Chapter 17: Digestive System I. Introduction



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Shier, Butler, and Lewis: Hole’s Human Anatomy and Physiology, 10th ed. Chapter 17: Digestive System

Chapter 17: Digestive System




I. Introduction


A. Digestion is the breakdown of foods into forms that cell membranes can absorb.

B. Mechanical digestion breaks large pieces of food into smaller ones without altering their chemical composition.

C. Chemical digestion breaks down food into simpler chemicals.

D. The organs of the digestive system carry out the processes of ingestion, propulsion, absorptions, defecation, and digestion.

E. The alimentary canal is composed of the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anal canal.

F. The accessory organs of the digestive system are salivary glands, liver, gallbladder, and pancreas.


II. General Characteristics of the Alimentary Canal


A. Introduction

1. The alimentary canal is a muscular tube that passes through the body’s ventral cavity.

2. The structure of its wall, how it moves food, and its innervation are similar throughout its length.

B. Structure of the Wall

1. The four layers of the alimentary wall are the mucosa, submucosa, muscular layer, and serosa.

2. The mucosa is located as the inner lining and is composed of epithelial tissue, a small amount of connective tissue, and some smooth muscle.

3. The functions of the mucosa are to secrete mucus and enzymes, and to absorb nutrients.

4. The submucosa is located deep to the mucosa and is composed of loose connective tissue, glands, blood vessels, lymphatic vessels, and nerves.

5. The functions of the submucosa are to nourish surrounding tissues and to carry away absorbed substances.

6. The muscular layer is located between the submucosa and serosa and is composed of two coats of smooth muscle tissue.

7. When the circular fibers contract, the diameter of the tube decreases.

8. When the longitudinal fibers contract, the tube shortens.

9. The serosa layer is located superficial to the muscular layer and is composed of the visceral peritoneum.

10. The functions of the serosa are to moisten and lubricate the outside of the organ.

C. Movements of the Tube

1. The two types of motor functions of the alimentary canal are mixing and propelling.

2. Mixing occurs when smooth muscles in small segments of the tube contract rhythmically.

3. Peristalsis is a wavelike motion.

4. Peristalsis occurs when a ring of contraction moves down the wall of the tube.

D. Innervation of the Tube

1. Branches of the sympathetic and parasympathetic nervous system innervate the alimentary canal.

2. The innervation of the alimentary canal maintains muscular tone

and regulates strength, rate, and velocity of muscular contractions.

3. The submucosal plexus is important for controlling secretions by the gastrointestinal tract.

4. The myenteric plexus is important for gastrointestinal motility.

5. The functions of parasympathetic impulses are to increase the activities of the digestive system.

6. The functions of sympathetic impulses are decrease the activities of the digestive system.

III. Mouth


A. Introduction

1. The functions of the mouth are to receive food and to begin digestion.

2. Mastication is chewing.

3. The mouth is surrounded by lips, cheek, tongue, and palate.

4. The oral cavity is the space between the tongue and palate.

5. The vestibule of the mouth is the space between the teeth, cheeks, and lips.

B. Cheeks and Lips

1. The cheeks form the lateral walls of the mouth and consist of skin, fat, muscles, and an inner moist lining.

2. The lips surround the mouth opening and consist of skeletal muscles, sensory receptors, and skin.

3. The reddish color of lips is due to the many blood vessels near their surfaces.

C. Tongue

1. The tongue is located in the floor of the oral cavity.

2. Mucous membranes cover the tongue.

3. The frenulum of the tongue is a membranous fold that anchors the tongue to the floor of the mouth.

4. The body of the tongue is composed of skeletal muscles.

5. Muscles of the tongue function to mix food particles and push food to the back of the throat during swallowing.

6. Papillae of the tongue are rough projections on the surface of the tongue.

7. Functions of papillae are to provide friction and to house taste buds.

8. The root of the tongue is anchored to the hyoid bone.

9. Lingual tonsils are located on the root of the tongue.

D. Palate

1. The palate forms the roof of the oral cavity and consists of a hard part and a soft part.

2. The hard palate is formed by the palatine processes of the maxillary bones and palatine bones.

3. The soft palate is formed by a mucous membrane and muscles.

4. The uvula is a downward extension of the soft palate.

5. The function of the uvula is to prevent food or liquids from entering the nasal cavity.

6. Palatine tonsils are located in the back of the mouth on either side of the palate.

7. Pharyngeal tonsils are located on the posterior wall of the throat above the soft palate.

E. Teeth

1. The primary teeth are the first set of teeth to develop.

2. The secondary teeth are the permanent teeth.

3. The secondary teeth consist of 32 teeth.

4. The arrangement of secondary teeth are two incisors, cuspid, two bicuspids, and three molars (from midline to back).

5. Wisdom teeth are the third set of molars.

6. Chewing increases the surface area of food particles.

7. Incisors are specialized to bite off large pieces of food.

8. Cuspids are specialized to grasp and tear food.

9. Bicuspids and molars are specialized to grind food.

10. The crown of a tooth is the portion of the tooth above the gum line.

11. The root of a tooth is the portion of the tooth below the gum line.

12. The neck of a tooth is the area where the crown and root meet.

13. Enamel consists of calcium salts.

14. Dentin is living cellular tissue beneath enamel.

15. The root canal is located in the root of a tooth and contains blood vessels and nerves.

16. The pulp cavity is located in the crown of the tooth and contains blood vessels, nerves and connective tissue called pulp.

17. Cementum is bonelike material that surrounds the root.

18. A periodontal ligament is a fibrous structure that surrounds cementum and anchors the tooth to the jaw.

IV. Salivary Glands


A. Introduction

1. Salivary glands secrete saliva.

2. The functions of saliva are to moisten food, bind food together, and begin the chemical digestion of carbohydrates.

3. The three pairs of major salivary glands are parotid glands, submandibular glands, and sublingual glands.

B. Salivary Secretions

1. Two cell types of salivary glands are serous and mucous.

2. Serous cells produce watery fluid that contains amylase.

3. Mucous cells produce mucus.

4. Amylase digests carbohydrates.

5. Salivary glands are innervated by both sympathetic and parasympathetic nerves.

6. Sympathetic fibers stimulate the glands to secrete a small amount of viscous saliva.

7. Parasympathetic fibers stimulate the glands to large amounts of watery saliva.

C. Major Salivary Glands

1. The largest of the major salivary glands is the parotid.

2. The parotid glands are located anterior and inferior to each ear.

3. A parotid duct is located within the buccinator muscle and opens into the inside of the cheek about the level of the second molar.

4. The parotid glands secrete a water fluid rich in amylase.

5. The submandibular glands are located in the floor of the mouth on the inside surfaces of the lower jaws.

6. The submandibular glands secrete primarily serous fluid.

7. Ducts of submandibular glands open inferior to the tongue.

8. The sublingual glands are located inferior to the tongue.

9. The sublingual glands secrete primarily mucus.

10. The ducts of sublingual glands open beneath the tongue.

V. Pharynx and Esophagus


A. Introduction

1. The pharynx is a cavity posterior to the nasal and oral cavities.

2. The pharynx and esophagus function in swallowing.

B. Structure of the Pharynx

1. The pharynx connects the nasal and oral cavities with the larynx and esophagus.

2. The three divisions of the pharynx are the nasopharynx, oropharynx, and laryngopharynx.

3. The nasopharynx is located behind the nasal cavity.

4. The nasopharynx provides a passageway for air.

5. The oropharynx is located behind the oral cavity.

6. The oropharynx is a passageway for food and air.

7. The laryngopharynx is located just inferior to the oropharynx.

8. The laryngopharynx is a passageway to the esophagus.

9. Constrictor muscles function to pull the pharyngeal walls inward during swallowing.

C. Swallowing Mechanism

1. The events of the first stage of swallowing are chewing of food and the mixing of food with saliva.

2. The events of the second stage of swallowing are pushing of food toward the pharynx and the triggering of the swallowing reflex.

3. The events of the third stage of swallowing are movements of food through the esophagus and to the stomach.

4. The actions of the swallowing reflex are raising of soft palate, elevation of larynx and hyoid bone, pressing of tongue against soft palate, contraction of pharyngeal muscles, opening of the esophagus, and movement of food into the esophagus.

D. Esophagus

1. The esophagus is a passageway for food.

2. The esophagus propels food from the pharynx to the stomach.

3. The esophagus descends through the thoracic cavity.

4. The esophageal hiatus is an opening in the diaphragm.

5. Mucous glands are scattered throughout the submucosa of the esophagus.

6. The lower esophageal sphincter is located where the esophagus and stomach join and functions to prevent regurgitation of food.

VII. Stomach


A. Introduction

1. The shape of the stomach is J-shaped.

2. The location of the stomach is just inferior to the diaphragm.

3. Rugae are thick folds in the lining of the stomach.

4. The functions of the stomach are to mix food with gastric juice, begin protein digestion, to begin a small amount of absorption, and movement of food into the small intestine.

B. Parts of the Stomach

1. The four parts of the stomach are cardiac portion, body, fundus, and plylorus.

2. The cardiac region is the region near the esophageal opening.

3. The fundic region is a pouch that extends superior to the cardiac portion.

4. The body of the stomach is the main part of the stomach.

5. The pyloric region is the narrow region that is continuous with the small intestine.

6. The pyloric sphincter is located between the pylorus and the duodenum and functions to control the movement of food into the small intestine.

C. Gastric Secretions

1. Gastric pits are openings of gastric glands.

2. The three cell types of gastric glands are parietal, chief, and mucous.

3. Mucous cells secrete mucus.

4. Chief cells secrete digestive enzymes.

5. Parietal cells secrete hydrochloric acid and intrinsic factor.

6. Gastric juice is a mixture of the secretions of mucous, parietal, and chief cells.

7. Pepsin is an enzyme that digests proteins.

8. The function of pepsinogen is to be converted to pepsin when needed.

9. The function of hydrochloric acid in the stomach is to convert pepsinogen into pepsin and to destroy pathogens.

10. The coating of the stomach is important for protecting the stomach wall from digestive enzymes and acids.

11. The function of intrinsic factor is enhance the absorption of vitamin B12.

D. Regulation of Gastric Secretions

1. Somatostatin is produced in the stomach and functions to inhibit acid secretion.

2. Parasympathetic innervation stimulates the release of gastric juice.

3. Gastrin is produced the stomach and functions to increase the secretory activity of gastric glands.

4. The three stages of gastric secretion are cephalic, gastric, and intestinal.

5. The events of the cephalic phase are secretion of gastric juice before food enters the stomach.

6. The events of the gastric phase are distension of the stomach and the release of more gastric juice.

7. The events of the intestinal phase are the movement of food into the small intestine.

8. Cholecystokinin is produced by the small intestine and functions to inhibit gastric secretions.

E. Gastric Absorption

1. The stomach absorbs alcohol, some drugs, salts, and a small amount of water.

2. Most nutrients are absorbed in the small intestine.

F. Mixing and Emptying Actions

1. A stomachache results from the rise of internal pressure in the stomach.

2. Chyme is food substances that have been mixed with gastric juice.

3. Peristaltic waves push chyme toward the pylorus of the stomach.

4. Stomach contractions push chyme a little at a time into the duodenum

and backwards into the stomach, mixing it further.

5. The lower esophageal sphincter prevents regurgitation of food.

6. The rate at which the stomach empties depends on the fluidity of the chyme and its contents.

7. Liquids usually pass first through the stomach.

8. The enterogastric reflex is a reflex involving the small intestine and the stomach. It is triggered by distension of the small intestine wall and inhibits peristalsis in the stomach to slow down movement of food into the duodenum.

9. Vomiting results from a complex reflex that empties the stomach in the reverse of the normal direction.

VIII. Pancreas

A. Structure of the Pancreas

1. The pancreas is located close to the duodenum posterior to the parietal peritoneum.

2. Pancreatic acinar cells produce digestive enzymes and make up the bulk of the pancreas.

3. Acini are clusters of acinar cells.

4. The pancreatic ducts extends the hepatopancreatic ampulla and empties into the duodenum.

5. Hepatopancreatic ampulla is a dilated tube that receives the pancreatic duct and hepatic duct.

6. The hepatopancreatic sphincter is the sphincter that surrounds the hepatopancreatic ampulla.

B. Pancreatic Juice

1. Pancreatic juice contains many enzymes and bicarbonate ions.

2. The function of pancreatic amylase is to digest carbohydrates.

3. The function of pancreatic lipase is digest lipids.

4. The functions of trypsin, chymotrypsin, and carboxypeptidase are to digest proteins.

5. Zymogen granules are granules that store pancreatic enzymes.

6. The function of trypsinogen is to be converted to trypsin.

7. The functions of nucleases are to digest nucleic acids.

C. Regulation of Pancreatic Secretion

1. Parasympathetic fibers cause the pancreas to release pancreatic juice.

2. The function of secretin is to stimulate the pancreas to release pancreatic juice with a high concentration of bicarbonate ions.

3. The release of cholecystokinin is triggered by the presence of chyme in the small intestine.

4. The action of cholecystokinin on the pancreas is to release pancreatic juice that has a high concentration of digestive enzymes.

IX. Liver

A. Introduction

1. The largest internal organ is the liver.

2. The liver is located in the upper right abdominal quadrant.

B. Liver Structure

1. The two large lobes of the liver are the right and left.

2. The falciform ligament is a fold that separates the lobes of the liver and anchors the liver to the posterior abdominal wall.

3. The two small lobes of the liver are caudate and quadrate.

4. The porta hepatis is where blood vessels and ducts enter or exit the liver.

5. The coronary ligament is a ligament that attaches the liver to the diaphragm.

6. Hepatic lobules are divisions of a liver lobe.

7. A hepatic lobule consists of many hepatic cells radiating outward from a central vein.

8. Hepatic sinusoids are vascular channels in hepatic lobules.

9. Kupffer cells are macrophages of the liver.

10. Bile canals are canals within liver lobules that receive secretions from hepatic cells.

11. Hepatic ducts are formed from bile canals of neighboring hepatic lobules.

C. Liver Functions

1. The liver carries on many important metabolic functions.

2. The liver plays a key role in carbohydrate metabolism by helping maintain the normal blood glucose concentrations.

3. The liver plays a key role in lipid metabolism by oxidizing fatty acids, synthesizing lipoproteins, phospholipids, and cholesterol.

4. The liver plays a key role in protein metabolism by deaminating amino acids, forming urea, synthesizing plasma proteins, and converting amino acids to other forms of amino acids.

5. The liver stores glycogen, iron, and vitamins A,D, and B12.

6. Liver cells help destroy worn out red blood cells.

7. The liver removes toxic substances from the blood.

8. The liver’s role in digestion is to secrete bile.

D. Composition of Bile

1. Bile is secreted by hepatic cells.

2. Bile contains water, bile salts, bile pigments, cholesterol, and electrolytes.

3. Hepatic cells use cholesterol to make bile salts.

4. Bile pigments are products of the breakdown of hemoglobin.

5. Jaundice results from an accumulation of bile pigments in the blood stream.

E. Gallbladder

1. The gallbladder is located inferior to the liver.

2. The cystic duct is the duct of the gallbladder and opens into the common bile duct.

3. The common bile duct is formed from the cystic duct and common hepatic duct and opens into duodenum.

4. Gallstones form when bile is too concentrated.

F. Regulation of Bile Release

1. Cholecystokinin triggers the gallbladder to release bile.

2. Cholecystokinin is released in response to presence of lipids and proteins in the small intestine.

G. Functions of Bile Salts

1. Functions of bile salts are to aid in the absorption of fatty acids, and certain vitamins, and to emulsify fats.

2. Emulsification is the breaking of fat globules into smaller droplets.

3. Lack of bile salts results in poor lipid absorption and vitamin deficiencies.

X. Small Intestine


A. Introduction

1. The small intestine extends from the stomach to the large intestine.

2. The small intestine receives secretions from the pancreas, gallbladder, and liver.

3. The functions of the small intestine are to complete digestion, absorption of nutrients, and movement of solid wastes to the large intestine.

B. Parts of the Small Intestine

1. The three parts of the small intestine are duodenum, jejunum, and ileum.

2. The duodenum is located posterior to the parietal peritoneum just beneath the stomach.

3. The jejunum is located in the abdominal cavity between the duodenum and ileum.

4. The ileum is located in the abdominal cavity between the jejunum and large intestine.

5. Mesentery is double-layered fold of peritoneum and supports the blood vessels, nerves, and lymphatic vessels that supply the intestinal wall.

6. The greater omentum is a double fold of peritoneal membrane that drapes like an apron from the stomach, over the transverses colon, and the small intestine.

7. The functions of the omentum are to prevent the spread of infections in the peritoneal cavity.

C. Structure of the Small Intestinal Wall

1. The velvety appearance of the inner wall of the small intestine is due to intestinal villi.

2. Intestinal villi are tiny projections of the mucosa of the small intestine.

3. The functions of villi are to increase the surface area of the lining of the small intestine.

4. Each villus consists of a layer of simple columnar epithelium and a core of connective tissue containing blood capillaries, a lacteal, and nerves.

5. A lacteal is a lymphatic capillary.

6. Microvilli increase the surface area intestinal cells.

7. Intestinal glands are between the bases of adjacent villi.

8. Plicae circulares are circular folds in the mucosa of the small intestine.

D. Secretions of the Small Intestine

1. Brunner’s glands are mucous-secreting glands and are located in the submucosa of the proximal portion of the duodenum.

2. Brunner’s glands secrete alkaline mucus.

3. The enzymes secreted by epithelial cells of the small intestine are peptidase, sucrase, maltase, lactase, lipase, and enterokinase.

4. The functions of peptidases are to digest proteins.

5. The functions of sucrase, maltase, and lactase are to digest sucrose, maltose, and lactose.

6. The functions of intestinal lipase are to digest lipids.

E. Regulation of the Small Intestinal Secretions

1. Mechanical digestion and the presence of gastric juice in the small intestine stimulate the duodenal mucous glands to release large amounts of mucus.

2. Chyme stimulates the goblet cells and intestinal glands to secrete their products.

3. Distension of the intestinal wall stimulates the parasympathetic reflex that causes intestinal secretions.

F. Absorption in the Small Intestine

1. The most important absorbing organ is the small intestine.

2. Carbohydrate digestion begins in the mouth and is completed in the small intestine.

3. Monosaccharides are absorbed by facilitated diffusion and active transport.

4. Protein digestion begins in the stomach and is completed in the small intestine.

5. Amino acids are absorbed by active transport.

6. Fat molecules are digested almost entirely by the small intestine.

7. Chylomicrons are lipoproteins that contain lipids and proteins.

8. Chylomicrons are carried to the blood by lymph.

9. The ions absorbed by the intestinal villi are sodium, potassium, chloride, nitrate, and bicarbonate.

10. Water is absorbed by osmosis.

G. Movements of the Small Intestine

1. Segmentation is the major mixing movement of the small intestine.

2. Chyme moves slowly through the small intestine.

3. Parasympathetics enhance mixing and peristaltic movements and sympathetics inhibits mixing and peristaltic movements.

4. A peristaltic rush is the rapid sweeping of chyme into the large intestine.

5. Diarrhea results from a peristaltic rush.

6. The ileocecal sphincter joins the ileum and cecum.


XI. Large Intestine


A. Introduction

1. The large intestine is located primarily in the abdominal cavity and part of the pelvic cavity.

2. The functions of the large intestine are to form feces, eliminate solid wastes, and to absorb remaining water and electrolytes from chyme.

B. Parts of the Large Intestine

1. The parts of the large intestine are cecum, colon, rectum, and anal canal.

2. The cecum is the initial portion of the large intestine.

3. The vermiform appendix is located off the cecum and consists of lymphatic tissue.

4. The four parts of the colon are ascending colon, transverse colon, descending colon, and sigmoid colon.

5. The ascending colon is located on the right side of the abdominal cavity.

6. The transverse colon is located between the ascending and descending colon.

7. The descending colon is located on the left side of the abdominal cavity.

8. The sigmoid colon is an s-shaped portion of the colon off the descending colon.

9. The rectum is the continuation of the sigmoid colon.

10. The anal canal is the continuation of the rectum.

11. Anal columns are folds of mucous membranes in the anal canal.

12. The anus is the opening of the anal canal.

13. Two sphincters of the anus are the internal and external.

14. The internal anal sphincter is composed of smooth muscle.

15. The external anal sphincter is composed of skeletal muscle.

C. Structure of the Large Intestinal Wall

1. Teniae coli are bands of smooth muscle that extend the length of the large intestine.

2. Haustra are pouches of the large intestinal wall created by teniae coli.

3. Epiploic appendages are collections of fat in the serosa on the outer surface of the large intestine.

D. Functions of the Large Intestine

1. Mucus secretion into the large intestine is controlled by mechanical stimulation and parasympathetic impulses.

2. The functions of mucus in the large intestine are to form and store feces, eliminate feces, and absorb remaining water and electrolytes from chyme.

3. Chyme entering the large intestine contains few nutrients, nondigestible materials, water, electrolytes, mucus, and bacteria.

4. The large intestine can absorb water and electrolytes.

5. Intestinal flora is a bacterial population that exists in the large intestine.

6. The functions of intestinal flora are to synthesize some vitamins and to produce gas.

E. Movements of the Large Intestine

1. The movements of the large intestine are less frequent than those of the small intestine.

2. Mass movements are produced when a large section of the intestinal wall constricts vigorously.

3. The defecation reflex is triggered by holding a deep breath and contracting the abdominal wall muscles.

4. The actions of the defecation reflex are to increase internal abdominal pressure and the forcing go feces into the rectum. Peristaltic waves are triggered and anal sphincters relax.

5. A person can inhibit defecation by contracting the external anal sphincter.

F. Feces

1. Feces are composed of materials that were not digested or absorbed, some water, electrolytes, mucus, and bacteria.


2. The pungent odor or feces results from a variety of compounds that bacteria produce.

XII. Life-Span Changes


A. Maintaining healthy teeth requires frequent dental checks, cleaning and plaque removal, plus care of the gums.

B. The effects of aging on teeth include thinning of enamel, thickening of cementum, receding of gums, and loosening of teeth.

C. Dry mouths in elderly people are usually a result of side effects of drugs.

D. Frequent heartburn may be the result of the slowing of peristalsis in the stomach.

E. Effects of aging on the small intestine include decreased efficiency in absorbing nutrients and vitamins.

F. The effects of aging on the large intestine include thinning of the lining and decreased mucus production that leads to constipation.



G. The effects of aging on the pancreas, liver, and gallbladder include a decline in their secretions.


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