Download Digestive System, Chapter 19

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Digestive System, Chapter 19
Outline of class notes
Objectives - After studying this chapter you should be able to:
1. Describe the two main divisions of the organs of the digestive system and explain its basic
functions.
2. Describe the layers that form the wall of the gastrointestinal tract.
3. Describe the structure of the peritoneum and its main folds (extensions).
4. Describe the structures of the mouth including the palate, tongue, and salivary glands.
5. Identify the parts of a typical tooth and compare deciduous and permanent dentitions.
6. Describe the structures and function of the pharynx
7. Describe the structures and function of the esophagus
8. Describe the general structures, histology, and functions of the stomach.
9. Describe the general structures, histology, and functions of the small intestine. Be sure to
include the 3 adaptations of the intestinal wall to increase surface area.
10. Describe the general structures, histology, and functions of the large intestine.
11. Describe the general structures, histology, and functions of the liver. Be sure to include the
structure/function of the gall bladder and the duct system for bile flow.
12. Describe the general structures, histology, and functions of the pancreas.
13. Discuss the following clinical conditions: Peritonitis, Ankyloglossia, GERD, Hiatal hernia,
Lactose intolerance, Jaundice, and gallstones.
Digestive System Topics
• Divisions of the digestive system
– Alimentary canal
– Accessory digestive organs
• Functions
• Digestive organs
Purpose of the Digestive System
• Take it in
• Break it down
• Absorb the good stuff
• Eliminate the rest (wastes)
Divisions of the Digestive System
• Digestive system can be divided into 2 groups of organs:
 Gastrointestinal (GI) tract (alimentary canal).
– Continuous tube (30’ in cadaver) that extends from the mouth to the anus.
– Organs: Oral cavity, pharynx, esophagus, stomach, small intestine, large
intestine and anus
 Accessory digestive organs
– Contribute to the processes of digestion and absorption; but no food or food
waste actually passes thru them.
– Organs: Teeth, tongue, salivary glands, liver, gallbladder, and pancreas
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Functions of Digestive System
• Ingestion: Taking food into the mouth
 Mastication: Chewing - process of breaking large food particles into smaller food
particles.
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Propulsion: Process of moving food thru the alimentary canal. Includes
 Deglutition, i.e., swallowing (voluntary)
 Peristalsis (involuntary)
– Peristalsis is the primary means of moving food thru the GI tract. It involves
waves of alternating contraction and relaxation of the smooth muscle in the organ
walls
Digestion: Includes mechanical and chemical
 Mechanical digestion: Initial breakdown that physically prepares food for chemical
digestion.
– Includes chewing, mixing of food and saliva by the tongue, churning of food in
the stomach, and segmentation to mix food with enzymes
• Segmentation: The rhythmic local contractions of the small intestine that
helps mix food.
– Chemical digestion: Hydrolytic breakdown of food molecules by enzymes secreted
into the alimentary canal.
• Begins in the mouth and is finished in the small intestine.
Absorption: Passage of digested end products (along with vitamins, mineral, and water) from
the lumen of the GI tract across the mucosa and into either blood or lymph.
Defecation: Elimination of indigestible substances from the body via the anus in the form of
feces.
– Includes indigestible substances, bacteria, sloughed epithelial cells.
Layers of the Alimentary Canal
• From the esophagus to the anal canal, the walls of the digestive tract have the same basic
four layers: Mucosa, submucosa, muscularis, and serosa.
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Peritoneum
• Peritoneum: Serous membrane that lines the wall and many organs of the abdominal cavity.
– Periotneal cavity: Space between the organs and abdominal wall
– Serous fluid produced by the peritoneum allows organs to slide past each other.
– Ascites: Distention of the periotoneal cavity due to accumulation of several liters of
serous fluid.
Folds of Peritoneum
• Folds of peritoneum:
– Bind organs to each other and to the abdominal walls.
• Contain blood vessels, lymphatic vessels, and nerves
– Include many different folds, but will focus on the greater omentum and mesentery
• Greater Omentum:
– Emerges from the stomach and duodenum and drapes over the intestines like a “fatty
apron” then returns superiorly to attach to the transverse colon.
– Contain a great amount of adipose tissue and can greatly expand with weight gain.
• Protects internal organs from blows
• Energy storage
• Gives rise to the characteristic “beer belly”
• Mesentary:
– Suspends the small intestine from the posterior abdominal wall.
– Provide a pathway thru which nerves, blood vessels, and lymph vessels can travel to
and from the small intestines
Peritonitis
• Peritonitis: Inflammation of the periotoneum
– Can be caused by:
• Infectious microbes due to accidental or surgical wounds, or rupture of abdominal
organs.
The Mouth and Associated Organs
• Oral cavity (mouth)
– Bounded by the lips anteriorly, palate superiorly, tongue inferiorly, and the cheeks
laterally
• Palate: Forms partition between the oral and nasal cavity
– Hard palate: Anterior portion of the roof of mouth - bone.
– Soft palate is the posterior portion of roof of mouth.
• Uvula: Posterior extension of the soft palate
• During swallowing, it rises and closes off the entry to the nasal cavity preventing
food from entering the nasal cavity.
– Palatine tonsils are situated between the arches.
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Salivary Glands
– Produce saliva (1-1.5 Liters/day) which:
• Cleans the mouth
• Dissolves food particles
• Moistens food facilitating its compaction into a bolus
• Contains important enzymes
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– Composition of saliva
• 99% water 1% solutes
• Solutes:
• Electrolytes
• Mucus
• Salivary amylase - an enzyme that chemically digests starch
• Lysozyme - an enzyme that provides immune defense.
– Location of salivary glands:
• Parotid Gland – found anterior to the ear.
• The largest of the salivary glands.
• Submandibular gland – lies beneath the
body of the mandible.
• In certain people, saliva can squirt
out of the mouth from the duct of
these glands.
• Sublingual gland – lies under the tongue
Clinical Considerations: Mumps
• Mumps : Inflammation of the parotid gland, caused by
a viral infection
– The inflamed parotid glands become swollen, often making the cheeks quite large.
• Tooth Regions = 3
– Crown: Visible portion above the level of the
gums
• Covered by enamel
– Neck: Boundary between the crown and root
• Covered by both enamel and cementum.
– Roots: Portion embedded into bony socket or
alveolus.
• Covered by cementum
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Tooth structure
– Composed of an internal pulp cavity filled with
soft tissue (pulp) and surrounded by two layers
of calcified tissues: Enamel and Dentin
– Dentin: Calcified CT, (similar to bone) that gives
the tooth its basic shape.
• Surrounds the pulp cavity and root canal.
– Pulp cavity
• Pulp consists of nerves, blood and lymphatic vessels.
• Root canals are narrow run through the root and open at their tips so vessels can
enter
– Enamel
• Covers the crown and part of the neck.
• Consists of calcium phosphate and calcium carbonate.
• Hardest substance in the body.
• Protects the tooth from wear and is a barrier against acids that can dissolve
dentin
• Cannot repair itself (acellular)
– Cementum
• Bone-like substance that attaches the root to the peridontal ligament.
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Deciduous vs. Permanent Teeth
• Two sets of teeth: deciduous and permanent teeth
• Deciduous Teeth (baby teeth, primary teeth
– 20 teeth (10/jaw)
– Erupt from 6 to 20 months and fall out between ages 6-12 years as permanent teeth
erupt
• Permanent Teeth (secondary teeth)
– 32 teeth (16teeth/jaw)
– Begin erupting ~ 6 years of life. By adolescence, all but the 3rd molars (erupt between
age 17 and 25) should be in place.
– Each row contains (top or bottom)
Pharynx (throat)
• Pharynx (throat): Extends from the internal nares to the esophagus and larynx.
– Consists of three parts: nasopharynx, oropharynx, and laryngopharynx
– During swallowing, the epiglottis closes off the larynx preventing food from entering the
respiratory tract.
Esophagus
• Esophagus: A muscular tube (~10” long) that functions if the transport of food from the
pharynx to the stomach.
– Lies posterior to the trachea
– Passes through the diaphragm.
• Lower Esophageal (cardiac) sphincter: Relaxes to allow a bolus of flood to enter the
stomach.
Clinical Considerations: GERD
• Gastroesophageal Reflux Disease (GERD): Incomplete closure of the LES allows stomach
contents to reflux (back up) into the esophagus.
• Causes:
– LES sphincter relaxation due to smoking, alcohol, lying down after a meal, hiatal hernia.
– Excess stomach acid production
• Can result from foods containing caffeine, alcohol, chocolate, tomatoes,
peppermint, spearmint, onions, and high amounts of fat.
• Heartburn results from stomach hydrochloric acid (HCl) irritating the esophageal wall - a
classic symptom of GERD.
Stomach
• Stomach: A J shaped expansion of the GI tract.
• Stomach: J shaped expansion of the GI tract.
• Contains Rugae: Mucosal folds seen when the stomach is empty
– Allows the stomach to stretch when filled.
• Pyloric sphincter relaxes to allow chyme to enter the duodenum
Stomach Histology
• Gastric Glands: Contain 4 types of cells (3 exocrine and 1 endocrine
type), which together produce 2-3 liters of gastric juice/day.
– Goblet Cells– line the surface of the stomach.
• Secrete mucus which lubricates the chyme and protects stomach lining from
digestive enzymes and acid.
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– Parietal cells –Secrete hydrochloric acid (HCl) and
intrinsic factor.
• HCl produces a pH of about 2 - 3 in the stomach –
functions to kill microoganisms and denature proteins.
• HCl is necessary to activate the enzyme pepsinogen
to become pepsin, which breaks down proteins.
• Intrinsic factor is necessary for absorption of vitamin
B12.
• Vitamin B12 is important for red blood cell
production; without it, pernicious anemia will
develop.
– Chief cells– secrete pepsinogen.
• Pepsinogen is converted to the enzyme pepsin by
HCl and pepsin digests proteins.
– G cells (Endocrine cells) – secretes the hormone gastrin
into the blood which stimulates:
• Increased production of gastric juice
• parietal cells to secrete HCl
• chief cells to secrete pepsinogen
• gastric motility
Stomach Functions
• Functions in the preparation of food to inter the small intestine.
– Food mixed with enzymes, HCl, and mucous
• Converted into a paste called chyme.
• Stomach contents are emptied 2-6 hours after eating a meal.
– Carbohydrate rich food empty fastest (~2 hrs)
– Protein rich foods take longer (3-5 hrs)
– Fats (triglycerides) take the longest (5-6 hrs)
• Absorption is limited to:
– Some water, electrolytes, certain drugs such as aspirin and alcohol.
Why doesn’t the stomach digest itself
• Protection of stomach from HCl acid and digestive enzymes is due to:
– A thick coating of bicarbonate-containing mucus lines the wall
– Damaged cells are quickly shed and replaced
Peptic Ulcers
• Peptic ulcers are erosions of the mucous membranes of the stomach or duodenum produced
by the action of HCl.
• Causes:
– Mechanisms that reduce the barriers of the gastric mucosa to self digestion.
– Infection by Helicobacter pylori cause most cases.
• Found in the GI tract of ~50% of humans
• Treated with antibiotics
– Excessive gastric acid secretion – especially affecting the small intestine.
• Not a major cause of stomach ulcers
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Small Intestine
• Small intestine: Site of most (90%) digestion and absorption.
– Divided into three sections: duodenum, jejunum, and ileum.
• Duodenum
– First portion of small intestine
– ~10” (12 cm) long (duodenum means 12)
– Receives the common bile duct (delivering bile from the liver and gallbladder) and the
main and accessory pancreatic ducts (delivering pancreatic juice from the pancreas)
• Jejunum
– Extends from the duodenum to the ileum
– Suspended by mesentery
– Primary site of digestion and absorption
• Ileum
– Last part of small intestine.
– Suspended by mesentery
– Primarily involved in absorption of electrolytes and vitamins
Large Intestine
• General anatomy
– Extends form the iliocecal valve to the anus (~ 5’ long).
– Divided into 4 principle regions:
Cecum, colon, rectum, and anal
canal.
Cecum
• Cecum: Saclike structure at the proximal
end of the large intestine.
– Ileocecal sphincter (valve) allows
materials from small intestine to
pass into large intestine
– Cecum merges with the colon
• Appendix: Tube (~3” long) attached to
cecum
– Contains large numbers of
lymphoid nodules and plays a role
in bacterial exposure and memory
cell generation.
Colon
• Colon consists of four parts:
– Ascending colon: Travels up from the cecum along the right side of the abdominal
cavity.
– Transverse colon: Continues across the abdominal cavity.
– Descending colon: Continues downward to merge with sigmoid colon
– Sigmoid colon: S-shaped tube that merges with the rectum
Rectum
• The rectum is a straight, muscular tube (~8”) that begins at the termination of the sigmoid
colon and ends at the anal canal.
Anal Canal
• The anal canal (~1” long) is the terminal portion of the rectum
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Functions of Large Intestine
• Functions:
– To absorb water, electrolytes, some vitamins, and prepare feces.
• Chyme remains in large intestine 3-10 hours.
– Resident bacteria breakdown indigestible carbohydrate residues and produce many B
vitamins including biotin and pantothenic acid, as well as most of the body’s supply of
vitamin K
• Create gases (hydrogen, carbon dioxide, methane, hydrogen sulfide) to
contribute to flatus (colon gas).
– Indigestible food is expelled as fecal matter by the process of defecation.
Colon Motility
• Peristalsis: Migrating waves sweep over the large areas of the colon and force its contents
towards the rectum.
– Large peristaltic waves called mass peristalsis, propel the colon contents along the
transverse colon thru the rest of the colon to the rectum, especially after eating a large
meal or during defecation.
• Practice the “wave”
• Gastrocolic reflex: Presence of food within the stomach often initiates mass peristalsis.
Colon Motility
• Haustral churning: Haustral contractions push fecal matter from haustrum to haustrum.
– A diverticulum is a pouch larger than a haustrum that forms when the wall of the
muscularis (externa) weakens and stretches.
– Diverticulitis: An infection of the diverticula -- may cause pain, fever and change in
bowel habits.
Accessory Organs
• Accessory organs: Consists of the teeth, tongue, salivary glands, liver, and pancreas.
– Liver, gallbladder, and pancreas lie outside the GI tract
– Will focus on the liver and pancreas which empty into the duodenum.
Liver
• The largest internal organ of the body (~3 lb) and heaviest gland.
• Located underneath the diaphragm and partially shielded by the ribcage on the right side of the
body.
• The gallbladder is a pear-shaped sac located on the inferior surface.
Gall Bladder
• Gallbladder: Functions in the storage and concentration of bile until it is expelled.
– The liver continuously produces bile (~1L/day). However the hepatopancreatic
sphincter is normally closed. This results in bile backing up into the common bile duct,
cystic duct, and ultimately into the gall bladder.
Functions of the Liver
• Carbohydrate metabolism
– Helps maintain normal blood glucose levels.
– During low blood glucose levels:
• Breaks down liver glycogen to glucose which is released into the bloodstream.
• Converts other molecules such as amino acids, lactic acid, fructose, and
galactose to glucose.
– During high blood glucose levels:
• Converts excess glucose to glycogen.
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Lipid Metabolism
– Synthesis of triglycerides and cholesterol
– Excretion of cholesterol in bile
Protein Metabolism
– Synthesize plasma proteins (albumins, globulins, and fibrinogens)
– Deamination: Removes the amino group (NH2) from amino acid so they can be used to
make sugar or fats.
• Results in the formation of ammonia (NH3) which is then converted to the less
toxic urea and is excreted in urine.
Detoxification
– Removes hormones and drugs from the blood and chemically alter them – making them
less toxic.
• Synthesis of bile
– Bile is used for the emulsification of fats, which is the act of separating large fat
globules into tiny fatty droplets.
• Increases the available surface area for lipases to work upon
– Hepatocytes produce about 800-1000 ml per day
• Storage of vitamins and minerals
– Stores vitamins (A, B12, D, E, and K) and minerals (iron) for red blood cell production
• Excretion of bilirubin
– Excretion of bilirubin in bile
– Bilirubin is derived from the heme of worn out red blood cells.
• Contributes to the brown color of feces and the yellow color of urine
Jaundice
• Due to a buildup of the yellow pigment bilirubin in the blood which gives a yellowish
appearance to the skin mucous membranes and sclera.
Gallstones
• Most due to crystallized cholesterol (~80%), some due to bilirubin pigment and calcium
– Can be caused by insufficient bile salts or excessive cholesterol, calcium, or bilirubin.
– Complete obstruction of ducts may result depending on stone size and quantity
• Treatment: Stone dissolving drugs, lithotripsy, or surgery
Pancreas
• The pancreas has both an exocrine and
endocrine function
• Endocrine Function
– Involved in the regulation of blood
glucose levels
– Pancreatic islets (islets of
Langerhans) consist of 2 primary cell
types.
• Alpha cells – secrete the
hormone glucagon
• Beta cells – secrete the
hormone insulin
– Glucagon is released in response to
low plasma glucose levels and acts to
increase plasma glucose
– Insulin is released in response to high
plasma glucose levels and acts to
lower plasma glucose
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Exocrine Function
– Exocrine secretory cells (acinar cells) produce pancreatic juice.
– Pancreatic juice consists of water, bicarbonate, and a mixture of digestive enzymes that
will enter the duodenum via the pancreatic and accessory ducts:
• Pancreatic Amylase: break down starches
• Pancreatic Lipase: break down lipids
• Ribonuclease and Deoxyribonuclease: break down nucleic acids
• Proteases such as trypsin, chymotrypsin, and carboxypeptidases: break
down large proteins into smaller peptides and amino acids
• Bicarbonate ions help neutralize the acidic pH of entering chyme
Overall Goal of Digestion
• To change food into forms that can pass through the epithelial cells of the mucosa into the
blood/lymphatic vessels.
• These forms are:
– Monsaccharides (glucose, fructose, galactose) from complex carbohydrates.
– Amino acids, dipeptides, and tripeptides from proteins.
– Fatty acids, glycerol and monoglycerides from lipids.
– Pentoses and nitrogenous bases from nucleic acids.
• About 90% of all absorption occurs in the small intestine and 10% in stomach and large
intestine.
Digestion and Absorption of
Carbohydrates
• Most carbohydrates
ingested are in the form
of:
– Starch:
Polysaccharide of
glucose.
– Sucrose (table
sugar):
Disaccharide of
glucose and
fructose.
– Lactose (milk
sugar):
Disaccharide of
glucose and
galactose.