Chapter 17 The Digestive System The Digestive System

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Chapter 17
The Digestive System

The Digestive System

  • Alimentary canal or GI tract

  • Extends from mouth to anus—9 m (29 feet)

  • Involved in digestion, absorption and metabolism of nutrients

  • System includes main and accessory organs

  • Main organs: mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anal canal

  • Accessory organs: teeth and tongue, salivary glands, liver, gallbladder, pancreas, and vermiform appendix


  • Roof—formed by hard palate (parts of maxillary and palatine bones) and soft palate (an arch-shaped muscle separating mouth from pharynx); uvula, a downward projection of soft palate


  • Floor—formed by tongue and its muscles; papillae, small elevations on mucosa of tongue; taste buds, found in many papillae; lingual frenulum, fold of mucous membrane that helps anchor the tongue to the floor of the mouth


  • Names of teeth—incisors, cuspids, bicuspids, and tricuspids

  • Twenty teeth in temporary set; average age for cutting first tooth about 6 months; set complete at about 2 years of age


  • Thirty-two teeth in permanent set; 6 years is average age for starting to cut first permanent tooth; set complete usually between ages of 17 and 24 years

  • Structures of a typical tooth—crown, neck, and root

Disorders of the
Mouth and Teeth

  • Problems include infections, cancer and congenital defects

  • Infections and cancer may spread

  • Leukoplakia—precancerous condition of mouth tissue

  • Snuff dipper’s pouch—caused by use of chewing tobacco

  • Squamous cell carcinoma—most common form of mouth cancer

Disorders of the
Mouth and Teeth

  • Dental caries

  • Tooth disease resulting in permanent defect called “cavity”

  • Infection may spread to other adjacent tissues or to blood

  • Lost or diseased teeth may be replaced by dentures or implants

  • Gingivitis—gum inflammation or infection

  • May result from poor oral hygiene, diabetes, vitamin deficiency, or pregnancy

Disorders of the
Mouth and Teeth

  • Periodontitis—inflammation of periodontal membrane

  • Often a complication of advanced or untreated gingivitis

  • Leading cause of tooth loss among adults

  • Thrush or oral candidiasis—caused by yeast-like fungal organism

  • Patches of “cheesy” looking exudate form over an inflamed tongue and oral mucosa which itches and bleeds easily

  • Common in immunosuppressed individuals (AIDS) or after antibiotic therapy

Disorders of the
Mouth and Teeth

  • Congenital defects of mouth

  • Cleft lip and cleft palate are most common types

  • May occur alone or together
  • Caused by failure of mouth structures to fuse during embryonic development

Salivary Glands

  • Three pairs of glands produce about 1 liter of saliva each day—located outside of GI tract—convey secretions via ducts into tract lumen

  • Parotid glands

  • Largest of salivary glands—located in front of ear at angle of jaw

  • Ducts open into mouth opposite second molars

  • Inflamed in mumps

Salivary Glands

  • Submandibular glands—ducts open on either side of lingual frenulum

  • Sublingual glands—ducts open into floor of mouth

  • Saliva contains salivary amylase—begins digestion of carbohydrates


  • Muscular tube lined with mucous membrane

  • Functions as part of both respiratory and digestive systems

  • Subdivided into three anatomical segments

Wall of the Digestive Tract

  • The wall of the digestive tube is formed by four layers of tissue

  • Mucosa—mucous epithelium

  • Submucosa—connective tissue

  • Muscularis—two layers of smooth muscle

  • Serosa—serous membrane that covers the outside of abdominal organs; it attaches the digestive tract to the wall of the abdominopelvic cavity by forming folds called mesenteries


  • Muscular, mucus-lined tube about 25 cm (10 inches) long

  • Connects pharynx with stomach

  • Muscular walls help push food toward stomach

  • Sphincters in GI tract help keep ingested material moving in one direction down the tube

  • Each end of esophagus “guarded” by a sphincter—upper esophageal sphincter (UES) and lower esophageal sphincter (LES)


  • GERD—gastroesophageal reflux disease (severe/ongoing heartburn); backflow of acidic stomach contents into esophagus

  • Causes include problem foods and hiatal hernia

  • Nonsurgical treatments include dietary changes, weight loss, acid blocking medications, and drugs which strengthen LES


  • Surgical treatments include fundoplication, Stretta, and Bard endoscopic suturing procedures


  • Size—expands after large meal; about size of large sausage when empty

  • Pylorus—lower part of stomach; pyloric sphincter muscle closes opening of pylorus into duodenum

  • Wall—many smooth muscle fibers; contractions produce churning movements (peristalsis)


  • Lining—mucous membrane; many microscopic glands that secrete gastric juice and hydrochloric acid into stomach; mucous membrane lies in folds (rugae) when stomach is empty

Disorders of the Stomach

  • Gastroenterology—study of stomach and intestines and their diseases; gastric diseases often exhibit these signs or symptoms: gastritis (inflammation), anorexia (appetite loss), nausea (upset stomach), and emesis (vomiting)

  • Pylorospasm—abnormal spasms of the pyloric sphincter; pyloric stenosis is similar, also narrowing the pyloric opening

Disorders of the Stomach

  • Ulcers—open wounds caused by acid in gastric juice

  • Often occur in duodenum or stomach

  • Associated with infection by the bacterium Helicobacter pylori and use of NSAIDs

  • Stomach cancer is associated with consumption of alcohol or preserved food and use of chewing tobacco

Small Intestine

  • Size—about 7 meters (20 feet) long but only 2 cm or so in diameter

  • Divisions

  • Duodenum

  • Jejunum

  • Ileum

  • Wall—contains smooth muscle fibers that contract to produce peristalsis

Small Intestine

  • Lining—mucous membrane; many microscopic glands (intestinal glands) secrete intestinal juice; villi (microscopic finger-shaped projections from surface of mucosa into intestinal cavity) contain blood and lymph capillaries

Disorders of the Small Intestine

  • Enteritis—intestinal inflammation; gastroenteritis—inflammation of stomach and intestines

  • Malabsorption syndrome—group of symptoms resulting from failure to absorb nutrients properly (e.g., anorexia, abdominal bloating, cramps, anemia, and fatigue)

Liver and Gallbladder

  • Size and location—liver is largest gland; fills upper right section of abdominal cavity and extends over into left side

  • Liver secretes bile

  • Ducts

  • Hepatic—drains bile from liver

Liver and Gallbladder

  • Cystic—duct by which bile enters and leaves gallbladder

  • Common bile—formed by union of hepatic and cystic ducts; drains bile from hepatic or cystic ducts into duodenum

Liver and Gallbladder

  • Gallbladder

  • Location—undersurface of the liver

  • Function—concentrates and stores bile produced in the liver

  • Disorders of the liver and gallbladder

  • Gallstones—calculi (stones) made of crystallized bile pigments and calcium salts

Liver and Gallbladder

  • Cholelithiasis—condition of having gallstones (Figure 17-13)

  • Cholecystitis—inflammation of the gallbladder; may accompany cholelithiasis

  • Can obstruct bile canals, causing jaundice

Liver and Gallbladder

  • Hepatitis—liver inflammation

  • Characterized by liver enlargement, jaundice, anorexia, discomfort, gray–white feces, and dark urine

  • Caused by a variety of factors—toxins, bacteria, viruses, and parasites

Liver and Gallbladder

  • Cirrhosis—degeneration of liver tissue involving replacement of normal (but damaged) tissue with fibrous and fatty tissue

  • Portal hypertension—high blood pressure in the hepatic portal veins caused by obstruction of blood flow in a diseased liver; may cause varicosities of surrounding systemic veins


  • Location—behind stomach

  • B Functions

  • Pancreatic cells secrete pancreatic juice into pancreatic ducts; main duct empties into duodenum

  • Pancreatic islets (of Langerhans)—cells not connected with pancreatic ducts; secrete hormones glucagons and insulin into the blood


  • Pancreatic disorders

  • Pancreatitis—inflammation of pancreas; acute pancreatitis results from blocked ducts that force pancreatic juice to backflow, digesting the gland

  • Cystic fibrosis—thick secretions block flow of pancreatic juice

  • Pancreatic cancer is very serious—fatal in the majority of cases

Large Intestine

  • Divisions

  • Cecum

  • Colon—ascending, transverse, descending, and sigmoid

  • Rectum

  • Opening to exterior—anus

Large Intestine

  • Wall—contains smooth muscle fibers that contract to produce churning, peristalsis, and defecation

  • Lining—mucous membrane

Large Intestine

  • Disorders of the large intestine often relate to abnormal motility (rate of movement of contents)

  • Diarrhea results from abnormally increased intestinal motility; may result in dehydration or convulsions

  • Constipation results from decreased intestinal motility

Large Intestine

  • Diverticulitis (inflammation of abnormal outpouchings called diverticula) may cause constipation

  • Colitis is the general name for any inflammatory condition of the large intestine

  • Colorectal cancer is a common malignancy of the colon and rectum associated with colonic polyps; advanced age; low-fiber, high-fat diets; and genetic predisposition

Appendix and Appendicitis

  • Blind tube off cecum; no important digestive function in humans

  • Appendicitis—inflammation or infection of appendix; if appendix ruptures, infectious material may spread to other organs


  • Definitions—peritoneum, serous membrane lining abdominal cavity and covering abdominal organs; parietal layer of peritoneum lines abdominal cavity; visceral layer of peritoneum covers abdominal organs; peritoneal space lies between parietal and visceral layers


  • Extensions—largest ones are the mesentery and greater omentum; mesentery is extension of parietal peritoneum, which attaches most of small intestine to posterior abdominal wall; greater omentum, or “lace apron,” hangs down from lower edge of stomach and transverse colon over intestines


  • Peritonitis—inflammation of peritoneum resulting from infection or other irritant; often a complication of ruptured appendix

  • Ascites—abnormal accumulation of fluid in peritoneal space, often causing bloating of abdomen


  • Meaning—changing foods so that they can be absorbed and used by cells

  • Mechanical digestion—chewing, swallowing, and peristalsis break food into tiny particles, mix them well with digestive juices, and move them along the digestive tract


  • Chemical digestion—breaks up large food molecules into compounds having smaller molecules; brought about by digestive enzymes


  • Carbohydrate digestion—mainly in small intestine

  • Pancreatic amylase—changes starches to maltose

  • Intestinal juice enzymes

  • Maltase—changes maltose to glucose

  • Sucrase—changes sucrose to glucose

  • Lactase—changes lactose to glucose


  • Protein digestion—starts in stomach; completed in small intestine

  • Gastric juice enzymes, rennin and pepsin, partially digest proteins

  • Pancreatic enzyme, trypsin, completes digestion of proteins to amino acids

  • Intestinal enzymes, peptidases, complete digestion of partially digested proteins to amino acids


  • Fat digestion

  • Bile contains no enzymes but emulsifies fats (breaks fat droplets into very small droplets)

  • Pancreatic lipase changes emulsified fats to fatty acids and glycerol in small intestine


  • Meaning—digested food moves from intestine into blood or lymph

  • Where absorption occurs—foods and most water from small intestine; some water also absorbed from large intestine

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