The Digestive System Dr. Holly Nash-Rule

Overview of the Digestive System Organs are divided into two groups Alimentary canal Mouth, pharynx, and esophagus Stomach, small intestine, and large intestine Accessory digestive organs Teeth and tongue (sort of) Gallbladder, salivary glands, liver, and pancreas Accessory organs are connected to the alimentary canal by ducts Secretions contribute to breakdown of foodstuffs

Digestive Processes Ingestion—occurs in the mouth Propulsion—movement of food Peristalsis—major means of propulsion Mechanical digestion—prepares food for chemical digestion Chewing, churning food in stomach, segmentation Segmentation is rhythmic local constrictions of intestine

Digestive Processes Chemical digestion—complex molecules broken down to chemical components Mouth Stomach Small intestine Absorption—transport of digested nutrients Small intestines, primarily jejunum Defecation—elimination of indigestible substances as feces

Peristalsis & Segmentation Major means of propulsion Adjacent segments of the alimentary canal relax and contract Segmentation Rhythmic local contractions of the intestine Mixes food with digestive juices Peristalsis Figure 23.3a

Abdominal Regions Four lines divide abdominal wall into nine regions Midclavicular lines—vertical lines of grid Subcostal plane—superior horizontal line Connects inferior points of costal margin Transtubercular plane—inferior horizontal line Connects tubercles of iliac crests

Abdominal Quadrants A simpler method of sectioning the anterior abdominal wall Right upper quadrant Left upper quadrant Right lower quadrant Left lower quadrant

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 Retroperitoneal organs Behind the peritoneum Peritoneal organs Digestive organs that keep their mesentery

The Peritoneal Cavity and Peritoneum Mesentery—a double layer of peritoneum Holds organs in place Sites of fat storage Provides a route for circulatory vessels and nerves

Mesenteries Lesser omentum attaches to lesser curvature of stomach Greater omentum—a “fatty apron” of peritoneum Greater omentum and transverse colon reflected Mesenteric Arteries Figure 23.6c

Intraperitoneal and Secondarily Retroperitoneal Organs Initially formed within peritoneum Become retroperitoneal Fuse to posterior abdominal wall Table 23.1

Histology of the Alimentary Canal Wall Same four layers from esophagus to anus The mucosa—innermost layer Consists of Epithelium Muscularis mucosae The submucosa—external to the mucosa Contains blood and lymphatic vessels, nerve fibers

Histology of the Alimentary Canal Wall The muscularis externa—external to the submucosa Two layers Circular muscularis—inner layer Longitudinal muscularis—outer layer The serosa—the outermost layer Is the visceral peritoneum

Smooth Muscle Primarily found in walls of viscera Fibers elongated Have one centrally located nucleus Grouped into sheets Longitudinal layer—parallel to long axis of organ Circular layer—deeper layer, fibers run around circumference of organ

Nerve Plexuses Myenteric nerve plexus Lies between circular and longitudinal muscularis Controls peristalsis and segmentation Submucosal nerve plexus Lies in submucosa Signals glands to secrete Innervation Sympathetic and parasympathetic motor fibers Visceral sensory fibers

The Mouth and Associated Organs The mouth—oral cavity Mucosal layer Stratified squamous epithelium The lips and cheeks Formed from orbicularis oris and buccinator muscles, respectively

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 Lingual frenulum Secures tongue to floor of mouth

The Superior Surface of the Tongue Tongue papillae Filiform papillae—no taste buds Fungiform papillae Circumvallate papillae Sulcus terminalis Marks border between mouth and pharynx Posterior one-third of tongue lies in oropharynx Lined with lingual tonsil Tongue papillar

The Teeth 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 2I, 1C, 2P, 3M Teeth

The Salivary Glands Produce saliva Parotid glands Parotid duct—parallel to zygomatic arch Submandibular glands Lay along medial surface of mandible Sublingual glands Lay in floor of oral cavity

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 Cardiac sphincter (lower esophageal sphincter) — closes lumen to prevent stomach acid from entering esophagus

The Stomach Site where food is churned into chyme Secretion of pepsin begins protein digestion Functions under acidic conditions Food remains in stomach approximately 4 hours Regions of the stomach Cardiac region Fundus Body Pyloric region Healthy stomach

Microscopic Anatomy of the Stomach Muscularis has three layers Circular and longitudinal layers and oblique layer Epithelium is simple columnar epithelium Mucosa dotted with gastric pits Gastric glands—deep in gastric pits

Microscopic Anatomy of the Stomach Gastric glands of fundus and body Mucous neck cells Secrete a special mucus Parietal cells Secrete hydrochloric acid and gastric intrinsic factor Chief (zymogenic) cells Secrete pepsinogen Pepsinogen is activated to pepsin when it encounters acid in the gastric glands

The Small Intestine—Gross Anatomy Longest portion of the alimentary canal Site of most enzymatic digestion and absorption Three subdivisions Duodenum Jejunum Ileum Innervation Parasympathetic fibers from vagus nerve Sympathetic from thoracic splanchnic nerves

The Duodenum Receives chyme, digestive enzymes and bile Main pancreatic duct and common bile duct enter duodenum Sphincters control entry of bile and pancreatic enzymes

The Small Intestine—Microscopic Anatomy Modifications for absorption Circular folds Transverse ridges of mucosa and submucosa Villi Finger-like projections of the mucosa Covered with simple columnar epithelium Microvilli Further increase surface area for absorption

Histology of the Intestinal Wall Absorptive cells Uptake digested nutrients Goblet cells Secrete mucus that lubricates chyme Enteroendocrine cells Secrete hormones Intestinal crypts Epithelial cells secrete intestinal enzymes

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 Cecum Blind pouch Beginning of large intestine Vermiform appendix Contains lymphoid tissue Neutralizes pathogens Appendix

Gross Anatomy of Large Intestine Colon Divided into distinct segments Ascending, transverse, descending, and sigmoid Rectum Descends along the inferior half of the sacrum Anal canal The last subdivision of the large intestine Lined with stratified squamous epithelium

Vessels and Nerves of the Large Intestine First half of large intestine Arterial supply—superior mesenteric artery Innervation Sympathetic innervation—superior mesenteric and celiac ganglia Parasympathetic innervation—vagus nerve

Vessels and Nerves of the Large Intestine Distal half of large intestine Arterial supply—inferior mesenteric artery Innervation Sympathetic innervation—inferior mesenteric and hypogastric plexuses Parasympathetic innervation—pelvic splanchnic nerves

Microscopic Anatomy of Large Intestine Villi are absent Contains numerous goblet cells Intestinal crypts Lined with simple columnar epithelial tissue Epithelium changes at anal canal Becomes stratified squamous epithelium

The Liver Largest gland in the body Performs over 500 functions Digestive function Bile production Performs many metabolic functions

Microscopic Anatomy of the Liver Hepatocyte—functional cells of the liver Portal triad composed of Bile duct tributary Branch of hepatic portal vein Branch of hepatic artery Kupffer cells—destroy bacteria

Microscopic Anatomy of the Liver Some functions of hepatocytes Rough ER manufactures blood proteins Smooth ER produces bile salts, detoxifies poisons Peroxisomes detoxify poisons (alcohol) Golgi apparatus packages secretory products Mitochondria provide energy for liver processes Glycosomes store sugar Great capacity for regeneration

The Gallbladder Stores and concentrates bile Expels bile into duodenum Bile emulsifies fats Cholecystokinin—released from enteroendocrine cells in response to fatty chyme Gallbladder

The Pancreas Exocrine function Acinar cells make, store, and secrete pancreatic enzymes Enzymes are activated in the duodenum Endocrine function Produces insulin and glucagon Regulates blood sugar

Peptic Ulcers Are erosions of the mucosa of a region of the alimentary canal Gastric ulcers Occur in pyloric region of the stomach Duodenal ulcers Occur in duodenum of the small intestine

Peptic Ulcers Caused by Helicobacter pylori H. pylori Acid-resistant Binds to gastric epithelium Induces oversecretion of acid and inflammation

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

The End Dr. Holly Nash-Rule