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© Endeavour College of Natural Health endeavour.edu.au
BIOH122Human Biological Science 2
Session 15
Digestive System 3 –
Small Intestine & Large Intestine
Bioscience Department
© Endeavour College of Natural Health endeavour.edu.au 2
Session Plan
o Absorption in Small intestine
o Large intestine – anatomy and histology
o Large intestine – mechanical and chemical digestion
o Absorption and Faeces Formation in the Large Intestine
• The Defecation Reflex
o Phases of digestion
o Aging and the digestive system
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Review: Digestion of
Carbohydrates
o Mouth: Salivary amylase
o Oesophagus and stomach: no enzyme released for
carbohydrates but salivary amylase active for about an
hour.
o Small intestine:
• Pancreatic amylase
• Brush border enzymes (ɑ-dextrinase, maltase,
sucrase and lactase) act on disaccharides
o Production of monosaccharides - fructose, glucose and
galactose for absorption
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Review: Digestion of
Carbohydrates
Marieb, and Hoehn, 2011
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Review: Digestion of Proteins
o Mouth and Oesophagus: No digestion
o Stomach:
• HCl
• Pepsin
o Small Intestine:
• Pancreatic peptidases: trypsin, chymotrypsin,
carboxypeptidase, and elastase
• Brush border peptidases: aminopeptidase and dipeptidase
o Digestive enzymes - split peptide bonds between different
amino acids for absorption.
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Review: Digestion of Proteins
Marieb, and Hoehn, 2011
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Review: Digestion of Lipids
o Mouth: Lingual lipase (activated in stomach)
o Stomach: Gastric lipase (has a limited role in the adult)
o Small Intestine:
• Bile salts
• Pancreatic lipase
• No enzymes in brush border
o Triglycerides are broken down into long/short chain fatty
acids and monoglycerides for absorption
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Review: Digestion of Lipids
Marieb, and Hoehn, 2011
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Digestion of Nucleic Acids
o Mouth, oesophagus and Stomach: No enzymes for
digestion
o Small intestine:
o Pancreatic nucleases: Ribonuclease and
deoxyribonuclease
o Brush border enzymes: nucleosidase and phosphatase
o Nucleic acids are broken down into nucleotides and
then into pentoses, phosphates, and nitrogenous bases
for absorption.
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Digestion of Nucleic Acids
Marieb, and Hoehn, 2011
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Absorption in Small intestine
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Absorption in the Small Intestine
o Absorption: The passage of the end products of
digestion from the GI tract into the blood or lymph
o Occurs by:
• Simple diffusion
• Facilitated diffusion
• Osmosis
• Active transport
o Most absorption occurs in Small intestine
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Absorption in Small Intestine
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Absorption of Monosaccharides
o Essentially all carbohydrates are absorbed as
monosaccharides.
o Absorption into epithelial cell
• Glucose and Galactose - sodium glucose symporter
(active transport)
• Fructose - facilitated diffusion
o Movement out of epithelial cell into bloodstream
• by facilitated diffusion
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Absorption of Monosaccharides
Marieb, and Hoehn, 2011
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Absorption of Amino Acids and
Dipeptides
o Absorption of Amino Acids, Dipeptides, and Tripeptides
o Absorption into epithelial cell
• active transport with Na+ or H+ ions (symporters)
o Movement out of epithelial cell into blood
• diffusion
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Absorption of Amino Acids
and Dipeptides
Marieb, and Hoehn, 2011
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Absorption of Lipids
o Dietary lipids are all absorbed by simple diffusion.
o Absorption into epithelial cell:
• Long-chain fatty acids and monoglycerides: absorbed as part of
micelles
• Small fatty acids: simple diffusion
o Movement out of epithelial cell
• Long-chain fatty acids and monoglycerides: chylomicrons into
the lacteal of a villus.
• Small fatty acids: simple diffusion into blood
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Absorption of Lipids
o From the lacteal the chylomicrons enter the lymphatic
system and then pass into the cardiovascular system,
finally reaching the liver or adipose tissue.
o The plasma lipids: fatty acids, triglycerides, cholesterol -
are insoluble in water and body fluids.
o Lipoproteins: The combination of lipid and protein
transporters in order to transport lipids in blood and
utilized by body cells.
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Absorption of Lipids
o Lipids enter cells by
simple diffusion leaving
bile salts behind in the
lumen.
o Bile salts are
reabsorbed into blood
and reformed into bile in
the liver
Marieb, and Hoehn, 2011
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Absorption of Electrolytes
o Many of the electrolytes absorbed by the small intestine
come from gastrointestinal secretions and some are part
of digested foods and liquids.
o Absorption: by diffusion and secondary active transport
• Sodium: diffusion, secondary active transport, Na+/K+
pumps (active transport)
• Chloride, iodide and nitrate: active or passively
transport
• Iron, magnesium, potassium and phosphate ions:
active transport
• Calcium: active transport, stimulated by calcitriol.
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Absorption of Vitamins
o Fat-soluble vitamins (A, D, E, and K):
• travel in micelles and are absorbed by simple
diffusion
o Water-soluble vitamins (B and C):
• absorbed by diffusion
o B12 combines with intrinsic factor before it is transported
into the cells
• receptor mediated endocytosis
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Absorption of Water
o Water absorption: Osmosis from the lumen of the
intestines through epithelial cells and into blood
capillaries.
o Depends on the absorption of electrolytes and nutrients
to maintain an osmotic balance with the blood.
o Small intestine reabsorbs 8.3 liters
o Absorption is by osmosis through cell walls into vascular
capillaries inside villi
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Absorption of
Water o 9.3 liters of fluid dumped
into GI tract each day
o Small intestine reabsorbs
8.3 liters
o Large intestine reabsorbs
90% of that last one liter
o Absorption is by osmosis
through cell walls into
vascular capillaries inside
villi
24
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Where will the absorbed nutrients go?
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Large intestine – anatomy and
histology
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Large Intestine
o The large intestine: The terminal portion of the GI tract
(1.5 m long by 6.5 cm in diameter), extends from the
ileocaecal sphincter to the anus.
o Functions:
• Haustral churning, peristalsis, and mass peristalsis drive
contents of colon into rectum.
• Bacteria in large intestine convert proteins to amino acids, break
down amino acids, and produce some B vitamins and vitamin K.
• Absorbing some water, ions, and vitamins.
• Forming faeces and defecating (emptying rectum).
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Anatomy of the Large Intestine
o Anatomical subdivisions:
• Caecum
• Colon
• Rectum
• Anal canal
o Appendix
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Anatomy of the Large Intestine
o Anal canal
• Anus:
– An internal anal
sphincter
– An external anal
sphincter
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Histology of the Large Intestine
o Layers of large intestinal wall:
• Mucosa
• Submucosa
• Muscularis
• Serosa
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Histology of the Large Intestine
o Mucosa: composed of:
o The epithelial layer: Simple columnar epithelium
• Contains Absorptive and Goblet cells
o Intestinal glands: Extend the full thickness of the mucosa
o Lamina propria: Areolar connective tissue
• Solitary lymphatic nodules
o Muscularis mucosae: smooth muscle
o Structural adaptations: No circular folds or villi; however,
microvilli are present on the absorptive cells
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Histology of the Large Intestine
Cells of Intestinal glands:
• Absorptive cell
• Goblet cell
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Histology of the Large Intestine
o Submucosa: Areolar connective tissue.
• Solitary lymphatic nodules: may extend through the
muscularis mucosae into the submucosa
o Muscularis: Consists of 2 layers of smooth muscles.
• An internal layer: circular smooth muscle.
• An external layer: longitudinal smooth muscle
– Taeniae coli
– Haustra
o Serosa: Visceral peritoneum
• Omental appendices
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Large intestine – mechanical and
chemical digestion
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Mechanical Digestion in the
Large Intestine
o Mechanical movements:
• Haustral churning: Slow haustral contractions that occur about
every 30 minutes and last approximately 1 minute.
• Peristalsis: 3 to 12 contractions per minute
• Mass peristalsis: 3 or 4 times a day, during or immediately after a
meal
• Gastroileal reflex: when stomach is full, gastrin hormone
relaxes ileocecal sphincter so small intestine will empty
and make room
• Gastrocolic reflex: when stomach fills, a strong peristaltic
wave moves contents of transverse colon into rectum
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Chemical Digestion in the
Large Intestine
o Large intestinal glands: only mucous, no enzymes are
secreted
o Bacterial fermentation:
• Undigested carbohydrates: into hydrogen, carbon dioxide and
methane gas
• Flatulence
• Undigested proteins: into indole, skatole, hydrogen sulfide, and
fatty acids
• Fecal odour and Urinary excretion
• Bilirubin: into simpler pigments- Stercobilin
• Fecal colour
• Vitamins K and B produced in the colon
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Absorption and Feces Formation in
the Large Intestine
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Absorption in the Large Intestine
o Absorption:
o Water: by osmosis through cell walls into vascular
capillaries inside villi
• Of the 0.5–1.0 litre, 90% reabsorbed Only 100 -200 ml of water
is excreted in the faeces each day.
o Some electrolytes: sodium and chloride ions
o Some vitamins
© Endeavour College of Natural Health endeavour.edu.au 39
Feces Formation in the Large
Intestine
o Feces formation: After 3 to 10 hours, due to water
absorption from chyme.
o Feces: Solid or semisolid chyme consist of
• water, inorganic salts, sloughed-off epithelial cells
from the mucosa of the gastrointestinal tract, bacteria,
products of bacterial decomposition, unabsorbed
digested materials, and indigestible parts of food.
© Endeavour College of Natural Health endeavour.edu.au 40
The Defaecation Reflex
o Defaecation: The elimination of faeces from the rectum
o The defaecation reflex: action aided by voluntary
contractions of
• The diaphragm,
• The abdominal muscles and
• The external anal sphincter
o The external anal sphincter can be voluntarily controlled
(except in infants) to allow or postpone defaecation.
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The Defaecation Reflex
Marieb, and Hoehn, 2011
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Defaecation Problems
o Diarrhoea: • Increased motility of the intestines.
• Caused by: lactose intolerance, stress, and microbes that irritate the gastrointestinal mucosa.
• Chyme passes too quickly through intestine
• Decreased absorption by the intestines.
o Constipation:• Decreased intestinal motility
• Faeces remain in the colon for prolonged periods, too much water is absorbed
• Caused by poor habits, spasms of the colon, insufficient fibre in the diet, inadequate fluid intake, lack of exercise, emotional stress, and certain drugs
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Dietary fibers
o Dietary fibers: indigestible plant carbohydrates
– cellulose, lignin, and pectin—found in fruits, vegetables,
grains, and beans
– affect the speed of food passage through the GI tract
o Insoluble fiber: insoluble in water
– woody parts of plants (wheat bran, veggie skins)
– may help protect against colon cancer
o Soluble fiber: soluble in water
– in beans, oats, barley, broccoli, prunes, apples, and citrus
fruits.
– lowers blood cholesterol by preventing reabsorption of bile
salts so liver has to use cholesterol to make more
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Phases of Digestion
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Phases of Digestion
o Three overlapping phases:
• Cephalic phase
• Gastric phase
• Intestinal phase
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Ph
ase
s o
f D
ige
stio
n
© Endeavour College of Natural Health endeavour.edu.au 48
Cephalic Phase
o The cephalic phase: prepares the mouth and stomach
for food that is about to be eaten.
o Activated by: The smell, sight, thought, or initial taste of
food
o Neural centres: in the cerebral cortex, hypothalamus,
and brain stem.
o The facial and glossopharyngeal nerves:
• stimulate the salivary glands to secrete saliva
o Vagus nerve:
o stimulate the gastric glands to secrete gastric juice
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Gastric Phase
o The gastric phase: Begins when food arrives in the
stomach.
o Promote gastric secretion and gastric motility.
o Two regulations:
• Neural control
• Hormonal control
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Gastric Phase
o Neural control:
• Stretch receptors and chemoreceptors: in the wall of stomach
• Submucosal plexus
• Vigorous peristalsis and gastric juice secretions
• Gastric emptying
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Gastric Phase
o Hormonal control:
o Gastrin: released from G cells secretion of gastrin into
bloodstream
o Stimuli:
• Distension of the stomach by chyme,
• Partially digested proteins in chyme,
• High pH of chyme due to the presence of food in the stomach*
• Acetylcholine released from parasympathetic neurons.
* Gastrin release inhibited when pH of gastric juice drops below 2.0
© Endeavour College of Natural Health endeavour.edu.au 52
Gastric Phase
o Roles of Gastrin:
• Stimulates gastric glands to secrete large amounts of
gastric juice.
• Strengthens the contraction of the lower oesophageal
sphincter to prevent reflux of acid chyme into the
oesophagus,
• Increases motility of the stomach, and relaxes the
pyloric sphincter, which promotes gastric emptying.
© Endeavour College of Natural Health endeavour.edu.au 53
Intestinal Phase
o The intestinal phase: Begins when food enters the small
intestine.
o Slow the exit of chyme from the stomach. Promote
digestion of food in small intestine
o Two regulations:
• Neural control
• Hormonal control
© Endeavour College of Natural Health endeavour.edu.au 54
Intestinal Phase
o Neural control:
o Stretch receptors, fatty acids or sugar: Signal Enteric NS
and CNS (medulla)
o ↑ Sympathetic nerves ↓Parasympathetic nerves: Slow
stomach motility / emptying
o The enterogastric reflex: stretch of duodenum causes
inhibition of gastric motility and increases the contraction
of the pyloric sphincter to decrease gastric emptying.
© Endeavour College of Natural Health endeavour.edu.au 55
Intestinal Phase:
Neural control
Start here
© Endeavour College of Natural Health endeavour.edu.au 56
Intestinal Phase
o Hormonal control:
• Secretin: Stimulates the flow of pancreatic juice rich in
bicarbonate, and inhibits the secretion of gastric juice.
• Cholecystokinin (CCK): decreases stomach emptying
and stimulates the secretion of pancreatic juice rich in
digestive enzymes, and increase the flow of bile.
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Aging and The Digestive System
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Aging and the Digestive System
o Changes associated with ageing:
• Decreased secretory mechanisms
• Decreased motility of the digestive organs
• Loss of strength and tone of muscular tissue
• Changes in neurosensory feedback
• Diminished response to pain and internal stimuli
© Endeavour College of Natural Health endeavour.edu.au 59
Aging and the Digestive System
Pathologies with Ageing:
o Upper GIT:
• mouth irritations and sores,
• loss of taste,
• periodontal disease,
• difficulty in swallowing,
• hiatal hernia, gastritis, and peptic ulcer disease.
o Small intestine:
• Duodenal ulcers,
• malabsorption, and maldigestion
© Endeavour College of Natural Health endeavour.edu.au 60
Aging and the Digestive System
Pathologies with Ageing:
o Large intestine:• Constipation
• Haemorrhoids
• Diverticular disease
• Cancer of the colon or rectum
• Bowel obstructions and impactions
o Other pathologies:• Appendicitis,
• Gallbladder problems,
• Jaundice, and cirrhosis,
• Acute pancreatitis
© Endeavour College of Natural Health endeavour.edu.au 61
Readings and Resources
o Tortora, GJ & Derrickson, B 2014. Principles of Anatomy and Physiology, 14th edn, Wiley.
o Harris, P, Nagy, S & Vardaxis, N 2010, Mosby’s Dictionary of Medicine, Nursing and Health Professions, 2nd edn, Mosby Elsevier.
o Guyton, AC & Hall, JE 2011, Textbook of Medical Physiology, 12th edn, Saunders Elsevier.
o Marieb, EN & Hoehn, K 2011, Human Anatomy and Physiology, 9th edn, Benjamin Cummings Pearson.
o Moore, KL, Dalley, AF & Agur, AMR 2010, Clinically Orientated Anatomy, 6th edn, Lippincott, Williams & Wilkins.
© Endeavour College of Natural Health endeavour.edu.au 62
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