Digestive System

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Digestive System Top to Bottom
Dr Usama ALAlami
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Digestion
Breakdown of huge food molecules into small ones
Digestive Process
1 Ingestion Taking food into the digestive
tract
2 Propulsion Move food through alimentary
canal
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Propulsion
Peristalsis Adjacent sections of the alimentary
canal alternately contract and relax
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Coordinated Muscular Contractions Produce
Peristaltic Movements
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Mechanical Digestion
Chewing Mixing of food with saliva using
tongue Churning of food in the stomach Segmentatio
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Chemical Digestion
Begins in the mouth and ends in the small
intestine.
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4 AbsorptionDigested end products (minerals,
vitamins and water) absorbed to blood
and lymph supply
5 DefecationNeed I explain this ?
Digestive Tract (Alimentary Canal)
ARSE
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Digestive Tract (Alimentary Canal)
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Key To The Diagram Of The Digestive System.
Key To The Diagram Of The Digestive System

• 1. Buccal cavity.
• 2. Tongue.
• 3. Oesophagus.
• 4. Diaphragm.
• 5. Stomach.
• 6. Pyloric sphincter.
• 7. Liver.
• 8. Gall bladder.

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Key To The Diagram Of The Digestive System

• 9. Bile duct.
• 10. Pancreas.
• 11. Duodenum.
• 12. Ileum.
• 13. Caecum.
• 14. Appendix.
• 15. Colon.
• 16. Rectum.
• 17. Anus.

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Accessory Digestive Organs
Teeth Tongue Gallbladder Salivary Glands
(Saliva) Liver (Bile) Pancreas (Enzymes)
Blood Supply
Arteries branch off the abdominal aorta
to a) Digestive Organs b) Hepatic Portal
Circulation
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Arterial Supply
Superior/inferior mesenteric arteries (supply
small and large intestine)
Digestive system receives approximately 25 of
cardiac Output.
This increases after a meal.
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Composite Micrograph Showing Variations In Gut
Wall Structure
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Histology of the Alimentary Canal
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Histology Of The Alimentary Canal
From innermost to outermost layer.
1 Mucosa
Lines the luminal surface of the digestive tract.
Three layers
a) Mucous Membrane
Epithelial cells for secretion, absorption and
protection
Exocrine and endocrine cells for secretion of
digestive Juices and GI hormones respectively.
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b) Lamina Propria
Well vascularized (nerves and lymph vessels)
Defence against intestinal bacteria bacteria
c) Muscularis Mucosa
Outer layer of smooth muscle
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2 Submucosa
Thick layer of connective tissue.
Provide distensibility and elasticity
Contain large blood and lymph vessels.
Contains nerve networks SUBMUCOUS
PLEXUX Therefore, controls local activity of each
gut region.
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3 Muscularis Externa
Major smooth muscle layer.
MYENTERIC PLEXUS between the two layers
regulate local gut activity.
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4 Serosa
Outer covering of digestive tract.
Secretes serous fluid ? lubricate and prevent
friction between digestive organs and surrounding
viscera.
Serosa is continuous with mesentery.
Mesenteric tearing ? HERNIA
Hernia Protrusion of an organ through the
muscular wall of the cavity that contains
them
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Regulation Of Digestive Function
Digestive function is controlled by four factors
Autonomous smooth muscle function.
Intrinsic nerve plexuses
Extrinsic nerves
Gastrointestinal hormones
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Autonomous Smooth Muscle Function
Smooth muscle cells of the digestive system
possess Basic Electrical Rhythm (BER).
This does not directly induce contraction
When a large group of cells reach excitation ?
contraction
Whether contraction is achieved depends
on _at_ Mechanical effects _at_ Nervous
system _at_ Hormonal effects
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Intrinsic Nerve Plexuses
Nerve Plexus Interconnecting network of nerve
cells

• Submucous (Meissners) plexus in submucosa
• Myenteric (Auerbachs) plexus between
  longitudinal and
• circular smooth muscle cell layers.

They run the entire length from oesophagus to
anus.
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• Intrinsic plexuses influence
• Smooth muscle contractility
• Exocrine cell secretion (digestive juices)
• c) Endocrine cell secretions (digestive hormones)

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Extrinsic Nerves
Sympathetic and parasympathetic nerve branches.
Sympathetic fight or flight slow digestive
function.
Parasympathetic dominant in quiet relaxed
situations.
Arrive by way of VAGUS nerve ? increase smooth
muscle contractility secretion of digestive
enzymes and hormones.

Autonomous nervous system also coordinates
between different organs of the digestive tract.
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Extrinsic Nerves
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Gastrointestinal Hormones
Endocrine cells within mucosa release hormones
into blood
They affect _at_ Exocrine gland
secretions _at_ Smooth muscle cells _at_ Pancreatic
endocrine cells ? influence food storage and
uptake.
GI hormones released in response to changes in
luminal content (protein, fat or acid).

Effect is direct on endocrine glands or indirect
on nerve plexuses or extrinsic autonomous nerves.
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Receptors Of The Digestive Tract
1 Chemoreceptors
2 Mechanoreceptors (Pressurereceptors)
3 Osmoreceptors

• Activities of these receptors results in
• Short neural reflex (via intrinsic nerve
  plexuses)
• Long neural reflexes (via autonomous nerves)

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Mouth
The oral cavity (buccal cavity) has four
boundaries
1 Lips (anterior) 2 Cheeks (lateral) 3 Pala
te (superior) 4 Tongue (inferior)
Anterior opening is the oral orifice
Posteriorly, the oral cavity is continuous with
the oropharynx

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Lips
Reddened area known as red margin (or scoring
zone !!!)
Lips help to guide and contain food in the mouth

• Non digestive functions
• Speech
• b) Snogging

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Palate
Arched roof separating mouth from nasal passage
Allows chewing, breathing and sucking to take
place Simultaneously.
Failure of this fusion causes CLEFT PALATE.
Anterior Hard palate bone Posterior Soft
palate skeletal muscle
Uvula Hangs from soft palate ? seals off nasal
passage during swallowing.

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Tongue
Composed of voluntary skeletal muscle
Houses taste buds, serous and mucous glands
Helps reposition food between teeth and mix it
with saliva (Bolus)
Non digestive functions Speech Snogging
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Teeth
First step in digestive process is mastication
(chewing).
Exposed portion of tooth enamel hardest
substance in body.
Occlusion Upper and lower teeth fit together
when jaws are closed

• Malocclusion due to
• Overcrowding of teeth too large to fit in the jaw
  space
• 2) One jaw displaced in relation to the other

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This results in inefficient chewing and pain in
the temporomandibular joint.
Purpose of mastication 1 Grind food into
smaller pieces to fascilitate swallowing 2 Mix
food with saliva 3 Stimulate taste buds
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Tooth and Gum Diseases
1 Dental Caries (Rottenness)
Also known as cavities
Due to dental decay
Decay due to dental plaque (film of sugar,
bacteria and mouth debris)
Bacteria metabolise sugar ? acid ? decay
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2 Calculus (Stone)
Effect of plaque on gums
Disrupt seal between gingivae and teeth
Risk of gum infection
Gums bleed, sore and swollen (Gingivitis)
(reversible)
If plaque not removed, this leads to ..
3 Peridontal Disease (Peridontitis)
Treated by antibiotics
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Salivary Glands
Saliva secreted by 1 Extrinsic salivary glands
(Major) 2 Intrinsic salivary glands (minor)
(in mucosa lining the cheeks)
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Extrinsic Salivary Glands
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Extrinsic Salivary Glands
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Lie outside oral cavity and discharge saliva
through small ducts into mouth.
Sublingual Below tongue Submandibular Below
mandible Parotid (parnear, otidear) anterior
to ear.
Mumps Inflammation of the parotid gland caused
by the mumps virus (myxovirus)
resulting in fever and pain upon chewing
Composed mainly of serous cells (watery secretion
of enzymes and ions) and mucous cells (viscous
secretion of mucus).
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Sublingual Mostly mucous cells Submandibular
and buccal Equal proportion of both Parotid
Only serous cells
Composition Of Saliva
99.5 water, 0.5 protein and electrolytes
Saliva begins digestion of carbohydrates in mouth
by salivary amylase.
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Saliva facilitates swallowing by moistening food
particles via mucus (thick and slippery)
Saliva possesses antibacterial action
through _at_ Lysozymes _at_ Rinsing away material
that may serve as food source for bacteria.
Saliva is neither sugary or salty ? important for
perception of sweet and salty tastes.
Bicarbonate in saliva neutralizes acid in food.
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Facilitates speech via moistening of lips and
tongue
Xerostomia Diminished saliva secretion Result
in difficulty in chewing, swallowing,
inarticulate speech and dental caries.
Control Of Salivary Secretion
1-2 litres daily
Basal secretion due to direct autonomic nerve
stimulation
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• Enhanced secretion of saliva due to
• Simple unconditional salivary reflex
• Acquired or conditional salivary reflex

Very little digestion action in mouth
Amylase action accomplished in body of stomach
No absorption of foodstuff
However, absorption of therapeutic agents occurs
via oral mucosa (e.g. nitroglycerine)
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Pharynx and Oesophagus
Motility associated with pharynx and oesophagus
is swallowing or deglutition.
Bolus from mouth through oesophagus into stomach.
Swallowing is initiated voluntarily.
But once initiated it cant be stopped
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Peristalsis In The Oesophagus (Anterior View)
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Mechanism Of Swallowing
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Swallowing
1 Oropharyngeal Stage
Lasts about 1 second
Mouth to pharynx to oesophagus
Food must be prevented from re-entering
a) Mouth Position of tongue against hard palate.
b) Nasal Passages Uvula elevated against back of
throat.
c) Trachea Cartilaginous flap epiglottis
seals trachea.
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2 Oesophageal Stage
Muscular tube (approx 25 cm long)
Connects pharynx to stomach.
Penetrates diaphragm at oesophageal hiatus
Stomach protrusion through this ? Hiatal Hernia

• Two locks or sphincters
• Pharyngooesophageal sphincter (top) Prevents
  large volumes of air entering digestive
  tract ? eructation (burping).
• Gastrooesophageal sphincter (lower)

Food moves down oesophagus by active process
(peristalsis)
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Diseases Of The Gastrooesophageal Sphincter
GES closed except during swallowing
When gastric contents (acidic) enter oesophagus
despite GES being closed ? HEARTBURN
This is followed by opening of sphincter to allow
contents back into stomach
If sphincter remains shut ? ACHALASIA
Complications of achalasia ASPIRATION PNEUMONIA
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Stomach
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Stomach
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Diagram Of The Stomach Showing The Three Muscle
Layers
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Stomach
J-shaped saclike chamber lying between the
oesophagus and the small intestine.
Divided according to anatomical and histological
parameters to
a Fundus Dome-shaped, this smooth muscle
portion of the stomach.
Lies above oesophageal opening
b Body Midportion of the stomach
This layer of smooth muscle
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c Antrum Pyloric antrum narrows to form
pyloric anal and ends in
pylorus (gatekeeper).
Main function of stomach is storing ingested food
until it can be emptied into small intestine at a
rate appropriate for optimal digestion and
absorption.
Second function is secretion of HCl and
protein-digesting enzymes
Final product from the stomach is CHYME
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Microscopic Anatomy
Extra oblique layer of muscle to allow churning
and mixing of food.
Muscularis mucosa has mucus-secreting goblet
cells.
Also contains gastric pits leading to gastric
glands with specialized cells (secrete gastric
juice)
1 Mucous Neck Cells
Secrete thin mucus
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2 Chief Cells
Secrete inactive pepsinogen (active pepsin
protein-digesting enzyme).
3 Parietal (Oxyntic) Cells
Secrete HCl and intrinsic factor
Pepsinogen activated by HCl
Intrinsic factor absorption of vitamin B12 in
small intestine
Gastric mucosa atrophy or gastrectomy ? loss of
chief and parietal cells ? treated by regular
vitamin B12 injections
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Surface View of the Gastric Mucosa Showing
Entrance To Gastric Pits SEM (x35)
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4 Enteroendocrine Cells
Secret hormones (Gastrin, Serotonin,
Endorphine,) into blood
Occur in antrum region
Gastrin secreted by G cells into blood
Travels back to oxyntic mucosa ? stimulate chief
and Parietal cells ? stimulate gastric juices
Also stimulates growth of stomach and small
intestine mucosa.
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Control Of Gastric Secretions
1 Cephalic Phase
Seeing/smelling food
Vagus stimulates HCl and pepsinogen secretion via
intrinsic nerve plexuses
Vagus stimulates Gastrin secretion by G cells ?
increased secretion of HCl and pepsinogen
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2 Gastric Phase
Stomach distension causes activation of stretch
receptors
As with cephalic phase, HCl and pepsinogen
secretion is stimulated via vagal pathways
Caffeine and alcohol ? stimulate gastric juices
even if stomach is empty ? aggravate existing
ulcer.
3 Intestinal Phase
Protein fragments entering duodenum ? stimulate
intestinal gastrin ? travel by blood to stomach
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Control Of Gastric Secretions The Cephalic Phase
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Control Of Gastric Secretions The Gastric Phase
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Control Of Gastric Secretions The Intestinal
Phase
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Summary Of Information Relating To Gastrin
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Gastric Mucosal Barrier (GMB)
1 Luminal membrane impermeable to HCl
2 Tight junctions between cells
Ulcer
Peptic ulcer in oesophagus, stomach or duodenum
Weakness in GMB.
Increased acidity leads to increased histamine
leading to increased acidity and a vicious cycle
Helicobacter Pylori ? 90 of peptic ulcers
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Treatment Of Peptic Ulcers
_at_ Antihistamine (Cimetidine) _at_ Cutting vagus
nerve supply to stomach _at_ Removal of stomach
antrum _at_ Diet void of caffeine and alcohol
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Gastric Ulcer
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A Donor Kebab, A Curry and Get Pissed.
WHY?
No food could be absorbed through the stomach
Alcohol can be however.
Alcohol more rapidly absorbed through small
intestine into blood.
Fat-rich food (kebab/curry) delays gastric
motility ? delay arrival of alcohol into duodenum
? delay alcohol from producing its effects
rapidly
Aspirin can also be absorbed through stomach ?
exert effect more quickly.
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Regulation Of Stomach Motility
1 Gastric Filling
Plasticity Ability of stomach smooth muscle to
be stretched without greatly increasing its
tension
As food is travelling down oesophagus ? Receptive
Relaxation.
As food enters stomach ? Adaptive Relaxation.
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2 Gastric Storage
BER means slow weak contraction of smooth muscle
in fundus and body while stronger in antrum
Therefore, food is stored in fundus and body.
3 Gastric Mixing
Peristaltic movement in antrum ? mixing ? chyme ?
push forward to pyloric sphincter
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4 Gastric Emptying
Expulsion of chyme into duodenum.
Amount of chyme emptied depends on strength of
peristalsis.

• Rate depends on
• Volume of chyme
• Fluidity of chyme
• Duodenal factors such as fat, acid and distension

Emotions may influence gastric motility via
autonomous nervous system (e.g. sad ? reduced
emptying)
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Vomiting
Induced by
a) Bacterial toxins
b) Unpleasant odours
c) Stressful situation
d) Excessive alcohol
e) Drugs
Mediated by emetic centre in the medulla
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Pancreas
Tadpole-shaped gland behind and below the stomach

Contains both endocrine and exocrine tissue
Acini Cluster of secretory cells that form
sacs
Acini empty into main pancreatic duct
Endocrine portion Islets of Langerhans (secrete
insulin and glucagon)
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Composition Of Pancreatic Juices
1 Enzymatic secretions
2 Aqueous secretions rich in sodium bicarbonate

1 Enzymatic Secretions
a) Proteolytic enzymes
b) Pancreatic amylase
c) Pancreatic lipase
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a) Proteolytic Enzymes

• Trypsinogen
• Chymotrypsinogen
• Procarboxypeptidase

Trypsinogen ? Trypsin in small intestine by
enterokinase
Cymotrypsinogen and procarboxypeptidase both
activated in the small intestine by the activated
trypsin
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b) Pancreatic Amylase
Secreted in the active form
Digest carbohydrates
c) Pancreatic Lipase
Secreted in the active form
Only enzyme in digestive system that can digest
fats to monoglycerides and fatty acids
Steatorrhea Pancreatic exocrine insufficiency
60-70 indigested fat in
faeces Protein/carbohydrat
e digestion impaired to a
lesser extent
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2 Pancreatic Aqueous Alkaline Secretions
Fact 1 Pancreatic enzymes work at neutral to
alkaline environment.
Fact 2 They start their function in the duodenum
Fact 3 Chyme emptied from stomach into duodenum
is highly acidic
OH SHIT WE HAVE A PROBLEM
Solution Neutralize acidity of chyme in duodenum
by alkaline secretions from the pancreas
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Regulation Of Pancreatic Secretions
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Summary Of Information Relating To Intestinal
Hormones
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Liver and Gallbladder
Liver and gallbladder form the biliary system ?
secrete bile into duodenum
Bile Breaks down bid fat molecules into smaller
ones that are accessible to digestive
enzymes
Liver Detoxifies waste and drugs
Removal of bacteria due to resident kupffer
cells.
Liver cells Hepatocytes and Kupffer cells.
Blood enters liver via hepatic artery
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All digestive organs drain venous blood into
hepatic portal vein of the liver for
a) Processing
b) Storage
c) Detoxification
Blood from hepatic portal vein drains into
hepatic vein and subsequently into the inferior
vena cava.
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Microscopic Structure
Liver made of four lobules
Each lobule is hexagonal

• Each hexagonal part has
• Hepatic artery branch
• Hepatic portal vein branch
• 3) Bile duct

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• Blood from hepatic artery branch and hepatic
  portal
• vein branch flow into an expanded capillary
  Sinusoids

b) Kupffer cells line inside of sinusoids and
hepatocytes on outside.
c) The blood from sinusoids from all six sections
of lobule drains into central vein
d) Central vein from all four lobules drains into
hepatic vein
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e) Bile secreted by hepatocytes flows into
canaliculi between cells.
f) Bile canaliculi carry bile to bile duct in
each lobule
g) Bile ducts from each of four lobules drains
into common bile duct
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Gallbladder Why?
Bile produced by liver enters duodenum ONLY
during digestion of a meal.
Therefore, must be stored somewhere before it is
released
Gallbladder is site for storage of bile
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Composition Of Bile
- Sodium bicarbonate from duct cells
- Bile salts
- Cholesterol
- Lecithin
- Bilirubin
Bile salts are derivatives of cholesterol
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Gallstones
Biliary calculi
Due to excess proportion of cholesterol compared
to bile salts and lecithin.
Obstruct flow of bile
Symptoms Pain radiating to right thoracic region
Treatment - Drugs to dissolve crystals
- Pulverising them with ultrasound
(Lithotripsy) - Vaporising with laser
- Surgical removal of gallbladder
75 due to cholesterol, 25 due to precipitation
of bilirubin
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Bilirubin
Pigment resulting from breakdown of haem portion
of haemoglobin.
Bilirubin converted to urobilinogen by small
intestine bacteria (this gives faeces its brown
colour).
Diseases Of The Liver
1 Jaundice
Prehepatic excessive breakdown of RBC.
Hepatic Liver is diseased and not able to deal
with normal levels of bilirubin.
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Posthepatic Obstructive jaundice due to bile
duct obstruction ? bilirubin cannot be eliminated
in faeces
Patients appear yellowish especially in the
whites of their eyes
2 Hepatitis
Due to toxins such as alcohol, tranquillisers and
mushroom poisoning.
Viruses Hepatitis A Transmitted through
sewage- contaminated water
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Hepatitis B Transmitted via blood transfusions
and could results in cancer.
Vaccines for hepatitis A and B have now been
developed.
Hepatitis C Treated by combination drug therapy
of immunosuppressing steroid prednisone and
genetically engineered interferon.
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3 Cirrhosis (Orange Coloured)
Due to chronic alcoholism or chronic hepatitis

Connective fibrous tissue mass of the liver
increases.
Blocks blood flow through hepatic portal system ?
portal hypertension
Hepatic portal vein drains into small veins
Excess blood ? small veins burst ? vomit blood
Snakelike network of veins surrounding the naval
(Caput medusae medusae head)
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Small Intestine
Site of digestion and absorption
6.3 m long and 2.5 cm wide
Coiled between stomach and large intestine
Duodenum, Jejunum and ileum
Food is mixed and moved along small intestine by
segmentation

• Segmentation influenced by
• Intestinal distension
• Gastrin
• Extrinsic nerve activity

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Segmentation more frequent in upper part of small
intestine compared to lower one
Allows food to move forward more than backwards

Also allows more time for absorption
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Structure/Function Relationship
Most absorption in duodenum and jejunum
Vitamin B12 and bile salt absorption in ileum
1 Inner surface of small intestine lined by
finger-like projections Villi (Increase
surface area)
2 Villi have mucous and epithelial cells
3 Epithelial cells have microvilli on their
surface (digestion of protein and carbohydrates
finished and absorption occurs)
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4 Crypts of Lieberkuhn between villi ?
regenerate epithelial cells on villi
Radiation and anticancer agents inhibit this
epithelial cell regeneration ? reduced absorption
? weak, lethargic patient.
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Structure Of The Ileum
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Villi Form A Dense Covering Over The Surface Of
The Ileum
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Longitudinal Section Of A Villus Light Microscope
Image (x252)
Lacteal
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Digestion and Absorption
Digestion
Exocrine glands in small intestinal mucosa
secrete 1.5 litres/day of water and mucus

• In the lumen
• Fat digestion is complete
• Proteins reduced to peptides and amino acids
• 3) Carbohydrates reduced to disaccharides.

How is protein and carbohydrate digestion
complete ?
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Epithelial cells contain
Enterokinase Activates pancreatic trypsinogen
Disaccharidase Reduced disaccharides to
monosaccharides
Aminopeptidase Reduces peptides to amino acids
Thus, carbohydrate and protein digestion is
completed in the epithelial cells of brush border
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Lactose Intolerance
Lactase deficiency
Lactose accumulates in small intestine lumen
Water, carbon dioxide and methane gas accumulate
as a result ? abdominal cramps and diarrhoea
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Absorption
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Salt and water Salt Active or passive
absorption Water Passive absorption
Carbohydrates Enter from lumen to brush border
by energy-consuming transport
Fats, vitamins, calcium and iron Only difference
is that iron could be absorbed into epithelial
cells and then stored as ferritin before release
into blood
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Diarrhoea
Excessive defecation of highly fluid faecal
material.
Eliminates harmful material from the body
Also eliminates water (dehydration), nutrient
material and HCO3 (metabolic acidosis)
Caused by _at_ Viral/bacterial infection leading
to excessive intestinal motility _at_ Lactase
deficiency _at_ Toxins of Vibrio Cholerae
(bacterium) ? secretion of vast amounts of fluid
by small intestinal mucosa
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Large Intestine
Cecum Pouch shaped
Appendix Lymphoid tissue housing lymphocytes
Colon Ascending, transverse, descending (last
section is sigmoid colon)
Rectum (Meaning straight)
Walls of large intestine pocketlike sacs
haustra
Material reaching large intestine indigestible
food (e.g. Cellulose), unabsorbed biliary
compounds and fluid.
Large intestine absorbs more water and salt and
stores faeces
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Absorptive Storage Functions Of The Colon
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Emptying At The Ileocaecal Valve
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Histological Detail Of The Colon
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Histological Detail Of The Colon
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Defecation
Haustral contractions slow allow bacteria to
brow in large intestine
Ascending and transverse colon contract
simultaneously to drive faeces to descending
colon.
Once faeces reaches the rectum, it stretches and
sphincters relax
External sphincter is skeletal voluntary muscle
Abdominal muscles contract and the individual
breaths a sigh of relief.
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Constipation and Appendicitis
Faeces remains in colon for large periods
More water absorbed ? dry faeces.
Can cause headache, depression, nausea and
haemorrhoids.
If faecal material is lodged in appendix ? loss
of blood supply ? inflammation ? Appendicitis
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Farting and Burping
Air swallowed during breathing or produced by
bacteria must exit, SOMEHOW
This gives rise to gurgling sounds (borborygmi)
Air either leaves by burping (eructation)
The alternate is to FART (flatus)
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Reading
Human Physiology from cells to systems. Lauralee
Sherwood. West Publishing Company
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THE END