity 26.2a 26.2b 26.2c 26.2d 780 781 Cheeks, Lips, and Palate Tongue 782 Salivary Glands 782 Teeth 784 781 26.3 Pharynx 786 26.4 General Arrangement of Abdominal GI Organs 787 26.4a Peritoneum, Peritoneal Cavity, and Mesentery 787 26.4b General Histology of GI Organs (Esophagus to Large Intestine) 788 26.4c Blood Vessels, Lymphatic Structures, and Nerve Supply 790 26.5 Esophagus 26 Digestive System 790 26.5a Gross Anatomy 791 26.5b Histology 791 26.6 The Swallowing Process 26.7 Stomach 793 792 26.7a Gross Anatomy 793 26.7b Histology 793 26.7c Gastric Secretions 794 26.8 Small Intestine 797 26.8a Gross Anatomy and Regions 26.8b Histology 799 26.9 Large Intestine 797 799 26.9a Gross Anatomy and Regions 799 26.9b Histology 801 26.9c Control of Large Intestine Activity 802 26.10 Accessory Digestive Organs 26.10a 26.10b 26.10c 26.10d 803 Liver 804 Gallbladder 805 Pancreas 807 Biliary Apparatus 808 26.11 Aging and the Digestive System 810 26.12 Development of the Digestive System 810 26.12a Stomach, Duodenum, and Omenta Development 810 26.12b Liver, Gallbladder, and Pancreas Development 810 26.12c Intestine Development 810 MODULE 12: DIGESTI V E SYSTEM mck78097_ch26_779-816.indd 779 2/25/11 2:20 PM 780 Chapter Twenty-Six Digestive System ach time we eat a meal and drink fluids, our bodies take in the nutrients necessary for survival. However, these nutrients must be digested and processed—broken down both mechanically and chemically—into components small enough for our cells to use. Once the nutrients are broken down sufficiently, they are absorbed from the digestive system into the bloodstream and transported to the body tissues. Not everything we eat can be used by the body. After the nutrients from foods are absorbed, some materials remain that cannot be digested or absorbed, such as cellulose and fiber. These materials must be expelled from the body via a process called defecation. All of these functions are the responsibility of the digestive system. E 26.1 General Structure and Functions of the Digestive System Learning Objectives: 1. 2. 3. 4. 5. Identify the GI tract organs and accessory digestive organs. Describe the basic functions of the digestive system. Compare and contrast mechanical and chemical digestion. Identify the processes of peristalsis and segmentation. Define the processes of secretion, absorption, and elimination of wastes. The digestive (di-jes′tiv, dı̄-; digestus = to force apart, divide, dissolve) system includes the organs that ingest the food, transport the ingested material, digest the material into smaller usable components, absorb the necessary digested nutrients into the bloodstream, and expel the waste products from the body. When you eat, you put food into your mouth and it mixes with saliva as you chew. The chewed food mixed with saliva is called a bolus (bō′lŭs; lump), and it is the bolus that is swallowed. The stomach processes the bolus and turns it into a pastelike substance called chyme. Hereafter in the chapter, we will simplify our discussion by referring to the ingested contents as “material.” The digestive system is composed of two separate categories of organs: digestive organs and accessory digestive organs (figure 26.1). The digestive organs collectively make up the gastrointestinal (GI) tract, also called the digestive tract or alimentary (al-i-men′ter-ē; alimentum = nourishment) canal. The GI tract organs are the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. These organs form a continuous tube from the mouth to the anus. The contraction of muscle in the GI tract wall propels materials through the tract. Accessory digestive organs are not part of the long GI tube, but often develop as outgrowths from and are connected to the GI tract. The accessory digestive organs assist the GI tract in the digestion of material. Accessory digestive organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. 26.1a Digestive System Functions The digestive system performs six main functions: ingestion, digestion, propulsion, secretion, absorption, and elimination of wastes. Accessory Digestive Organs Gastrointestinal Tract (Digestive Organs) Parotid salivary gland Teeth Tongue Oral cavity Pharynx Sublingual salivary gland Submandibular salivary gland Esophagus Figure 26.1 Digestive System. The digestive system is composed of the gastrointestinal (GI) tract and accessory digestive organs that assist the GI tract in the process of digestion. Liver Stomach Gallbladder Pancreas Large intestine Small intestine Anus mck78097_ch26_779-816.indd 780 2/25/11 2:20 PM Chapter Twenty-Six Wave of contraction Wall of GI tract Lumen Relaxation Bolus (a) Peristalsis Mixing Further mixing Peristalsis and Segmentation. A swallowed bolus is propelled through the GI tract by the coordinated contraction and relaxation of the musculature in the GI tract wall. (a) Peristalsis is a wave of contraction that moves material ahead of the wave through the GI tract toward the anus. (b) Segmentation is a back-and-forth movement in the small intestine whereby ingested material mixes with secretory products to increase the efficiency of digestion and absorption. Ingestion (in-jes′chŭn; ingero = to carry in) is the introduction of solid and liquid materials into the oral cavity. It is the first step in the process of digesting and absorbing nutrients. Digestion is the breakdown of large food items into smaller structures and molecules. There are two aspects to digestion: Mechanical digestion physically breaks down ingested materials into smaller pieces, and chemical digestion breaks down ingested material into smaller molecules by using enzymes. (Review enzymes in chapter 2.) The first part of mechanical digestion is mastication (mas′ti-kā′shu n̆ ; mastico = to chew), the chewing of ingested material by the teeth in the oral cavity. After the materials are swallowed, they move through the GI tract by a process termed propulsion (prō-pŭl′shŭn; propello = to move forth). Two types of movement are involved in propulsion: peristalsis and segmentation. Peristalsis (per-i-stal′sis; stalsis = constriction) is the process of muscular contraction that forms ripples along part of the GI tract and forces material to move further along the tract (figure 26.2a). Peristalsis is like pushing toothpaste through a toothpaste tube: As you push on the flat end of the tube, a portion of toothpaste moves toward the opening. As you push the middle of the tube, the segment of toothpaste moves even closer to the opening. Churning and mixing movements in the small intestine, called segmentation (seg′men-tā′shŭn), help disperse the material being digested and combine it with digestive organ secretions (figure 26.2b). The material within the lumen of the small intestine is moved back-andforth to mix it with the secretory products that are being introduced into that region of the GI tract. Thus, contraction and relaxation of the digestive wall musculature creates a “blender” effect, mixing ingested materials with digestive enzymes and mechanically breaking down larger digested particles into smaller ones. Secretion (se-krē′shŭn; secerno = to separate) is the process of producing and releasing mucin or fluids such as acid, bile, and digestive enzymes. When these products are secreted into the lumen of the GI tract, they facilitate chemical digestion and the passage of material through the GI tract. Some of these products (e.g., acid, bile, digestive enzymes) help digest food. Mucin mck78097_ch26_779-816.indd 781 781 secretions serve a protective function. Mucin mixes with water to form mucus, and the mucus coats the GI wall to protect and lubricate it against acidic secretions and abrasions by passing materials. Absorption (ab-sōrp′shŭn; absorptio = to swallow) involves either passive movement or active transport of electrolytes, digestion products, vitamins, and water across the GI tract epithelium and into GI tract blood and lymph vessels. The final function of the digestive system is the elimination of wastes. Our bodies use most, but not all, of the components of what we eat. All undigestible materials as well as the waste products secreted by the accessory organs into the GI tract are compacted into feces (fē′sēz; faex = dregs), or fecal material, and then eliminated from the GI tract by the process of defecation (defĕ-kā′shŭn; defaeco = to remove the dregs). W H AT D I D Y O U L E A R N? (b) Segmentation Figure 26.2 Digestive System 1 ● 2 ● What structures compose the GI tract? How do secretion and absorption differ? 26.2 Oral Cavity Learning Objective: 1. Identify and describe the structure and function of the tongue, salivary glands, and teeth. The oral cavity, or mouth, is the entrance to the GI tract (figure 26.3). The mouth is the initial site of mechanical digestion (via mastication) and chemical digestion (via an enzyme in saliva). The epithelial lining of the oral cavity is a nonkeratinized stratified squamous epithelium that protects against the abrasive activities associated with digestion. This lining is moistened continually by the secretion of saliva. The oral cavity is bounded anteriorly by the teeth and lips and posteriorly by the oropharynx. The superior boundary of the oral cavity is formed by the hard and soft palates. The floor, or inferior surface, of the oral cavity is formed by the mylohyoid muscle covered with a mucous membrane. The tongue attaches to the floor of the oral cavity. The oral cavity has two distinct regions: The vestibule is the space between the cheeks or lips and the gums. The oral cavity proper lies central to the alveolar processes of the mandible and maxillae. 26.2a Cheeks, Lips, and Palate The lateral walls of the oral cavity are formed by the cheeks, which are covered externally by the integument and contain the buccinator muscles. The buccinator muscles compress the cheeks against the teeth to hold solid materials in place during chewing. The cheeks terminate at the fleshy lips (or labia), which form the anterior wall of the oral cavity. The lips are formed primarily by the orbicularis oris muscle and covered with keratinized stratified squamous epithelium. Lips have a reddish hue because of their abundant supply of superficial blood vessels and the reduced amount of keratin within their outer epithelial layer. The gingivae (jin′ji-vē), or gums, are composed of dense irregular connective tissue, with an overlying nonkeratinized stratified squamous epithelium that covers the alveolar processes of the upper and lower jaws and surrounds the necks of the teeth. The internal surfaces of the superior and inferior lips each are attached to the gingivae by a thin mucosa fold in the midline, called the labial (lā′bē-ăl) frenulum (fren′ū-lŭm; frenum = bridle). The palate (pal′ăt) forms the roof of the oral cavity and acts as a barrier to separate it from the nasal cavity. The anterior two-thirds 2/25/11 2:20 PM 782 Chapter Twenty-Six Digestive System Superior lip Superior labial frenulum Transverse palatine folds Hard palate Soft palate Nasopharynx Palatoglossal arch Hard palate Oral cavity Soft palate Palatoglossal arch Palatopharyngeal arch Palatine tonsil Tongue Uvula Palatine tonsil Oropharynx Vestibule Lingual tonsil Uvula Fauces Epiglottis Tongue Salivary duct orifices Laryngopharynx Lingual frenulum Sublingual Submandibular Teeth Esophagus Gingivae Inferior labial frenulum Larynx Trachea Inferior lip (a) Oral cavity, anterior view (b) Sagittal section Figure 26.3 Oral Cavity. (a) Ingested food and drink enter the GI tract through the oral cavity, shown here in anterior view. (b) A diagrammatic sagittal section shows the structures of the oral cavity and the pharynx. of the palate is hard and bony (called the hard palate), while the posterior one-third is soft and muscular (called the soft palate). The hard palate is formed by the palatine processes of the maxillae and the horizontal plates of the palatine bones. It is covered with dense connective tissue and nonkeratinized stratified squamous epithelium and exhibits prominent transverse palatine folds, or friction ridges, that assist the tongue in manipulating ingested materials prior to swallowing. The arching soft palate is primarily composed of skeletal muscle and covered with nonkeratinized stratified squamous epithelium. Extending inferiorly from the posterior part of the soft palate is a conical median projection called the uvula (ū′vū-lă; uva = grape). When you swallow, the soft palate and the uvula elevate to close off the posterior entrance into the nasopharynx and prevent ingested materials from entering the nasal region. The fauces represent the opening between the oral cavity and the oropharynx. The fauces are bounded by paired muscular folds: the palatoglossal arch (anterior fold containing the palatoglossus muscle) and the palatopharyngeal arch (posterior fold containing the palatopharyngeal muscle). The palatine tonsils are housed between the arches (see chapter 24). These tonsils serve as an “early line of defense” as they monitor ingested food and drink for antigens, and initiate an immune response when necessary. 26.2b Tongue The tongue (tŭng) is an accessory digestive organ that is formed primarily from skeletal muscle and covered with stratified squamous epithelium. As described in chapter 19, numerous small projections, termed papillae (pă-pil′ē; sing., papilla; papula = pimple), cover the superior (dorsal) surface of the tongue. The tongue’s stratified squamous epithelium is keratinized over the filiform papillae but nonkeratinized over the rest of the tongue. Chapter 25 described the tongue as a participant in sound production. In addition, the tongue manipulates and mixes ingested materials during chewing and helps compress the materials against the palate to turn them into a bolus. The tongue also performs important functions in swallowing. The inferior surface of the tongue attaches to mck78097_ch26_779-816.indd 782 the floor of the oral cavity by a thin, vertical mucous membrane, the lingual frenulum (figure 26.3a). In addition, the posteroinferior surface of the tongue contains lingual tonsils. Two categories of skeletal muscles move the tongue (see chapter 11). 26.2c Salivary Glands The salivary glands collectively produce and secrete saliva (să-lı̄′vă), a fluid that assists in the initial activities of digestion. The volume of saliva secreted daily ranges between 1.0 and 1.5 liters. Most saliva is produced during mealtime, but smaller amounts are produced continuously to ensure that the oral cavity mucous membrane remains moist. Water makes up 99.5% of the volume of saliva and is its primary ingredient. Saliva also contains a mixture of other components (table 26.1). Saliva has various functions. It moistens ingested food and helps it become a slick, semisolid bolus that is more easily swallowed. Saliva also moistens, cleanses, and lubricates the oral cavity structures. The first step in chemical digestion occurs when amylase in saliva begins to break down carbohydrates. Saliva contains antibodies and an antibacterial substance called lysozyme that help inhibit bacterial growth in the oral cavity. Finally, saliva is the watery medium into which food molecules are dissolved so that taste receptors on the tongue can be stimulated. Three pairs of multicellular salivary glands are located external to the oral cavity: the parotid, submandibular, and sublingual glands (figure 26.4a). The parotid (pă-rot′id; para = beside, ot = ear) salivary glands are the largest salivary glands. Each parotid gland is located anterior and slightly inferior to the ear, partially overlying the masseter muscle. The parotid salivary glands produce about 25–30% of the saliva, which is conducted through the parotid duct to the oral cavity. The parotid duct extends from the gland, parallel to the zygomatic arch, before penetrating the buccinator muscle and opening into the vestibule of the oral cavity near the second upper molar. As their name suggests, the submandibular (sŭb-man-dib′ūlă r) salivary glands are inferior to the body of the mandible. They 2/25/11 2:20 PM Chapter Twenty-Six Digestive System 783 Table 26.1 Saliva Characteristics Production Rate pH Range Composition of Saliva Solute Components Neural Control of Saliva Secretion 1–1.5 L/day Slightly acidic (pH 6.4 to 6.8) 99.5% water; 0.5% solutes Ions (e.g., Na+, K+, chloride, bicarbonate) Immunoglobulin A (helps decrease bacterial infections) Lysozyme (antibacterial enzyme) Mucin Salivary amylase (enzyme that breaks down carbohydrates) Parasympathetic axons in CN IX stimulate parotid salivary gland secretions Parasympathetic axons in CN VII stimulate submandibular and sublingual salivary gland secretions Sympathetic activation from cervical ganglia stimulates mucin secretion Parotid salivary gland Figure 26.4 Parotid duct Masseter muscle Salivary Glands. Saliva is produced by three pairs of salivary glands. (a) The relative locations of the parotid, submandibular, and sublingual salivary glands are shown in a diagrammatic sagittal section. (b) Serous and mucous alveoli are shown in a diagrammatic representation of salivary gland histology. (c) A photomicrograph reveals the histologic detail of the submandibular salivary gland. Mucosa (cut) Sublingual ducts Submandibular duct Sublingual salivary gland Mylohyoid muscle (cut) Submandibular salivary gland (a) Salivary glands Salivary duct Mucous cell Mucous cells Salivary duct Mucous alveolus Duct epithelium Mixed alveoli Serous alveolus LM 200x Serous cell (b) Salivary gland histology mck78097_ch26_779-816.indd 783 (c) Submandibular salivary gland Serous cells 2/25/11 2:20 PM 784 Chapter Twenty-Six Digestive System Table 26.2 Salivary Gland Characteristics Salivary Gland Structure and Location Types of Secretion Percentage of Saliva Produced Parotid Largest of the salivary glands; located anterior and slightly inferior to ears; parotid duct conducts secretions into the vestibule near upper 2nd molar Only serous secretions 25–30% Submandibular Located inferior to mandibular body; submandibular duct opens lateral to lingual frenulum Both mucous and serous secretions 60–70% Sublingual Smallest of the salivary glands; located inferior to tongue; tiny sublingual ducts open into floor of oral cavity Both mucous and serous secretions 3–5% produce most of the saliva (about 60–70%). A submandibular duct transports saliva from each gland through a papilla in the floor of the mouth on the lateral sides of the lingual frenulum. The sublingual (sŭb-ling′gwăl) salivary glands are inferior to the tongue and internal to the oral cavity mucosa. Each sublingual salivary gland extends multiple tiny sublingual ducts that open onto the inferior surface of the oral cavity, posterior to the submandibular duct papilla. These tiny glands contribute only about 3–5% of the total saliva. Two types of secretory cells are housed in the salivary glands: mucous cells and serous cells (figure 26.4b, c). Mucous cells secrete mucin, which forms mucus upon hydration, while serous cells secrete a watery fluid containing ions, lysozyme, and salivary amylase. The proportion of mucous cells to serous cells varies among the three types of salivary glands. The submandibular and sublingual glands produce both serous and mucous secretions, whereas the parotid glands produce only serous secretions. Table 26.2 summarizes the structure of the salivary glands and their secretions. The salivary glands are primarily innervated by the parasympathetic division of the autonomic nervous system. In particular, the facial nerve (CN VII) innervates the submandibular and sublingual glands, while the glossopharyngeal nerve (CN IX) innervates the parotid gland. Parasympathetic innervation stimulates salivary gland secretion, which is why your mouth “waters” when you see a delicious dinner in front of you. Your salivary glands are preparing the body for the start of the digestion process. In contrast, sympathetic stimulation inhibits normal secretion from these glands, which is why you may experience a dry mouth after a fight-or-flight response. W H AT D O Y O U T H I N K ? 1 ● Research suggests that a “dry mouth” (inadequate production of saliva) is correlated with an increase in dental problems, such as cavities. What are the possible reasons for this correlation? 26.2d Teeth The teeth are collectively known as the dentition (den-tish′ŭn; dentition = teething). Teeth are responsible for mastication, the first part of the mechanical digestion process. A tooth has an exposed crown, a constricted neck, and one or more roots that anchor it to the jaw (figure 26.5). The roots of the teeth fit tightly into dental alveoli, which are sockets within the alveolar processes of both the maxillae and the mandible. Collectively, the roots, the dental alveoli, and the periodontal ligaments that bind the roots to the alveolar processes form a gomphosis joint (described in chapter 9). Each root of a tooth is ensheathed within hardened material called cementum (se-men′tūm; rough quarry stone). A tough, mck78097_ch26_779-816.indd 784 Enamel Crown Gingiva Neck Dentin Pulp cavity Root canal Root Cementum Periodontal ligaments Dental alveolus Blood vessels and nerves in apical foramen Figure 26.5 Anatomy of a Molar. Ingested material is chewed by the teeth in the oral cavity. durable layer of enamel (ē-nam′ĕl) forms the crown of the tooth. Enamel, the hardest substance in the body, is composed primarily of calcium phosphate crystals. Dentin (den′tin) forms the primary mass of a tooth. Dentin is comparable to bone but harder, and is deep to the cementum and the enamel. The center of the tooth is a pulp cavity that contains a connective tissue called pulp. A root canal opens into the connective tissue through an opening called the apical foramen and is continuous with the pulp cavity. Blood vessels and nerves pass through the apical foramen and are housed in the pulp. The mesial (mē′zē-ăl; mesos = middle) surface of the tooth is the surface closest to the midline of the mouth, while the distal surface of the tooth is farthest from the mouth midline. Other tooth surfaces include: the buccal surface, adjacent to the internal surface of the cheek; the labial surface, adjacent to the internal surface of the lip; the lingual surface, facing the tongue; and the occlusal (ŏ -kloo′zăl) surface, where the teeth from the opposing superior and inferior arches meet. Two sets of teeth develop and erupt during a normal lifetime. In an infant, 20 deciduous (dē-sid′ū-ŭs; deciduus = falling off) teeth, also called milk teeth, erupt between 6 months and 30 months after birth. These teeth are eventually lost and replaced by 32 permanent teeth. As figure 26.6 shows, the more anteriorly placed permanent 2/25/11 2:20 PM Chapter Twenty-Six Digestive System 785 Central incisor (7–9 mos.) Lateral incisor (9–11 mos.) Canine (18–20 mos.) 1st molar (14–16 mos.) Upper teeth 2nd molar (24–30 mos.) Permanent teeth 2nd molar (20–22 mos.) Lower teeth Deciduous teeth 1st molar (12–14 mos.) Canine (16–18 mos.) Lateral incisor (7–9 mos.) (a) Child’s skull Central incisor (6–8 mos.) (b) Deciduous teeth Right Upper (Maxillary) Quadrant Left Upper (Maxillary) Quadrant Central incisor (7–8 yrs.) Lateral incisor (8–9 yrs.) Canine (11–12 yrs.) 1st premolar (10–11 yrs.) 2nd premolar (10–12 yrs.) Upper teeth 7 1st molar (6–7 yrs.) 8 9 10 6 11 12 13 5 2nd molar (12–13 yrs.) 3rd molar (17–25 yrs.) 4 3 Hard palate 3rd molar (17–25 yrs.) 2nd molar (11–13 yrs.) 1st molar (6–7 yrs.) Lower teeth 14 2 15 1 16 32 17 31 18 30 29 28 27 19 20 21 22 26 25 24 23 2nd premolar (11–12 yrs.) 1st premolar (10–12 yrs.) (d) Teeth numbering Canine (9–10 yrs.) Lateral incisor (7–8 yrs.) Central incisor (6–7 yrs.) Right Lower (Mandibular) Quadrant Left Lower (Mandibular) Quadrant (c) Permanent teeth Figure 26.6 Teeth. (a) Both deciduous teeth and unerupted permanent teeth are visible in this cutaway section of a child’s skull. (b, c) Comparison of the dentition of deciduous and permanent teeth, including the approximate age at eruption for each tooth. Also shown in (c) are the dental quadrants, which are formed by a sagittal plane that passes between the central incisors in the maxilla and mandible. (d) Teeth may be precisely identified using the Universal Numbering System. mck78097_ch26_779-816.indd 785 2/25/11 2:21 PM 786 Chapter Twenty-Six Digestive System Table 26.3 Oral Cavity Structures and Their Functions Structure Description Function Gingivae Composed of dense irregular connective tissue and nonkeratinized stratified squamous epithelium Surround necks of teeth and cover alveolar processes Hard palate Anterior roof of mouth; bony shelf covered by dense connective tissue and nonkeratinized stratified squamous epithelium Forms anterior two-thirds of roof of mouth; separates oral cavity from nasal cavity Lips Form part of anterior walls of oral cavity; covered with keratinized stratified squamous epithelium Close oral cavity during chewing Salivary glands Three pairs of large multicellular glands: parotid glands, sublingual glands, and submandibular glands Produce saliva Soft palate Posterior roof of mouth formed from skeletal muscle and covered with nonkeratinized stratified squamous epithelium; the uvula hangs from it Forms posterior one-third of roof of mouth; helps close off entryway to nasopharynx when swallowing Teeth Hard structures projecting from the alveolar processes of the maxillae and mandible: incisors, canines, premolars, and molars Mastication (chewing food) Tongue Composed primarily of skeletal muscle and covered by stratified squamous epithelium; surface covered by papillae Manipulates ingested material during chewing; pushes material against palate to turn it into a bolus; detects tastes (via taste buds) Tonsils Aggregates of partially encapsulated lymphatic tissue Detect antigens in swallowed food and drink and initiate immune response if necessary Uvula Conical, median, muscular projection extending from the soft palate Assists soft palate in closing off entryway to nasopharynx when swallowing Vestibule Space between cheek and gums Space between lips/cheeks and gums where ingested materials are mixed with saliva and mechanically digested teeth tend to appear first, followed by the posteriorly placed teeth. (The major exception to this rule are the first molars, which appear at about age 6 and are sometimes referred to as the “6-year molars.”) The last teeth to erupt are the third molars, often called wisdom teeth, in the late teens or early 20s. Often the jaw lacks space to accommodate these final molars, and they may either emerge only partially or grow at an angle and become impacted (wedged against another structure). Impacted teeth cannot erupt properly because of the angle of their growth. left side of the maxilla (number 16). Number 17 is the most posterior tooth on the left side of the mandible, and it continues along the mandibular arch to the most posterior tooth on the right side of the mandible (number 32). This system is based on 32 teeth so—if someone is missing tooth number 1 (wisdom tooth)—the first number will be 2 instead of 1, acknowledging the missing tooth. Table 26.3 summarizes the structures of the oral cavity and their functions. W H AT D I D Y O U L E A R N? W H AT D O Y O U T H I N K ? 2 ● If the deciduous teeth are eventually replaced by permanent teeth, why do humans have deciduous teeth in the first place? The most anteriorly placed permanent teeth are called incisors (in-sı̄′zŏr; incido = to cut into). They are shaped like a chisel and have a single root. They are designed for slicing or cutting into food. Immediately posterolateral to the incisors are the canines (kā′nı̄n; canis = dog), which have a pointed tip for puncturing and tearing food. Premolars are located posterolateral to the canines and anterior to the molars. They have flat crowns with prominent ridges called cusps that are used to crush and grind ingested materials. Premolars may have one or two roots. The molars (mō′lăr; molaris = millstone) are the thickest and most posteriorly placed teeth. They have large, broad, flat crowns with distinctive cusps, and three or more roots. Molars are also adapted for grinding and crushing ingested materials. If the mouth is divided into quadrants, each quadrant contains the following number of permanent teeth: two incisors, one canine, two premolars, and three molars (figure 26.6c). In the United States, general dentists have adopted the Universal Numbering System (recognized by the American Dental Association) to identify teeth (figure 26.6d). Tooth number 1 is the most posterior tooth on the right side of the maxilla. Numbering continues anteriorly and around to the most posterior tooth on the mck78097_ch26_779-816.indd 786 3 ● 4 ● What are the main components of saliva, and what functions do they serve? What are the types of permanent teeth, and what is each tooth’s main function in mastication? 26.3 Pharynx Learning Objectives: 1. Describe the structure of the pharynx. 2. Explain the action of the pharyngeal constrictors. The common space used by both the respiratory and digestive systems is the pharynx (see figure 26.1 and chapter 25). The nonkeratinized stratified squamous epithelial lining of the oropharynx and the laryngopharynx provides protection against the abrasive activities associated with swallowing ingested materials. Three skeletal muscle pairs, called the superior, middle, and inferior pharyngeal constrictors, form the wall of the pharynx. Sequential contraction of the pharyngeal constrictors decreases the diameter of the pharynx, beginning at its superior end and moving toward its inferior end, thus pushing swallowed material toward the esophagus. As the pharyngeal constrictors constrict, the epiglottis of the larynx closes over the laryngeal opening to prevent ingested materials from entering the larynx and trachea. 2/25/11 2:21 PM Chapter Twenty-Six The vagus nerves (CN X) innervate most pharyngeal muscles. The principal arteries supplying the pharynx are branches of the external carotid arteries. The pharynx is drained by the internal jugular veins. How do pharyngeal constrictors move swallowed material to the esophagus? 26.4 General Arrangement of Abdominal GI Organs Liver Lesser omentum Pancreas Stomach Duodenum Mesocolon Jejunum Parietal peritoneum Mesentery proper Identify and describe the peritoneum location and function. Explain the derivation of specific mesenteries. Compare and contrast the four tunics in the GI tract wall. Describe the blood vessels, lymphatic structures, and nerves that supply the GI tract. The abdominal organs of the GI tract are supported by serous membranes, and the walls of these organs have specific layers, called tunics. 26.4a Peritoneum, Peritoneal Cavity, and Mesentery The abdominopelvic cavity is lined with moist serous membranes (figure 26.7). The portion of the serous membrane that lines the inside surface of the body wall is called the parietal peritoneum (per′i-tō-nē′ŭm; periteino = to stretch over). The portion of the serous membrane that folds back (reflects) to cover the surface of internal organs is called the visceral peritoneum. Between these two layers is the peritoneal cavity, a potential space where the peritoneal layers that face each other secrete a lubricating serous fluid. The thin layer of fluid in the peritoneal cavity lubricates both the body wall and the internal organ surfaces, allowing the abdominal organs to move freely, and reducing any friction resulting from this movement. Study Tip! The serous peritoneum is very similar to the serous pleura and the serous pericardium. These membranes have an outer parietal layer that lines the body wall and a visceral layer that covers the organ. The space between the parietal and visceral layers is where serous fluid is secreted, and this fluid acts as a lubricant to prevent friction as the organ moves. So, if you remember the basics about the pleura and the pericardium, you will know the basics about the peritoneum too. Within the abdomen, organs that are completely surrounded by visceral peritoneum are called intraperitoneal (in′tră-per′i-tōnē′ăl) organs. They include the stomach, part of the duodenum of the small intestine, the jejunum and the ileum of the small intestine, the cecum, the appendix, and the transverse and sigmoid colon of the large intestine. Retroperitoneal (re-trō-per′i-tō-nē′ăl) organs typically lie directly against the posterior abdominal wall, so only their anterolateral portions are covered with peritoneum. Retroperitoneal organs include most of the duodenum, the pancreas, the ascending and descending colon of the large intestine, and the rectum. mck78097_ch26_779-816.indd 787 Transverse colon Greater omentum Learning Objectives: 1. 2. 3. 4. 787 Diaphragm W H AT D I D Y O U L E A R N? 5 ● Digestive System Visceral peritoneum Ileum Peritoneal cavity Rectum Urinary bladder Figure 26.7 Peritoneum and Mesenteries. Many abdominal organs are held in place by double-layered serous membrane folds called mesenteries. The peritoneum is the serous membrane lining the internal abdominal wall (parietal layer) and covering the outer surface of the abdominal organs (visceral layer). This sagittal view through the abdominopelvic cavity shows the relationship between the peritoneal membranes and the abdominal organs they ensheathe. The mesenteries (mes′en-ter-ē; mesos = middle, enteron = intestine) are folds of peritoneum that support and stabilize the intraperitoneal GI tract organs. Blood vessels, lymph vessels, and nerves are sandwiched between the two folds and supply the digestive organs. There are several different types of mesenteries. The greater omentum (ō-men′tŭm) extends inferiorly like an apron from the greater curvature of the stomach and covers most of the abdominal organs (figure 26.8a). It often accumulates large amounts of adipose connective tissue. The lesser omentum connects the lesser curvature of the stomach and the proximal end of the duodenum to the liver. The lesser omentum may be subdivided into a hepatogastric ligament, which runs from the liver to the stomach, and a hepatoduodenal ligament, which runs from the liver to the duodenum. The mesentery proper is a fan-shaped fold of peritoneum that suspends most of the small intestine from the internal surface of the posterior abdominal wall (figure 26.8b). The peritoneal fold that attaches parts of the large intestine to the internal surface of the posterior abdominal wall is called the mesocolon (mez′ōkō′lon). Essentially, a mesocolon is a mesentery for parts of the large intestine. There are several distinct sections of the mesocolon, each named for the portion of the colon it suspends. For example, transverse mesocolon is associated with the transverse colon, while sigmoid mesocolon is associated with the sigmoid colon. 2/25/11 2:21 PM 788 Chapter Twenty-Six Digestive System Greater omentum (reflected) Liver Falciform ligament Round ligament of the liver Lesser omentum Stomach Transverse colon Transverse mesocolon Greater omentum Mesentery proper Small intestine (a) Omenta (b) Mesentery proper and mesocolon Figure 26.8 Omenta and Mesentery. Cadaver photos show anterior views of (a) the greater and lesser omenta, and (b) the mesentery proper and some of the mesocolon. A peritoneal ligament is a peritoneal fold that attaches one organ to another organ, or attaches an organ to the anterior or lateral abdominal wall. Some examples of peritoneal ligaments include: the coronary ligament, a peritoneal fold attaching the superior surface of the liver to the diaphragm at the margins of the bare area of the liver; the falciform (fal′si-fōrm; falx = sickle) ligament, a peritoneal fold that attaches the liver to the anterior internal abdominal wall; and the lienorenal ligament, a fold of peritoneum between the spleen and the kidney. 26.4b General Histology of GI Organs (Esophagus to Large Intestine) The GI tract from the esophagus through the large intestine is a tube composed of four concentric layers, called tunics. From deep (the lining of the lumen) to superficial (the external covering), these tunics are the mucosa, the submucosa, the muscularis, and the adventitia or serosa (figure 26.9). The general pattern of the tunics is described next. There are variations in the general pattern, which will be described in detail when we discuss the specific organs in which they appear. Mucosa The mucosa (mū-kō′să), or mucous membrane, has three components: (1) an inner (superficial) epithelium lining the lumen of the GI tract; (2) an underlying areolar connective tissue, called the lamina propria; and (3) a relatively thin layer of smooth muscle, termed the muscularis mucosae. The muscularis mucosae is very thin (or absent) at the level of the laryngopharynx and thickens progressively as it approaches the stomach. mck78097_ch26_779-816.indd 788 For most of the abdominal GI tract organs, the lining epithelium is a simple columnar epithelium. The exception to this rule is the esophagus, which is lined with nonkeratinized stratified squamous epithelium. Submucosa The submucosa is composed of either areolar or dense irregular connective tissue and has far fewer cells than the lamina propria. Submucosa components include: accumulations of lymphatic tissue in some submucosal regions; mucin-secreting glands that project ducts across the mucosa and open into the lumen of the tract in the esophagus and duodenum; many large blood vessels and lymph vessels; and nerves that extend fine branches into both the mucosa and the muscularis. These nerve fibers and their associated ganglia are collectively referred to as the submucosal nerve plexus (or Meissner plexus). It contains sensory neurons, sympathetic postganglionic axons, and parasympathetic ganglia. Muscularis The muscularis (mŭ s-kū′lā′ris) typically contains two layers of smooth muscle. Exceptions to this pattern include the esophagus (which contains a mixture of skeletal and smooth muscle) and the stomach (which contains three layers of smooth muscle). The fibers of the inner layer of smooth muscle are oriented circumferentially around the GI tract, and are called the inner circular layer. The fibers of the outer layer are oriented lengthwise along the GI tract, and are called the outer longitudinal layer. 2/25/11 2:21 PM Chapter Twenty-Six Digestive System 789 Mucosa Epithelium Lamina propria Muscularis mucosae Mesentery Vein Artery Lymph vessel Submucosa Submucosal gland Lumen Blood vessel Submucosal nerve plexus Muscularis Inner circular layer Myenteric nerve plexus Outer longitudinal layer Serosa Figure 26.9 Tunics of the Abdominal GI Tract. The wall of the abdominal GI tract has four tunics. From the lining of its lumen to the external covering, the tunics are the mucosa, submucosa, muscularis, and adventitia or serosa. If you think of the GI tract as a “tube,” then contractions of the circular layer constrict the diameter of the tube lumen, while contractions of the longitudinal layer shorten the tube. At specific locations along the GI tract, the inner circular muscle layer is greatly thickened to form a sphincter. A sphincter closes off the lumen opening at some point along the GI tract, and in so doing it can help control the movement of materials through the GI tract. The nerve fibers and the associated ganglia located between the two layers of smooth muscle control its contractions and are collectively referred to as the myenteric (mı̄-en-ter′ik; mys = muscle, enteron = intestine) nerve plexus (or Auerbach plexus). mck78097_ch26_779-816.indd 789 Adventitia or Serosa The outermost tunic may be either an adventitia or a serosa. An adventitia (ad-ven-tish′ă) is composed of areolar connective tissue with dispersed collagen and elastic fibers. A serosa (se-rō′să) has the same components as the adventitia, but is covered by a visceral peritoneum. Intraperitoneal organs have a serosa, because they are completely surrounded by visceral peritoneum. Retroperitoneal organs primarily have an adventitia, since these organs are only partially covered by visceral peritoneum. For example, the ascending colon (which is retroperitoneal) has an adventitia, while the stomach (which is intraperitoneal) has a serosa. 2/25/11 2:21 PM 790 Chapter Twenty-Six Digestive System and smooth muscle in the middle region, and smooth muscle in its inferior region. Study Tip! You have just learned that the “default” pattern of the tunics is as follows: 1. 2. 3. 4. Mucosa (typically lined with simple columnar epithelium) Submucosa Muscularis (typically formed from two layers of smooth muscle) Adventitia or serosa As you learn the basic GI organs, determine how these organs follow or deviate from this default pattern. For example: ■ The esophagus has these tunics, but it deviates from the pattern in two ways: Its mucosa has a stratified squamous epithelium, and its muscularis has a skeletal muscle in its superior region, skeletal 26.4c Blood Vessels, Lymphatic Structures, and Nerve Supply The GI tract has a rich blood and nerve supply. In addition, extensive lymphatic structures along the length of the GI tract act as sentinels to monitor for antigens that may have been ingested. The blood vessels, lymphatic structures, and nerves enter the GI tract from either the surrounding structures (e.g., nearby organs, abdominal wall) or via the supporting mesentery. Blood Vessels Branches of the celiac trunk, superior mesenteric artery, and inferior mesenteric artery supply the abdominal GI tract (see chapter 23). These artery branches split into smaller branches that extend throughout the walls of the GI organs. Branches travel within the tunics, and the mucosa contains capillaries that have fenestrated endothelial cells to promote absorption. The veins arising in the mucosa form anastomoses in the submucosa before exiting the wall of the GI tract adjacent to their companion arteries. Eventually, the veins merge to form the hepatic portal system of veins, to be discussed later in this chapter. Lymph Vessels and Tissues Lymphatic capillaries arise as blind tubes in the mucosa of the GI tract. In the small intestine, each villus usually contains a single, blind-ended, central lymphatic capillary called a lacteal. Recall from chapter 24 that lacteals are responsible for absorbing dietary lipids and lipid-soluble vitamins (vitamins that can be absorbed only if they are dissolved in lipids first). Outside the organ walls, lymphatic capillaries merge to form lymphatic vessels. These vessels enter and exit the many lymph nodes scattered near the organs and within the mesentery. Eventually, this lymph will be transported to the cisterna chyli, which drains into the thoracic duct. The lymphatic structures within the GI tract lie primarily in the lamina propria of the mucosa. Lymphatic structures called MALT (mucosa-associated lymphatic tissue) are found in the small intestine and appendix (see chapter 24). In the small intestine, these aggregate nodules are called Peyer patches. They appear to the naked eye as oval bodies about the size of a pea, but are often much larger structures. Less commonly, lymphatic structures are found external to the simple epithelium throughout the stomach and intestines where they are known as diffuse lymphatic tissue. Also, solitary lymphatic nodules may occur in the esophagus, the pylorus of the stomach, and along the entire length of the small and large intestines. mck78097_ch26_779-816.indd 790 ■ The stomach has these tunics, but it deviates from the pattern in that its muscularis has three layers of smooth muscle, not two. ■ The small intestine follows the basic “default” pattern of tunics. ■ The large intestine has these tunics, but in its muscularis, the outer longitudinal layer of muscle forms three distinct bands called teniae coli (described later in this chapter). Knowing the basic tunic pattern, and then figuring out how an organ may deviate from this pattern, will help you better distinguish the histology of the esophagus, stomach, small intestine, and large intestine. Nerves The nerves associated with the GI tract consist of both autonomic motor and sensory axons. There are three main groups of autonomic plexuses: ■ ■ ■ The celiac plexus contains sympathetic axons (from the T5–T9 segments of the spinal cord) and parasympathetic axons (from the vagus nerve). This plexus supplies structures that receive their blood supply from branches of the celiac trunk. The superior mesenteric plexus contains sympathetic axons (from the T8–T12 segments of the spinal cord) and parasympathetic axons (from the vagus nerve). This plexus transmits autonomic innervation to structures that receive their blood supply from branches of the superior mesenteric artery. The inferior mesenteric plexus contains sympathetic axons (from the L1–L2 segments of the spinal cord) and parasympathetic axons (from the pelvic splanchnic nerves). This plexus supplies structures that receive blood from branches of the inferior mesenteric artery. In general, parasympathetic innervation promotes digestive system activity by stimulating GI gland secretions and peristalsis, and by relaxing GI sphincters. These changes in activity induce vasodilation and an increase in GI blood flow. Sympathetic innervation inhibits digestive system activity, and it tends to do the opposite of parasympathetic innervation. So sympathetic innervation inhibits some GI gland secretions, inhibits peristalsis, closes the GI sphincters, and vasoconstricts the blood vessels to the GI tract. W H AT D I D Y O U L E A R N? 6 ● 7 ● Compare the terms intraperitoneal and retroperitoneal, and give examples of each type of organ. What are the four main tunics of the abdominal GI tract, and what is the “default” pattern in each tunic? 26.5 Esophagus Learning Objective: 1. Describe the structure and function of the esophagus. The esophagus (ē-sof′ă-gŭs; gullet) is a tubular passageway for swallowed materials being conducted from the pharynx to the stomach (see figure 26.1). The inferior region of the esophagus 2/25/11 2:21 PM Chapter Twenty-Six Digestive System 791 Mucosa Stratified squamous epithelium Muscularis mucosae Submucosa Muscularis Mucosa Lamina propria Adventitia LM 11x (a) Esophagus, transverse section Muscularis mucosae LM 65x (b) Esophageal mucosa Figure 26.10 Histology of the Esophagus. The esophagus extends inferiorly from the pharynx and conducts swallowed materials to the stomach. (a) A photomicrograph of a transverse section through the esophagus identifies the tunics in its wall. (b) A photomicrograph shows the esophageal mucosa. connects to the stomach by passing through an opening in the diaphragm called the esophageal hiatus (hı̄-ā′tŭ s; to yawn). 26.5a Gross Anatomy The esophageal wall is thick and composed of concentric tunics that are continuous superiorly with those of the pharynx and inferiorly with those of the stomach. In the living adult human, the esophagus is about 25 centimeters (about 10 inches) long, and most of its length is within the thorax, directly anterior to the vertebral bodies. Only the last 1.5 centimeters of the esophagus are located in the abdomen. The empty esophagus is flattened; thus, there is no continuously open lumen. Only a passing bolus of food slightly expands the esophagus. 26.5b Histology The esophageal mucosa is different from that of the abdominal GI tract organs in that it is composed of thick, nonkeratinized stratified squamous epithelium (figure 26.10). The stratified squamous epithelium is better suited to withstand the abrasions of the bolus as it moves through the esophagus. Since the esophagus does not absorb any nutrients, this thicker, protective epithelium supports its function. The submucosa is thick and composed of abundant elastic fibers that permit distension during swallowing. It houses numerous mucous glands that provide a thick, lubricating mucus for the epithelium. The muscularis is composed of an inner circular layer and an outer longitudinal layer. The muscularis of the esophagus is unique in that it contains a blend of both skeletal and smooth muscle fibers. The two layers of muscle in the superior one-third mck78097_ch26_779-816.indd 791 of the muscularis layer are skeletal, rather than smooth, to ensure that the swallowed material moves rapidly out of the pharynx and into the esophagus before the next respiratory cycle begins. (Remember that skeletal muscle contracts more rapidly than does smooth muscle.) Striated and smooth muscle fibers intermingle in the middle one-third of the esophagus, and only smooth muscle is found within the wall of the inferior one-third. This transition marks the beginning of a continuous smooth muscle muscularis that extends throughout the stomach and the small and large intestines to the anus. The outermost layer of the esophagus is an adventitia. The esophagus adheres to the posterior body wall via its adventitia. At the superior end of the esophagus, the superior esophageal sphincter (or pharyngoesophageal sphincter) is a thickened ring of circular skeletal muscle marking the area where the esophagus and the pharynx meet. This sphincter is closed during inhalation of air, so that air doesn’t enter the esophagus and enters the larynx and trachea instead. The orifice between the esophagus and the stomach is bounded by a thin band of circular smooth muscle, the inferior esophageal sphincter (esophagealgastric or cardiac sphincter). This sphincter isn’t strong enough alone to prevent materials from refluxing back into the esophagus; instead, the esophageal opening of the diaphragm assumes the duty of the “stronger sphincter” to prevent materials from regurgitating from the stomach into the esophagus. W H AT D I D Y O U L E A R N? 8 ● How do the esophageal tunics differ from the “default” tunic pattern? 2/25/11 2:21 PM 792 Chapter Twenty-Six Digestive System CLINICAL VIEW Reflux Esophagitis and Gastroesophageal Reflux Disease The inferior esophageal sphincter and the diaphragm usually prevent acidic stomach contents from regurgitating into the esophagus. However, sometimes acidic chyme refluxes into the esophagus, causing the burning pain and irritation of reflux esophagitis. Because the pain is felt posterior to the sternum and can be so intense that it is mistaken for a heart attack, this condition is commonly known as “heartburn.” Unlike the stomach epithelium, the esophageal epithelium is poorly protected against acidic contents and easily becomes inflamed and irritated. Other symptoms include abdominal pain, difficulty in swallowing, increased belching, and sometimes bleeding. Reflux esophagitis can occur in anyone, but is seen most frequently in overweight individuals, smokers, those who have eaten a very large meal (especially just before bedtime), and/or people with hiatal hernias (hı¯-ā′tăl her′nē-ă; rupture), in which a portion of the stomach protrudes through the diaphragm into the thoracic cavity. Eating spicy foods or ingesting too much caffeine may exacerbate the symptoms in people affected by reflux esophagitis. Preventive treatment includes lifestyle changes such as limiting meal size, not lying down until at least 2 hours after eating, quitting smoking, and losing weight. Sleeping with the head of the bed elevated 4 to 6 inches, so that the body lies at an angle rather than flat, appears to alleviate symptoms. Chronic reflux esophagitis can lead to gastroesophageal reflux disease (GERD). In this condition, frequent gastric reflux erodes the esophageal tissue, so over a period of time, scar tissue builds up in the esophagus, leading to narrowing of the esophageal lumen. An endoscope (an optical tubular instrument used to visualize the lumina of internal organs) may be inserted into the laryngopharynx and esophagus to visualize the damage. In advanced cases of GERD, the esophageal epithelium may change from stratified squamous to columnar secretory epithelium, a condition known as Barrett esophagus. The specific reasons why Barrett esophagus develops are not known. Barrett esophagus is associated with an increased risk of cancerous growths in the esophagus, as these new secretory cells can develop into cancer. The continual reflux and injury may also lead to esophageal ulcers. GERD can be treated with a series of medications. Proton pump inhibitors (e.g., omeprazole [Prilosec], esomeprazole [Nexium]) limit acid secretion in the stomach by acting on the proton (hydrogen ion) “pumps” that help produce acid. Histamine (H2) blockers (e.g., famotidine [Pepcid], nizatidine [Axid], ranitidine [Zantac]) also help limit acid secretion in the stomach. Antacids (e.g., Tums, Rolaids) help neutralize stomach acid. Columnar secretory epithelium (a) Endoscopic view of a normal esophagus. (b) An endoscopic view of the esophagus shows the signs of reflux esophagitis. An endoscopic view of the esophagus shows the signs of reflux esophagitis. 26.6 The Swallowing Process Learning Objective: 1. List and explain the three phases of swallowing. Swallowing, also called deglutition (dē-gloo-tish′ŭn; degluto = to swallow), is the process of moving ingested materials from the oral cavity to the stomach. There are three phases of swallowing: the voluntary phase, the pharyngeal phase, and the esophageal phase (figure 26.11). The voluntary phase occurs after ingestion. Food and saliva mix in the oral cavity. Chewing forms a bolus that is mixed and manipulated by the tongue and then pushed superiorly against the hard palate. Transverse palatine folds in the hard palate help direct the bolus posteriorly toward the oropharynx. mck78097_ch26_779-816.indd 792 LM 400x The condition known as Barrett esophagus. The appearance of the bolus at the entryway to the oropharynx initiates the pharyngeal phase, when tactile sensory receptors trigger the swallowing reflex, which is controlled by the swallowing center in the medulla oblongata. The bolus passes quickly and involuntarily through the pharynx to the esophagus. During this phase, (1) the soft palate and uvula elevate to block the passageway between the nasopharynx and oropharynx; (2) the bolus enters the oropharynx; and (3) the larynx and laryngeal opening elevate toward the epiglottis, ultimately covering and sealing the glottis to prevent swallowed materials from entering the trachea. Pharyngeal constrictors are skeletal muscle groups that contract involuntarily and sequentially to ensure that the swallowed bolus moves quickly (1 second elapses in this phase) through the pharynx and into the esophagus. The esophageal phase is involuntary. It is the time (about 5 to 8 seconds) during which the bolus passes through the 2/25/11 2:21 PM Chapter Twenty-Six Hard palate Bolus Soft palate and uvula elevate and cover nasopharynx Digestive System 793 Soft palate and uvula return to pre-swallowing position Direction of bolus movement Uvula Oropharynx Tongue Epiglottis Bolus Epiglottis closes over laryngeal opening Larynx Esophagus Trachea 1 Voluntary phase: Bolus of food is pushed by tongue against hard palate and then moves toward oropharynx. 2 Pharyngeal phase (involuntary): As bolus moves into oropharynx, the soft palate and uvula close off nasopharynx, and the larynx elevates so the epiglottis closes over laryngeal opening. Esophagus Bolus 3 Esophageal phase (involuntary): Peristaltic contractions of the esophageal muscle push bolus toward stomach; soft palate, uvula, and larynx return to their pre-swallowing positions. Figure 26.11 Phases of Swallowing. Swallowing, or deglutition, occurs as a result of coordinated muscular activities that force the bolus (1) into the pharynx from the oral cavity, (2) through the pharynx, and (3) into the esophagus on the way to the stomach. esophagus and into the stomach. This phase begins when the superior esophageal sphincter relaxes to allow ingested materials into the esophagus. The presence of the bolus within the lumen of the esophagus stimulates peristaltic waves of muscular contraction that assist in propelling the bolus toward the stomach. The soft palate, uvula, and larynx return to the pre-swallowing positions. W H AT D I D Y O U L E A R N? 9 ● Briefly describe the three phases of swallowing. 26.7 Stomach Learning Objectives: 1. Describe the gross anatomy of the stomach. 2. Explain the histology of the stomach wall. 3. Compare and contrast the secretions of the stomach. The stomach (stŭm′ŭk; stomachus = belly) is a muscular, J-shaped sac that occupies the left upper quadrant of the abdomen, immediately inferior to the diaphragm (figure 26.12). It continues the mechanical and chemical digestion of the bolus. After the bolus has been completely processed in the stomach, the product is called chyme (kı̄m; chymos = juice). Chyme has the consistency of a pastelike soup. The stomach facilitates mechanical digestion by the contractions of its thick muscularis layer, which churns and mixes the bolus and the gastric secretions. The stomach facilitates chemical digestion through its gastric secretions of acid and enzymes. 26.7a Gross Anatomy The stomach is composed of four regions: ■ The cardia (kar′dē-ă; heart) is the small, narrow, superior entryway into the stomach lumen from the esophagus. The internal opening where the cardia meets the esophagus is called the cardiac orifice. mck78097_ch26_779-816.indd 793 ■ ■ ■ The fundus (fŭn′dŭs; bottom) is the dome-shaped region lateral and superior to the esophageal connection with the stomach. Its superior surface contacts the diaphragm. The body (corpus) is the largest region of the stomach; it is inferior to the cardiac orifice and the fundus. The pylorus (pı̄-lōr′ŭs; gatekeeper) is a narrow, medially directed, funnel-shaped pouch that forms the terminal region of the stomach. The pyloris is divided into two parts: the pyloric antrum (more expanded region near the body) and the pyloric canal (more narrow region that attaches to the duodenum). Its opening with the duodenum of the small intestine is called the pyloric orifice. Surrounding this pyloric orifice is a thick ring of circular smooth muscle called the pyloric sphincter. The pyloric sphincter regulates the entrance of chyme into the small intestine by closing upon sympathetic innervation and opening upon parasympathetic innervation. The inferior convex border of the stomach is the greater curvature, while the superior concave border is the lesser curvature. The greater omentum attaches to the greater curvature edge of the stomach, and the lesser omentum extends between the lesser curvature and the liver. Internally, the stomach lining is composed of numerous gastric folds, or rugae (roo′gē; ruga = wrinkle). These gastric folds, which are observed only when the stomach is empty, allow the stomach to expand greatly when it fills and then return to its normal J-shape when it empties. 26.7b Histology The stomach is lined by a simple columnar epithelium, although little absorption occurs in the stomach. This epithelium does not contain goblet cells; instead, surface mucous cells (described later) secrete mucin onto the epithelial lining. Another feature that distinguishes the stomach mucosa from the default pattern is that the stomach lining is indented by numerous depressions called gastric 2/25/11 2:21 PM 794 Chapter Twenty-Six Digestive System Fundus Esophagus Cardia Pyloric orifice Pyloric Duodenum sphincter Lesser curvature Longitudinal layer (outer) Circular layer (middle) Oblique layer (inner) Three layers of smooth muscle Body Greater curvature Pyloric canal Pylorus Pyloric antrum Gastric folds (a) Stomach regions, anterior view Figure 26.12 Liver (cut) Lesser curvature Diaphragm Gross Anatomy of the Stomach. The stomach is a muscular sac where mechanical and chemical digestion of the bolus occurs. (a) The major regions of the stomach are the cardia, the fundus, the body, and the pylorus. Three layers of smooth muscle make up the muscularis. (b) A cadaver photo shows an anterior section of the stomach, illustrating the gastric folds and the cardiac orifice. Esophagus Cardiac orifice Gastric folds Body of stomach Pylorus of stomach Greater curvature (b) Gross anatomy of stomach (cut open) pits. At the base of each pit are the openings of several branched tubular glands, called gastric glands, which extend through the length of the mucosa to its base (figure 26.13). The mucosa of the stomach is only 1.5 millimeters at its thickest point (about the thickness of a nickel). The muscularis mucosae lies between the base of the gastric glands and the submucosa, and it helps expel gastric gland secretory products when it contracts. The muscularis of the stomach varies from the default pattern in that it is composed of three smooth muscle layers instead of two: an inner oblique layer, a middle circular layer, and an outer longitudinal layer. The oblique layer is best-developed at the cardia and the body of the stomach. The presence of a third layer of smooth muscle assists the continued churning and blending of the bolus. The stomach is intraperitoneal, so its outermost tunic is a serosa. mck78097_ch26_779-816.indd 794 W H AT D O Y O U T H I N K ? 3 ● The stomach secretes gastric juices, which are highly acidic and can break down and chemically digest food. What prevents the gastric juices from eating away at the stomach itself? 26.7c Gastric Secretions Gastric juices are produced by cells in the gastric glands, and their secretions are released into gastric pits, which funnel them to the lumen of the stomach. Five types of secretory cells form the gastric epithelium (figure 26.13b, c). Surface mucous cells line the stomach lumen and extend into the gastric pits. They continuously secrete mucin onto the gastric luminal surface to prevent ulceration of the lining upon 2/25/11 2:21 PM Chapter Twenty-Six Digestive System 795 Opening to gastric pit Gastric pit Simple columnar epithelium Lamina propria Mucosa Lymph vessel Muscularis mucosae Submucosa Submucosal nerve plexus Oblique layer Muscularis Circular layer Longitudinal layer Serosa Myenteric nerve plexus Artery Vein (a) Stomach wall, sectional view Opening to gastric pit Gastric pit Surface mucous cell (secretes mucin) Simple columnar epithelium Gastric pit Mucous neck cell (secretes acidic mucin) Parietal cell (secretes hydrochloric acid and intrinsic factor) Gastric gland Gastric glands Chief cell (secretes pepsinogen) Enteroendocrine cell (secretes gastrin) LM 60x (b) Stomach mucosa (c) Gastric pit and gland Figure 26.13 Histology of the Stomach Wall. (a) The stomach lumen contains invaginations within the mucosa called gastric pits that lead into gastric glands. (b) A photomicrograph shows gastric glands and the cells lining the gastric pit. (c) A diagrammatic section of a gastric pit and gland shows their structure and the distribution of different types of secretory cells. mck78097_ch26_779-816.indd 795 2/25/11 2:21 PM 796 Chapter Twenty-Six Digestive System CLINICAL VIEW Peptic Ulcers Normally there is a balance in the stomach between the acidic gastric juices and the protective regenerative nature of the mucosa lining. When this balance is thrown off, the stage is set for the development of a peptic ulcer, which is a chronic, solitary erosion of a portion of the lining of either the stomach or the duodenum. Annually, over 4 million people in the United States are diagnosed with an ulcer. Gastric ulcers are peptic ulcers that occur in the stomach, whereas duodenal ulcers are peptic ulcers that occur in the superior part of the duodenum, which is the first segment of the small intestine. Duodenal ulcers are common because the first part of the duodenum receives the chyme from the stomach but has yet to receive the alkaline pancreatic juice that can neutralize chyme’s acidic content. Symptoms of an ulcer include a gnawing, burning pain in the epigastric region, which may be worse after eating, as well as nausea, vomiting, and extreme belching. Bleeding may also occur, and the partially digested blood results in dark and tarry stools. If left untreated, an ulcer may erode the entire organ wall and cause perforation, which is a medical emergency. Irritation of the gastric mucosa (gastritis) has been linked to many cases of peptic ulcer. Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen and aspirin, are a common cause of gastritis, and these drugs also impair healing of the gastric lining. However, the major player in peptic ulcer formation is a bacterium called Helicobacter pylori, which is present in over 70% of gastric ulcer cases and well over 90% of duodenal ulcer cases. H. pylori resides in the stomach and produces enzymes that break down the components in the gastric mucus, weakening its protective effects. As leukocytes enter the stomach to destroy the bacteria, they also destroy the mucous neck cells. This further irritates the stomach lining and creates an ideal environment for continued H. pylori colonization. Thus, the bacteria initiate a cascade of events that lead to erosion of the gastric lining and eventual perforation of the stomach wall if not treated. Treatment for a gastric ulcer involves eliminating the bacteria as well as reducing gastric acidity to promote healing. Categories of medications that help include an antibiotic taken for 2 weeks to eradicate H. pylori, an antacid (Tums, Rolaids) to help neutralize stomach acid, a proton-pump inhibitor (e.g., omeprazole, esomeprazole), and/or a histamine (H2) blocker (e.g., famotidine, nizatidine, ranitidine) to help limit acid secretion in the stomach. Stomach Duodenal ulcer Gastric ulcers Mucosa Submucosa Muscularis Serosa Duodenum (a) Common locations of gastric and duodenal ulcers (a) A sectioned view of the stomach and duodenum illustrates some common locations for gastric and duodenal ulcers. (b) A gross anatomic specimen shows a perforated gastric ulcer. (b) Perforated gastric ulcer exposure to the high acidity of the gastric fluid and protect the epithelium from gastric enzymes. Mucous neck cells are located immediately deep to the base of the gastric pit and are interspersed among the parietal cells. These cells produce an acidic mucin that differs structurally and functionally from the mucin secreted by the surface mucous cells. mck78097_ch26_779-816.indd 796 The acidic mucin helps maintain the acidic conditions resulting from the secretion of hydrochloric acid by parietal cells. Parietal cells (also called oxyntic cells) are located primarily in the proximal and middle parts of the gastric gland. Their distinctive features are small intracellular channels called canaliculi, which are lined by microvilli. Hydrochloric acid secreted across 2/25/11 2:21 PM Chapter Twenty-Six the parietal cells’ vast internalized surface helps denature proteins to facilitate chemical digestion. Parietal cells also produce intrinsic factor, a molecule that binds vitamin B12 in the stomach lumen and assists in B12 absorption in the ileum of the small intestine. Chief cells (also called zymogenic cells or peptic cells) are housed primarily in the distal part of the gastric gland. These cells synthesize and secrete enzymes, primarily inactive pepsinogen, into the lumen of the stomach. The acid content of the stomach then converts inactive pepsinogen into the active enzyme pepsin, which chemically digests denatured proteins in the stomach into smaller fragments. Enteroendocrine (en′ter-ō -en′dō -krin; enteron = gut, intestine) cells are endocrine cells widely distributed in the gastric glands of the stomach. These cells secrete gastrin, a hormone that enters the blood and stimulates the secretory activities of the chief and parietal cells and the contractile activity of gastric muscle. Enteroendocrine cells also produce other hormones, such as somatostatin, that modulate the function of nearby enteroendocrine and exocrine cells. There are numerous gastric pits in the mucosa of the stomach, but the types of secretory cells differ in the various regions of the stomach. The glands of the fundus and body contain all of the cell types previously discussed; however, the glands of the cardia and pylorus contain mostly mucous neck cells. Parietal cells are abundant in the glands of the fundus and body, but they are virtually absent in pyloric glands. Thus, the glands of the fundus and body produce highly acidic secretions, whereas those in the cardia and pylorus produce mainly protective, alkaline, mucin. This makes functional sense, since alkaline mucus can help protect the adjacent regions of the digestive tract (esophagus and duodenum) from damage by stomach acid and digestive enzymes. Surface mucous cells protect the stomach lining, but the esophagus and duodenum lack such protection. Saliva, mucin, and gastric secretions contribute substantially to the volume of ingested material. In fact, if a person eats 1 liter of food, the amount of chyme that enters the small intestine is 3 to 4 liters, due to the volume of the secretions. W H AT D I D Y O U L E A R N? 10 ● 11 ● What are the four regions of the stomach, and where is each located? What are the five types of secretory cells in the stomach, and what does each secrete? 26.8 Small Intestine Learning Objectives: 1. Describe the gross anatomy of the small intestine. 2. Compare and contrast the three regions of the small intestine. 3. Explain the microscopic structure of the small intestine. The small intestine finishes the chemical digestion process and is responsible for absorbing up to 90% of the nutrients and water. Ingested nutrients spend at least 12 hours in the small intestine as chemical digestion and absorption are completed. 26.8a Gross Anatomy and Regions The small intestine, also called the small bowel, is a coiled, thinwalled tube about 6 meters (20 feet) in length in the unembalmed cadaver. (It is much shorter in a living individual due to muscle tone. We do not know the exact length in a living human because it would be physically difficult to measure the intestine, and would cause severe discomfort to a living individual.) It extends from the mck78097_ch26_779-816.indd 797 Digestive System 797 Duodenum Duodenojejunal flexure Large intestine Jejunum Ileocecal valve Ileum Figure 26.14 Gross Anatomy of the Small Intestine. The three regions of the small intestine—duodenum, jejunum, and ileum—are continuous and “framed” within the abdominal cavity by the large intestine. pylorus of the stomach to the cecum of the large intestine, and thus occupies a significant portion of the abdominal cavity. The small intestine receives its blood supply primarily from branches of the superior mesenteric artery, and it is innervated by the superior mesenteric plexus. The small intestine consists of three specific segments: the duodenum, the jejunum, and the ileum (figure 26.14). The duodenum (doo-ō-dē′nŭm, doo-od′ĕ-nŭm; breadth of twelve fingers) forms the initial or first segment of the small intestine. It is approximately 25 centimeters (10 inches) long and originates at the pyloric sphincter. The duodenum is arched into a C-shape around the head of the pancreas and becomes continuous with the jejunum at the duodenojejunal flexure (flek′sher; fleksura = bend). Most of the duodenum is retroperitoneal, although the very proximal part is intraperitoneal and connects to the liver by the lesser omentum. Within the wall of the duodenum is the major duodenal papilla, through which bile and pancreatic juice enter the duodenum. A minor duodenal papilla also receives an additional small amount of pancreatic juice via an accessory pancreatic duct. The jejunum (jĕ-joo′nŭm; jejunus = empty) is the middle region of the small intestine. Extending approximately 2.5 meters (7.5 feet in an unembalmed cadaver), it makes up approximately two-fifths of the small intestine’s total length. The jejunum is the primary region within the small intestine for chemical digestion and nutrient absorption. It is intraperitoneal and suspended in the abdomen by the mesentery proper. The ileum (il′ē-ŭm; eiles = twisted) is the final or last region of the small intestine. At about 3.6 meters (10.8 feet) in length in an unembalmed cadaver, the ileum forms approximately three-fifths of the small intestine. Its distal end terminates at the ileocecal (il′ē-ō-sē′kăl) valve, a sphincter that controls the entry of materials into the large intestine. The ileum is intraperitoneal and suspended in the abdomen by the mesentery proper. Internally, the mucosal and submucosal tunics of the small intestine are thrown into circular folds (also called plicae [plı̄′kē; fold] circulares) (figure 26.15a, b). Circular folds, which can be seen with the naked eye, help increase the surface area through which nutrients can be absorbed. In addition, the circular folds 2/25/11 2:21 PM 798 Chapter Twenty-Six Digestive System Simple columnar epithelium with microvilli (absorbs nutrients) Circular folds Capillary network Mucosa Submucosa Goblet cells Muscularis Inner circular layer Outer longitudinal layer Lacteal Enteroendocrine cells (secrete hormones) Circular fold Serosa (a) Intestinal gland Lymphatic nodule Intestinal villi Muscularis mucosae Venule Lymph vessel Arteriole Submucosa (c) Intestinal villus Inner circular layer Muscularis Outer longitudinal layer Serosa (b) Section of small intestine Simple columnar cell Intestinal lumen Microvilli Villi Simple columnar epithelium Intestinal lumen Lamina propria TEM 18,000x (e) Microvilli Goblet cells Figure 26.15 LM 70x (d) Intestinal villi mck78097_ch26_779-816.indd 798 Histology of the Small Intestine. The surface area of the small intestine wall is vastly increased by specific structural modifications. (a) The wall is composed of four concentric tunics. (b) Circular folds formed from the mucosa and submucosa are lined by a dense covering of fingerlike projections called intestinal villi, which are formed from mucosa only. (c) The structure of a single villus. (d) A photomicrograph shows the internal structure of villi projecting into the intestinal lumen. (e) The plasma membrane along the apical surface of the epithelium contains microvilli. 2/25/11 2:22 PM Chapter Twenty-Six Digestive System 799 Table 26.4 Small Intestine Structures Involved in Digestion and Absorption Structure Anatomy Function Circular folds Circular folds of mucosa and submucosa Slow down the passage of materials undergoing digestion; increase surface area for both absorption and chemical digestion Villi Fingerlike projections of mucosa Increase surface area for both absorption and chemical digestion Microvilli Folded, fingerlike projections of plasma membrane on apical surface of columnar epithelial cells Increase surface area for both absorption and chemical digestion Intestinal glands Invaginations into mucosa between villi Increase surface area for both absorption and chemical digestion; enteroendocrine cells lining intestinal glands secrete digestive hormones Submucosal glands Coiled tubular glands within submucosa with ducts opening into intestinal lumen Secrete alkaline mucin to protect and lubricate lining of small intestine act like “speed bumps” to slow down the movement of chyme and ensure that it remains within the small intestine for maximal nutrient absorption. Circular folds are more numerous in the duodenum and jejunum, and least numerous in the ileum. W H AT D O Y O U T H I N K ? 4 ● Why are the circular folds much more numerous in the duodenum and least numerous in the ileum? How does the abundance of circular folds relate to the main functions of the duodenum and ileum? jejunum. The number of goblet cells in the mucosa increases from the duodenum to the ileum, due to the increased need for lubrication as digested materials are absorbed and undigested materials are left behind. Peyer patches are abundant primarily in the ileum. Table 26.4 summarizes all of the small intestine structures that aid in digestion and absorption. W H AT D I D Y O U L E A R N? 12 ● Compare and contrast the gross anatomic and histologic characteristics that distinguish the duodenum, jejunum, and ileum. 26.8b Histology When circular folds are viewed at the microscopic level, smaller, fingerlike projections of mucosa only, called villi, can be seen along their surface. These villi further increase the surface area for absorption and secretion. Increasing the absorptive surface area even further are microvilli (mı̄-krō-vil′ı̄; mikros = small) along the free surface of the simple columnar cells (figure 26.15e). Individual microvilli are not clearly visible in light micrographs of the small intestine; instead, they collectively appear as a brush border that resembles a brightly staining surface on the apical end of the simple columnar cells (figure 26.15d). Each villus contains an arteriole and a venule, with a rich capillary network between them. The capillaries absorb most nutrients. In the center of the villus is a lacteal, which is responsible for absorbing lipids and lipid-soluble vitamins, which are too large to be absorbed by the capillaries. Between some of the intestinal villi are invaginations of mucosa called intestinal glands (also known as intestinal crypts or crypts of Lieberkuhn). These glands extend to the base of the mucosa and slightly resemble the gastric glands of the stomach. Lining them are simple columnar epithelial cells (with goblet cells) and enteroendocrine cells. These enteroendocrine cells release hormones such as secretin, cholecystokinin, and gastric inhibitory peptide. Some of these hormones temporarily slow down digestive activities as material from the stomach begins to enter the small intestine, thereby prolonging the time for stomach emptying into the small intestine. The goblet cells produce mucin to lubricate and protect the intestinal lining as materials being digested pass through. Distinctive histologic features characterize the three small intestine regions. The proximal duodenum contains submucosal glands (or Brunner glands), which produce a viscous, alkaline mucus that protects the duodenum from the acidic chyme. Circular folds are best-developed in the jejunum and nearly absent in the ileum. Additionally, villi are larger and more numerous in the mck78097_ch26_779-816.indd 799 26.9 Large Intestine Learning Objectives: 1. 2. 3. 4. Describe the gross anatomy of the large intestine. Compare and contrast the large intestine regions. Explain the microscopic structure of the large intestine. Trace the movement of material through the large intestine. The large intestine, also called the large bowel, forms a three-sided perimeter in the abdominal cavity around the centrally located small intestine (figure 26.16). From its origin at the ileocecal junction to its termination at the anus, the large intestine has an approximate length of 1.5 meters (5 feet) and a diameter of 6.5 centimeters (2.5 inches). It is called the “large” intestine because its diameter is greater than that of the small intestine. Recall that the small intestine absorbs much, but not all, of the digested material. On average, about 1 liter of remaining material passes from the small intestine to the large intestine daily. The large intestine absorbs most of the water and ions from the remaining digested material. In so doing, the watery material that first enters the large intestine soon solidifies and becomes feces. The large intestine stores the feces until the body is ready to defecate (expel the feces). The large intestine also absorbs a very small percentage of nutrients still remaining in the digested material. 26.9a Gross Anatomy and Regions The initial or first region of the large intestine is a blind sac called the cecum (sē′kŭm; caecus = blind), which is located in the right lower abdominal quadrant. This pouch extends inferiorly from the ileocecal valve, which represents the attachment of the distal end of the small intestine to the proximal region of the large 2/25/11 2:22 PM 800 Chapter Twenty-Six Digestive System Transverse colon Ascending colon Descending colon Cecum Vermiform appendix Rectum Sigmoid colon Anal canal Left colic flexure Transverse mesocolon Right colic flexure Omental appendices Haustrum Tenia coli Superior mesenteric artery Descending abdominal aorta Inferior mesenteric artery Rectum Ileocecal valve Rectal valve Cecum Levator ani muscle Anal canal Ileum Vermiform appendix Veins Rectum Sigmoid mesocolon Internal anal sphincter Anus Anal canal Anal columns (a) Large intestine, anterior view External anal sphincter Anal sinuses (b) Anal canal Figure 26.16 Gross Anatomy of the Large Intestine. (a) Anterior view of the large intestine, which forms the distal end of the GI tract. (b) Details of the anal canal. intestine. Projecting inferiorly from the posteromedial region of the cecum is the vermiform (ver′mi-fōrm; vermis = worm) appendix (ă-pen′diks; appendage), a thin, hollow, fingerlike sac lined by lymphocyte-filled lymphatic nodules. Both the cecum and the vermiform appendix are intraperitoneal organs. At the level of the ileocecal valve, the colon begins and forms an inverted U-shaped arch. The colon is partitioned into four segments: the ascending colon, transverse colon, descending colon, and sigmoid colon. The ascending colon originates at the ileocecal valve and extends superiorly from the superior edge of the cecum along the right lateral border of the abdominal cavity. The ascending colon is retroperitoneal, since its posterior wall directly adheres to the posterior abdominal wall, and only its anterior surface is covered mck78097_ch26_779-816.indd 800 with peritoneum. As it approaches the inferior surface of the liver, the ascending colon makes a 90-degree turn toward the left side of the abdominal cavity. This bend in the colon is called the right colic (kol′ik) flexure, or the hepatic flexure (figure 26.16a). The transverse colon originates at the right colic flexure and curves slightly anteriorly as it projects horizontally across the anterior region of the abdominal cavity. A type of mesentery called the transverse mesocolon connects the transverse portion of the large intestine to the posterior abdominal wall. Hence, the transverse colon is intraperitoneal. As the transverse colon approaches the spleen in the left upper quadrant of the abdomen, it makes a 90-degree turn inferiorly. The resulting bend in the colon is called the left colic flexure, or the splenic (splen′ik) flexure. 2/25/11 2:22 PM Chapter Twenty-Six The descending colon is retroperitoneal and found along the left side of the abdominal cavity. It originates at the left colic flexure and descends vertically until it terminates at the sigmoid colon. The sigmoid (sig′moyd; resembling letter S) colon originates at the sigmoid flexure, where the descending colon curves and turns inferomedially into the pelvic cavity. The sigmoid colon is intraperitoneal and has a mesentery called the sigmoid mesocolon. The sigmoid colon terminates at the rectum. The rectum (rek′tŭm; rectus = straight) is a retroperitoneal structure that connects to the sigmoid colon. The rectum is a muscular tube that readily expands to store accumulated fecal material prior to defecation. Three thick, transverse folds of the rectum, called rectal valves, ensure that fecal material is Study Tip! Many of the segments of the colon are named for the direction in which materials travel. So material ascends (travels superiorly) in the ascending colon, material travels across in the transverse colon, and material descends (travels inferiorly) in the descending colon. Digestive System 801 retained during the passing of gas. The rectum then terminates at the anal canal. The terminal few centimeters of the large intestine are called the anal (ā′năl) canal (figure 26.16b). The anal canal passes through an opening in the levator ani muscles of the pelvic floor and terminates at the anus. The internal lining of the anal canal contains relatively thin longitudinal ridges, called anal columns, between which are small depressions termed anal sinuses. As fecal material passes through the anal canal during defecation, pressure exerted on the anal sinuses causes their cells to release excess mucin. As a result, this extra mucus lubricates the anal canal during defecation. At the base of the anal canal are the internal and external anal sphincters, which close off the opening to the anal canal and relax (open) during defecation. The internal anal sphincter is involuntary, whereas the external anal sphincter is voluntary. 26.9b Histology The mucosa of the large intestine is lined with simple columnar epithelium and goblet cells (figure 26.17). Unlike the small intestine, the large intestine mucosa lacks intestinal villi; however, it contains numerous intestinal glands that extend to Goblet cells Opening to intestinal gland Opening to intestinal gland Simple columnar epithelium Goblet cells Simple columnar epithelium Mucosa Intestinal gland Intestinal gland Lamina propria Submucosa Lymphatic nodule Muscularis mucosae Muscularis Inner circular layer of muscularis Serosa (or adventitia) Nerves (a) Large intestine tunics Arteriole Venule Outer longitudinal layer of muscularis (tenia coli) Muscularis mucosae LM 80x (b) Large intestine mucosa and submucosa Figure 26.17 Histology of the Large Intestine. (a) The luminal wall of the large intestine is composed of several layers. (b) A photomicrograph shows the histology of the wall of the large intestine. mck78097_ch26_779-816.indd 801 2/25/11 2:22 PM 802 Chapter Twenty-Six CLINICAL VIEW: Digestive System In Depth Colorectal Cancer Colorectal cancer is the second most common type of cancer in the United States, with over 140,000 cases occurring annually and 60,000 of those resulting in death. In recent years, high-profile individuals such as baseball players Eric Davis and Darryl Strawberry and Supreme Court Justice Ruth Bader Ginsburg have been treated for colorectal cancer. These well-publicized cases have helped increase awareness of this deadly disease. The term colorectal cancer refers to a malignant growth anywhere along the colon or rectum. The majority of colorectal cancers appear in the rectum, sigmoid colon, and distal descending colon, which are the segments of the large intestine that have the longest contact with fecal matter before it is expelled from the body. Most colorectal cancers arise from polyps (pol′ip; polys = many), which are outgrowths from the colon mucosa. Note, however, that colon polyps are very common, and most of them never become cancerous. Low-fiber diets have also been implicated in increasing the risk of colon cancer, because decreased dietary fiber leads to decreased stool (section of fecal matter) bulk and longer time for stools to remain in the large intestine, thus theoretically exposing the large intestine mucosa to toxins in the stools for longer periods of time. Other risk factors include a family history of colorectal cancer, personal history of ulcerative colitis, and older age (since most patients are over the age of 40). should see their doctor if they experience rectal bleeding or any persistent change in bowel habits. By age 50 (or earlier, if you have symptoms or a family history of colorectal cancer), individuals should take advantage of the following screening methods: 1. Take a yearly fecal occult (ŏ-kŭlt′, ok′ŭlt) blood test, which checks for the presence of blood in the stools. 2. Every 5 years, have a sigmoidoscopy (sig′moy-dos′kŏ-pē). In this procedure, which is done in the doctor’s office, an endoscope is inserted into the anus, rectum, and sigmoid colon to check for polyps or cancer. 3. Every 10 years, undergo a colonoscopy (kō-lon-os′kō-pē; skopeo = to view). A colonoscopy is more extensive than a sigmoidoscopy. The endoscope is inserted through the anus and into the large intestine at least up to the right colic flexure of the colon, and sometimes as far proximally as the ileocecal valve. In addition, people are advised to eat a high-fiber diet to reduce the risk of developing colorectal cancer. Polyps Colon cancer Initially, most patients are asymptomatic. Later, they may notice rectal bleeding (often evidenced as blood in the stool or on the toilet paper) and a persistent change in bowel habits (typically constipation). For some, the bleeding may not be noticeable, but can be detected in a stool sample. Eventually, the person may experience abdominal pain, fatigue, unexplained weight loss, and anemia. A cancerous growth in the colon must be removed surgically, and sometimes radiation and/or chemotherapy are used as well. Colorectal cancers that are limited to the mucosa have a 5-year survival rate, but the prognosis is poor for cancers that have spread into deeper colon wall tunics or metastasized to the lymph nodes. The key to an increased survival rate for colorectal cancer is early detection. If caught early, colorectal cancer is very treatable. People the muscularis mucosae. The glands’ goblet cells secrete mucin to lubricate the undigested material as it passes through, and the simple columnar epithelial cells continue to absorb nutrients that were not absorbed during passage through the small intestine. Many lymphatic nodules and lymphatic cells occupy the lamina propria of the large intestine. The muscularis of the colon and cecum has two layers of smooth muscle, but the outer longitudinal layer does not completely surround the colon and cecum. Instead, these longitudinal smooth muscle fibers form three thin, distinct, longitudinal bundles called teniae (tē′nē-ē; ribbons, band) coli (kō′lı̄). mck78097_ch26_779-816.indd 802 Polyps in the large intestine sometimes lead to colorectal cancer. The teniae coli act like elastic in a waistband—they help bunch up the large intestine into many sacs, collectively called haustra (haw′stră; sing., haustrum; haustus = to drink up) (see figure 26.16a). Hanging off the external surface of the haustra are lobules of fat called omental appendices, or epiploic (ep′i-plō′ik; membrane-covered) appendages. 26.9c Control of Large Intestine Activity Large intestine movements are regulated by local reflexes in the autonomic nervous system. With the ingestion of more food, peristaltic movements in the ileum increase, as does the frequency 2/25/11 2:22 PM Chapter Twenty-Six CLINICAL VIEW Appendicitis Diverticulosis and Diverticulitis Inflammation of the appendix is called appendicitis (ă-pendi-sı¯′tis). Most cases of appendicitis occur because fecal matter obstructs the appendix, although sometimes an appendix becomes inflamed without any obstruction. As the tissue in its wall becomes inflamed, the appendix swells, the blood supply is compromised, and bacteria may proliferate in the wall. Untreated, the appendix may burst and spew its contents into the peritoneum, causing a massive infection called peritonitis, and possibly leading to death. Diverticulosis is the presence of out-pocketings of the intestinal wall known as diverticula. Diverticula are acquired protrusions of the mucosa through the colonic wall that have evaginated through the submucosa and a weakened and/or diminished muscularis mucosa. Diverticula occur in weakened areas in the bowel wall where the nutrient vessels pierce the muscularis near the teniae coli and the omental appendices. These defects can become weaker with age. Diverticula are most common in the sigmoid colon (95%) but may be concentrated along the course of the teniae coli for the entire length of the colon. As the inflammation worsens and the parietal peritoneum becomes inflamed as well, the pain becomes sharp and localized to the right lower quadrant of the abdomen. Individuals with appendicitis typically experience nausea or vomiting, abdominal tenderness in the inferior right quadrant, a low fever, and an elevated leukocyte count. An inflamed appendix is surgically removed in a procedure called an appendectomy. of the opening of the ileocecal valve. This so-called gastroileal reflex results in the accumulation of more chyme in the cecum and ascending colon. Segmentation movements are infrequent in the colon. Instead, three types of movements are typically associated with the passage of digested material through the large intestine, the absorption of fluid and ions, and the packaging of waste materials for defecation: peristaltic movements, haustral churning, and mass movement. Peristaltic movements of the large intestine are usually weak and sluggish, but otherwise they resemble those that occur in the wall of the small intestine. Haustral churning occurs after a relaxed haustrum fills with digested or fecal material until its distension stimulates reflex contractions in the muscularis, causing churning and movement of the material to more distal haustra. Mass movements are powerful, peristaltic-like contractions involving the teniae coli that propel fecal material toward the rectum. Generally, mass movements occur two or three times a day, often during or immediately after a meal. This is the gastrocolic (gas′trō-kol′ik) reflex. W H AT D I D Y O U L E A R N? 14 ● Identify the retroperitoneal and intraperitoneal large intestine structures. What specific movements and reflexes propel material through the large intestine? mck78097_ch26_779-816.indd 803 803 CLINICAL VIEW During the early stages of acute appendicitis, the smooth muscle wall contracts and goes into spasms. Because this smooth muscle is innervated by the autonomic nervous system, pain is referred to the T10 dermatome around the umbilicus. 13 ● Digestive System The exact etiology of diverticulosis is poorly understood; however, high intraluminal pressures caused by straining in people with motility problems or constipation may be a significant factor. Most scientists agree that a low-fiber diet is an underlying cause of diverticulosis. Most individuals with diverticulosis are asymptomatic, without evidence of complications. Complications of diverticulosis include bleeding, peridiverticular abscess, perforation, stricture, and fistula formation. If some of the diverticula become infected or inflamed, the condition is known as diverticulitis. Diverticulitis occurs in only about 10-20% of patients with diverticulosis. Treatment for diverticulitis requires the use of a special diet, antibiotics, and occasionally surgery. (a) (b) Diverticulosis (a) An external view of the sigmoid colon showing diverticula. (b) An endoscopic view of diverticula. 26.10 Accessory Digestive Organs Learning Objectives: 1. Identify liver anatomy and blood supply. 2. Describe bile secretion. 3. Explain the gross anatomy and microanatomy of the pancreas. 4. Compare and contrast pancreatic acinar cell function. 5. Trace and describe how secretory products travel through the biliary apparatus. 2/25/11 2:22 PM 804 Chapter Twenty-Six Right lobe Inferior vena cava Digestive System Anterior Left lobe Quadrate lobe Round ligament of liver Gallbladder Porta hepatis Hepatic artery proper Common hepatic duct Hepatic portal vein Cystic duct Right lobe Falciform ligament Round ligament of liver Left lobe Inferior vena cava Coronary ligament Gallbladder Ligamentum venosum Bare area Caudate lobe Posterior (a) Anterior view (b) Posteroinferior view Figure 26.18 Gross Anatomy of the Liver. The liver is in the upper right quadrant of the abdomen. (a) Anterior and (b) posteroinferior views show the four lobes of the liver, as well as the gallbladder and the porta hepatis. The accessory digestive organs produce secretions that facilitate the chemical digestive activities of GI tract organs. Important accessory digestive organs are the liver, the gallbladder, and the pancreas. 26.10a Liver The liver (liv′er) lies in the right upper quadrant of the abdomen, immediately inferior to the diaphragm. Weighing 1 to 2 kilograms (2.25 to 4.5 pounds), it constitutes approximately 2% of an adult’s body weight. The liver is covered by a connective tissue capsule and a layer of visceral peritoneum, except for a small region on its diaphragmatic surface called the bare area. Gross Anatomy The liver is composed of four incompletely separated lobes and supported by two ligaments (figure 26.18). The major lobes are the right lobe and the left lobe. The right lobe is separated from the smaller left lobe by the falciform ligament, a peritoneal fold that secures the liver to the anterior abdominal wall. In the inferior free edge of the falciform ligament lies the round ligament of the liver (or ligamentum teres), which represents the remnant of the fetal umbilical vein. Subdivisions of the right lobe include the caudate (kaw′dāt; cauda = tail) lobe and the quadrate (kwah′drāt; quadratus = square) lobe. The caudate lobe is adjacent to the inferior vena cava, and the quadrate lobe is adjacent to the gallbladder (figure 26.18b). Along the inferior surface of the liver are several structures that collectively resemble the letter H. The gallbladder and the round ligament of the liver form the vertical superior parts of the H; the inferior vena cava and the ligamentum venosum form the vertical inferior parts. (Recall from chapter 23 that the ligamentum venosum was a remnant of the ductus venosus in the mck78097_ch26_779-816.indd 804 embryo. This vessel shunted blood from the umbilical vein to the inferior vena cava.) Finally, the porta (pōr′tă; gate) hepatis (hep′ătis) represents the horizontal crossbar of the H and is where blood and lymph vessels, bile ducts, and nerves enter and leave the liver. In particular, the hepatic portal vein and branches of the hepatic artery proper enter at the porta hepatis. Histology A connective tissue capsule branches through the liver and forms septa that partition the liver into thousands of small, polyhedral hepatic lobules, which are the classic structural and functional units of the liver (figure 26.19). Within hepatic lobules are liver cells called hepatocytes (hep-a′tō-sı̄t). At the periphery of each lobule are several portal triads, composed of branches of the hepatic portal vein, the hepatic artery, and the bile duct. A dual blood supply serves the liver. The hepatic portal vein carries blood from the capillary beds of the GI tract, spleen, and pancreas. It brings approximately 75% of the blood volume to the liver. This blood is rich in nutrients and other absorbed substances but relatively poor in oxygen. The hepatic artery proper, a branch of the celiac trunk, splits into left and right hepatic arteries. These arteries carry well-oxygenated blood to the liver. Blood from branches of the hepatic arteries and hepatic portal vein mixes as it passes to and through the hepatic lobules. At the center of each lobule is a central vein that drains the blood from the lobule. Central veins collect venous blood and merge throughout the liver to form numerous hepatic veins that eventually empty into the inferior vena cava. In cross section, a hepatic lobule looks like a side view of a bicycle wheel. The hub of the wheel is represented by the central vein. At the circumference of the wheel where the tire would be are 2/25/11 2:22 PM Chapter Twenty-Six Digestive System 805 Hepatic sinusoid Central vein Hepatocytes Bile canaliculi Hepatic lobule Reticuloendothelial cell Central vein Hepatic sinusoid Bile canaliculi Hepatocyte Portal triad Branch of bile duct (a) Hepatic lobules Branch of hepatic portal vein Branch of hepatic artery Portal triad Branch of bile duct Hepatic sinusoid Branch of hepatic portal vein Hepatocytes Branch of hepatic artery (b) Hepatocytes and sinusoids LM 220x (c) Portal triad Figure 26.19 Histology of the Liver. (a) The functional units of the liver are called hepatic lobules. (b) A central vein projects through the center of a hepatic lobule, and several portal triads define its periphery. (c) A photomicrograph depicts the portal triad and hepatocytes. several portal triads that are usually equidistant apart. The numerous spokes of the wheel are the hepatic sinusoids (si′nū-soyd; sinus = cavity), which are bordered by cords of hepatocytes. Hepatic sinusoids are thin-walled, porous or “leaky” capillaries where venous and arterial blood are mixed and then flow slowly through the hepatic lobule toward the central vein. The sinusoids are lined with stellate cells called reticuloendothelial cells (or Kupffer cells), which are phagocytic cells that have an immune function. Hepatocytes absorb nutrients from the sinusoids, and they also produce bile, a greenish fluid that breaks down fats to assist in their chemical digestion. Between each cord of hepatocytes is a small bile canaliculus. Bile canaliculi conduct bile from the hepatocytes to the bile duct in the portal triad. Liver Function The liver has numerous functions. Besides producing bile, hepatocytes detoxify drugs, metabolites, and poisons. Hepatocytes mck78097_ch26_779-816.indd 805 also store excess nutrients and vitamins and release them when they are needed. Finally, hepatocytes synthesize blood plasma proteins such as albumins, globulins, and proteins required for blood clotting. Reticuloendothelial cells in the liver sinusoids phagocytize debris in the blood as well as help break down and recycle components of aged erythrocytes and damaged or wornout formed elements. 26.10b Gallbladder Attached to the inferior surface of the liver, a saclike organ called the gallbladder (see figure 26.18) concentrates bile produced by the liver and stores this concentrate until it is needed for digestion. The cystic (sis′tik; cysto = bladder) duct connects the gallbladder to the common bile duct. The gallbladder can hold approximately 40 to 60 milliliters of concentrated bile. The gallbladder has three regions: the neck, body, and fundus (see figure 26.21). At the neck of the gallbladder, a sphincter valve controls the flow of bile into 2/25/11 2:22 PM 806 Chapter Twenty-Six Digestive System CLINICAL VIEW Cirrhosis of the Liver Chronic injury to the liver inevitably leads to liver cirrhosis (sir-rō′sis; kirrhos = yellow). Liver cirrhosis results when hepatocytes have been destroyed and are replaced by fibrous scar tissue. This scar tissue often surrounds isolated nodules of regenerating hepatocytes. The fibrous scar tissue also compresses the blood vessels and bile ducts in the liver, leading to hepatic portal hypertension (high blood pressure in the hepatic portal venous system) and bile flow impediments. Liver cirrhosis is caused by chronic injury to the hepatocytes, as may result from chronic alcoholism, liver disease, or certain drugs or toxins. Chronic hepatitis (hep-ǎ-tı̄′tis) is a long-term inflammation of the liver that leads to necrosis of liver tissue. Most frequently, viral infections from either hepatitis B or hepatitis C produce chronic hepatitis. Other disorders that result in liver cirrhosis include some inherited diseases, chronic biliary obstruction, and biliary cirrhosis. Early stages of liver cirrhosis may be asymptomatic. However, once liver function begins to falter, the patient complains of fatigue, weight loss, and nausea, and may have pain in the right hypochondriac region. During a physical, the doctor may palpate an abnormally small and hard liver. To confirm the diagnosis, a liver biopsy is done by obtaining a small portion of liver tissue through a needle passed into the liver and then examining the cells. The fibrosis and scarring of liver cirrhosis are irreversible. However, further scarring can be slowed or prevented by treating the cause of the cirrhosis (hepatitis, alcoholism, etc.). Advanced liver cirrhosis may have a variety of complications: ■ ■ ■ ■ ■ Jaundice (yellowing of the skin and sclerae of the eyes) occurs when the liver can’t eliminate enough bilirubin. (Bilirubin is a yellowish product formed when aged erythrocytes are broken down. See chapter 21 for further information.) It is accompanied by darkening of the urine. Edema (accumulation of fluid in body tissues) and ascites (ā-sı¯′tēz) (fluid accumulation in the abdomen) develop because of decreased albumin production. Intense itching occurs when bile products are deposited in the skin. Toxins in the blood and brain accumulate because the liver cannot effectively process them. Hepatic portal hypertension can lead to dilated veins of the inferior esophagus (esophageal varices). End-stage liver cirrhosis can be treated only with a liver transplant. Otherwise, death results either from progressive liver failure or from the complications. In the United States, over 25,000 people die each year of liver cirrhosis. Fibrous scar tissue LM 100x (a) Nodular cirrhosis of the liver (b) Histology of liver cirrhosis Normal hepatocytes (a) This gross specimen depicts a type of nodular cirrhosis of the liver. (b) A photomicrograph shows how fibrous scar tissue infiltrates and replaces hepatocytes. and out of the gallbladder. The gallbladder has three tunics: an inner mucosa, a middle muscularis, and an external serosa. Folds in the mucosa permit distension of the wall as the gallbladder fills with bile. mck78097_ch26_779-816.indd 806 W H AT D O Y O U T H I N K ? 5 ● If your gallbladder were surgically removed, how would this affect your digestion of fatty meals? What diet alterations might you have to make after this surgery? 2/25/11 2:22 PM Chapter Twenty-Six Digestive System 807 Body of pancreas Main pancreatic duct Common bile duct Tail of pancreas Duodenum Accessory pancreatic duct Duodenojejunal flexure Hepatopancreatic ampulla Pancreatic acini Major duodenal papilla Pancreatic islet Jejunum Head of pancreas (a) Duodenum and pancreas, anterior view LM 75x Acinar cell LM 200x Pancreatic acinus (b) Histology of pancreas Figure 26.20 Anatomy and Histology of the Pancreas. (a) The components of the pancreas are shown in relationship to the pancreatic duct and the duodenum. (b) Photomicrographs depict the histology of acinar cells within the pancreas. 26.10c Pancreas The pancreas is referred to as a mixed gland because it exhibits both endocrine and exocrine functions. The endocrine functions are performed by the cells of the pancreatic islets (see chapter 20). Exocrine activity results in the secretion of digestive enzymes and bicarbonate, collectively called pancreatic juice, into the duodenum. The pancreas is a retroperitoneal organ that extends horizontally from the medial edge of the duodenum toward the left side of the abdominal cavity, where it touches the spleen. It exhibits a wide head adjacent to the curvature of the duodenum, a central, elongated body projecting toward the left lateral abdominal wall, and a tail that tapers as it approaches the spleen (figure 26.20). The pancreas contains modified simple cuboidal epithelial cells called acinar cells. These cells, which are organized into large clusters termed acini (sing., acinus), or lobules, secrete mck78097_ch26_779-816.indd 807 the mucin and digestive enzymes of the pancreatic juice. The simple cuboidal epithelial cells lining the pancreatic ducts secrete bicarbonate (alkaline fluid) to help neutralize the acidic chyme arriving in the duodenum from the stomach. Most of the pancreatic juice travels through ducts that merge to form the main pancreatic duct, which drains into the major duodenal papilla in the duodenum. A smaller accessory pancreatic duct drains a small amount of pancreatic juice into a minor duodenal papilla in the duodenum. Both hormonal and neural mechanisms control pancreatic juice secretions. Enteroendocrine cells in intestinal glands release cholecystokinin to promote the secretion of pancreatic juices from acinar cells, and secretin to stimulate the release of alkaline fluid from pancreatic duct cells. Pancreatic juice secretion is also stimulated by parasympathetic (vagus nerve) activity, while sympathetic activity inhibits pancreatic juice secretion. 2/25/11 2:22 PM 808 Chapter Twenty-Six Digestive System CLINICAL VIEW Gallstones (Cholelithiasis) High concentration of certain materials in the bile can lead to the eventual formation of gallstones. Gallstones occur twice as frequently in women as in men, and are more prevalent in developed countries. Obesity, increasing age, female sex hormones, Caucasian ethnicity, and lack of physical activity are all risk factors for developing gallstones. The term cholelithiasis (kō′lē-li-thı¯′ă-sis; chole = bile, lithos = stone, iasis = condition) refers to the presence of gallstones in either the gallbladder or the biliary apparatus. Gallstones are typically formed from condensations of either cholesterol or calcium and bile salts. These stones can vary from the tiniest grains to structures almost the size of golf balls. The majority of gallstones are asymptomatic until a gallstone becomes lodged in the neck of the cystic duct, causing the gallbladder to become inflamed (cholecystitis) and dilated. The most common symptom is severe pain (called biliary colic) perceived in the right hypochondriac region or sometimes in the area of the right shoulder. Nausea and vomiting may occur, along with indigestion and bloating. Symptoms are typically worse after eating a fatty meal. Treatment consists of surgical removal of the gallbladder, called cholecystectomy (kō′lē-sis-tek′tō-mĕ; kystis = bladder, ektome = excision). 26.10d Biliary Apparatus The biliary (bil′ē-ā r-ē; bilis = bile) apparatus is a network of thin ducts that carry bile from the liver and gallbladder to the duodenum (figure 26.21). The left and right lobes of the liver drain bile into the left and right hepatic ducts, respectively. The left and right hepatic ducts merge to form a single common hepatic duct. The cystic duct is attached to the common hepatic duct and carries bile to and from the gallbladder. The cystic duct allows bile to enter and to leave the gallbladder; the direction of flow is controlled by hormonal influences. Bile travels from the common hepatic duct through the cystic duct to be stored in the gallbladder; stored bile travels back through the cystic duct for conduction to the small intestine. The union of the cystic duct and the common hepatic duct forms the common bile duct that extends inferiorly to the duodenum. The hepatopancreatic ampulla is a posteriorly placed swelling on the duodenal wall where the common bile duct and main pancreatic duct merge and pierce the duodenal wall. Bile and pancreatic juice mix in the hepatopancreatic ampulla prior to emptying into the duodenum via the major duodenal papilla. W H AT D I D Y O U L E A R N? 15 ● 16 ● 17 ● Following surgery, the liver continues to produce bile, even in the absence of the gallbladder, but there is no means of concentrating the bile, so further gallstones are unlikely. Photo of gallstones in a gallbladder. Left and right hepatic ducts 1 Cystic duct Common hepatic duct Neck 2 Common bile duct Gallbladder Body Fundus Main pancreatic duct Hepatopancreatic ampulla Major duodenal papilla 3 4 Duodenum Identify the structural components of a portal triad. What is the function of the gallbladder? What is the function of pancreatic acini? Figure 26.21 Biliary Apparatus. Bile flows through the biliary apparatus (green), and pancreatic juice flows through the main pancreatic duct until the two vessels merge at the hepatopancreatic ampulla. Numbers indicate the pathway sequence for bile and pancreatic juice. mck78097_ch26_779-816.indd 808 1 Left and right hepatic ducts merge to form a common hepatic duct. 2 Common hepatic and cystic ducts merge to form a common bile duct. 3 Pancreatic duct merges with common bile duct at the hepatopancreatic ampulla. 4 Bile and pancreatic juices enter duodenum at the major duodenal papilla. 2/25/11 2:22 PM Chapter Twenty-Six Digestive System 809 CLINICAL VIEW Intestinal Disorders Celiac disease (also known as celiac sprue or gluten-sensitive enteropathy) is an autoimmune disorder of the small intestine. This disease runs in families and may be triggered after surgery, pregnancy, or a viral infection. Affected individuals cannot tolerate a protein called gluten, which is found in all forms of wheat, rye, and barley. When an individual with celiac disease ingests food containing gluten, the body’s immune cells attack the small intestine mucosa and damage the villi, severely impairing nutrient absorption. Symptoms of celiac disease vary, but may include gas and bloating, diarrhea, fatigue, and weight loss (due to malnutrition). Because these symptoms are similar to those of other bowel disorders, diagnosis may be difficult. The only treatment for celiac disease is to follow a gluten-free diet, but this is challenging because many commercially prepared foods use gluten as a preservative or stabilizer. Thus, the patient with celiac disease must carefully read all ingredient lists on prepared foods. tissue can lead to bowel obstruction. Other patients develop selective malabsorption of certain vitamins. The age distribution and symptoms of ulcerative colitis are similar to those of Crohn disease, but ulcerative colitis involves only the large intestine. The rectum and descending colon are the first to show signs of inflammation and are generally the most severely affected. Also, in ulcerative colitis the inflammation is confined to the mucosa, instead of the full thickness of the intestinal wall. Finally, unlike Crohn disease, ulcerative colitis is associated with a profoundly increased risk of colon cancer. Historically, patients who have had ulcerative colitis for more than 10 years develop colon cancer at a frequency 20 or 30 times that seen in the rest of the population. A section of large intestine showing the signs of ulcerative colitis. A section of small intestine showing the signs of Crohn disease. The term inflammatory bowel disease (IBD) applies to two autoimmune disorders, Crohn disease and ulcerative colitis. In both of these disorders, selective regions of the intestine become inflamed. Crohn disease is a condition of young adults characterized by intermittent and relapsing episodes of intense abdominal cramping and diarrhea. Although any region of the gastrointestinal tract, from esophagus to anus, may be involved, the distal ileum is the most frequently and severely affected site. Inflammation involves the entire thickness of the intestinal wall, extending from the mucosa to the serosa. For reasons that are not clear, lengthy regions of the intestine having no trace of injury or inflammation may be followed abruptly by several inches of markedly diseased intestine. In some patients scarring of the mck78097_ch26_779-816.indd 809 Treatment of either Crohn disease or ulcerative colitis is complex. Anti-inflammatory drugs, as well as stress reduction and possibly nutritional supplementation, help control symptoms. Surgery may be necessary in both forms of inflammatory bowel disease. Crohn disease, ulcerative colitis, and celiac disease are distinctly different from a much more common disorder called irritable bowel syndrome. Irritable bowel syndrome is characterized by abnormal function of the colon with symptoms of crampy abdominal pain, bloating, constipation, and/or diarrhea. Irritable bowel syndrome occurs in approximately one in every five people in the United States, and is more common in women than men. Irritable bowel syndrome may be diagnosed if a medical evaluation has ruled out Crohn disease and ulcerative colitis. Although neither a cause nor a cure for irritable bowel syndrome is known, most people can control their symptoms by reducing stress, changing their diet, and using certain medicines. 2/25/11 2:22 PM 810 Chapter Twenty-Six Digestive System 26.11 Aging and the Digestive System Learning Objective: 1. Explain how the digestive system changes as we age. Age-related changes in digestion system function usually progress slowly and gradually. Overall, reduced secretions accompany normal aging. Mucin secretion decreases, reducing the thickness and amount of mucus in the layer that protects the GI tract and making internal damage to digestive organs more likely. Decreased secretion of enzymes and acid results in diminished effectiveness of chemical digestion, which in turn can diminish nutrient absorption. The reduction of glandular secretion is compounded by decreased replacement of epithelial cells. Another change that occurs with age is a reduction in the thickness of the smooth muscle layers in the muscularis. Consequently, both muscular tone and GI tract motility may be lowered significantly. Additionally, smooth muscle ensheathing the GI tract may adversely affect the function of sphincters or valves that help move materials through the GI tract. As a result, significant changes in nutrient absorption, as well as gastroesophageal reflux disease (GERD), may occur. Improper dental care or sometimes simply aging can result in either the loss of teeth or periodontal disease. Both conditions adversely affect normal eating habits and nutrition. As food intake dwindles, changes occur in GI tract motility, secretion, and absorption. Marked dietary alterations may also increase the chance of developing cancer in either the stomach or the intestines. Finally, a direct result of aging is a reduction in olfactory and gustatory sensations. When a person loses the ability to smell or taste food, his or her dietary intake usually suffers. W H AT D I D Y O U L E A R N? 18 ● How might the decrease in mucin secretion due to aging affect GI tract organs? 26.12 Development of the Digestive System Learning Objective: 1. Identify and describe the major events in digestive system development. At the end of the third week of development, the disc-shaped embryo undergoes lateral folding, which transforms the endoderm and some lateral plate mesoderm into a cylindrical primitive gut tube. The gut tube eventually connects to the primitive mouth and the developing anus, while still maintaining a connection to the yolk sac via a thin stalk called the vitelline (vı̄-tel′in, -ēn) duct. A double-layered membrane called dorsal mesentery wraps around the gut tube and helps anchor it to the posterior body wall. A portion of the gut tube called the abdominal foregut has ventral mesentery that attaches it to the anterior body wall. The gut tube may be divided into three components: a superior or cephalic foregut, a centrally located midgut, and an inferior mck78097_ch26_779-816.indd 810 or caudal hindgut (figure 26.22a). Table 26.5 lists the structures derived from the foregut, midgut, and hindgut. Portions of the gut tube expand to form some of the abdominal organs. Some accessory digestive system organs develop as buds, or outgrowths, from the gut tube. Many of these structures rotate or shift before assuming their final positions in the body. 26.12a Stomach, Duodenum, and Omenta Development By the fourth week of development, a portion of the foregut expands and forms a spindle-shaped dilation that will become the stomach. Between the fifth and seventh weeks, the posterior (dorsal) part of this dilation grows faster than the anterior (ventral) part, so the anterior wall becomes the concave lesser curvature, and the posterior wall becomes the convex greater curvature (figure 26.22b). The dorsal mesentery that attaches to the stomach becomes thinner and stretches out from the greater curvature. This mesentery will form the greater omentum, while the ventral mesentery that attaches to the lesser curvature will form the lesser omentum. During the seventh week, the developing stomach rotates 90 degrees clockwise about a longitudinal axis, as viewed from the superior aspect of the organ (figure 26.22c). Thus, the posterior side of the stomach (the greater curvature) rotates so that it faces the left side of the body, while the anterior side (the lesser curvature) rotates and faces the right side. One week later, the stomach and duodenum rotate about an anterior-posterior axis. This rotation pulls the pyloric region of the stomach and the duodenum superiorly while moving the fundus and the cardia of the stomach slightly inferiorly (figure 26.22d). 26.12b Liver, Gallbladder, and Pancreas Development The liver parenchyma, gallbladder, pancreas, and biliary apparatus develop from buds or outgrowths from the endoderm of the duodenum. (The liver stroma or connective tissue framework is formed from nearby mesoderm.) The liver bud appears first, during the third week, develops within the ventral mesentery, and grows superiorly toward the developing diaphragm. By the fourth week, the liver continues to grow, and its attachment to the duodenum becomes a thinned tube that forms the common bile duct. An outgrowth forms off the bile duct that becomes the gallbladder. The connection to the gallbladder and bile duct thins and becomes the cystic duct. As the liver enlarges, the ventral mesentery thins and becomes the falciform ligament. The pancreas starts to form at week 4 from two separate outgrowths of the bile duct, called the ventral pancreatic bud and the dorsal pancreatic bud. By week 6, the ventral pancreatic bud and the biliary apparatus rotate behind the duodenum. The ventral and dorsal pancreatic buds fuse to form the pancreas. Due to this rotation, much of the biliary apparatus now lies posterior to the duodenum and drains into the major duodenal papilla. 26.12c Intestine Development The small and large intestines are formed from the midgut and hindgut, as shown in figure 26.23. During the fifth week, the midgut rapidly elongates and forms a primary intestinal loop. The cranial portion of the loop forms the jejunum and most of the ileum, while the caudal portion of the loop forms the very distal 2/25/11 2:22 PM Chapter Twenty-Six Ventral mesentery Digestive System 811 Esophagus Dorsal mesentery Gallbladder (cystic bud) Liver Lesser curvature of stomach Falciform ligament Liver buds Greater curvature of stomach Dorsal pancreatic bud Ventral pancreatic bud Foregut Cystic bud Vitelline duct Dorsal pancreatic bud Ventral pancreatic bud Vitelline duct Primary intestinal loop Midgut Hindgut (a) Week 4: Liver, gallbladder, and pancreatic buds develop (b) Week 5: Greater and lesser curvatures of stomach form Direction of stomach rotation Gallbladder Falciform ligament Esophagus Falciform ligament Liver Liver Greater omentum Stomach Lesser omentum Gallbladder Direction of duodenum rotation Ventral pancreatic bud Direction of pancreas, bile duct rotation Common bile duct Pancreas Dorsal pancreatic bud (c) Weeks 6–7: Rotation of stomach, pancreatic buds Duodenum (d) Week 8: Postnatal position of organs attained Figure 26.22 Development of the Digestive System Foregut. (a) The foregut of the digestive system begins to develop from the primitive gut during week 4 of development. Stages of foregut development continue at (b) week 5, (c) weeks 6–7, and (d) week 8. Table 26.5 Anatomic Derivatives of the Primitive Gut Tube Organs Formed Foregut Midgut Hindgut Digestive organs Pharynx, esophagus, stomach, proximal half of duodenum Distal half of duodenum; jejunum, ileum, cecum, appendix, ascending colon, proximal (right) two-thirds of transverse colon Distal (left) one-third of transverse colon; descending colon, sigmoid colon, rectum, superior portion of anal canal Accessory digestive organs Most of liver; gallbladder, biliary apparatus, pancreas Other organs Lungs mck78097_ch26_779-816.indd 811 Urinary bladder epithelium, urethra epithelium 2/25/11 2:22 PM 812 Chapter Twenty-Six Digestive System Ventral mesentery Liver Dorsal mesentery Vitelline duct Stomach Ventral mesentery Descending abdominal aorta Primary intestinal loop (midgut) Caudal loop Dorsal mesentery Cranial loop Caudal loop Cranial loop Hindgut 90 counterclockwise rotation Superior mesenteric artery (a) Week 5: Primary intestinal loop forms (b) Week 6: Herniation of loop; 90 counterclockwise rotation Caudal loop (forms much of large intestine) Duodenum Transverse colon Direction of rotation 180 counterclockwise Colon Vitelline duct Cranial loop (forms most of small intestine) Small intestine Ascending colon Descending colon (c) Weeks 10–11: Retraction of intestines back into abdominal cavity; 180 counterclockwise rotation Cecum Sigmoid colon Appendix Rectum (d) Postnatal position Figure 26.23 Development of the Digestive System Midgut. The midgut of the digestive system forms almost all of the small intestine and the proximal region of the large intestine. Development of these structures is shown at (a) week 5, (b) week 6, and (c) weeks 10–11. (d) The postnatal position of the digestive organs. part of the ileum and much of the large intestine. The loop apex connects to the yolk sac via the vitelline duct. Due to rapid growth of the liver and the resulting limited space in the developing abdominal cavity, the primary intestinal loop herniates into the umbilicus during the sixth week. As this loop herniates, it undergoes a 90-degree counterclockwise rotation, as viewed from the anterior surface of the body. The cranial loop rotates to the right of the body, while the caudal loop rotates to the left of the body. For the next several weeks, the intestinal loop grows and expands in the umbilicus since there is no available space inside the abdominal cavity. By weeks 10 and 11, the midgut retracts into the abdomen as the abdominal cavity becomes spacious enough to house all the intestines. As the midgut retracts, it undergoes another 180-degree mck78097_ch26_779-816.indd 812 counterclockwise rotation, as viewed from the front of the body. Now the cranial loop is on the left side of the body, and the caudal loop is on the right. Thus, the net intestinal rotation is 270 degrees counterclockwise. The cranial loop (forming the jejunum and most of the ileum) retracts and positions itself in the left side of the abdominal cavity. The caudal loop (forming the distal ileum and the proximal part of the large intestine) is pulled to the right side of the abdominal cavity. The vitelline duct begins to regress and disappears before the baby is born. W H AT D I D Y O U L E A R N? 19 ● What structures develop from endoderm outgrowths of the duodenum? 2/25/11 2:23 PM Chapter Twenty-Six Digestive System 813 CLINICAL TERMS bariatric surgery (gastric bypass) Surgically narrowing or blocking off a large portion of the stomach; typically performed in severely obese people who have had trouble losing weight by more traditional methods. caries (kār ′ez; dry rot) Cavities in the tooth enamel caused by plaque and bacterial buildup on the tooth. colostomy A surgical procedure that moves and attaches a portion of the colon so it exits through the abdominal wall. Materials moving through the large intestine drain into a bag (ostomy pouch) attached to the abdomen. diarrhea (dı̄-ă-rē′ă; dia = through, rhoia = a flow) Frequent, watery stools. dysentery (dis-en-tēr-ē; dys = bad, entera = bowels) Painful, bloody diarrhea due to an infectious agent. gastroenteritis (gas′trō-en-ter-ı̄′tis) Inflammation and irritation of the GI tract, often associated with vomiting and diarrhea. hemorrhoids (hem′ŏ-roydz; rhoia = flow) Dilated, tortuous veins around the rectum and/or anus. intussusception (in-tŭs-sŭ-sep′shŭn; intus = within, suscipio = to take up) Type of intestinal obstruction in which one part of the intestine constricts and gets pulled into the immediately distal segment of intestine; most commonly seen near the ileocecal junction. Symptoms include bloody stools and severe abdominal pain. mumps (a lump) Viral infection of the parotid glands, resulting in painful swelling of the glands. pancreatitis (pan′krē-ă-tı̄′tis) Inflammation of the pancreas; most often caused by alcoholism or biliary apparatus disease (including gallstones). volvulus Twisting or torsion of an intestinal segment around itself. If left untreated, obstruction of digestive materials results, and the segment necroses due to poor blood supply. Symptoms include intense abdominal pain and vomiting. Chapter Summary 26.1 General Structure and Functions of the Digestive System 780 26.2 Oral Cavity 781 ■ The digestive system breaks down ingested food into usable nutrients. ■ The digestive organs are the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive organs are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. 26.1a Digestive System Functions ■ Mechanical digestion is the physical breakdown of ingested materials, and chemical digestion is the enzymatic breakdown of molecules. ■ Propulsion moves materials through the GI tract. Peristalsis moves materials from the mouth toward the anus; segmentation mixes ingested materials. ■ Secretion is the production and release of fluids; absorption is the movement or transport of materials across the wall of the GI tract. ■ The oral cavity is the entryway into the GI tract. 26.2a Cheeks, Lips, and Palate The cheeks form the lateral walls of the oral cavity, and the lips frame the anterior opening formed by the orbicularis oris muscle. ■ The palate is the oral cavity roof: The hard palate is the anterior, bony portion, and the soft palate is the posterior, muscular portion. ■ 782 The tongue moves and mixes ingested materials; its dorsal surface has papillae. 26.2c Salivary Glands ■ 782 Saliva is a fluid secreted by three pairs of multicellular salivary glands. It moistens food and provides lubrication. 26.2d Teeth 26.4 General Arrangement of Abdominal GI Organs 787 781 ■ 26.2b Tongue 26.3 Pharynx 786 780 784 ■ Adults have 32 permanent teeth of four types: incisors, canines, premolars, and molars. ■ Pharyngeal constrictors contract sequentially during swallowing. 26.4a Peritoneum, Peritoneal Cavity, and Mesentery 787 ■ Parietal peritoneum lines the internal body wall; visceral peritoneum covers abdominal organs. ■ Intraperitoneal organs are completely surrounded by visceral peritoneum; retroperitoneal organs are only partially ensheathed. ■ Mesenteries are double layers of peritoneum. 26.4b General Histology of GI Organs (Esophagus to Large Intestine) 788 ■ The mucosa is composed of epithelium, an underlying lamina propria, and the muscularis mucosae. ■ The submucosa is a dense irregular connective tissue layer containing blood vessels, lymph vessels, and nerves. ■ The muscularis usually has two smooth muscle layers: an inner circular layer and an outer longitudinal layer. ■ The adventitia is the outermost covering; when covered by visceral peritoneum, it is called a serosa. (continued on next page) mck78097_ch26_779-816.indd 813 2/25/11 2:23 PM 814 Chapter Twenty-Six Digestive System Chapter Summary (continued) 26.4c Blood Vessels, Lymphatic Structures, and Nerve Supply ■ 26.5 Esophagus 790 26.5a Gross Anatomy ■ 26.7 Stomach 793 791 The esophagus conducts swallowed materials from the pharynx to the stomach. 26.5b Histology 26.6 The Swallowing Process 792 791 ■ The esophagus is lined by nonkeratinized stratified squamous epithelium. ■ The muscularis has all skeletal fibers superiorly, changes to intermingled skeletal and smooth fibers in the middle, and contains all smooth fibers inferiorly. ■ Swallowing has three phases: The voluntary phase moves a bolus into the pharynx; the pharyngeal phase moves the bolus through the pharynx into the esophagus; and the esophageal phase conducts the bolus to the stomach. 26.7a Gross Anatomy 793 ■ Mechanical digestion and chemical digestion occur in the stomach. ■ The four stomach regions are the cardia, fundus, body, and pylorus. 26.7b Histology 793 ■ The stomach mucosa is composed of a simple columnar epithelium containing gastric pits and gastric glands. ■ The muscularis has three smooth muscle layers: inner oblique, middle circular, and outer longitudinal. 26.7c Gastric Secretions ■ 26.8 Small Intestine 797 794 Five types of secretory cells form the gastric glands: surface mucous cells (secrete mucin), mucous neck cells (secrete acidic mucin), parietal cells (secrete hydrochloric acid and intrinsic factor), chief cells (secrete pepsinogen), and enteroendocrine cells (secrete gastrin and other hormones). 26.8a Gross Anatomy and Regions 797 ■ The small intestine finishes chemical digestion and absorbs most of the nutrients. ■ The small intestine is divided into the duodenum, jejunum, and ileum. 26.8b Histology ■ 26.9 Large Intestine 799 799 The small intestine has circular folds with fingerlike villi projecting from them. The villi are lined with cells with microvilli on their apical surface. The circular folds, villi, and microvilli increase the surface area of the small intestine. 26.9a Gross Anatomy and Regions 799 ■ The large intestine absorbs fluids and ions, and compacts undigestible wastes. ■ The large intestine is composed of the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. 26.9b Histology 801 ■ The large intestine mucosa lacks villi, but contains intestinal glands. ■ The outer longitudinal layer of the muscularis forms three discrete bands called teniae coli. The teniae coli cause the wall of the colon to bunch into sacs called haustra. 26.9c Control of Large Intestine Activity ■ 26.10 Accessory Digestive Organs 803 802 Local autonomic reflexes regulate large intestine movements. 26.10a Liver 804 ■ The liver receives venous blood from the hepatic portal vein and oxygenated blood from the hepatic artery proper. ■ Liver functions are bile production, drug detoxification, storage of nutrients and glycogen, and synthesis of plasma proteins. 26.10b Gallbladder ■ ■ 805 The gallbladder stores and concentrates bile produced by the liver. 26.10c Pancreas 807 Pancreatic acini produce pancreatic juice that neutralizes acidic chyme. 26.10d Biliary Apparatus mck78097_ch26_779-816.indd 814 790 GI tract organs have extensive blood vessels, lymphatic structures, and nerves. 808 ■ The hepatic ducts drain bile into a single common hepatic duct, and the cystic duct merges with the common hepatic duct to form the common bile duct. ■ The common bile duct and the pancreatic duct merge to empty their contents into the duodenum via the major duodenal papilla. 2/25/11 2:23 PM Chapter Twenty-Six Digestive System 26.11 Aging and the Digestive System 810 ■ Age-related changes in the digestive system lead to reduced secretions and diminished absorption of nutrients. 26.12 Development of the Digestive System 810 ■ The gut tube has a superior foregut, a central midgut, and an inferior hindgut. 26.12a Stomach, Duodenum, and Omenta Development ■ 810 The liver parenchyma, gallbladder, and pancreas develop from endoderm outgrowths of the duodenum. 26.12c Intestine Development ■ 810 Stomach formation begins by week 4 of development. Differential growth rates cause the greater and lesser curvatures of the stomach to form. 26.12b Liver, Gallbladder, and Pancreas Development ■ 815 810 The small and large intestines form from the midgut and hindgut beginning in week 5. Challenge Yourself Matching Match each numbered item with the most closely related lettered item. ______ 1. circular folds ______ 2. falciform ligament ______ 3. pyloric sphincter ______ 4. haustra ______ 5. segmentation ______ 6. simple columnar ______ 7. muscularis ______ 8. stratified squamous ______ 9. Peyer patches ______ 10. submucosa a. typically contains two layers of smooth muscle b. epithelium lining small intestine c. contains dense irregular connective tissue and blood vessels d. lymphatic nodules in wall of ileum e. attaches liver to anterior abdominal wall f. restricts chyme entry into small intestine g. epithelium lining the esophagus h. sacs of large intestine wall i. increase surface area of small intestine j. process to mix digested materials in small intestine Multiple Choice Select the best answer from the four choices provided. ______ 1. Which organ is located in the right upper quadrant of the abdomen? a. liver b. spleen c. descending colon d. appendix mck78097_ch26_779-816.indd 815 ______ 2. The ______ cells of the stomach secrete hydrochloric acid (HCl). a. chief b. parietal c. mucous d. enteroendocrine ______ 3. Material leaving the ascending colon next enters the a. cecum. b. descending colon. c. sigmoid colon. d. transverse colon. ______ 4. Which of these organs is retroperitoneal? a. stomach b. transverse colon c. descending colon d. ileum ______ 5. Sympathetic innervation of the GI tract is responsible for a. closing the pyloric sphincter. b. stimulating peristalsis. c. stimulating secretion of the pancreatic acinar cells. d. vasodilating the major digestive system blood vessels. ______ 6. The ______ is derived from the cranial part of the primary intestinal loop. a. jejunum b. cecum c. ascending colon d. appendix ______ 7. The main pancreatic duct merges with the ______, and their contents empty into the duodenum through the major duodenal papilla. a. left hepatic duct b. common bile duct c. cystic duct d. common hepatic duct 2/25/11 2:23 PM 816 Chapter Twenty-Six Digestive System ______ 8. Which statement is false about pancreatic juice? a. It is secreted through the main pancreatic duct into the duodenum. b. It is responsible for emulsifying (breaking down) fats. c. It is produced by the acinar cells of the pancreas. d. The juice has an alkaline pH. ______ 9. The “living” part of a tooth is the a. dentin. b. cementum. c. pulp. d. enamel. 7. What is the function of the gallbladder, and what role does it play in digestion? 8. List in order the organs of the GI tract through which ingested material travels, and describe the type(s) of digestion (mechanical/chemical) that takes place in each organ. At what point is the material called a bolus, chyme, and feces? 9. Why are there so many mucin-producing glands along the length of the GI tract? 10. Describe how the small and large intestine form. ______ 10. Most of the chemical digestion of our food occurs within the a. large intestine. b. pancreas. c. small intestine. d. esophagus. Content Review 11. Take a fantastic voyage. Imagine that you are a molecule of fat that’s just about to be eaten. Trace your path as you travel from the mouth to the skin (dermis) of the foot. Include all major structures, vessels, tubes, and so on that you will pass by during your journey. Pay attention to any specific enzymes and any modifications to your structure that may play a role in your successful travels. (Remember that you will be traveling through parts of the digestive system and parts of the circulatory system.) Developing Critical Reasoning 1. What initial stages of digestion occur in the oral cavity? 2. The GI tract from the esophagus to the anal canal is composed of four tunics. Describe the general histology of the tunics and the specific features of the stomach tunics. 3. Compare and contrast gastric juice and pancreatic juice with respect to their composition and function. 4. Compare the anatomy and functions of the circular folds, villi, and microvilli in the small intestine. 5. What are the teniae coli and haustra, and how are they associated? 1. Alexandra experienced vomiting and diarrhea, and was diagnosed with gastroenteritis (stomach flu). What specific digestive system organs were affected by the illness, and how did the illness interfere with each organ’s function? 2. Most cases of colorectal cancer occur in the most distal part of the large intestine (the rectum, sigmoid colon, and descending colon). Why do fewer instances of colon cancer tend to occur in the proximal part of the large intestine? Include the anatomy and function of the colon components in your explanation. 6. What is the function of each structure in the portal triad of the hepatic lobule, and how do they work together to contribute to the overall functioning of the liver? Answers to “What Do You Think?” 1. Saliva cleanses the mouth in a variety of ways. As saliva washes over the tongue and teeth, it helps remove foreign materials and buildup. Lysozyme in saliva is an antibacterial enzyme. A person who has a dry mouth is not able to cleanse the mouth well and is more likely to develop dental problems as a result of built-up bacterial and foreign material. 4. The duodenum has many circular folds so that the digested materials can be slowed down and adequately mixed with pancreatic juice and bile. The jejunum also has many circular folds because the slowing down of the materials aids in absorbing the maximum amount of nutrients. By the time chyme reaches the distal ileum, most nutrient absorption has occurred, and so the circular folds are not as essential. 2. A young child’s mouth is too small to support adult-sized teeth. Deciduous teeth are much smaller and fit a child’s mouth better. As the mouth increases in size by about age 6, it is able to support the first adult-sized teeth. 5. When the gallbladder is removed, bile is still produced by the liver, but it is secreted in a “slow drip.” A fatty meal requires much more bile for digestion than the amount supplied by a “slow drip,” causing the patient to experience gas, pain, bloating, and diarrhea. Thus, surgical patients are initially told to limit their fat intake. The body gradually adjusts so that more bile is secreted, and thus a person may eventually be able to introduce more fat back into his or her diet. 3. The stomach is lined with specialized surface mucous cells that secrete mucus onto the lining. This mucus prevents the acid from eating away the stomach wall. In addition, the stomach epithelial lining is constantly regenerating to replace any cells that are damaged. www.mhhe.com/mckinley3 Enhance your study with practice tests and activities to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more. mck78097_ch26_779-816.indd 816 2/25/11 2:23 PM

Top subcategories

Top subcategories

Top subcategories

Top subcategories

Top subcategories

Top subcategories

Top subcategories

Top subcategories

Download 26. Digestive System