C H A P T E R 23 The Digestive System Overview of the Digestive System Organs are divided into two groups The alimentary canal Mouth, pharynx, and esophagus Stomach, small intestine, and large intestine (colon) Accessory digestive organs Teeth and tongue Gallbladder, salivary glands, liver, and pancreas The Alimentary Canal and Accessory Digestive Organs Mouth (oral cavity) Tongue Parotid gland Sublingual gland Salivary glands Submandibular gland Esophagus Liver Gallbladder Duodenum Small intestine Jejunum Ileum Anus Pharynx Stomach Pancreas (Spleen) Transverse colon Descending colon Ascending colon Cecum Large intestine Sigmoid colon Rectum Vermiform appendix Anal canal Figure 23.1 1
Digestive Processes Ingestion Propulsion Mechanical digestion Chemical digestion Absorption Defecation Peristalsis Major means of propulsion Adjacent segments of the alimentary canal relax and contract Figure 23.3a Segmentation Rhythmic local contractions of the intestine Mixes food with digestive juices Figure 23.3b 2
The Peritoneal Cavity and Peritoneum Peritoneum a serous membrane Visceral peritoneum surrounds digestive organs Parietal peritoneum lines the body wall Peritoneal cavity a slit-like potential space Falciform ligament Liver Parietal peritoneum Anterior Posterior Visceral peritoneum Peritoneal cavity (with serous fluid) Stomach Kidney (retroperitoneal) Wall of body trunk Figure 23.5 Mesenteries Lesser omentum attaches to lesser curvature of stomach Liver Gallbladder Lesser omentum Stomach Duodenum Transverse colon Small intestine Cecum Urinary bladder (b) Figure 23.6b Mesenteries Greater omentum a fatty apron of peritoneum Greater omentum and transverse colon reflected Greater omentum Transverse colon Transverse mesocolon Descending colon Jejunum Mesentery Sigmoid mesocolon Sigmoid colon Ileum (c) Figure 23.6c 3
Mesenteries Sagittal section through the abdominopelvic cavity Mesenteries attach to posterior abdominal wall Liver Lesser omentum Pancreas Stomach Duodenum Transverse mesocolon Transverse colon Mesentery Greater omentum Jejunum Ileum Visceral peritoneum Parietal peritoneum Urinary bladder Rectum (d) Figure 23.6d Histology of the Alimentary Canal Wall Same four layers from esophagus to anus The mucosa innermost layer Consists of: Epithelium Lamina propria Muscularis mucosae The submucosa external to the mucosa Contains blood and dlymphatic vessels, nerve fibers The muscularis externa external to the submucosa Two layers Circular muscularis Longitudinal muscularis The serosa the outermost layer The visceral peritoneum Nerve plexuses Myenteric nerve plexus Submucosal nerve plexus Histology of the Alimentary Canal Intrinsic nerve plexuses Myenteric nerve plexus Submucosal nerve plexus Glands in submucosa Mucosa Epithelium Lamina propria Muscularis mucosae Submucosa Muscularis externa Longitudinal muscle Circular muscle Serosa Epithelium Connective tissue Nerve Artery Gland in mucosa Lumen Vein Duct of gland outside Mucosa-associated Mesentery Lymphatic vessel alimentary canal lymphoid tissue (a) Longitudinal and cross-sectional views through the small intestine Figure 23.7a 4
Histology of the Alimentary Canal Mucosa Submucosa Muscularis externa Serosa (b) Light micrograph cross section through the small intestine (30 ) Figure 23.7b Smooth Muscle Longitudinal layer of smooth muscle (shows smooth muscle fibers in cross section, 145 ) (a) Location and plane of section shown in (b) Small intestine Mucosa (b) Cross section of the intestine showing the smooth muscle layers (one circular and the other longitudinal) running at right angles to each other Circular layer of smooth muscle (shows longitudinal views of smooth muscle fibers, 145 ) Figure 23.8a, b The Mouth and Associated Organs The mouth oral cavity Mucosal layer Stratified squamous epithelium Lamina propria The lips and cheeks Formed from orbicularis oris and buccinator muscles, respectively 5
Anatomy of the Mouth Soft palate Palatoglossal arch Uvula Hard palate Oral cavity Palatine tonsil Tongue Oropharynx Lingual tonsil Epiglottis Hyoid bone Laryngopharynx Esophagus Trachea (a) Sagittal section of the oral cavity and pharynx Figure 23.11a Anatomy of the Mouth The labial frenulum Connects lips to gum The palate Forms the roof of the mouth Gingivae (gums) Palatine raphe Hard palate Soft palate Uvula Palatine tonsil Sublingual fold with openings of sublingual ducts Vestibule Lower lip (b) Anterior view Upper lip Superior labial frenulum Palatoglossal arch Palatopharyngeal arch Posterior wall of oropharynx Tongue Lingual frenulum Opening of submandibular duct Gingivae (gums) Inferior labial frenulum Figure 23.11b The Tongue Interlacing fascicles of skeletal muscle Grips food and repositions it Helps form some consonants Intrinsic muscles within the tongue Extrinsic muscles external to the tongue 6
The Superior Surface of the Tongue Tongue papillae Filiform papillae Fungiform papillae Vallate papillae Figure 23.12 The Teeth Dentition and dental formula Deciduous teeth 20 teeth First appear at 6 months of age Permanent teeth 32 teeth Most erupt by the end of adolescence Dental formula shorthand Way to indicate number and position of teeth The Teeth Incisors Central (7 yr) Lateral (8 yr) Incisors Central (6 8 mo) Lateral (8 10 mo) Canine (eyetooth) (16 20 mo) Molars First molar (10 15 mo) Second molar (about 2 yr) (a) Deciduous (milk) teeth Canine (eyetooth) (11 yr) Premolars (bicuspids) First premolar (11 yr) Second premolar (12 13 yr) Molars First molar (6 7 yr) Second molar (12 13 yr) Third molar (wisdom tooth) (17 25 yr) Permanent teeth Figure 23.13a 7
The Teeth (b) Deciduous teeth Permanent teeth Figure 23.13b Tooth Structure Enamel Longitudinal section of tooth in alveolus Crown Neck Dentin Dentinal tubules Pulp cavity (contains blood vessels and nerves) Gingiva (gum) Cementum Root Root canal Periodontal ligament Apical foramen Bone Figure 23.14 The Salivary Glands Tongue Teeth Ducts of sublingual gland Frenulum of tongue Sublingual gland Mylohyoid muscle (cut) Anterior belly of digastric muscle (a) Parotid gland Parotid duct Masseter muscle Body of mandible (cut) Posterior belly of digastric muscle Submandibular duct Submandibular gland Mucous cells (b) Serous cells forming demilunes Figure 23.15 8
The Pharynx Oropharynx and laryngopharynx passages for air and food Lined with stratified squamous epithelium External muscle layer Consists of superior, middle, and inferior pharyngeal constrictors The Esophagus Gross anatomy muscular tube Begins as a continuation of the pharynx Joins the stomach inferior to the diaphragm Microscopic anatomy Epithelium is stratified squamous epithelium When empty mucosa and submucosa in longitudinal folds Mucous glands primarily compound tubuloalveolar glands Muscularis externa skeletal muscle first third of length Most external layer adventitia Microscopic Structure of the Esophagus Mucosa (contains a stratified squamous epithelium) Submucosa (areolar connective tissue) Lumen Muscularis externa Circular layer Longitudinal layer Adventitia (fibrous connective tissue) (a) Cross section through esophagus (5 ) (b) Gastroesophageal junction, longitudinal section (85 ) Figure 23.16 9
The Stomach Site where food is churned into chyme Protein digestion begins Secretes pepsin Functions under acidic conditions The Stomach Cardia Fundus Esophagus Muscularis externa Longitudinal layer Circular layer Oblique layer Lesser curvature Serosa Body Lumen Rugae of mucosa Greater curvature Duodenum (a) Pyloric canal Pyloric sphincter (valve) at pylorus Pyloric antrum Figure 23.17a The Stomach Microscopic Anatomy Gastric pits Surface epithelium (mucous cells) Surface epithelium Gastric pit Mucous neck cells Mucosa Lamina propria Muscularis Submucosa (contains mucosae submucosal plexus) Oblique layer Muscularis externa Circular layer (contains myenteric Longitudinal plexus) layer Serosa Stomach wall (a) Layers of the stomach wall, longitudinal section Parietal cell Gastric gland Chief cell Enteroendocrine cell (b) Enlarged view of gastric pits and gastric glands Pepsinogen HCl Pepsin Gastric pits Mucus-secreting cells Surface mucous cell Mitochondria Mucus neck cells Parietal cell Gastric gland HCl secreting parietal cells Chief cell Enteroendocrine cell (c) Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland Enzyme secreting chief cells Muscularis mucosae (d) Micrograph of the stomach mucosa, view similar to part (b) (115 ) Figure 23.18 10
The Small Intestine Gross Anatomy Longest portion of the alimentary canal Site of most enzymatic digestion and absorption Three subdivisions Duodenum Brunner s glands Jejunum Ileum Peyer s patches The Small Intestine Structural Features Vein carrying blood to hepatic portal vessel Muscle layers Circular folds Villi Lumen Absorptive cells Microvilli (brush border) Lacteal Goblet cell (a) Blood capillaries Mucosa associated lymphoid tissue Intestinal crypt Muscularis mucosae Duodenal (b) gland Vilus Enteroendocrine cells Venule Lymphatic vessel Submucosa Goblet cells Villi Absorptive cells (c) Intestinal crypt Figure 23.20 The Duodenum Receives digestive enzymes and bile Main pancreatic duct and common bile duct enter duodenum Sphincters control entry of bile and pancreatic juices 11
The Duodenum and Related Organs Right and left hepatic ducts of liver Common hepatic duct Cystic duct Bile duct and sphincter Accessory pancreatic duct Mucosa with folds Gallbladder Major duodenal papilla Hepatopancreatic ampulla and sphincter Duodenum Tail of pancreas Pancreas Jejunum Main pancreatic duct and sphincter Head of pancreas Figure 23.19 The Large Intestine Digested residue contains few nutrients Small amount of digestion by bacteria Main functions absorb water and electrolytes Mass peristaltic movements force feces toward the rectum Gross Anatomy of Large Intestine Subdivided into: Cecum, vermiform appendix, colon, rectum, anal canal Special features of large intestine Teniae coli, haustra, and epiploic appendages Cecum a blind pouch Vermiform appendix contains lymphoid tissue Neutralizes pathogens Colon divided into distinct segments Ascending, transverse, descending, and sigmoid colon Rectum descends along the inferior half of the sacrum Anal Canal the last subdivision of the large intestine 12
Gross Anatomy of Large Intestine Right colic (hepatic) flexure Transverse colon Superior mesenteric artery Haustrum Ascending colon IIeum IIeocecal valve Left colic (splenic) flexure Transverse mesocolon Epiploic appendages Descending colon Cut edge of mesentery Teniae coli Cecum Sigmoid colon Vermiform appendix (a) Rectum Anal canal External anal sphincter Figure 23.21a Gross Anatomy of Large Intestine Rectal valve Rectum Hemorrhoidal veins Levator ani muscle Anal canal External anal sphincter Internal anal sphincter Anal columns Anal valves Pectinate line Anal sinuses Anus (b) Figure 23.21b Microscopic Anatomy of Large Intestine Villi are absent Contains numerous goblet cells Intestinal crypts simple tubular glands Lined with simple columnar epithelial tissue Epithelium changes at anal canal Becomes stratified squamous epithelium 13
The Liver Largest gland in the body Performs over 500 functions Digestive function bile production Performs many metabolic functions Visceral Surface of the Liver Left lobe Ligamentum venosum in fissure Porta hepatis Left hepatic artery Hepatic portal vein Caudate lobe Hepatic veins Inferior vena cava Bare area Right hepatic artery Right hepatic duct Left hepatic duct (b) Common hepatic duct Fissure Falciform ligament Ligamentum teres Quadrate lobe Cystic duct Gallbladder Right lobe Figure 23.25b Microscopic Anatomy of Liver (a) Lobule (b) Central vein Connective tissue septum Interlobular veins (to hepatic vein) Central vein Sinusoids Plates of hepatocytes Bile canaliculi Bile duct (receives bile from bile canaliculi) Fenestrated lining (endothelial cells) of sinusoids Portal vein Hepatic macrophages in sinusoid walls Bile duct Portal venule Portal triad Portal arteriole (c) Figure 23.26 14
The Gallbladder Stores and concentrates bile Expels bile into duodenum Bile emulsifies fats Cholecystokinin released from enteroendocrine cells in response to fatty chyme The Gallbladder Right and left hepatic ducts of liver Common hepatic duct Cystic duct Bile duct and sphincter Accessory pancreatic duct Mucosa with folds Gallbladder Major duodenal papilla Hepatopancreatic ampulla and sphincter Duodenum Tail of pancreas Pancreas Jejunum Main pancreatic duct and sphincter Head of pancreas Figure 23.19 The Pancreas Pancreatic duct penetrates duodenal wall Endocrine functions Insulin (beta cells) and glucagons (alpha cells) Exocrine functions Acinar cells make, store, and secrete pancreatic enzymes Majority of pancreatic secretions Pancreatic juice secreted into small intestine ti Carbohydrases Lipases Nucleases Proteolytic enzymes Figure 23.27 15
The Pancreas Figure 23.27 Disorders of the Digestive System Intestinal obstruction Mechanical obstructions Adhesions, tumors, or foreign objects Nonmechanical obstruction Halt in peristalsis Trauma Intestines touched during surgery Disorders of the Digestive System Inflammatory bowel disease Inflammation of intestinal wall Crohn s disease Ulcerative colitis Viral hepatitis jaundice and flu-like symptoms Major types A, B, C, and G Cystic Fibrosis and the Pancreas 16
The Digestive System in Later Life Middle age gallstones and ulcers Old age activity of digestive organs decline Fewer digestive juices and enzymes produced Absorption is less efficient Dehydration of fecal mass leads to constipation Diverticulosis and cancer of digestive organs 17