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Anatomy & Physiology 34B
Chapter 23 - Digestive System
I. Overview
A. Introduction to the Digestive System
B. Serous Membranes & Tunics of the GI Tract
C. Mouth, Pharynx, & Associated Structures
D. Esophagus & Stomach
E. Small Intestine
F. Large Intestine
G. Liver, Gallbladder, & Pancreas
II. Introduction to the Digestive System
A. The food we eat must be mechanically and chemically reduced to
smaller molecules that can be
1. Absorbed through the intestinal wall
2. Transported to the cells by the blood
B. The digestive system functions to process food, extract nutrients
from it, and eliminate the residue. The stages include:
1. Ingestion - taking food into the mouth
2. Mechanical digestion - chewing food, churning it in the
stomach and small intestine
3. Motility – movement of food through the alimentary canal;
includes swallowing and peristalsis (wavelike contractions of
the GI tract)
4. Chemical digestion - chemical hydrolysis of macromolecules to
their absorbable monomers (proteins  amino acids,
carbohydrates  monosaccharides, nucleic acids 
nucleotides, and fats  fatty acids, & glycerol)
5. Glandular Secretion of enzymes, hormones and other products
that carry out or regulate digestion.
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6. Absorption - passage of food molecules through the small
intestine mucous membrane into blood and lymph capillaries for
distribution to cells
7. Defecation - discharge of indigestible wastes (feces) from the
GI tract
C. General Anatomy - the digestive system can be divided into two
anatomical subdivisions
1. Alimentary canal (GI tract) - tubular structure composed of
(from mouth to anus):
a. oral cavity and pharynx
b. esophagus and stomach
c. small & large intestines
2. Accessory organs, which include the following:
a. teeth, tongue, and salivary glands
b. liver and gall bladder
c. pancreas
D. It takes about 24-48 hrs for food to pass through the GI tract.
E. Each region of the GI tract has specific functions in preparing food
for utilization
1. Oral cavity - grinds food and mixes it with saliva; initiates
carbohydrate digestion and forms a food bolus which is
swallowed and passed from pharynx to
2. Esophagus - transports bolus to stomach by peristalsis; lower
esophageal (cardiac) sphincter prevents food backflow
3. Stomach - churns the bolus with gastric juices; initiates protein
digestion; has some absorption; moves partially digested food
(chyme) into the duodenum
4. Small intestine - mechanically & chemically breaks down
chyme with secretions from the liver & pancreas; absorbs
nutrients; transports wastes via peristalsis to the
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5. Large intestine - receives undigested wastes, absorbs water &
electrolytes; forms, stores, and expels feces
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III. Serous membranes & Tunics of the GI tract
A. Serous membrane characteristics
1. Is composed of simple squamous epithelium and some CT
2. Secretes serous fluid that lubricates the organs
3. Lines the abdominopelvic cavity and covers the organs
a. Parietal peritoneum lines the wall of the cavity
1) Forms a double layered peritoneal fold called the
mesentary in the posterior abdominal cavity
2) Mesocolon is part of the mesentary that supports the large
intestine
3) Falsiform ligament attaches the liver to the diaphragm &
anterior abdominal wall
4) Greater omentum - fatty peritoneum extension from
stomach to transverse colon that covers, cushions, and
protects the viscera
5) Lesser omentum - passes from the stomach & upper
duodenum to the inferior liver
6) The omenta above also contain many nerves, blood and
lymphatic vessels, and lymph nodes that fight abdominal
infections
b. Visceral peritoneum covers the intestinal organs
1) Peritoneal cavity is the space between the parietal and
visceral peritoneum
2) Organs such as the kidneys, adrenal glands, and parts of
the pancreas, duodenum, and abdominal aorta lie behind
the peritoneum (retroperitoneal)
4. Peritonitis is an inflammation of the peritoneum, often caused
by a burst appendix
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B. Tunics of the GI Tract, from the inside out, include the mucosa,
submucosa, muscularis externa, and serosa
1. Mucosa - thin layer that lines the GI tract lumen; consists
mainly of simple columnar epithelium with goblet cells (except
the mouth, esophagus and lower anus, which are lined with
stratified squamous epithelium) plus
a. Lamina propria - layer of aerolar or reticular CT that
contains lymphoid tissue and lies deep to the epithelial layer
b. Muscularis mucosa – deep to the lamina propria; smooth
muscle layer; has many folds (plicae circularis) in the small
intestine
2. Submucosa - thick, CT & vascular layer that serves the mucosa
a. Absorbed food molecules pass through the mucosa into the
blood vessels or lymph vessels of the submucosa
b. Also contains glands & nerve plexuses, such as the
submucosal (Meissner) plexus, that provides ANS
innervation to the muscularis mucosa
3. Muscularis externa – consists of an inner circular & outer
longitudinal layers of smooth muscle
a. Rhythmic contraction of these muscles (peristalsis) moves,
pulverizes, and churns food with digestive enzymes
b. Myenteric plexus between the muscle layers provides the
major ANS nerve supply to the GI tract
4. The outermost binding & protective layer may be the serosa or
adventitia, depending on where it is located
a. Serosa – is found on intraperitoneal organs & consists of
1) Simple squamous epithelium (visceral peritoneum)
sandwiched between
2) Aerolar CT
b. Adventitia is composed of fibrous CT and found on organs
not associated with the peritoneal cavity (e.g.: esophagus)
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c. Retroperitoneal organs have a serosa on their peritoneal side
& an adventitia on their posterior abdominal cavity side
C. Regulation of the digestive tract – motility and secretion is
controlled by neural, hormonal, and paracrine mechanisms
1. Neural controls include autonomic reflexes called short reflexes
and long reflexes
a. Short reflexes involve stretching or chemical stimulation of
the GI tract acting through the myenteric plexux to stimulate
the muscularis externa for peristaltic contractions
b. Long (vasovagal) reflexes act through ANS nerve fibers that
carry sensory signals to the CNS and motor commands back to
the GI tract. Digestive motility and secretion are stimulated
2. Hormones produced by the GI tract include gastrin and secretin,
as well as paracrine secretions such as histamine and
prostaglandins that stimulate digestive function
3. Paracrine secretions diffuse through the tissue fluids and
stimulate nearby target cells
IV. Mouth, Pharynx & Associated Structures
A. The oral cavity (mouth) is formed by the cheeks, lips, hard & soft
palates
1. Cheeks consist of skin, fat, muscles, and are lined with
nonkeratinized stratified squamous epithelium
2. Lips are formed from the obicularis oris muscle, CT, and skin; a
labial frenulum attaches each lip to the gum medially
3. The palate forms the roof of the oral cavity and consists of
a. Hard palate - formed by the palatine bones and palatine
process of the maxillae
b. Soft palate - posterior skeletal muscle arch continuous with
the hard palate anteriorly
c. Uvula - cone-shaped medial projection of the soft palate;
folds upward during swallowing to prevent food from
entering the nasal cavity
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d. Palatine tonsils are found on the lateral oropharynx walls
between the palatoglossal and palatopharyngeal arches
B. Tongue - moves food in the mouth for mastication and assists in
swallowing
1. Is attached to the hyoid bone and consists of intrinsic skeletal
muscle covered with a mucous membrane
2. Extrinsic tongue muscles move the tongue from side to side, in
and out
3. Lingual tonsils are found on the root of the tongue
4. A lingual frenulum connects the tongue medially to the
anterior floor of the mouth
5. Three types of papillae are on the tongue surface: filiform,
fungiform, & circumvallate; all but the filiform have taste
buds with chemoreceptors for the perception of taste
C. Teeth (dentition) - involved in mechanical breakdown of food; the
32 permanent teeth and their associated structures include:
1. Incisors – 8 front teeth that cut and shear food
2. Canines – 4 pointed side teeth for holding and tearing; both
canines and incisors have single roots
3. Premolars (8 bicuspids) & Molars (8) - posterior to the
canines; have dental cusps for grinding & crushing food
4. 3rd Molars (4 “wisdom teeth) are the last to emerge, may
become impacted is there is not enough room in the jaw
5. A tooth consists of:
a. A crown, above the gingiva (gum = mucous membrane
surrounding the alveolar processes)
b. Neck supports the crown where the it meets the gingiva
c. Roots extend through the gingiva and into the dental
alveolus (socket)
1) Each socket is lined with a CT periosteum (periodontal
ligament)
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2) Roots are covered with bonelike cementum and attach to
the periodontal membrane via the periodontal ligament
3) A root canal, continuous with the pulp cavity, opens to
the CT surrounding the root via an apical foramen
d. Layers of the tooth include:
1) outer enamel layer of the crown & neck, composed of
calcium phosphate (toughest substance in the body)
2) middle bone-like dentin layer, secreted by odontoblasts
that line the pulp cavity
3) inner pulp cavity with pulp containing loose CT, blood &
lymph vessels, and nerves
D. Salivary Glands
1. Secrete saliva, a solvent to cleanse teeth and dissolve food
particles. Solutes include
a. Salivary amylase - enzyme that hydrolyzes starches to
disaccharides
b. Lingual lipase - enzyme that is activated by stomach acid
and digests fats after food is swallowed
c. Mucus, which lubricates the food mass and aids swallowing
d. Lysozyme – enzyme that kills bacteria
e. Immunoglobulin A (IgA), an antibody that inhibits bacterial
growth
f. Electrolytes, including Na+, K+, Cl-, phosphate, and HCO3g. pH is about neutral
2. Salivary glands include:
a. Numerous intrinsic minor glands in the mucous membranes
of the palatal region of the mouth keep the mouth moist
b. Glands outside the mouth produce most saliva and transport
it to the oral cavity via salivary ducts. These glands include
1) Parotid gland - largest, found below and in front of the
ear auricle between the skin and masseter muscle; drains
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through the parotid duct into mouth. Mumps, caused by
a virus, is an inflammation of this gland
2) Submandibular gland - located inferior to the mandible
body and covered by the mylohyoid muscle; drains
through the submandibular duct
3) Sublingual gland - under the mucous membrane in the
floor of the mouth; contains several small ducts that empty
into the floor
3. Two types of secretory cells are found in all salivary glands
a. Serous cells produce a watery fluid containing digestive
enzymes
b. Mucus cells secrete thick, stringy mucus
c. Cuboidal epithelia cells line salivary gland duct lumina
4. Food stimulates receptors in the mouth, which transmit signals
to salivary nuclei in the medulla oblongata and pons, which
signal salivary glands to secrete saliva via cranial nerves VII and
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a. Parasympathetic nerves stimulate the production of
abundant, thin saliva rich in enzymes
b. Sympathetic nerves cause the glands to produce less
abundant, thicker saliva with more mucus
E. Pharynx (throat) - has both digestive and respiratory functions;
composed of skeletal muscle lined with mucous membrane
1. The deeper muscle layer is longitudinally oriented
2. The superficial layer is circular muscle, which is divided into
superior, middle, and inferior pharyngeal constrictors that force
food downward during swallowing
V. Esophagus & Stomach
A. Esophagus - portion of the GI tract that connects the pharynx to
the stomach. Features:
1. A collapsible tube, about 10 in long, containing smooth and/or
skeletal muscle lined with stratified squamous epithelium,
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2. Starts at the laryngopharynx, posterior to the trachea
3. Is located in the mediastinum and passes through the
esophageal hiatus (opening in the diaphragm) just above the
stomach
4. Lower esophageal (cardiac) sphincter between esophagus &
stomach usually prevents regurgitation of food from stomach
5. Gastroesophageal reflux disease (GERD) involves weakness of
the cardiac sphincter; allows stomach acids into the esophagus
6. Deglutition (swallowing) - the swallowing center of the
medulla oblongata and pons coordinates and integrates the
pharyngeal and esophageal muscles involved. The two phases
are
a. Buccal phase – tongue collects food, forms a bolus, then
pushes it into the oropharynx
b. Pharyngeal-esophageal phase – the bolus is driven
downward by gravity, pharyngeal muscle contractions and
esophageal peristalsis mediated by the myenteric nerve plexus
B. Stomach - continuous with the esophagus superiorly and with the
duodenum inferiorly. Features are:
1. Receives food bolus from the esophagus, churns it with gastric
secretions (e.g.: pepsin & HCl) to form chyme, which then
moves it to the duodenum
2. Major regions of the stomach include:
a. Cardia - narrow upper region below the cardiac sphincter
b. Fundus - dome-shaped region to the left of and in contact
with the diaphragm
c. Body (corpus) - large central portion
d. Pyloris - funnel-shaped terminal portion; a pyloric sphincter
regulates the passage of chyme from the stomach into the
duodenum
3. Borders of the stomach are:
a. Anterior and posterior borders
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b. Lesser curvature - medial concave border
c. Greater curvature – left lateral convex border
4. Stomach walls have all 4 GI tract tunics plus 2 additions:
a. An extra oblique muscle layer in the muscularis externa
b. Gastric folds (rugae) in the mucosa, which permit stomach
distension. The mucosa also contains gastric pits and 5
types of cells in gastric glands within the pits:
1) Mucous cells – found in the neck of the gland; secrete
mucous
2) Parietal cells – found mostly in the upper gland; secrete
a) HCl a strong acid with a pH as low as 0.8
b) Intrinsic factor, which is necessary for vit. B12
absorption in the small intestine
3) Chief (zymogenic) cells – in lower glands; secrete
pepsinogen, the precursor to protein-digesting pepsin
4) Enteroendocrine cells – in lower glands; secrete several
hormones, including gut-brain peptides, which coordinate
different parts of the GI tract with each other
5) Regenerative (stem) cells – divide to replace cells that die
c. B12 deficiency or deficient gastric intrinsic factor leads to
pernicious anemia due to insufficient RBC production
5. Gastric secretions produced by the gastric glands total 2-3 L
per day, and are composed mostly of water, HCl and pepsin
a. HCl is produced with H+ ions liberated from H2CO3 and Clions exchanged for HCO3- ions during a Cl- ion shift. Four
functions of HCl are
1) Activates pepsin and lingual lipase
2) Breaks up connective tissues and plant cell walls
3) Converts ingested iron ions to a form that can be absorbed
4) Destroys ingested bacteria and other pathogens
b. Pepsinogen is converted to protein digesting pepsin by HCl
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c. Chemical messengers produced by gastric enteroendocrine
cells include
1) Gastrin – stimulates gastric glands to secrete HCl and
enzymes
2) Seratonin - stimulates gastric motility
3) Somatostatin – inhibits gastric secretion and motility
6. Gastric motility – stomach movements have two phases
a. Receptive-relaxation response – arriving food stretches the
stomach, causing it to relax
b. Peristaltic contractions of the smooth muscle churns the bolus
and mixes it with gastric juices, then moves the chyme through
the pyloric sphincter into the duodenum
7. A small amount of digestion and absorption occur in the stomach
a. Salivary and gastric enzymes partially digest protein and some
starch and fat in the stomach, but most digestion and nutrient
absorption occurs in the small intestine
b. The stomach does absorb aspirin and some lipid soluble drugs
8. Protection of the stomach is accomplished by 3 mechanisms
a. Mucous coat lining the stomach resists acids and enzymes
b. Epithelial cell replacement – cells are replaced every 3-6 days
c. Tight junctions between the epithelial cells prevent gastric juices
from seeping between them and digesting the CT
9. Peptic ulcers can occur if HCL and pepsin erode the stomach wall.
The most common cause is infection by the bacterium
Heliobacter pyloris, which invades the stomach mucosa
10. Regulation of gastric function is accomplished by the nervous
and endocrine systems during three phases
a. Cephalic phase – mental and sensory stimuli lead to gastric
secretion and motility via the hypothalamus, medulla oblongata,
and vagus nerves
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b. Gastric phase – swallowed food and semidigested protein
stimulate gastric activity through short and long reflexes; ACh,
histamine, and gastrin stimulate the release of HCl, intrinsic
factor, and pepsin
c. Intestinal phase – chyme in the duodenum stimulates hormonal
and nervous reflexes that
1) Initially stimulate the stomach, then
2) Activate an enterogastric reflex that inhibits the stomach
3) Duodenal enteroendocrine cells secrete secretin,
cholecystokinin, and gastric inhibitory peptide, which inhibit
gastric activity to slow gastric emptying into the duodenum
VI. Liver, Gall Bladder, & Pancreas (accessory organs)
A. Liver - largest organ of the body, located beneath the diaphragm
1. The liver is composed of 4 lobes & 2 ligaments
a. Falciform ligament separates left & right lobes
b. Caudate lobe is near the inferior vena cava
c. Quadrate lobe is adjacent to the gallbladder
d. Ligamentum teres (round ligament = fetal vein remnant)
extends from the falciform ligament to the umbilicus
2. Hepatocytes form hepatic plates that are 1-2 cells thick and
separated by capillary spaces (sinusoids)
a. Sinusoids are lined with Kupffer cells, macrophagic cells
that remove bacteria and debris from the blood
b. The plate structures and highly permeable sinusoids allow
each hepatocyte to be in direct contact with the blood
3. Hepatic plates form functional units called liver lobules
a. At the periphery of each lobule are branches of the hepatic
portal vein and hepatic artery; blood from each mixes in
the sinusoids and passes from the lobule periphery to the
central vein
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b. The hepatic portal vein carries nutrient rich blood from the
digestive organs to the liver lobules, where the blood is
processed
1) After a meal, the liver removes glucose, amino acids, iron,
vitamins and other nutrients from the blood for metabolism
or storage
2) It also removes and degrades hormones, toxins bile
pigments, and drugs
3) The liver secretes albumin, lipoproteins, clotting factors,
angiotensinogen and other products into the blood
4) Between meals, the liver breaks down glycogen to release
glucose into circulation
5) Bile is produced, stored in the gall bladder, the released
into the duodenum to emulsify fats after eating
d. Bile produced by hepatocytes is secreted into bile canaliculi,
then drains into bile ducts at the periphery of each lobule,
then into hepatic ducts that carry bile to the gall bladder
e. A hepatic triad is formed by a bile duct, hepatic portal vein
branch, & hepatic artery branch
f. A central vein is in the center of each lobule; central veins of
each lobule merge to form the hepatic vein, which carries
blood from the liver to the inferior vena cava
B. Gallbladder - saclike organ attached inferiorly to the liver.
Features include:
1. Bile is secreted by the liver through the hepatic ducts 
common hepatic duct  bile duct  ampulla of Vater 
sphincter of Oddi  duodenum after a meal
2. When chyme from the stomach enters the duodenum, bile is
secreted there via the bile duct
3. When the small intestine empties, the sphincter of Oddi leading
into the duodenum constricts, and bile is forced up the bile duct
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to the cystic duct and into the gallbladder, which stores and
concentrates the bile until it is needed
4. Bile is a greenish fluid containing minerals, cholesterol, fats,
phospholipids, bile pigments (e.g., bilirubin), and bile acids
(salts). Bile acids emulsify fats in the duodenum
5. Gallstones (biliary calculi) sometimes form & block the bile
duct, causing jaundice and poor fat digestion. The gallbladder
may require surgical removal (cystectomy)
C. Pancreas - positioned transversely along the posterior abdominal
wall, between the stomach and duodenum. Features are:
1. The pancreas has 3 sections:
a. Head - expanded portion near the duodenum
b. Body - central portion inferior and posterior to the stomach
c. Tail - tapered part near the spleen
d. All but the tail are retroperitoneal
2. It’s a mixed gland, with both endocrine & exocrine functions
a. Endocrine hormones are secreted by the pancreatic islets
1) Alpha cells secrete glucagon
2) Beta cells secrete insulin
b. Exocrine functions are performed by the pancreatic acini
cells
1) Each acinus consists of a single layer of epithelial acinar
cells surrounding a lumen into which pancreatic digestive
enzymes are secreted
2) Secreted digestive enzymes move through the pancreatic
duct to the ampulla of Vater, past the sphincter of Oddi,
into the duodenum
3. Pancreatic juice is an alkaline mixture of water, enzymes,
sodium bicarbonate, and other electrolytes. Enzymes include
a. Trypsinogen, which is converted to trypsin, an enzyme that
breaks down proteins to amino acids
b. Pancreatic amylase digests starches to monosaccharides
c. Pancreatic lipase breaks down fats to fatty acids and glycerol
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d. Ribonuclease & Dexoyribonuclease digest RNA and DNA
D. Regulation of the secretion of bile and pancreatic juice is in
response to
1. Sympathetic nerves inhibit secretion
2. Parasympathetic nerves, and the hormones cholecystokinin
(CCK), gastrin, and secretin all stimulate secretion
a. CCK is secreted by the duodenum in response to acid and fat
arriving from the stomach, which triggers 3 responses
1) Gallbladder contracts, forcing bile into the bile duct
2) Pancreatic enzymes are secreted
3) Sphincter of Oddi relaxes, allowing bile and enzymes into
the duodenum
b. Gastrin from the stomach and duodenum also stimulates
gallbladder contraction and pancreatic juice release
c. Secretin from the duodenum stimulates the bile duct and
pancreatic duct to secrete bicarbonate, which neutralizes
stomach acid in the duodenum
VII. Small Intestine - located in the abdominal cavity between the
stomach pyloric sphincter & ileocecal valve leading into the large
intestine
A. Mesentery supports the SI and contains blood vessels, nerves, &
lymphatic vessels that supply the intestine
B. The living small intestine is about 12 ft. long (in a cadaver, it is
about 20 ft. long)
C. The SI is the main site of nutrient absorption
1. It contains digestive enzymes such as peptidase, sucrase, lipase,
amylase, nuclease, & enterokinase
2. It receives digestive enzymes from the salivary glands, gastric
glands, and pancreas, as well as bile from the liver
D The 3 major regions of the SI are: duodenum, jejunum, & ileum
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1. Duodenum - first 10 in., from the pyloric sphincter to the
duodenojejunal flexure; this is the main site of chemical
digestion
a. Bile duct delivers bile here from the liver & gall bladder
b. Pancreatic duct delivers pancreatic digestive enzymes
c. Both ducts unite to form a common entry called the
hepatopancreatic ampulla (ampulla of Vater), which drains
into the duodenum via the duodenal papilla
d. The duodenal papilla is opened and closed by the sphincter
of Oddi
e. Duodenal (Brunner’s) glands in the submucosa secrete
bicarbonate rich mucus and are found only in the duodenum
2. Jejunum - is about 3 ft. long & extends from duodenum to
ileum, and has a histological structure like the ileum; this is the
main region of absorption of digested food molecules into the
blood stream
3. Ileum - terminal small intestine is about 6 ft. long and empties
into the cecum through the iliocecal valve. Walls contain many
Peyer’s patches lymph nodules, which produce lymphocytes to
combat bacteria in the ileum
E. Structural Modifications & Microscopic Anatomy of the SI
1. Digested products are absorbed rapidly across the epithelial
linings of the intestinal mucosa, primarily in the jejunum, due to
modifications that increase the internal surface area
a. Plicae circulares are large macroscopic folds of the mucosa
b. Villi are fingerlike macroscopic projections of the mucosa
that extend into the lumen of the SI
1) Simple columnar epithelium(absorptive cells) & goblet
cells cover the villi
2) The lamina propria contains lymphocytes, capillaries (that
absorb monosaccharides & amino acids), and lymphatic
lacteals (that absorb fatty acids)
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c. Microvilli (brush border) are microscopic extensions formed
by the folding of each epithelial cell membrane
2. Brush border enzymes are integral proteins in the microvilli
membrane.
a. Enterokinase is one enzyme that activates pancreatic
enzymes
b. Other enzymes carry out the final stages of enzymatic
digestion
3. Intestinal crypts (of Liberkuhn), similar to gastric glands, are
found on the floor of the SI, between the bases of the villi
a. The upper half of the crypts consists of absorptive and goblet
cells
b. The lower half contains dividing stem cells, which replace
sloughed off epithelial cells every 3-6 days
F. Intestinal Motility –
1. Movements of the SI include segmentation and peristalsis
a. Segmentation is the formation of ringlike constrictions in the
intestine that churns the contents to assist digestion
b. Peristalsis is rhythmic contractions of the smooth muscle
layers that moves the contents toward the large intestine
2. Contractions of the SI serve three functions
a. Mixes chyme the intestinal juice, bile, and pancreatic juice,
allowing the fluids to neutralize acid and digest the nutrients
b. Churns chyme to bring it into contact with the brush border
enzymes for digestion and absorption
c. Moves the undigested residue toward the colon
VIII. Chemical Digestion & Absorption are complete by the time the
food residue leaves the SI and enters the cecum. Carbohydrates,
proteins, and fats are degraded and absorbed in the following manner
A. Carbohydrate (starch) digestion is accomplished by the
following enzymes
1. Salivary amylase begins starch digestion in the mouth and
stomach
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2. Pancreatic amylase continues starch digestion in the small
intestine, hydrolyzing it into oligosaccharides and maltose
3. Brush border enzymes dextrinase, glucoamylase, and maltase
finish digesting the breakdown products of starch; sucrase and
lactase break down sucrose and lactose, respectively
B. Carbohydrate absorption of the resulting monosaccharides
(glucose, fructose, and galactose) is accomplished in the following
manner
1. Monosaccharides can be transported across the apical membrane
via the sodium-glucose transport protein (SGLT), facilitated
transport, or by solvent drag, in which the molecules are pulled
between the cells with water
2. Once inside the epithelial cells, monosaccharides are transported
through the basal membrane via facilitated diffusion into the
blood capillaries of the villus
3. The nutrients are then delivered to the liver in the hepatic portal
vein
C. Protein digestion
1. Pepsin begins protein (polypeptide) digestion in the stomach
2. Trypsin and chymotrypsin from the pancreas continue digestion
in the SI
3. Brush border enzymes called peptidases (proteases) disassemble
the resulting peptides one amino acid at a time
D. Amino acid absorption is accomplished in a manner similar to
that of monosaccharides (sodium cotransport  facilitated
diffusion  blood capillaries  hepatic portal vein  liver).
E. Fat digestion
1. Begins with lingual lipase in saliva, activated in the stomach
2. Most fat (triglyceride) is digested by pancreatic lipase in the SI
3. Bile acids and lecithin help break fat into emulsification
droplets
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4. Lipase hydrolyzes the fats into free fatty acids and
monoglycerides, which collect with cholesterol and fat soluble
vitamins in bile-coated droplets called micelles.
F. Fat absorption
1. Micelles transport the lipids into the intestinal absorptive cells
2. The absorptive cells repackage the fats into chylomicron
droplets coated with phospholipid and protein
3. Chylomicrons are secreted from the absorptive cells and taken
into the lacteal in the villus
4. Chylomicrons travel in lymph in the lymphatic vessels and enter
the bloodstream at the subclavian veins
G. Dietary DNA & RNA are
1. Hydrolyzed into nucleotides (composed of sugar, phosphate,
and nitrogenous base) by pancreatic nucleases, then by brush
border nucleosidases and phosphatases
2. Ribose, deoxyribose, phosphate, and nitrogenous bases are
transported across the cell membrane and into the blood
capillaries of the villus
H. Vitamins are not digested; they are absorbed unchanged
1. Fat-soluble vitamins (A, D, E, K) are absorbed with other
lipids
2. Most water-soluble vitamins (B complex and C) are absorbed
by simple diffusion;
3. Vitamin B12 binds to intrinsic factor, and is then absorbed by
receptor mediated endocytosis
I. Minerals (electrolytes) are absorbed by
1. Na+ is cotransported with sugars and amino acids
2. Cl- & Fe2+ are absorbed by active transport
3. K+ & Ca2+ are absorbed via simple diffusion
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J. Water is absorbed by osmosis, following an osmotic gradient
established by the absorption of salts and organic nutrients
IX. Large Intestine
A. About 5 ft. long, it begins at the ileocoecal valve, and ends at the
anus
B. Functions to absorb water & electrolytes; form, store, & expel
feces
C. The Large Intestine is divided structurally into the cecum, colon,
rectum, and anal canal
1. Cecum is a dilated pouch slightly below the ileocoecal valve;
the fingerlike appendix extends from the inferior medial cecum
2. The colon extends from the cecum and consists of 4 parts:
a. Ascending colon - ascends from the cecum along the right
abdominal wall to the liver
b. Transverse colon - extends across the body from the right
colic (hepatic) flexure to the left colic (splenic) flexure
c. Descending colon - descends along the left abdominal wall
to the pelvic region
d. Sigmoid colon - S-shaped colon from the descending colon
to the rectum
3. Rectum - about 7.5 in. terminal portion of the GI tract. The last
inch is called the
4. Anal canal, which has the anus opening regulated by two
sphincters
a. Internal anal sphincter - composed of smooth muscle
b. External anal sphincter - composed of skeletal muscle
5. The large intestine has the 4 GI tract tunics with some
modifications:
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a. Lacks villi but has many goblet cells in the mucosal layer
b. The longitudinal muscle layer forms 3 bands called taeniae
coli that run the length of the LI
c. Bulges in the LI walls form sacculations (haustra)
d. Fat-filled pouches called epiploic appendages are attached
to the taeniae coli
D. Bacterial Flora & Intestinal Gas
1. The large intestine contains numerous species of bacteria,
collectively called bacterial flora
2. Bacteria ferment cellulose, other undigested carbohydrates, and
fats; they also synthesize B vitamins and vitamin K, necessary
for blood clotting
3. The average person expels about 500 ml of flatus (gas) per day
E. Defecation is facilitated by the following reflexes
1. Stretching of the rectum triggers the intrinsic defecation reflex,
mediated by the nyenteric nerve plexus, which drives feces
downward and relaxes the internal anal sphincter
2. A stronger parasympathetic defecation reflex involves a reflex
arc through the spinal cord and parasympathetic fibers of the
pelvic nerve
3. These reflexes cause defecation, although voluntary control of
the external anal sphincter can prevent defecation
IX. Disorders of the Digestive System
A. Viral Hepatitis – long term liver inflammation; 3 major types:
1. Hepatitis A – spread by fecal-oral route; does not usually cause
permanent damage; preventive vaccines available
2. Hepatitis B – transmitted by infected blood or body fluids;
infection can lead to liver cancer; preventive vaccine available
3. Hepatitis C – transmitted by body fluids; no short term
symptoms; can lead to cirrohosis and liver cancer; no vaccine
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B. Pancreatitis – inflammation of the pancreas
C. Diverticulitis – inflammed herniations (diverticula) of the colon,
which can rupture. Associated with low fiber diets
D. Crohn disease – inflammation of the small and large intestines,
producing granular lesions and fibrosis of the intestines, diarrhea,
and lower abdominal pain