The Digestive System
Functions to break down food
substances into forms that can be
absorbed
Types of Digestion
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Mechanical
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Chemical
Digestive Processes
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Ingestion
Digestion
Absorption
Elimination
Divisions of the Digestive
System
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Gastrointestinal tract
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Oral cavity, pharynx, esophagus,
stomach, large and small intestines
Accessory structures
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Teeth, tongue, salivary glands, liver,
pancreas, and gallbladder
Layers of the GI Tract
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Mucosa - innermost layer
Epithelium - stratified squamous
(pharynx and esophagus) and simple
columnar
 Lamina propria - areolar connective
tissue
 Muscularis - smooth muscle layer causing
small folds
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Layers of GI Tract
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Submucosa - loose connective tissue
containing blood vessels, nerves, glands, and
lymphatics
Muscularis - two layers of smooth muscle
 inner circular layer and outer longitudinal
one
 some skeletal muscle
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Serosa - outer mesothelium
 Peritoneum (parietal and visceral)
 Extensions: mesentery, mesocolon,
greater and lesser omentum, falciform
ligament
Organs and Structures
Mouth
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Vestibule
Tongue
Teeth
Salivary glands
Mouth or Buccal Cavity
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Vestibule- area bounded by the lips and
cheeks externally/internally by the
gums and teeth
Tongue - striated skeletal muscle that
manipulates food, help form words, and
serves as a sense organ
Teeth - breaks food into smaller pieces
increasing surface area for digestion
Teeth
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Found in alveolar sockets
Primary (deciduous or baby) teeth
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Usually consists of 20 teeth)
Permanent teeth (32 teeth)
Incisors - chisel-shaped
Canines (Cuspids) - fang-like
Premolars (bicuspids) - grinding and crushing
Molars - grinding and crushing
Tooth Structure
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Crown - exposed portion above gum
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Covered in enamel
Dentin - fills interior of tooth
Neck - hidden by gumline
Root - embedded in jawbone, number of roots
varies
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Cementum - attaches tooth to periodontal
ligament
Salivary Glands
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Parotid glands
Submandibular glands
Sublingual glands
Functions:
 cleanses, dissolves, moistens, digests
 97% water
 Slightly acidic
 Contains electrolytes, salivary amylase,
mucin, lysozymes, and immunoglobulins
Esophagus
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Hollow muscular tube that functions to carry
food to the stomach through an opening in
the diaphragm called the esophageal hiatus
Upper third composed of skeletal musle;
lower regions made of smooth muscle
Hiatal hernia
Digestive Processes of the
mouth, pharynx, & esophagus
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Mastication - chewing
Deglutition - swallowing
 chewed food formed into bolus
 tongue blocks mouth, soft palate blocks
nasopharynx, epiglottis blocks trachea
Peristalsis - rhythmic contractions of circular
muscles causing food to move down GI tract
Stomach
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Serves as a reservoir for food and to mix
food with gastric juice
Regions: cardia, fundus, body, and pylorus
Greater and lesser curvatures
Pyloric sphincter
Rugae
Additional oblique smooth muscle layer
Gastric gland cells
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Mucous goblet cells - secrete mucosal layer
Parietal cells (oxyntic) - secrete HCl and
intrinsic factor
Chief cells (zymogenic) - secrete pepsinogen
 Pepsinogen converted to pepsin which breaks
proteins into peptides
Enteroendocrine cells (G-cells)- secrete
hormones:
 gastrin, histamine, endorphins, serotonin,
cholecystokinin, somatostatin
Stomach ulcers - caused by Heliobacter pylori
Digestive Processes in the
Stomach
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Mechanical & chemical digestion turns food
into chyme
Pepsin breaks down proteins into peptides
Rennin (in children) breaks down milk
Intrinsic factor - required for absorption of
vitamin B12 in the intestines
Gastric motility & emptying
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Stomach contractions produce chyme,
increase mixing of food
Entire stomach can hold up to 4L
Approx. 3 ml squirts through pylorus with
each contraction
Stomach contracts 3X/min
Usually empties within 4 hours but may be
delayed depending on contents
Pancreas
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Secretes pancreatic juice formed by acini
Pancreatic juice
 sodium bicarbonate that serves to buffer HCl
 proteases such as trypsin, chymotrypsin, and
carboxypeptidase
 pancreatic amylase
 pancreatic lipases
 pancreatic nucleases
Liver and Gallbladder
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Largest internal organ in the body
Consists of right and left lobe separated by the
falciform ligament; caudate and quadrate lobes
on the posterior surface
Digestive function includes the production of
bile; protein, carbohydrate & lipid metabolism;
removal of drugs & hormones
Gallbladder - stores and concentrates bile
Small Intestines
Duodenum, Jejunum, and Ileum
Small Intestine
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Duodenum - (10 inches) receives chyme
from the stomach and digestive secretions
from the pancreas and liver ; major digestion
occurring here
Secretin
Cholecystokinin (CCK)
Gastric inhibitory peptide (GIP)
Jejunum and Ileum
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8 + 12 = 20 feet long
Region where most absorption occurs
Plica circulares = folds
Villi lined with simple columnar epithelium
Each villus contains an arteriole, venule,
capillary bed and a lacteal
Microvilli form brush border
Peristalsis and segmentation occurs
Enzymes of the small intestine
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Dextrinase
break down
Glucoamylase
oligosaccharides
Maltase
break
Sucrase
down
Lactase
disaccharides
Carboxypeptidase
breaks
Aminopeptidase
down
Dipeptidase
proteins
Large Intestine
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5.5 feet long
Functions:
 reabsorption of remaining water and
electrolytes
 absorption of vitamins B and K (produced by
the bacterial flora)
 elimination of feces (may take 12-24 hours)
Large Intestine
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Cecum
Ileocecal valve
Appendix
Ascending, transverse, and descending colon
Sigmoid colon
Taeniae coli / Haustra
Rectum / Anus
Hemorrhoids/Diverticulitis
Products of Digestion
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Carbohydrates
Lipids
Proteins
Carbohydrates
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Monosaccharides - single sugars
Glucose
 Fructose
 Galactose
 Deoxyribose
 Ribose
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Carbohydrates
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Disaccharides - double sugars
Sucrose
 Lactose
 Maltose
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Polysaccharides - starches (many sugars)
Amylose
 Cellulose
 Glycogen
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Formation of a disaccharide
+
Formation of a fat
Carbohydrate Digestion
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Carbohydrates are broken down into
disaccharides and then into
monosaccharides
Starch (Amylose)+ amylase --> Maltose
Maltose + Maltase --> 2 Glucose molecules
Lactose + Lactase --> Galactose & Glucose
Sucrose + Sucrase --> Glucose & Fructose
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Lipid Digestion
Lipids (fats) + Lipase ---> Glycerol & Fatty Acids
This process is made more efficient by the
emulsifying action of bile.
Protein Digestion
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Protein + Pepsin --> large polypeptides
Protein + Rennin --> large polypeptides
Protein + Trypsin --> small polypeptides
Protein + Chymotrypsin --> small polypeptides
Protein + Carboxypeptidase --> sm. Plypeptds
Aminopeptidases - to single amino acids
Carboxypeptidase - to single amino acids
Dipeptidase - to single amino acids
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Overview of Digestive Diseases
Digestive diseases range from the occasional
upset stomach to the more life-threatening
colon cancer and encompass disorders of the
gastrointestinal tract, the liver, the gallbladder,
and the pancreas.
What Causes a Digestive Disease?
Digestive disease may develop congenitally or
from multiple factors such as stress, fatigue,
diet, or smoking. Abusing alcohol imposes the
greatest risk for digestive diseases, particularly
increasing the risk of esophageal, colorectal,
and liver cancers.
Who Develops a Digestive Disease?
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Each year 62 million Americans are diagnosed
with a digestive disorder.
The incidence and prevalence of most
digestive diseases increase with age.
Women are more likely than men to report a
digestive condition, particularly non-ulcer
dyspepsia and irritable bowel syndrome (IBS).
How Are Digestive Diseases Diagnosed?
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Most digestive diseases are very complex, with
subtle symptoms. Because of this, patients
may undergo extensive and expensive
diagnostic tests which may include a blood
test, an upper or lower GI series, an
ultrasound, and endoscopic examinations of
the colon, esophagus, stomach, or small
intestine, or more sophisticated tests such as
a CAT (computerized axial tomography) scan
or MRI.
Significant Digestive Conditions
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Gallstones affect 20 percent of women and 10
percent of men, or approximately 20 million adult
Americans.
are solid masses, primarily of cholesterol that
develop in the gallbladder or less often in the bile
ducts leading from the liver to the small intestine.
Symptoms may include mid- or right-upper
abdominal pain that may lead to complications such
as acute cholecystitis and pancreatitis. Gallstones are
rarely associated with gallbladder cancer.
Treatment options include surgery by open
cholecystectomy or laparoscopic cholecystectomy,
watchful waiting, or oral bile acid therapy in patients
who cannot tolerate surgery.
Gastroesophageal Reflux Disease
and Related Disorders
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Gastroesophageal reflux disease affects nearly one-third of the American
population. GERD is the backward flow of the stomach's contents into the
esophagus. Heartburn is the most common symptom of GERD.
Doctors also believe that diet and lifestyle habits, hiatal hernia, obesity, and
pregnancy contribute to GERD. Certain foods,including chocolate, fried or
fatty foods, and alcohol may weaken the LES, permitting reflux and
heartburn.
Antacids such as Tums and Gaviscon neutralize stomach acid for relatively
short periods of time.
Histamine 2 (or H2-blockers), which suppress acid, are also prescribed to
relieve symptoms of GERD. The H2-remedies currently available include
cimetidine (Tagamet), famotidine (Pepcid), nizatidine (Axid), and ranitidine
HCl (Zantac).
Omeprazole (Prilosec, Losec, or Antra), are pump inhibitors that
dramatically inhibit an enzyme, H+(hydrogen), K+(potassium)-ATPase, from
producing stomach acid; lansoprazole (Prevacid) has also recently been
approved.
Inflammatory Bowel Disease
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Inflammatory bowel disease (IBD) refers to
two chronic intestinal disorders: Crohn's
disease and ulcerative colitis. IBD affects
between 2 to 6 percent of Americans or an
estimated 300,000 to 500,000 people. The
causes of Crohn's disease and ulcerative colitis
are not known. A leading theory suggests that
some agent, perhaps a virus or bacterium,
alters the body's immune response, triggering
an inflammatory reaction in the intestinal wall.
The onset for both diseases peaks during
young adulthood. An individual with either
disease may suffer persistent abdominal pain,
bowel sores, diarrhea, fever, intestinal
bleeding, or weight loss.
Crohn's Disease
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Crohn's disease primarily involves the small
bowel and the colon. It may cause the
intestinal wall to thicken, which may narrow
the bowel channel and block the intestinal
tract.
Although surgery relieves chronic symptoms,
Crohn's disease often recurs at the location
where the healthy parts of the bowel were
rejoined. The length of time that a Crohn's
patient is in remission is not predictable.
Ulcerative Colitis
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Ulcerative colitis (UC) is an
inflammatory disorder affecting the
inner lining of the large intestine. The
inflammation originates in the lower
colon and spreads through the entire
colon. Blood in the stool is the most
common and distinct symptom of
ulcerative colitis.
Irritable Bowel Syndrome
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Irritable bowel syndrome (IBS) is a common
functional disorder of the intestines estimated
to affect 5 million Americans. The cause of IBS
is not yet known. Doctors refer to IBS as a
functional disorder because there is no sign of
disease when the colon is examined. However,
doctors believe that people with IBS
experience abnormal patterns of colonic
movement. The IBS colon is highly sensitive,
overreacting to any stimuli such as gas, stress,
or eating high-fat or fiber-rich foods.
Peptic Ulcer Disease
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Peptic ulcer disease, estimated to affect 4.5 million
people in the United States, is a chronic inflammation
of the stomach and duodenum.
Peptic ulcers result from the breakdown of the lining
of the stomach and duodenum caused by increased
stomach acid and pepsin and Helicobacter pylori (H.
pylori). One type of ulcer occurs in the stomach, the
other in the duodenum, the first part of the small
intestine. Duodenal ulcers are much more common
than stomach ulcers, which have a greater risk of
malignancy.