GI Anatomy and
Physiology
Presented by Derrick Anderson
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Definitions: Digestive Process
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Ingestion  Taking food in
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Propulsion  moving food through alimentary canal
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Peristalsis
Mechanical digestion
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Chewing, churning by the stomach, segmentation
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Chemical digestion: Breaking down molecules
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Absorption: Taking in molecules
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Defecation  eliminating indigestible substances
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Where does the GI system
start???
Mouth
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Gastrointestinal
Tract
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Peritoneum
S The serous membrane of the abdominopelvic cavity
S Peritonitis  inflammation of the peritoneum
S Two Parts
S Visceral Peritoneum
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Covers external surface of most digestive organs
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Continuous with the parietal peritoneum
S Parietal Peritoneum
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Lines the body wall
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Continuous with the visceral peritoneum
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Peritoneal Cavity
S Potential space containing slippery fluid secreted by the
serous membranes
S Filled with serous fluid  lubrication
S Following diagram depicts the “potential space” of the
peritoneal cavity
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Mesentery
S Double layer of peritoneum extending to the digestive
organs from the body wall
S Provide routes for blood vessels, lymphatics and nerves to
reach digestive viscera
S Hold organs in place
S Stores fat
S Can be dorsal or ventral
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Continuous Pathway
S Alimentary canal – continuous, muscular canal
S aka gastrointestinal
S Mouth, pharynx, esophagus, stomach, small intestine, large
intestine
S Accessory digestive organs
S Teeth, tongue, gallbladder, salivary glands, liver and pancreas
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Histology of the Alimentary
Canal*
S Four basic layers (“tunics”) from the esophagus to the anal canal
S Mucosa
S Submucosa
S Muscularis externa
S Serosa
* All layers are presented starting from the innermost layer
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Alimentary Canal
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Mucosa
S Aka mucus membrane
S Innermost layer
S Epithelial membrane
S Lines the lumen of the alimentary canal
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Functions of the Mucosa
S Secrete mucus, digestive enzymes and hormones
S Absorb end products of digestion into the blood
S Protect against infections
S Depending on the region of the mucosa one or all of these
functions may be occur
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Mucosa Sublayers
S Epithelieum
S Lamina propria
S Muscularis mucosae
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Mucosal Epithelium
S Simple columnar epithelium
S Contains mucus-secreting cells
S Protects the GI tract from the digestive enzymes
S Eases passage of substances
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Lamina Propria
S Loose areolar connective tissue
S Capillaries nourish epithelium and absorb nutrients
S Contains lymphoid follicles
S MALT  mucosa-associated lymphatic tissue
S Defend against pathogens
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Muscularis Mucosa
S Smooth muscle cells
S Local movements of the mucosa
S Exist as many folds in the small intestine
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Alimentary Canal
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Submucosa
S Areolar connective tissue
S Rich supply of blood and lymphatic vessels, lymphoid
follicles and nerve fibers
S Contains elastic fibers
S Allows stomach to return to normal shape after being stretched
from storing food
S Extensive vascular network supplies surrounding tissue
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Alimentary Canal
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Muscularis Externa
S Aka muscularis
S Responsible for peristalsis and segmentation
S Inner circular layer of smooth muscle
S Thickens in some areas to form sphincters which prevent backflow
S Contraction = smaller lumen
S Outer longitudinal layer of smooth muscle
S Contraction = shorter tube
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Alimentary Canal
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Serosa
S Protective outermost layer
S Same as Visceral peritoneum
S Areolar connective tissue covered with mesothelium
S Mesothelium = single layer of squamous epithelial cells
S Replaced by adventitia in the esophagus
S Fibrous connective tissue binding the esophagus to the surrounding
structures
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Alimentary Canal
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Enteric Nervous System
S Nerve supply for the GI
S Regulate digestive system activity
S Semiautonomous
S 2 major intrinsic nerve plexuses
S Submucosal
S Myenteric
S Also subject to control by the CNS and ANS
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Submucosal Nerve Plexus
S In the submucosa
S Sensory and motor neurons
S Regulates activity of glands and smooth muscle in the
mucosa
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Myenteric Nerve Plexus
S Between the circular and longitudinal smooth muscle of the
muscularis
S Provide major nerve supply to GI tract wall and control GI
motility
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Mouth and Associated Organs
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Mouth
S Lined with stratified squamous epithelium
S Withstand friction
S Epithelium of the gums, hard palate and dorsal tongue are
slightly keratinized
S Contains antimicrobial peptides called defensins
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Salivary glands
S Paired tubuloalveolar glands
S Saliva
S Cleanses mouth
S Dissolves food chemicals so that they can be tasted
S Moistens food and aids in forming a bolus
S Contains enzymes that begin chemical breakdown of starchy
food
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Salivary Glands
S Parotid gland
S Largest
S Anterior to ear
S Mumps
S Submandibular gland
S Sublingual gland
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Salivary Glands
S 2 types of secretory cells
S Serous
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Produce watery secretion containing enzymes ions and mucin
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Parotid gland only has these
S Mucus
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Produce mucus
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Sublingual contains mostly these
S Submandibular contains equal amounts of both cell types
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Esophagus
S Joins stomach at cardiac orifice
S Cardiac orifice surrounded by gastroesophageal or cardiac
sphincter
S Physiological sphincter  slight thickening
S Not a true sphincter  emesis; heartburn
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Esophagus
S Nonkeratinized stratified squamous mucosa
S Changes to simple columnar epithelium at the esophagus-stomach junction
S Specialized for secretion
S Submucosa contains mucus-secreting esophageal glands
S “greases” the lumen of the esophagus to ease passage of the bolus
S Muscularis externa (Muscle type transition)
S Upper (skeletal)Middle(skeletal and smooth)Lower (smooth) muscle
S Adventitia (serosa layer)
S Fiborus Connective tissue
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Stomach Tissue
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Digestive Process: Mouth to
Stomach
S Mastication = chewing
S Mechanical breakdown
S Voluntary and reflexive
S Deglutination = swallowing
S 2 phases
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Buccal phase
S In the mouth; voluntary
S Tongue forces bolus down
Pharyngeal-esophageal phase
S Triggered by bolus reaching receptors in the pharynx
S Involuntary; controlled by swallowing center located in the brain stem
S Respiration temporarily inhibited
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Stomach
S Upper left quadrant of the peritoneal cavity
S “Storage tank”
S Size
S Empty = 50 ml
S Can expand to a volume of 4 L (about 1 gallon)
S Chemical breakdown of proteins begins
S Food converted to creamy paste called chyme
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Stomach
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Stomach
S Contains folds called rugae
S Regions
S Cardiac region (near the heart) or cardia
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Surrounds cardiac orifice
S Fundus Body
S Pyloric region
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Pyloric antrum  wider and more superior
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Pyloric canal  narrower
S Pylorus
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Pyloric sphincter  a true sphincter
S Controls stomach emptying
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Stomach
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Stomach
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S Omenta  mesenteries extending from the greater and lesser
curvatures
S Tether stomach to abdominal wall
S Lesser omentum
S Greater omentum  extends down to cover the small intestine
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Contains a lot of fatty tissue and lymph nodes
S Arterial supply comes form celiac trunk
S Subject to para control
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Abdominal Omentum
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Microscopic Anatomy
S Modified muscularis and mucosa
S Muscularis has an additional innermost layer of smooth
muscle fibers running obliquely
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Allows stomach to mix, churn move and pummel food
S Mucosa is simple columnar epithelium comprised entirely of
mucus cells
S Mucus cells produce protective two-layer coat of alkaline
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Stomach Mucosa
S Deep gastric pits
S Mucous cells form the walls of the gastric pits
S Pits lead into gastric glands  produce gastric juice
S Mucus neck cells  upper region of gastric glands
S Produce thin soluble mucus different from mucus from the mucus
cells
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Parietal And Chief Cells
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S Parietal Cells  middle region of glands
S Secrete hydrochloric acid (HCl) and intrinsic factor
S Have microvilli  increased surface are for more HCl secretion
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HCl creates extremely acidic environment of the stomach which is
required for optimal activity of pepsin (enzyme)
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Intrinsic factor is a glycoprotein required for vitamin B12 absorption in
the small intestine
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S Chief Cells  mostly basal region of gastric glands
S Produce pepsinogen (inactive form of pepsin)
S Pepsin digests protein
S At first, pepsinogen is converted to pepsin due to presence of HCl
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However, once pepsin is present, pepsin itself catalyzes activation of
pepsinogen
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Positive feedback
S Also release lipase
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Parietal and Chief Cells
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S Enteroendocrine cells  deep in gastric glands
S Release variety of chemical messengers directly into the lamina
propria
S Histamine, serotonin, somatastatin and gastrin
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Vulnerable Stomach?
S No!
S Mucosal Barrier
S Thick coating of bicarbonate rich mucus on stomach wall
S Tight junctions between epithelial cells  prevent leaking of
gastric juices into underlying tissue
S Damaged epithelial cells are shed quickly and quickly replaced
by undifferentiated stem cells
S Stomach mucus cells renewed every 3-6 days
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H. pylori and Gastric Ulcers
S Acid resistant bacteria that burrow through mucus and
destroy protective mucosal layer
S Release…
S ammonia to neutralize the environment
S Cytotoxin that damages stomach epithelium
S Detected by a breath test
S Treated with 2 week long antibiotic course
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Gastric ulcers
S For ulcers not caused by H. pylori  usually caused by
long-term use of NSAIDs (eg. Aspirin, ibuprofen)
S Treated with H2 (histamine) receptor blockers
S Cimetidine (Tagamet)
S Ranitidine (Zantac)
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Digestive Process in the
Stomach
S Protein digestion is the primary digestion that occurs in the
stomach
S HCl
S Pepsin
S In infants, stomach also secretes enzyme called rennin
S Acts on milk protein casein
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Alcohol and Aspirin
S Lipid soluble substance
S Absorbed directly from the stomach into the blood
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Intrinsic Factor
S Secreted by parietal cells
S Required for intestinal absorption of vitamin B12
S B12 important for production of mature erythrocytes
S Deficiency can cause anemia
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Regulation of Gastric
Secretions
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S Both neural and hormonal control
S Neural control
S Long nerve reflexes (vagus nerve)
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Vagus nerve stimulation = increased activity of nearly all glands
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Sympathetic nerve stimulation = opposite
S Short nerve reflexes (local enteric)
S Hormonal control is mostly mediated with gastrin (stomach) and
hormones from the small intestine
S Stimulates secretion of enzymes and HCl
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Phases of gastric secretion
S Cephalic phase
S Gastric phase
S Intestinal phase
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Cephalic phase
S Occurs before food enters the stomach
S Few minutes long
S Stimulated by smell, taste, sight or thought of food and gets
stomach ready
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Cephalic Phase- Extrinsic
S Activated olfactory receptors and taste buds send signals to
the hypothalamus
S Hypothalamus stimulates vagal nuclei of the medulla
oblongata
S Vagus nerves transmits motor impulses to parasympathetic
enteric ganglia
S Enteric ganglionic neurons stimulate the stomach glands
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Gastric Phase
S Occurs once food enters stomach
S 3-4 hours
S Provides 2/3rd of gastric juices released
S Most important stimuli:
S Distension  activates stretch receptors and local (myenteric) and
long (vagovagal) reflexes
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Both reflexes cause acetylcholine (ACh) to be released which stimulates
gastric juice secretion
S Peptides
S Low acidity
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Gastrin
S Chemicals from partially digested proteins, caffeine, and rising
pH directly activate enterendocrine cells called G cells
S G cells release gastrin
S Gastrin stimulates HCl secretion
S Highly acidic environment inhibits gastrin secretion
S G cells also stimulated by neural reflexes
S G cells inhibited by sympathetic nervous system
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HCl and Parietal Cells
S HCl secretion stimulated by 3 chemicals all of which work
via 2nd messengers
S ACh from parasympathetic fibers increase intracellular Ca2+
levels
S Gastrin from G cells increase intracellular Ca2+ levels
S Histamine from enterochromaffin-like cells acts through cAMP
S When all 3 chemicals bind to parietal cells  much HCl
secretion
S When only one binds  not much HCl
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HCl formation
S H+ actively pumped into stomach lumen by H+ K+ ATPase for
exchange of K+ ions into the cell
S Cl- ions follow H+ ions into the lumen in order to maintain an
electrical balance
S Cl- comes from blood plasma
S H+ comes from carbonic acid within the parietal cells
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S As H+ is pumped into the stomach, bicarbonate ion (HCO3-)
accumulates in cell and moves into the blood
S HCO3- moves into blood via the HCO3- Cl- antiporter
S As HCO3- moves into blood, Cl- moves in opposite direction
into cell
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Stomach-Blood vessel Interaction
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Intestinal Phase
S Has an excitatory and inhibitory component
S Excitatory component
S As food enters the duodenum intestinal mucosal cells are
stimulated to release intestinal (enteric) gastrin, a hormone that
encourages gastric glands to continue activity
S As intestine distends, inhibitory component takes over
because intestines can’t handle harsh acidic chyme
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Enterogastric Reflex
S Begins the inhibitory component of the intestinal phase
S Inhibits vagal nuclei in medulla
S Inhibits local reflexes
S Activates sympathetic fibers to cause pyloric sphincter to
tighten and prevent more food entering the small intestine
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S Causes release of intestinal hormones called
enterogastrones
S Secretin
S Cholescystokinin (CCK)
S Vasoactive intestinal peptide (VIP)
S All inhibit gastric secretion when stomach is very active
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Small Intestine
S Convoluted tube extending from pyloric sphincter to
ileocecal valve where it joins the large intestine
S Longest part of the alimentary canal
S 3 subdivisions: duodenum, jejunum, ileum
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Duodenum
S Shortest
S Bile duct and pancreatic duct unit at wall of the duodenum
 hepatopancreatic ampulla
S Hepatopancreatic sphincter controls entry of bile and
pancreatic juices
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Liver
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Jejunum and Ileum
S Jejunum is in between duodenum and ileum
S Ileum is longest
S Ileum joins long intestine at the ileocecal valve
S Jejunum and ileum are suspended by mesentery
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Nerve Supply
S Parasympathetic from the vagus nerve
S Sympathetic from the thoracic splanchnic nerves
S Both relayed through the superior mesenteric plexus
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Arterial Supply
S Mostly from the superior mesenteric artery
S Veins parallel the arteries and drain into the superior
mesenteric vein
S Superior mesenteric vein then drains into the hepatic portal
vein which carries nutrient-rich blood to the liver
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Structural Modifications of the
Small Intestine
S Circular folds
S Villi
S Microvilli
S All increase surface area
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Circular Folds
S Aka plicae circulares
S Deep permanent folds of the mucosa and submucosa
S Slow chyme movement to allow time for full nutrient
absorption
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Small Intestine
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Villi
S Fingerlike projections of the mucosa
S Epithelial cells of villi are mostly columnar cells for
absorption
S Lacteal  lymph capillary + dense capillary bed at the core
of each villus
S Nutrients are absorbed into the blood and the lymph
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Microvilli
S Densely packed
S “brush border” of mucosa
S Contain brush border enzymes which complete digestion of
carbohydrates and proteins
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Histology
S Mucosa is mostly made up of simple columnar cells for
absorption
S Epithelium also contain mucus-secreting goblet cells
S Pits between villi that lead into tubular glands called
intestinal crypts
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Intestinal Crypts
S Secrete intestinal juices (watery with mucus)
S Carrier fluid for nutrients
S Enteroendocrine cells scattered through crypts
S Source of enterogastrones (secretin and CCK)
S Intraepithelial lymphocytes
Type of T-cell
S Immediately kill infected cells
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S Paneth Cells
S Secretory cells that release defensins and and lysozyme
(antimicrobial enzyme)
S Secretions kill some bacteria and keep others
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Intestinal Submucosa
S Contains typical areolar connective tissue and er
peyer’s patches
S Peyer’s patches
S Aggregated lymphoid follicles
S Increase in number along the length of the small intestine (more in the
ileum)
S Duodenal glands
S Mucus secreting
S Only in submucosa of duodenum
S Produce alkaline mucus to neutralize acidic chyme
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Peyer’s patch component of Gut Associated lymphoid tissue (GALT)
Follicles lined by
specialized M
cells, short
microfolds (the
name M Cell)
Lymphoid
follicles with
germinal
centers
Domed areas
lacking villi
Basolateral
pockets of B cells
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H&E Ileum
SEM of surface of ileum
Accessory Organs of the Small
Intestine
S Liver
S Gallbladder
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Liver
S Largest gland in the body
S Has 4 primary lobes
S Ventral mesentery anchors liver to lesser curvature of the stomach
S Digestive role = produce bile
S Bile is a fat emulsifier
S Also has metabolic functions
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Liver Anatomy
S Hepatic artery and hepatic portal vein enter liver at porta
hepatis
S Bile leaves liver through several bile ducts which fuse to
form the common hepatic duct
S Common hepatic duct travels towards duodenum and fuses
with the cystic duct (drains gallbladder) to form the bile
duct
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Liver lobules as structural and functional units
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Portal triad exist at each of the six corners to the lobules
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Hepatic artery
Hepatic portal vein
Bile duct
Liver sinusoids  leaky capillaries found between hepatocyte cells
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Hexagonal shaped
Consist of hepatocytes “stacked” upon each other
Hepatocytes radiate from a central vein
Blood from hepatic artery and hepatic portal vein goes through liver sinusoids to the central vein
Contains Kupffer cells which remove debris (bacteria and old blood cells)
Secreted bile flows through bile canaliculi between hepatocytes towards the bile duct of the
portal triads
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Central Vein of Liver
Bile
S Yellow-green alkaline solution
S Contains bile salts, bile pigments, cholesterol, triglycerides,
phospholipids and electrolytes
S Only bile salts and phospholipids help in the digestive process
S Major mechanism of cholesterol elimination from the body
S Too much cholesterol or low levels of bile salts  cholesterol
crystallization = gallstones
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Bile salts
S Cholesterol derivatives
S Emulsify fats
S Facilitate fat and cholesterol absorption
S Help solubilize cholesterol
S Not excreted but rater recycled via enterohepatic circulation process
S Bile salts reabsorbed by the ileum
S Returned to liver via hepatic portal blood
S Resecreted in newly formed bile
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Bilirubin
S Primary bile pigment
S Waste product of the heme of hemeglobin during breakdown of
old erythrocytes
S Metabolized in small intestine by resident bacteria
S One of its breakdown products is responsible for the brown color
of feces
S Absence of bile = grey-white color of feces
S Not fats are being digested or absorbed
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Gallbladder
S Thin-walled, green muscular sac
S Stores bile
S Concentrates bile (by absorbing water and some of it ions)
S Contraction expels bile into the cystic duct which then flows
into bile duct
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Pancreas
S Gland
S Accessory digestive organ
S Produces enzymes that breakdown all categories of foodstuffs
S Pancreatic juice drains pancreas via main pancreatic duct which
fuses with the bile duct (hepatopancreatic ampulla) as it empties
into the duodenum
S Smaller accessory pancreatic duct empties directly into
duodenum
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S Acini  clusters of secretory cells surrounding ducts
S Full of rough endoplasmic reticulum
S Islets of Langerhans
S Scattered among the acini cells
S Endocrine glands
S Release insulin and glucagon
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Pancreas Cells
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Pancreas
Highly vascularized clusters of endocrine cells called
Islets of Langerhans (endocrine pancreas) surrounded by more
abundant acinar cells (exocrine pancreas)
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Immunocytochemistry Identifies Which Islet
Cell Secretes a Given Hormone
Beta Cells
Delta Cells
Alpha Cells
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F Cells
Pancreatic Juice
S Mostly water, enzymes and electrolytes
S Acinar cells produce enzyme-rich component
S Epithelial cells lining pancreatic duct release bicarbonate
ions to make it alkaline
S Amount of HCl produced in the stomach is exactly
matched to the amount of bicarbonate secreted by the
pancreas
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S Pancreatic proteases are released in inactive form and activated
in the duodenum
S Prevents pancreas from digesting itself
endopeptidase
S Trypsinogen
trypsin
S Endopeptidase is an intestinal brush border protease
S Trypsin activates more trypsinogen
S Trypsin also activates procarboxypeptidase and
chymotrypsinogen to carboxypeptidase and chymotrypsin,
respectively
S Amylases, lipases and nucleases released in active form
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Small Intestine
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Regulation of Bile and Pancreatic
secretions
S Bile salts are major stimulus for bile secretion
S As more bile salts are recycled, more bile is secreted
S Secretin released from intestine also stimulates liver cells to secrete
bile
S Live continuously makes bile, but it’s not released until gallbladder
contracts
S CCK (intestinal enzyme) is major stimulus for gallbladder
contraction
S CCK released into blood when acidic fatty chyme enters
duodenum
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CCK
S Stimulates gallbladder to contract
S Stimulates secretion of pancreatic juice
S Relaxes hepatopancreatic sphincter so that bile and
pancreatic juice enters duodenum
S Stimulates acini to release enzyme-rich pancreatic juice
S Potentiates effects of secretin
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Secretin
S Released in response to HCl in intestine
S Targets pancreatic duct cells for a bicarbonate-rich
pancreatic juice
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Roles of gastrin, secretin and cholecystokinin in digestion
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Large Intestine
S Frames small intestine
S Extends from ileocecal valve to the anus
S Major digestive function is to absorb remaining water
S Stores food residues temporarily
S Eliminates remaining food residues in the form of feces
S Not essential for life (nor major digestion occurs here)
S Removal (which may occur in cases of colon cancer) does not inhibit
life
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Anatomical modifications of the
Large Intestine
S Teniae coli
S The three bands which represent the reduction of the longitudinal layer of
the muscularis
S Haustra
S Pocket-like sacs
S Tone of teniae colie causes haustra
S Epiploic appendages
S Small fat-filled pouches of visceral peritoneum that hang from the surface
of the large intestine
S Significance is unknown
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Large Intestine
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Subdivisions
S Cecum
S Appendix
S Colon
S Rectum
S Anal canal
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Appendix
S Contains masses of lymphoid tissue (part of MALT)
S Small and twisted  infection likely
S Appendicitis  inflammation of the appendix
S Blockage that traps bacteria in the appendix
S Appendix swells  cuts off blood supply  appendix decays
S If rupture  peritonitis (infection of peritoneum)
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Colon
S Ascending colon
S Right colic/hepatic flexure
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bend between ascending and transvers colon
S Transverse colon
S Left colic/splenic flexure
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Bend between transverse and descending colon
S Descending colon
S Sigmoid colon
S S-shaped
S In the pelvis
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Rectum
S Located in the pelvis
S Three lateral curves
S Internally, these curves present as three transverse folds
called rectal valves
S Rectal valves separate feces from flatus (gas)
S Prevent feces from being passed along with gas
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Anal Canal
S Located in the perineum
S 2 sphincters
S Internal anal sphincter
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Involuntary
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Smooth muscle
S External anal sphincter
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Voluntary
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Skeletal muscle
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Microscopic Anatomy
S Mucosa is simple columnar epithelium except in the anal canal
S No villi
S No cells that secrete digestive enzymes
S Thicker mucosa
S Deeper crypts
S Large number of goblet cells in crypts
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S Mucosa of anal canal is stratified squamous epithelium
(increased abrasion)
S Anal canal hangs in longitudinal folds called anal columns
S Anal sinuses are between the anal columns
S Secrete mucus to ease feces movement
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Motility
S Fecal matter stays in large intestine for 12-24 hours
S Haustral contractions  slow segmenting movements
S More frequent (every 30 minutes)
S Mass movements  long, slow-moving, powerful contractile
waves over large areas of the colon
S Less frequent (3-4 times a day)
S Usually occur during or just after eating
S Fiber increases strength of contractions and softens stool
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Defecation
S When mass movements force feces into rectum, rectal wall
stretches and initiates defecation reflex
S Sigmoid colon and rectum contract
S Internal anal sphincter relaxes
S If defecation is delayed reflex contractions end
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Diarrhea and Constipation
S Diarrhea
S Watery stools
S Results from any condition that rushes food through the large
intestine (reduced time to absorb water)
S Dehydration and electrolyte imbalance
S Constipation
S Hard stools
S When feces remains in the large intestine too long
S Lack of fiber, improper bowel habits, lack of exercise, emotional
upset or laxative abuse
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Chemical Digestion and Absorption
S Chemical digestion is a metabolic process in which large
food molecules are broken down to monomers
S Hydrolysis  enzymatic breakdown of food molecule
S Involves addition of water molecule to each bond being broken
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Carbohydrates
S Broken down to monosaccharaides
S Includes starch
S Indigestible carbohydrates (cellulose) act as bulk and help move
foodstuffs along the GI tract
S Chemical digestion begins in the mouth with salivary amylase
S Amylase works best in slightly acidic to neutral environment
S Inactivated by stomach acid
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S Digestion continues in small intestine with aid of pancreatic
amylase
S Amylases convert starch to oligosaccharides
S Intestinal brush border enzymes convert oligosaccharides to
monosaccharaides
S Dextrinase, glucoamylase, maltase, sucrase, lactase
S Digestion of carbohydrates ends with the small intestine (no
digestion in large intestine)
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Proteins
S Digested to its amino acid monomers
S Digestion begins in the stomach
S Pepsinogen from chief cells is activated to pepsin which digests
proteins
S Optimal functioning in acidic environment (low pH)
S Inactivated by high pH in duodenum
S Activity restricted to stomach
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S Trypsin and chymotrypsin from the pancreas continue to
cleave protein bonds into smaller peptide bonds in the
intestine
S Carboxypeptidase (brush border enzyme) cleaves amino
acids one-by-one at the carboxyl end
S Aminopeptidase does the same from the amino end
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Lipids
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Digestion only occurs in the small intestine
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Pancreas provides lipase
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Triglycerides are insoluble in water, therefore “pre-treated” with bile salts
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Bile salts emulsify and increase surface area of the triglycerides to aid in fat
digestion
Bile salts have both polar and nonpolar ends
Polar and faces aqueous environment. Nonpolar end faces fat molecules
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Digestion occurs with LIPASE. BILE SALTS DO NOT DIGEST
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Breakdown into fatty acids and monoglycerides
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Nucleic Acids
S DNA and RNA in the nuclei of cells of food hydrolyzed to
their nucleotide monomers
S Pancreatic nucleases responsible
S Nucleotides later broken down by brush border enzymes
nucleosidases and phosphatases
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Absorption
S Most absorption complete by the time the chyme reaches
the ileum
S Ileum mostly responsible for reclaiming bile salts
S Most nutrients absorbed via active transport
S Tight junctions exist between mucosal epithelial cells,
therefore nutrients must move through the cell
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Carbohydrate Absorption
S Glucose and galactose use secondary active transport (with
Na+) to move into epithelial cells
S Move out of epithelial cells and into blood via facilitated
diffusion
S Fructose moves entirely by facilitated diffusion
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Protein Absorption
S Several types of protein transporters move different amino
acids
S Most transporters are coupled to the active transport with
Na+
S Short-chained amino acids are absorbed with H+
cotransport into the epithelial cell
S Broken down to single amino acids before moving into
circulation
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Lipid Absorption
S Monoglycerides and fatty acids associate with bile salts and
lecithin (phospholipid) to form micelles
S Micelles  collections of fatty elements clustered together
with bile salts
S Micelles easily diffuse between microvilli
S Fat absorption complete in the ileum
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S Once inside the epithelial cells, they are resynthesized into
triglycerides by the smooth ER
S Triglycerides coated with a “skin” of proteins to form
chylomicrons (water-soluble lipoprotein droplet)
S Chylomicrons leave via exocytosis and enter lacteals to join
the lymph (not blood)
S Later emptied into venous blood in the neck region
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S In the blood, chylomicrons liberate triglycerides
S Triglycerides hydrolyzed to fatty acids and glycerol by
lipoprotein lipase
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Nucleic Acid Absorption
S Pentose sugars, nitrogenous bases and phosphate ions from
nucleotide breakdown are actively transported across
epithelium by special carriers
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Vitamin Absorption
S Small intestine absorbs dietary vitamins
S Large intestine absorbs K and some B vitamins
S Fat soluble vitamins (A, D, E and K) dissolve in dietary fats and
are absorbed in micelles
S Water soluble vitamins (B and C) absorbed via diffusion or
specific active or passive transporters
S Exception: Vitamin B12 is very large. Requires intrinsic factor
(stomach)
S
Intrinsic factor binds to B12. Intrinsic factor binds to its receptor in the
ileum allowing for endocytosis of B12
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Water Absorption
S Mostly absorbed in the small intestine by osmosis
S Osmosis in the large intestine
S Water moves freely across intestinal mucosa in both
directions but net osmosis occurs when there is a
concentration gradient established by active transport of
solutes into mucosal cells
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160
161
162
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General Organization and Structure
of the Digestive/Alimentary tube
From Kierszenbaum,
164
Basic mucosal
forms:
Secretory-only in stomach
Protective-esophagus
Absorptive/Protective- Large intestine
Absorptive- entire small intestine
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