dgs.pdf

T his chapter deals with the anatomy of the diges- h state so that nutrients can be absorbed and used by the tive system. The organs of the digestive system...

T his chapter deals with the anatomy of the diges- h state so that nutrients can be absorbed and used by the tive system. The organs of the digestive system body cells. The complete process of altering the physical together perform a vital function—that of prepar- and chemical composition of ingested food material so ing food for absorption and for use by the millions of that it can be absorbed and used by the body cells is body cells. Most food when eaten is in a form that cannot called digestion. This complex process is the function of reach the cells (because it cannot pass through the intes- both the digestive tract and accessory organs that make tinal mucosa into the bloodstream), nor could it be used up the digestive system. The physiology of the digestive by the cells even if it could reach them. It must therefore system is discussed in Chapter 29. be modified in both chemical composition and physical ORGANIZATION OF THE DIGESTIVE SYSTEM It is important to realize that ingested food material passing through the lumen of the GI tract is actually outside the internal Organs of Digestion environment of the body, even though the tube itself is inside the The main organs of the digestive system (Figure 28-1) form ventral body cavity. a tube that goes all the way through the ventral cavities of the Box 28-1 lists the main organs of the digestive system, that is, body. It is open at both ends. This tube is usually referred to as the segments of the alimentary canal, and the accessory organs the alimentary canal (tract). The term gastrointestinal (GI) tract located in the main digestive organs or opening into them. Organs refers only to the stomach and intestines but is sometimes used in such as the larynx, trachea, diaphragm, and spleen are labeled in reference to the entire alimentary canal. Figure 28-1, but they are not digestive organs. They are shown to assist in orienting the digestive organs to other important body structures. S R L Parotid gland Wall of the GI Tract I The GI tract is essentially a tube with walls fash- Submandibular salivary gland Tongue ioned of four layers of tissues: a mucous lining, a submucous coat of connective tissue in which are Sublingual salivary gland embedded the main blood vessels of the tract, a Pharynx muscular layer, and a fibroserous layer (Figure 28-2). Larynx Blood vessels and nerves travel through the mesentery to reach the digestive tube throughout most of its length. Trachea Esophagus Common Cystic hepatic Liver duct duct Spleen Diaphragm Stomach Transverse colon UNIT 5 Spleen Liver Hepatic flexure of colon Splenic flexure Stomach of colon Ascending colon Descending colon Ileum Sigmoid colon Gallbladder Pancreas Cecum Duodenum Vermiform Anal canal appendix Rectum FIGURE 28-1 Location of the digestive organs. 862 tahir99-VRG & vip.persianss.ir  Chapter 28 Anatomy of the Digestive System 863 MUCOSA The innermost layer of the GI wall—the layer facing the lumen, Box 28-1 | Organs of the or open space, of the tube—is called the mucosa or mucous layer. Digestive System Note in Figure 28-2 that the mucosa is made up of three layers— an inner mucous epithelium, a layer of loose fibrous connective tis- Segments of the Accessory sue called the lamina propria, and a thin layer of smooth muscle Gastrointestinal Tract Organs called the muscularis mucosae. Mouth Salivary glands SUBMUCOSA Pharynx Parotid The submucosa layer of the digestive tube is composed of con- Esophagus Submandibular nective tissue that is thicker than the mucosal layer. It contains Stomach Sublingual numerous small glands, blood vessels, and parasympathetic nerves Small intestine Tongue that form the submucosal plexus (Meissner plexus). Duodenum Teeth Jejunum Liver MUSCULARIS Ileum Gallbladder The muscularis—or muscular layer—is a thick layer of muscle Large intestine Pancreas tissue that wraps around the submucosa. This portion of the wall Cecum Vermiform appendix is characterized by an inner layer of circular and an outer layer Colon of longitudinal smooth muscle. Like the submucosa, the muscu- Ascending colon laris contains nerves organized into a plexus called the myenteric Transverse colon plexus (Auerbach plexus). This plexus lies between the two mus- Descending colon cle layers. Note in Figure 28-2 that the term intramural plexus is Sigmoid colon used to describe both plexuses. Together they play an important Rectum role in the regulation of digestive tract movement and secretion. Anal canal Mesentery Nerve SEROSA Connective tissue layer Blood vessels Peritoneum Myenteric plexus Intramural plexus Submucosal plexus SUBMUCOSA Gland in submucosa Duct from gland MUCOSA Mucous epithelium UNIT 5 Lamina propria Muscularis mucosae Lymph nodule MUSCULARIS Circular muscle layer Longitudinal muscle layer FIGURE 28-2 Wall of the GI tract. The wall of the gastrointestinal (GI) tract is made up of four layers, shown here in a generalized diagram of a segment of the GI tract. Notice that the serosa is continuous with a fold of serous membrane called a mesentery. Notice also that digestive glands may empty their products into the lumen of the GI tract by way of ducts. tahir99-VRG & vip.persianss.ir  864 UNIT 5 Respiration, Nutrition, and Excretion SEROSA | QUICK CHECK | The serosa—or serous layer—is the outermost layer of the GI wall. It is made up of serous membrane (see Figures 6-39 and 6-40 on 1. What is another name for the digestive tract? pp. 639 and 640). The serosa is actually the visceral layer of the peri- 2. Name the four layers of the digestive tract wall. toneum—the serous membrane that lines the abdominopelvic cav- ity and covers its organs. The lining attached to and covering the walls of the abdominopelvic cavity is called the parietal layer of the peritoneum. The fold of serous membrane shown in Figure 28-2 MOUTH that connects the parietal and visceral portions is called a mesentery. Structure of the Oral Cavity MODIFICATIONS OF LAYERS The mouth is also called the oral cavity. The following structures Although the same four tissue layers form the various organs of form the oral cavity (buccal cavity): the lips, which surround the the GI tract, their structures vary in different regions of the tube orifice of the mouth and form the anterior boundary of the oral throughout its length. Variations in the epithelial layer of the mu- cavity, the cheeks (side walls), the tongue and its muscles (floor), cosa, for example, range from stratified layers of squamous cells and the hard palate and soft palate (roof) (Figure 28-3). that provide protection from abrasion in the upper part of the esophagus to the simple columnar epithelium, designed for ab- LIPS sorption and secretion, which is found throughout most of the The lips are covered externally by skin and internally by mucous tract. Notice in Figure 28-2 that exocrine glands empty their se- membrane that continues into the oral cavity and lines the mouth. cretions into the lumen of the GI tract through ducts. Some of The junction between skin and mucous membrane is highly sen- these modifications are listed in Table 28-1; refer back to this table sitive and easily irritated. The upper lip is marked near the mid- when each of these organs is studied in detail. line by a shallow vertical groove called the philtrum, which ends TA B L E 2 8 - 1 Modifications of Layers of the Digestive Tract Wall ORGAN MUCOSA MUSCULARIS SEROSA Esophagus Stratified squamous epithelium resists Two layers—inner one of circular fibers Outer layer, fibrous (adventitia); serous abrasion and outer one of longitudinal fibers; around part of the esophagus in the striated muscle in the upper part and thoracic cavity smooth muscle in the lower part of the esophagus and in the rest of the tract Stomach Arranged in flexible longitudinal folds Has three layers instead of the usual Outer layer, visceral peritoneum; hangs in called rugae; allow for distention; two—circular, longitudinal, and oblique a double fold from the lower edge of contains gastric pits with microscopic fibers; two sphincters—lower the stomach over the intestines and gastric glands esophageal sphincter at the entrance forms an apronlike structure; greater of the stomach and pyloric sphincter at omentum; lesser omentum connects its exit, formed by circular fibers the stomach to the liver Small intestine Contains permanent circular folds, plicae Two layers—inner one of circular fibers Outer layer, visceral peritoneum, circulares and outer one of longitudinal fibers continuous with the mesentery Microscopic fingerlike projections, villi UNIT 5 with brush border Crypts (of Lieberkühn) Microscopic duodenal (Brunner) mucous glands Aggregated lymphoid nodules (Peyer patches) Numerous single lymphoid nodules called solitary nodules Large intestine Solitary lymph nodes Outer longitudinal layer condensed to Outer layer, visceral peritoneum, Intestinal mucous glands form three tapelike strips (taeniae coli); continuous with mesocolon small sacs (haustra) give the rest of the Anal columns form in the anal region wall of the large intestine a puckered appearance; internal anal sphincter formed by circular smooth fibers; external anal sphincter formed by striated fibers tahir99-VRG & vip.persianss.ir  Chapter 28 Anatomy of the Digestive System 865 S R L Philtrum CHEEKS I The cheeks form the lateral boundaries of the oral cavity. They Upper lip are continuous with the lips in front and are lined by mucous membrane that is reflected onto the gingiva, or gums, and the Hard palate soft palate. The cheeks are formed in large part by the buccina- tor muscle, which is sandwiched with a considerable amount of adipose, or fat, tissue between the outer skin and mucous mem- Soft palate brane lining. Numerous small mucus-secreting glands are located Uvula between the mucous membrane and the buccinator muscle; their Palatine ducts open opposite the last molar teeth. tonsil Tongue HARD PALATE AND SOFT PALATE Fauces The hard palate consists of portions of four bones: two maxillae (opening) and two palatines (see Figure 8-5. B on p. 205). The soft palate, which forms a partition between the mouth and nasopharynx (see Lower lip Figure 26-3), is fashioned of muscle arranged in the shape of an arch. The opening in the arch leads from the mouth into the oropharynx and is named the fauces. Suspended from the midpoint of the poste- rior border of the arch is a small cone-shaped process, the uvula. FIGURE 28-3 TONGUE The oral cavity. The tongue is a solid mass of skeletal muscle components (intrin- sic muscles) covered by a mucous membrane. Note in Figure 28-4, A, that the tongue has a blunt root, a tip, at the junction between skin and mucous membrane in a slight and a central body. The upper, or dorsal, surface of the tongue prominence called the tubercle. The term fissure is often used to is normally moist, pink, and covered by rough elevations, called describe a cleft or groove between or separating anatomical struc- papillae (Figure 28-4, B). Recall from Chapter 17 that papillae tures. Therefore, when the lips are closed, the line of contact be- possess sensory organs called taste buds. tween them is called the oral fissure. Besides keeping food in the The four types of papillae—circumvallate, fungiform, foliate, mouth while it is being chewed, the lips help sense the tempera- and filiform—are all located on the sides or upper surface (dor- ture and texture of food before it enters the mouth. The lips are sum) of the tongue. Note in Figure 28-4, A, that the large circum- also needed to form many speech sounds (syllables). vallate papillae form an inverted V-shaped row extending from a Epiglottis Uvula Lingual tonsil Palatine tonsil Fauces Root Circumvallate papillae UNIT 5 Foramen Foliate cecum Fungiform papillae papillae Filiform Body papillae Filiform Fungiform papillae papillae P R L Tip A A B FIGURE 28-4 Dorsal surface of tongue. A, Sketch showing the three divisions of the tongue (see also Figure 17-8 on p. 516). B, Photograph showing rough texture of tongue produced by papillae. tahir99-VRG & vip.persianss.ir  866 UNIT 5 Respiration, Nutrition, and Excretion Circumvallate papillae Stratified squamous epithelium Moat Taste bud Taste buds Taste bud pore A B FIGURE 28-5 Circumvallate papillae on the surface of the tongue. A, Taste buds are located on the lateral surfaces of the papillae. Several taste buds can be seen opening into the moat from the sides of the papillae. (×35.) B, Enlargement of the photomicrograph of taste buds in A. The arrow points to a pore in the outer surface of the taste bud. (×40.) Compare to Figure 17-8 on p. 516. median pit named the foramen cecum on the posterior part of the coloration and are distributed over the anterior two thirds of the tongue. There are 10 to 14 of these large, mushroomlike papillae. tongue. Filiform papillae do not contain taste buds. Refer back to You can readily distinguish them if you look at your own tongue. Chapter 17, p. 516, for more discussion of papillae and taste buds. Figure 28-5 shows two micrographs of circumvallate papillae. To The lingual frenulum (Figure 28-6, A) is a fold of mucous be tasted, a dissolved substance must enter a moatlike depression membrane in the midline of the undersurface of the tongue that surrounding the papillae where it contacts taste buds located on helps anchor the tongue to the floor of the mouth. If the frenulum the lateral surface. is too short and hinders tongue movement—a congenital condi- Taste buds are also located on the sides of the fungiform papil- tion called ankyloglossia—the individual is said to be tongue-tied; lae, which are found chiefly on the sides and tip of the tongue. Foli- this causes faulty speech (Figure 28-6, B). ate papillae are leaflike ridges on the posterior, lateral edges of the A fold of mucous membrane called the fimbriated fold (or plica tongue that also posses taste buds. The numerous filiform papillae fimbriata) (see Figure 28-6, A) extends toward the apex of the tongue are filamentous and threadlike in appearance. They have a whitish on either side of the lingual frenulum. The floor of the mouth and Fimbriated fold Shortened (plica fimbriata) UNIT 5 lingual Lingual frenulum frenulum Lingual vein Sublingual gland (under the mucosa) Submandibular duct (opening) S R L A I B FIGURE 28-6 Floor of mouth. A, Floor of mouth and ventral surface of tongue. B, Photo showing ankyloglossia—characterized by an abnormally short lingual frenulum. tahir99-VRG & vip.persianss.ir  External acoustic meatus (of Superior longitudinal Median groove temporal bone) muscle Styloid Lingual Body of tongue process Mucous aponeurosis membrane Styloglossus Transverse Orbicularis muscle Vertical muscle oris muscle muscle Hyoglossus muscle Lingual Mandible septum Thyrohyoid Inferior membrane Deep lingual longitudinal Genioglossus artery muscle muscle Thyroid cartilage Deep lingual vein Sublingual Cricoid S salivary S cartilage gland R L A P First ring Genioglossus muscle (extrinsic) of trachea A I B I FIGURE 28-7 Muscles of the tongue. A, Intrinsic muscles of the tongue shown in a frontal section. B, Extrinsic muscles of the tongue. undersurface of the tongue are richly supplied with blood vessels. Figure 28-7, A, intrinsic muscles have their fibers oriented in all The deep lingual vein can be seen (see Figure 28-6, A) shining directions, thus providing a basis for extreme maneuverability. through the mucous membrane between the lingual frenulum and Changes in the size and shape of the tongue caused by intrinsic fimbriated fold. In this region many vessels are extremely superficial muscle contraction assist in placement of food material between and are covered only by a very thin layer of mucosa. Soluble drugs, the teeth during mastication (chewing). Such movements are such as aspirin or nitroglycerin used also necessary for forming speech syllables properly. during a heart attack, are ab- Extrinsic tongue muscles are those that insert into the tongue but sorbed into the circulation have their origin on some other structure, such as the hyoid bone or rapidly if placed under one of the bones of the skull. Examples of extrinsic tongue muscles the tongue. are the genioglossus, which protrudes the tongue, and the hyoglossus, The intrinsic which depresses it (see Figure 28-7, B). Contraction of the extrinsic muscles of the muscles is important during deglutition, or swallowing, and speech. tongue have, by definition, both their Salivary Glands origin and their in- The salivary glands are typical of the accessory glands associated sertion in the tongue with the digestive system. They are located outside the alimentary itself. As you can see in canal and convey their exocrine secretions by way of ducts from the glands into the lumen of the tract (Figure 28-8, A). The mucous and serous cells seen in the compound tubuloalveolar gland pictured Parotid gland in Figure 28-8, B, together secrete a mixture of fluids that are then Parotid duct Submandibular gland UNIT 5 Submandibular duct Duct A Serous Mucous cells Sublingual gland cells Lumen Duct of duct epithelium Mucous S cells A P Serous cells I B C FIGURE 28-8 Salivary glands. A, Location of the salivary glands. B and C, Detail of submandibular salivary gland. This mixed- or compound-type gland produces mucus from mucous cells and enzymatic secretion from serous cells. Duct cross sections are also visible. (×140.) tahir99-VRG & vip.persianss.ir  868 UNIT 5 Respiration, Nutrition, and Excretion Cusp Enamel modified by the duct cells on their way out of the salivary gland. The functions of saliva in the digestive process are discussed in Chapter 29. Three pairs of major salivary glands (see Figure 28-8)—the Crown Dentin parotid, submandibular, and sublingual glands—secrete a major amount (about 1 liter) of the saliva produced each day. The minor salivary glands (buccal glands) that occur in the mucosa lining the Pulp cavity cheeks and mouth contribute less than 5% of the total salivary with nerves Neck and vessels volume. Buccal gland secretion is important, however, to the hy- giene and comfort of the mouth tissues. Gingiva (gum) PAROTID GLANDS Root canal The pyramidal parotids are the largest of the paired salivary glands (see Figure 28-8, A). They are located between the skin and underly- Periodontal ligament ing masseter muscle in front of and below the external ear. The parot- Root Periodontal ids produce a watery, or serous, type of saliva containing enzymes but membrane not mucus. The parotid (Stensen) ducts are about 5 cm (2 inches) long. They penetrate the buccinator muscle on each side and open Cementum into the mouth through the parotid papilla opposite the upper second molars. Inflammation of the parotids is called mumps or parotitis and Bone is caused by paramyxovirus (see Mechanisms of Disease, p. 888). SUBMANDIBULAR GLANDS Submandibular glands (see Figure 28-8, A) are called mixed or FIGURE 28-9 compound glands because they contain both serous (enzyme) and Typical tooth. A molar tooth sectioned to show its bony socket and mucus-producing elements (see Figure 28-8, B and C). These details of its three main parts: crown, neck, and root. Enamel (over glands are located just below the mandibular angle. You can feel the crown) and cementum (over the neck and root) surround the the gland by placing your index finger on the posterior part of the dentin layer. The pulp contains nerves and blood vessels. floor of the mouth and your thumb medial to and just in front of the angle of the mandible. The gland is irregular in form and about The root of a tooth may be a single peglike structure or consist of the size of a walnut. The ducts of the submandibular glands (Wharton two or three separate conical projections. The root is not rigidly ducts) open into the mouth on either side of the lingual frenulum. anchored to the alveolar process by cement but is suspended in the socket by the fibrous periodontal membrane (see Figure 28-9). SUBLINGUAL GLANDS In addition to enamel, the outer shell of each tooth is com- Sublingual glands are the smallest of the main salivary glands (see posed of two additional dental tissues—dentin and cementum (see Figure 28-8, A). They lie in front of the submandibular glands, Figure 28-9). Dentin makes up the greatest proportion of the tooth under the mucous membrane covering the floor of the mouth. shell. It is covered by enamel in the crown and by cementum in the Each sublingual gland is drained by 8 to 20 ducts (ducts of Rivi- neck and root area. The dentin contains a pulp cavity consisting of nus) that open into the floor of the mouth. Unlike the other salivary connective tissue, blood and lymphatic vessels, and sensory nerves. glands, the sublingual glands produce only a mucous type of saliva. TYPES OF TEETH Teeth Dentition is the type, number, and arrangement of teeth in the jaws. The teeth are the organs of mastication, or chewing. They are de- Twenty primary or deciduous teeth, or so-called baby teeth, signed to cut, tear, and grind ingested food so that it can be mixed appear early in life and are later replaced by 32 permanent teeth UNIT 5 with saliva and swallowed. During the process of mastication, food (Figure 28-10). The names and numbers of teeth in both sets are is ground into small bits. This increases the surface area that can given in Table 28-2. The first deciduous tooth usually erupts at be acted on by the digestive enzymes. about 6 months of age. The remainder follow at the rate of one or more a month until all 20 have appeared. There is, however, great TYPICAL TOOTH individual variation in the age at which teeth erupt. Deciduous A typical tooth (Figure 28-9) can be divided into three main parts: teeth are generally shed between the ages of 6 and 13 years. The crown, neck, and root. The crown is the exposed portion of a third molars (wisdom teeth) are the last to appear, and usually tooth. It is covered by enamel—the hardest and chemically most erupt sometime after 17 years of age. stable tissue in the body. Enamel consists of approximately 97% Teeth in the upper jaw are called maxillary teeth because they calcified (inorganic) material and only 3% organic material and are in the maxilla bone of the skull. Teeth in the lower jaw are called water. It is ideally suited to withstand the very abrasive process of mandibular teeth because they are anchored in the mandible bone. mastication. The neck of a tooth is the narrow portion shown in Deciduous teeth are identified by either name or capital Roman Figure 28-9 that is surrounded by the gingivae, or gums. It joins letters (A, B, C, etc.) applied in a clockwise direction, as you can see the crown of the tooth to the root. It is the root that fits into the in Figure 28-10. Permanent teeth are likewise identified by name or socket of the alveolar process of either the upper or lower jaw. by Arabic numeral (1, 2, 3, etc.) in a clockwise direction. tahir99-VRG & vip.persianss.ir  Chapter 28 Anatomy of the Digestive System 869 Deciduous (primary) teeth Adult teeth Eruption Central incisor (months) Lateral incisor Central incisor 6-8 Canine Lateral incisor First premolar 7-12 Canine 7 8 9 10 Second 6 11 16-20 A premolar First D E F G 5 12 molar C H R L 4 First molar 12-16 13 Second B I P 3 Second molar Maxillary teeth 14 Maxillary teeth 20-30 molar (upper jaw) A (upper jaw) J 2 15 Third molar (wisdom 1 16 tooth) T K Mandibular teeth 18-22 P Eruption (lower jaw) (years) S L R L 32 17 R 16-24 17-24 M A Q P N 14-18 31 18 O Mandibular teeth 10-14 7-9 (lower jaw) 30 19 6-8 5-8 29 20 28 21 10-14 27 22 9-13 26 25 24 23 S 9-14 L R 7-10 7-8 I FIGURE 28-10 Dentition. In the deciduous set, where letters are used in place of numbers, there are no premolars and only two pairs of molars in each jaw. Generally, the lower teeth erupt before the corresponding upper teeth. The photo inset is a “Panorex” dental x ray. It displays the full dentition in a single “flattened-out” image. Arrows show the third molars or “wisdom teeth” that have not yet erupted. |A&P CONNECT | TA B L E 2 8 - 2 UNIT 5 Dentition An epidemic of methamphetamine abuse has resulted in a number of health problems including “meth mouth.” See an NUMBER PER JAW (UPPER OR LOWER) example of this disorder and learn about its causes in Meth NAME OF TOOTH DECIDUOUS SET PERMANENT SET Mouth online at A&P Connect. Central incisors 2 2 Lateral incisors 2 2 | QUICK CHECK | Canines (cuspids) 2 2 Premolars (bicuspids) 0 4 3. What are the boundaries of the oral cavity? First molars (tricuspids) 2 2 4. Describe the location of the taste buds in the mouth. Second molars 2 2 5. What are the names of the three types of salivary glands? Third molars (wisdom 0 2 6. How do the names of the salivary glands describe their teeth) locations? 7. What are the three main parts of a typical tooth? TOTAL (per jaw) 10 16 TOTAL (per set) 20 32  870 UNIT 5 Respiration, Nutrition, and Excretion PHARYNX ESOPHAGUS The act of swallowing, or deglutition, moves a rounded mass The esophagus (eh-SOFF-ah-gus), a collapsible, muscular, mucosa- of food, called a bolus, from the mouth to the stomach. As the lined tube about 25 cm (10 inches) long, extends from the phar- food bolus passes from the mouth, it enters the oropharynx ynx to the stomach and pierces the diaphragm in its descent from by passing through a constricted, archlike opening called the the thoracic cavity to the abdominal cavity (Figure 28-11). It lies fauces. The oropharynx is the second division of the pharynx (see posterior to the trachea and heart and serves as a dynamic pas- Figure 26-3). During respiration, air passes through all three sageway for food, pushing the food toward the stomach. The short pharyngeal divisions. However, only the terminal portions of portion of the esophagus in the neck is called the cervical part, the the pharynx serve the digestive system. Once a bolus has passed portion in the thorax is called the thoracic part, and the short por- through the pharynx, it enters the digestive tube proper—the tion in the abdomen is called the abdominal part. portion of the digestive tract that serves only the digestive sys- The esophagus is the first segment of the digestive tube proper, tem. The anatomy of the pharynx is discussed in more detail in and the four layers that form the wall of the GI tract organs can Chapter 26 on pp. 802–803. be identified there (Figure 28-12, A). The esophagus is normally flattened, and thus the lumen is practically nonexistent in the rest- |A&P CONNECT | ing state. The stratified squamous epithelium of the esophageal mucosa seen in Figure 28-12 provides a thick, abrasion-resistant The ring of tonsils in the pharynx defends both the digestive lining that protects the esophagus from injury. The inner circular tract and respiratory tract from infection. Visualize the tonsils and outer longitudinal layers of the muscular layer are striated and review their defensive role in Protective Strategies of the (voluntary) in the upper third, mixed (striated and smooth) in Respiratory Tract online at A&P Connect. the middle third, and smooth (involuntary) in the lower third of the tube. Cricoid cartilage Cricoesophageal (of larynx) tendon Cricopharyngeus Cervical part muscle (cut) of esophagus Trachea Pharynx Lung Aortic arch Cervical part of esophagus Bronchus Upper esophageal sphincter (UES) Thoracic part Bronchoesophageal of esophagus muscle Esophageal hiatus Pleuroesophageal (of diaphragm) Thoracic part muscle of esophagus UNIT 5 Abdominal part of Diaphragm esophagus Stomach Esophageal hiatus Lower esophageal (of diaphragm) sphincter (LES) Diaphragm S S R L Abdominal aorta L R A I B I FIGURE 28-11 Esophagus. A, Diagram showing the major features of the esophagus. B, View of the muscular wall of the esophagus from behind, showing its position relative to other structures.  Chapter 28 Anatomy of the Digestive System 871 Lumen Stratified squamous epithelium Esophagus (abdominal Mucosa Lamina part) propria Muscularis mucosae Esophagus- stomach Submucosa junction Muscularis externa Circular muscle layer Stomach (cardiac part) Longitudinal muscle layer A Adventitia (fibrous coat) B FIGURE 28-12 Wall of esophagus. A, The four layers of the gastrointestinal (GI) wall are easily identified in this microscopic cross section of the esophagus (see Table 28-1). B, Cadaver dissection photograph of the mucosal lining at the junction between the lower part of the esophagus and the stomach—a frequent site of irritation caused by reflux of acidic gastric secretions. Each end of the esophagus is encircled by muscular sphinc- The muscles of the diaphragm at the esophageal hiatus—an open- ters that act as valves to regulate passage of material. The ing in the diaphragm located near the junction between the termi- upper esophageal sphincter (UES) in the cervical part of nal portion of the esophagus and the stomach—form the extrinsic the esophagus helps prevent air from entering the esophagus part of the LES. during respiration. The UES is made up of several muscles, The esophageal hiatus in the diaphragm, which permits pas- UNIT 5 but the cricopharyngeus muscle (see Figure 28-11, B) that sage of the esophagus into the abdomen, may become stretched wraps around the back of the cervical esophagus plays the or otherwise enlarged. Such enlargement may permit bulging primary role. Relaxation of the UES is what permits belching of the lower segment of the esophagus and intrinsic LES and (or burping), which is the sudden escape of air trapped in the part or even all of the stomach upward through the diaphragm stomach and esophagus. and into the chest. The condition is called a hiatal (hye-AYT-al) The lower esophageal sphincter (LES) is also called the car- hernia. diac sphincter or cardial sphincter. The intrinsic part of the LES Gastroesophageal reflux disease, or GERD, is the term used is located at the junction with the stomach and is made up of to describe the backward flow of stomach acid up through the layers of circular muscle that are thicker than in other parts of LES and into the lower part of the esophagus. It often causes a the esophagus. Slinglike oblique muscles from the stomach wall painful sensation called heartburn. Being a potentially serious also form part of the LES, adding to its strength in containing medical condition, GERD is treated by elimination of the under- the stomach contents while the stomach is full and churning. lying causes, such as a hiatal hernia, by drugs to reduce excess  872 UNIT 5 Respiration, Nutrition, and Excretion Box 28-2 | DIAGNOSTIC study Upper Gastrointestinal X Ray Study An upper GI (UGI) study consists of a series of x rays of the lower part of the esophagus, stomach, and duodenum, usually with barium sulfate used as the contrast medium. The test is used to detect ulcerations, tumors, inflammation, or anatomical malpositions such as hiatal hernia (distention of the diaphragm that allows the stomach to protrude). Ob- struction of the upper GI tract is also easily detected. Stomach Pyloris In this test the patient is asked to drink a flavored liquid containing (of stomach) (body) barium sulfate. As the contrast medium travels through the system, the lower portion of the esophagus, gastric wall, pyloric channel, and duo- denum are each evaluated for defects. Benign peptic ulcer is a common pathological condition affecting these areas. Tumors, cysts, or enlarged organs near the stomach can also be identified by an anatomical distor- tion of the outline of the upper GI tract. Can you identify the outline of Duodenum the stomach in the figure? S R L X ray film of the lower part of the esophagus, stomach, and duodenum. I stomach acid, or by surgery to reduce the lumen size or strengthen mass to the left of the median line (see Figure 28-1). In other words, the LES (see Mechanisms of Disease on p. 890). Box 28-2 de- it is described as lying in the epigastrium and left hypochondrium scribes a method for imaging a hiatal hernia and other problems (see Figure 1-7, p. 14). Its position, however, alters frequently. For of the esophagus and stomach. example, it is pushed downward with each inspiration and upward The junction between the lower part of the esophagus and with each expiration. When it is greatly distended from an unusu- stomach (see Figure 28-12, B) is an important site for a number ally large meal, its size interferes with descent of the diaphragm of pathologic conditions, many associated with repeated exposure on inspiration, thereby producing the familiar feeling of dyspnea to acid gastric secretions. In the last 1- to 1.5-cm-long abdominal (breathing difficulty) that accompanies overeating. In this state, the part of the esophagus below the diaphragm, stratified squamous stomach also pushes upward against the heart and may give rise to epithelium is replaced by columnar epithelium. It is this area of the sensation that the heart is being crowded. transition that is often damaged by exposure to acid and diges- tive enzymes from the stomach. The area of transition between Divisions of the Stomach the lower part of the esophagus and stomach is clearly visible in The fundus, body, and pylorus are the major divisions of the Figure 28-12, B. The stratified squamous epithelial lining of the stomach. The fundus is the enlarged portion to the left and above esophagus appears pale, whereas the columnar gastric epithelium the opening of the esophagus into the stomach. The body is the appears brown. UNIT 5 central part of the stomach, and the pylorus is its lower portion (see Figure 28-13). The small collar or margin of the stomach STOMACH at its junction with the esophagus is often called the cardia or cardiac part (or cardial part). Size and Position of the Stomach Just below the diaphragm, the digestive tube dilates into an elon- Curves of the Stomach gated pouchlike structure, the stomach (Figure 28-13), the size The curve formed by the upper right surface of the stomach is of which varies according to several factors, notably the amount known as the lesser curvature; the curve formed by the lower left of distention. For some time after a meal, the stomach is enlarged surface is known as the greater curvature (see Figure 28-13). because of distention of its walls, but as food passes out of the stomach, the walls partially collapse, leaving the organ about the size of a large sausage. In adults, the stomach usually holds a Sphincter Muscles volume of up to 1 to 1.5 liters. Sphincter muscles regulate passage of material at both stomach The stomach lies in the upper part of the abdominal cavity openings. A sphincter muscle, as you know, consists of circular under the liver and diaphragm, with approximately five sixths of its fibers arranged to form an opening in the center of them (like the  Chapter 28 Anatomy of the Digestive System 873 FIGURE 28-13 Cardia Stomach. A portion of the anterior wall has been cut away to reveal Fundus the muscle layers of the stomach wall. Notice that the mucosa lining the stomach forms folds called rugae. Pylorus Esophagus Fundus Body Gastroesophageal opening Lower esophageal sphincter (LES) Body of stomach Cardia Serosa Pyloric sphincter Pylorus Longitudinal muscle layer re atu rv u Circular muscle layer Muscularis Duodenal Lesser c bulb Oblique muscle layer Submucosa Mucosa Duodenum S re tu urva R L r c at e Rugae Gre I hole in a doughnut) when they are relaxed and no opening when Surface mucous cells Gastric pit they are fully contracted. The lower esophageal sphincter (LES), or cardiac sphincter, Lamina propria controls the opening of the esophagus into the stomach, and the pyloric sphincter controls the opening from the pyloric portion of Gastric glands: the stomach into the first part of the small intestine (duodenum). Mucous neck cell Stomach Wall Mucosa Chief cell Each of the four layers of the stomach wall suits the function of this organ, as summarized in Table 28-1 (p. 863) and shown in Parietal cell Figures 28-13 and 28-14. Of particular interest are the modifica- UNIT 5 tions to the stomach mucosa and muscularis, both of which are Endocrine cell briefly described below. Muscularis Submucosa GASTRIC MUCOSA mucosae The epithelial lining of the stomach is thrown into folds, called rugae, and marked by depressions called gastric pits. Numerous coiled Blood vessels Oblique muscle layer Muscularis FIGURE 28-14 Circular muscle layer Gastric pits and gastric glands. Gastric pits are depressions in the epithelial lining of the stomach. At the bottom of each pit is one or Serosa Longitudinal more tubular gastric glands. Chief cells produce the enzymes of muscle layer gastric juice, and parietal cells produce stomach acid. Endocrine Connective tissue cells secrete the appetite-boosting hormone ghrelin. Visceral peritoneum  874 UNIT 5 Respiration, Nutrition, and Excretion tubular-type glands, gastric glands, are found below the level of the that stimulates the hypothalamus to secrete growth hormone and pits, particularly in the fundus and body of the stomach. Figure 28-14 increase appetite—and gastrin, which influences digestive func- illustrates the anatomical relationship of the gastric pits and gas- tions. The roles of the gastric secretory cells are explored further tric glands. The glands secrete most of the gastric juice, a mucous in Chapter 29. fluid containing digestive enzymes and hydrochloric acid (HCl). Figure 28-15, A, a low-power micrograph of the mucosal lining in GASTRIC MUSCLE the body of the stomach, shows numerous gastric pits and a uniform The thick layer of muscle in the stomach wall—the muscularis— underlying layer of coiled gastric glands. The mucosal lining is eas- is made of three distinct sublayers of smooth muscle tissue. As ily differentiated from the deeper submucosal layer in this section. Figure 28-13 shows, there is the usual layer of longitudinal mus- Figure 28-15, B, shows an enlarged view of gastric pits and gastric cles and circular muscles, as well as an additional, underlying glands isolated from the submucosa and surrounding tissues. oblique layer. The crisscrossing pattern of smooth muscle fibers In addition to the mucus-producing cells that cover the en- formed by this arrangement gives the stomach wall the ability to tire surface of the stomach and line the pits, the gastric glands contract strongly at many angles—thus making the mixing action contain three major secretory cells—chief cells, parietal cells, of this organ very efficient. and endocrine cells (see Figure 28-14). Chief cells (zymogenic cells) secrete the enzymes of gastric juice. Parietal cells secrete Functions of the Stomach hydrochloric acid and are also thought to produce the important The stomach performs the following functions: substance known as intrinsic factor. Intrinsic factor binds to vi- tamin B12 molecules to protect them from digestive juices until Serves as a food reservoir, its main function; food is stored in they reach the small intestine—and then facilitates the absorp- the stomach until it can be partially digested and moved tion of B12. Endocrine cells secrete ghrelin (GHRL)—a hormone farther along the gastrointestinal tract. Lumen Gastric glands Submucosa Muscularis Gastric glands UNIT 5 A FIGURE 28-15 Gastric mucosa. A, Low-power light micrograph (×4) showing that folds of gastric mucosa (rugae) have numerous gastric pits (arrows) and underlying gastric glands. B, Scanning electron micrograph (×500) showing epithelium that has been isolated from the gastric mucosa. Again, notice the gastric pits that have gastric glands at their bases. The outer surfaces of the parietal cells B are seen as prominent dome-shaped bulges.