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Marieb’s Human
Anatomy and Physiology
Marieb w Hoehn
Chapter 23
Digestive System
Lecture 11
Lecture Overview
• Liver
• Gall Bladder
• Small Intestine
• Large Intestine
2
Liver [ Hepat(o)- ]
Round ligament is part of the
falciform ligament that divides
the lobes; remnant of fetal
umbilical vein.
Note that the vena cava
does not enter the liver;
it passes by
Figure from: Martini, Anatomy & Physiology, Pearson Education, 2004
3
Arterial Supply and Venous Drainage of Liver
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
4
Hepatic Lobule
Hepatic lobules are the functional units of the liver (>100,000)
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
5
Paths of Blood and Bile in Hepatic Lobule
Liver’s role
in digestion is
production of
bile
Sinusoid
Hepatic portal vein → sinusoids → central vein → hepatic veins → inferior vena cava
Hepatic artery
6
Liver Functions (over 200!)
• Three general categories of function
1) Metabolic regulation
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•
•
•
•
Interconversion of carbohydrates, lipids, amino acids
Removal of wastes
Vitamin and mineral metabolism
Drug inactivation
Know items
Storage of fats, glycogen, iron, vit A/B12/D/E/K
2) Hematological regulation
•
•
•
•
•
in red
Phagocytosis and antigen presentation; ab removal
Synthesis of plasma proteins
Removal of circulating hormones
Removal of worn-out RBCs (Kupffer cells)
Removal or storage of toxins
3) Synthesis and secretion of bile (role in digestion)
7
Gallbladder [Cyst(o)-]
Figure from: Martini, Anatomy &
Physiology, Prentice Hall, 2001
Main function is to store and concentrate bile between
meals, and release bile under the influence of CCK
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Composition of Bile (Chole-)
Yellowish-green liquid continually secreted by hepatocytes
• water
• bile salts (bile acids)
• derived from cholesterol
• emulsification of fats (increases surface area for digestive enzymes)
• helps absorption of fatty acids, cholesterol, and fat-soluble vitamins
• 80% are recycled (reabsorbed and reused) – enterohepatic
circulation of bile
• 20% excreted in feces (disposes of excess cholesterol)
• bile pigments (bilirubin and biliverdin from breakdown of
RBCs)
• electrolytes
10
Regulation of Bile Release from GB
• fatty chyme
entering duodenum
stimulates the GB to
release bile (via
CCK)
Secretin causes the bile ducts (and pancreatic ducts) to
secrete bile rich in HCO3-
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Actions of Cholecystokinin (CCK) on Digestion
Figure adapted from: Barrett, K.,
Gastrointestinal Physiology, Lange,
2006
CCK
Contraction of
Gallbladder
Secretion of
pancreatic
enzymes
Reduced
emptying of
stomach
Relaxation of
hepatopancreatic
sphincter
Protein, CHO, lipid absorption and digestion
Matching of nutrient delivery to digestive and absorptive capability
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Three Parts of Small Intestine
“Mixing bowl”; acid
neutralization
Bulk of
chemical
digestion
and
nutrient
absorption
occurs here
The ‘bowel’
consists of the
small and large
intestines.
Vitamin B12 absorption
Main functions of small intestine: 1) chemical digestion 2) absorption of
nutrients (90%) from chyme
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Blood Supply and Drainage of Small Intestine
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
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Wall of Small Intestine
Plicae circulares –
permanent circular
folds of mucosa that
further increase
surface area for
absorption – do not
flatten out with
distention like rugae of
stomach.
Especially prominent
in lower duodenum
and upper jejunum
Submucosa of duodenum contains mucus-secreting glands
(Brunner’s glands) that protect the small intestine
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Intestinal Villi & Glands
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
Enterocyte = Intestinal Cell
Intestinal glands secrete an abundant watery fluid that helps absorb
products of digestion. They also contain enteroendocrine cells
(enterokinase, gastrin, secretin, CCK)
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Intestinal Epithelium
Microvilli further increase
the surface area available
for absorption in the small
intestine
Form a ‘brush border’ on
the intestine
Digestive enzymes are
embedded in the
membrane of microvilli
Main function of plicae, villi, and microvilli is to increase the
surface area for absorption (from about 3.6 ft2 to about 2200 ft2!)
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Secretions of Small Intestine
• peptidase – breaks down peptides into amino acids
• sucrase, maltase, lactase – break down disaccharides into
monosaccharides
Brush
border
• intestinal lipase – breaks down fats into fatty acids and glycerol
• enterokinase – converts trypsinogen to trypsin
• gastrin/somatostatin – hormones that stimulate/inhibit acid secretion by
stomach
• cholecystokinin (CCK) – hormone that inhibits gastric glands, stimulates
pancreas to release enzymes in pancreatic juice, stimulates gallbladder to
release bile, and relaxes hepatopancreatic sphincter (of Oddi)
• secretin – stimulates pancreas to release bicarbonate ions in pancreatic
juice; stimulates gall bladder to release bicarbonate-rich bile
See Table 23.32 in Marieb for summary of digestive enzymes – great to use for XC!!
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Movements of the Small Intestine
Movements in local segments can occur without
stimulation by parasympathetic NS. However, nervous
stimulation accelerates segmentation and peristalsis.
• peristalsis – pushing movements
• segmentation – ringlike contractions that aid in mixing and
slowing peristalsis
• overdistended or irritated wall triggers “peristaltic rush”
resulting in diarrhea
“Long distance” movements are triggered by stomach
filling:
- gastroenteric reflex (↑ motility and secretion along length of
small intestine)
- gastroileal reflex (relaxation of ileocecal sphincter)
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Absorption in the Small Intestine
• monosaccharides and
amino acids
• through facilitated
diffusion and active
transport
• absorbed into blood
• electrolytes and water
• through diffusion,
osmosis, and active
transport
• absorbed into blood
• vitamins
• fat-soluble dissolve in dietary fats (vit A,D,E,K)
•Water-soluble through diffusion, except B12
(active transport)
• Vitamin K (large intestine) – with other lipids
• absorbed into blood
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Absorption of Fats in the Small Intestine
• fatty acids and
glycerol
• several steps
• absorbed into
lymph into
blood
Chylomicrons contain TG, cholesterol, and phospholipids
22
Large Intestine
Figure from:
Martini, Anatomy
& Physiology,
Prentice Hall,
2001
*
23
Histology of the Large Intestine
Walls of large intestine
are much thinner than
the small intestine,
however, the lumen is
larger
Note lack of villi and
presence of numerous
goblet cells (mucus)
No enzymes produced;
any digestion is from
previously introduced
enzymes or bacteria
*
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Functions of Large Intestine
• little or no digestive function
• absorbs water, bile salts, and electrolytes
• secretes mucus (lubrication, binding, protection, pH)
• conversion of bilirubin (uro- and stercobilinogen)
• houses intestinal flora (~800 species of bacteria) and
absorbs vitamins liberated by bacterial action (K, B5,
and Biotin); produces intestinal gas (flatus)
• forms and stores feces
• carries out defecation
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The Rectum, Anal Canal, and Anus
Rectal
valves
Temporary storage of
fecal material in
rectum triggers the
urge to defecate
Internal anal
sphincter is usually
contracted but relaxes
in response to
distension. External
sphincter must be
tensed reflexively to
retain feces
Procto- = anus or rectum
(Keratinzed strat. squamous epithelium)
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Movements of Large Intestine
• slower and less frequent than those of small intestine
• mixing movements (haustral churning every 30 min)
• mass movements - usually follow meals (stimulated
by distension of stomach and duodenum)
- gastrocolic reflex
- duodenocolic reflex
- peristaltic wave from transverse colon through
rest of large intestine
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All You Need to Know???
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Parasympathetic Defecation Reflex
Note that this reflex
opens the internal
sphincter and closes the
external sphincter
Need voluntary
relaxation of the external
sphincter for defecation 29
Feces
• water (75%), solids (25%)
• electrolytes
• mucus
• bacteria (30% of solids) and sloughed epithelial cells
• bile pigments altered by bacteria provide color (mainly
urobilins and stercobilins)
• odor produced by bacterial compounds (indoles and
skatoles, phenols, H2S, ammonia)
• indigestible materials
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Review
• Liver
– Two major lobes
– Blood supply and drainage
– Functional unit is the lobule
• Plates of hepatocytes
• Portal triads
• Blood/bile flow in lobules
– Three major categories of function
• Metabolic regulation
• Hematological regulation
• Bile production
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Review
• Gall Bladder (GB)
– Major function – storage and concentration of bile
– Stimulated to contract by CCK
– Path of bile from GB
•
•
•
•
Cystic duct
Common bile duct
Hepatopancreatic sphincter (Oddi) – CCK relaxes
Duodenal ampulla
– Bile
• Water, bile salts, bile pigments, electrolytes, cholesterol
• Function is emulsification of fats for digestion
• Absorption of fats
33
Review
• Small Intestine
– Three major parts
• Duodenum – mixing chamber; mucus
• Jejunum – digestion
• Ileum – connects to cecum of large intestine
– Blood supply and drainage
– Suspended from the posterior abdominal wall
by the mesentery
– Surface area greatly increased by
• Plicae
• Villi
• Microvilli
34
Review
• Small Intestine (cont’d)
– Secretions
– Control of secretion and absorption
– Absorption
• Protein, CHO, electrolytes – facilitated diffusion
• Fats via chylomicrons and lacteals -> circulation
– Movements
• Local via myenteric plexuses
• Long distance via stomach filling
– Gastroenteric reflex
– Gastroileal reflex
35
Review
• Large Intestine
– Three major parts
• Cecum (veriform appendix)
• Colon (ascending, transverse, descending)
• Rectum
– Blood supply and drainage
– Haustra (pouches)
– Longitudinal muscle of external muscularis
reduced to long strips (taeniae coli)
– Wall is thinner than small intestine, less
muscle, no villi
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Review
• Large Intestine (cont’d)
– Rectum
• Temporary storage of feces – urge to defecate
• Internal sphincter contracted until distended
• External sphincter is voluntary
– Major Functions
• Absorption of water, bile salts, and electrolytes
• Absorbs vitamins produced by bacteria (K, biotin, B5)
• Converts bilirubin
– Movements slower and less frequent than small
intestine
• Mass movements
• Stimulated by gastric and duodenal distension
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