Transcript Document

PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
23
The Digestive
System:
Part C
Copyright © 2010 Pearson Education, Inc.
Pancreas
• Location
• Mostly retroperitoneal, deep to the greater
curvature of the stomach
• Head is encircled by the duodenum; tail abuts
the spleen
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Pancreas
• Endocrine function
• Pancreatic islets secrete insulin and glucagon
• Exocrine function
• Acini (clusters of secretory cells) secrete
pancreatic juice
• Zymogen granules of the secretory cells
(Acinar) contain digestive enzymes
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Small
duct
Acinar cells
Basement
membrane
Zymogen
granules
Rough
endoplasmic
reticulum
(a)
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Figure 23.26a
Pancreatic Juice
• Watery alkaline solution (pH 8) neutralizes chyme
and allows pancreatic secreted enzymes to work
• The epithelial cells lining the small pancreatic
ducts secrete the electrolytes (primarily HCO3–)
• The bicarbonate is made in the epithelial cells –
for every bicarbonate secreted a H+ is returned to
the blood – thus the alkaline tide in the venous
blood return from the stomach is balanced by the
acidic venous blood from the pancreas
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Pancreatic Juice
• Acinar cells produce the enzyme rich secretion
• Enzymes
• Amylase, lipases, nucleases are secreted in active form
but require ions or bile for optimal activity
• Proteases secreted in inactive form
• Protease activation in duodenum
• Trypsinogen is activated to trypsin by brush border
enzyme enteropeptidase
• Procarboxypeptidase and chymotrypsinogen are activated
by trypsin
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Stomach
Pancreas
Epithelial
cells
Membrane-bound
enteropeptidase
Trypsinogen
Trypsin
(inactive)
Chymotrypsin
Chymotrypsinogen
(inactive)
Carboxypeptidase
Procarboxypeptidase
(inactive)
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Figure 23.27
Regulation of Bile Secretion
• Gallbladder contraction is stimulated by
• Cholecystokinin (CCK) from intestinal cells
exposed to proteins and fat in chyme
• Vagal stimulation (minor stimulus)
• CKK also causes the hepatopancreatic
sphincter to relax
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Regulation of Bile Secretion
• Bile secretion is stimulated by
• Bile salts in enterohepatic circulation – the
more bile salts in the enterohepatic circulation
the more bile is secreted.
• Secretin from intestinal cells exposed to HCl
and fatty chyme
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Regulation of Pancreatic Secretion
• Bile and pancreatic secretions are regulated
by the same factors (neural and hormonal)
• CCK induces the secretion of enzyme-rich
pancreatic juice by acini
• Secretin causes secretion of bicarbonate-rich
pancreatic juice by duct cells
• Vagal stimulation also causes release of
pancreatic juice (minor stimulus)
Copyright © 2010 Pearson Education, Inc.
Slide 1
1
Chyme entering duodenum
causes release of
cholecystokinin
(CCK) and
secretin from
duodenal
enteroendocrine
cells.
2
CCK (red
dots) and
secretin (yellow
dots) enter the
bloodstream.
3
CCK induces
secretion of
enzyme-rich
pancreatic juice.
Secretin causes
secretion of
HCO3–-rich
pancreatic juice.
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4
Bile salts and,
to a lesser extent,
secretin
transported via
bloodstream
stimulate liver to
produce bile
more rapidly.
5
CCK (via
bloodstream)
causes
gallbladder to
contract and
hepatopancreatic
sphincter to
relax; bile enters
duodenum.
6 During
cephalic and
gastric phases,
vagal nerve
stimulation
causes weak
contractions of
gallbladder.
Figure 23.28
1
Chyme entering duodenum
causes release of
cholecystokinin
(CCK) and
secretin from
duodenal
enteroendocrine
cells.
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Figure 23.28, step 1
1
Chyme entering duodenum
causes release of
cholecystokinin
(CCK) and
secretin from
duodenal
enteroendocrine
cells.
2
CCK (red
dots) and
secretin (yellow
dots) enter the
bloodstream.
Copyright © 2010 Pearson Education, Inc.
Figure 23.28, step 2
1
Chyme entering duodenum
causes release of
cholecystokinin
(CCK) and
secretin from
duodenal
enteroendocrine
cells.
2
CCK (red
dots) and
secretin (yellow
dots) enter the
bloodstream.
3
CCK induces
secretion of
enzyme-rich
pancreatic juice.
Secretin causes
secretion of
HCO3–-rich
pancreatic juice.
Copyright © 2010 Pearson Education, Inc.
Figure 23.28, step 3
1
Chyme entering duodenum
causes release of
cholecystokinin
(CCK) and
secretin from
duodenal
enteroendocrine
cells.
4
Bile salts and,
to a lesser extent,
secretin
transported via
bloodstream
stimulate liver to
produce bile
more rapidly.
2
CCK (red
dots) and
secretin (yellow
dots) enter the
bloodstream.
3
CCK induces
secretion of
enzyme-rich
pancreatic juice.
Secretin causes
secretion of
HCO3–-rich
pancreatic juice.
Copyright © 2010 Pearson Education, Inc.
Figure 23.28, step 4
1
Chyme entering duodenum
causes release of
cholecystokinin
(CCK) and
secretin from
duodenal
enteroendocrine
cells.
2
CCK (red
dots) and
secretin (yellow
dots) enter the
bloodstream.
3
CCK induces
secretion of
enzyme-rich
pancreatic juice.
Secretin causes
secretion of
HCO3–-rich
pancreatic juice.
Copyright © 2010 Pearson Education, Inc.
4
Bile salts and,
to a lesser extent,
secretin
transported via
bloodstream
stimulate liver to
produce bile
more rapidly.
5
CCK (via
bloodstream)
causes
gallbladder to
contract and
hepatopancreatic
sphincter to
relax; bile enters
duodenum.
Figure 23.28, step 5
1
Chyme entering duodenum
causes release of
cholecystokinin
(CCK) and
secretin from
duodenal
enteroendocrine
cells.
2
CCK (red
dots) and
secretin (yellow
dots) enter the
bloodstream.
3
CCK induces
secretion of
enzyme-rich
pancreatic juice.
Secretin causes
secretion of
HCO3–-rich
pancreatic juice.
Copyright © 2010 Pearson Education, Inc.
4
Bile salts and,
to a lesser extent,
secretin
transported via
bloodstream
stimulate liver to
produce bile
more rapidly.
5
CCK (via
bloodstream)
causes
gallbladder to
contract and
hepatopancreatic
sphincter to
relax; bile enters
duodenum.
6 During
cephalic and
gastric phases,
vagal nerve
stimulation
causes weak
contractions of
gallbladder.
Figure 23.28, step 6
Digestion in the Small Intestine
• Chyme from stomach contains
• Partially digested carbohydrates and proteins
• Undigested fats (minimally worked on by
salivary lipase and gastric lipase)
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Requirements for Digestion and Absorption
in the Small Intestine
• The intestine must get slow delivery of hypertonic
acidic chyme from the stomach
• 3cc or less per peristaltic wave and 3 waves per
minute so 9 cc or less per minute into small
intestine from stomach
• If the hypertonic chyme was delivered to the small
intestine too quickly it would pull in too much
water from the bloodstream
• Additionally the chyme is quite acidic – thus ulcer
formation if it enters the small intestine too fast
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• The small intestine only provides brush border
enzymes – most digestive chemicals in the
small intestine come from the liver and
pancreas
• Delivery of bile, enzymes, and bicarbonate
from the liver and pancreas
• Intestinal motility mixes chyme with
pancreatic, bile and intestinal juices as a
result of its segmentation waves
• The small intestine mainly uses segmentation
waves – peristaltic waves begin after most of
the materials have been absorbed
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Motility of the Small Intestine
• Segmentation Waves
• Make the intestinal contents appear as if they are
being massaged- the chyme is moved back and
forward in the lumen a few centimeters at a time
by alternating contraction and relaxation of rings
of smooth muscle.
• Initiated by intrinsic pacemaker cells located in
circular muscles – but unlike stomach pacemaker
cells which have only one rhythm – the
pacemakers in the duodenum depolarize more
frequently (12 -14 contractions per minute) than
those in ileum ( 8 or 9 contractions per minute)
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• Mixes and moves contents slowly and steadily
toward the ileocecal valve – giving plenty time
to complete digestion and absorption
• The intensity of the waves is altered by long
and short reflexes
•
The segmentation waves wane in the late
intestinal (fasting) phase after most of the
small intestinal contents have been absorbed
• Once the segmentation waves wane the
peristaltic waves begin and a result of
secretion of motilin from the duodenal
mucosae
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Motility of the Small Intestine
• Peristalsis
• Initiated by motilin in the late intestinal phase
• As the motilin blood level rises peristaltic waves are
initiated in the proximal duodenum every 90 – 120 minutes
and sweep slowly along the intestines – dying out in
approximately 2 feet from its initiation area.
• The next wave starts distal to the previous wave thus
termed the MMC ( migrating motility complex)
• A complete trip from duodenum to ileum takes
approximately two hours
• The process repeats itself thus meal remnants, bacteria,
sloughed off mucosal cells and debris are moved to the
large intestine
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• This housekeeping function is critical for
preventing the overgrowth of bacteria that
migrate from the large intestine. As food
enters the stomach with the next meal,
peristalsis is replaced by segmentation
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Motility of the Small Intestine
• The local enteric neurons coordinate intestinal
motility and it depends on which neurons are
activated or inhibited
• Cholinergic sensory neurons may activate the
myenteric plexus
• Causes contraction of the circular muscle
proximally and of longitudinal muscle distally
• Forces chyme along the tract
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Motility of the Small Intestine
• Most of the time the ileocecal sphincter is
closed. Two mechanisms open it
1. The stomach initiates a gastroileal reflex – a
long reflex that enhances the force of
segmentation in the ileum
2. Gastrin increases the motility of the ileum
and relaxes the ileocecal valve
• Ileocecal valve flaps close when chyme exerts
backward pressure
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Microvilli
(b)
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Absorptive
cell
Figure 23.3b
From mouth
(a) Peristalsis: Adjacent segments of alimentary
tract organs alternately contract and relax,
which moves food along the tract distally.
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Figure 23.3a
Large Intestines
• Approximately twice the diameter of the small
intestines – 3 inches wide
• Approximately 5 feet long
• Function (absorb most of the remaining water
from indigestible food residues and
temporarily store the residues before
elimination as feces)
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Functions of the Large Intestine
• Vitamins, water, and electrolytes are
reclaimed
• Major function is propulsion of feces toward
the anus
• Colon is not essential for life
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Large Intestine
• Unique features
• Teniae coli
• Three bands of longitudinal smooth muscle in the
muscularis
• Haustra
• Pocketlike sacs caused by the tone of the teniae coli
• Epiploic appendages
• Fat-filled pouches of visceral peritoneum
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Large Intestine
• Regions
• Cecum (pouch with attached vermiform
appendix)
• Colon
• Rectum
• Anal canal
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Left colic
(splenic) flexure
Transverse
mesocolon
Epiploic
appendages
Right colic
(hepatic)
flexure
Transverse
colon
Superior
mesenteric
artery
Haustrum
Descending
colon
Ascending
colon
IIeum
Cut edge of
mesentery
Teniae coli
IIeocecal
valve
Cecum
Vermiform appendix
Sigmoid
colon
Rectum
Anal canal
(a)
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External anal sphincter
Figure 23.29a
Colon
• Ascending colon and descending colon are
retroperitoneal
• Transverse colon and sigmoid colon are
anchored via mesocolons (mesenteries)
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Greater omentum
Transverse colon
Transverse
mesocolon
Descending colon
Jejunum
Mesentery
Sigmoid
mesocolon
Sigmoid colon
Ileum
(c)
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Figure 23.30c
Liver
Lesser omentum
Pancreas
Stomach
Transverse
mesocolon
Duodenum
Transverse colon
Mesentery
Greater omentum
Jejunum
Ileum
Visceral peritoneum
Parietal peritoneum
(d)
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Urinary bladder
Rectum
Figure 23.30d
Rectum and Anus
• Rectum
• Three rectal valves stop feces from being
passed with gas
• Anal canal
• The last segment of the large intestine
• Sphincters
• Internal anal sphincter—smooth muscle
• External anal sphincter—skeletal muscle
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Rectal valve
Rectum
Hemorrhoidal
veins
Levator ani
muscle
Anal canal
External anal
sphincter
Internal anal
sphincter
Anal columns
Pectinate line
Anal sinuses
Anus
(b)
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Figure 23.29b
Large Intestine: Microscopic Anatomy
• Mucosa of simple columnar epithelium except
in the anal canal (stratified squamous)
• Abundant deep crypts with goblet cells
• Extensive mucus eases passage of feces and
protects the intestinal wall from irritating acids
and gases released by resident bacteria in the
colon
• Low folds give anal columns and anal sinuses
are between the folds – the sinuses exude
mucus when defecate
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• The horizontal tooth-shaped line that parallels
the inferior margins of the anal sinuses is
called the pectinate line. Superior to this line,
the mucosa is innervated by visceral sensory
fibers and is relatively insensitive to pain. The
area inferior to this line is innervated by
somatic sensory fibers – thus very sensitive to
pain.
• Superficial venous plexuses of the anal canal
form hemorrhoids if inflamed
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Bacterial Flora
• 10 million different types
• Enter from the small intestine or anus
• Metabolize some host products (mucin, heparin, and
hyaluronic acid)
• Ferment some indegestible carbohydrates (cellulose,
xylan, and others
• Release irritating acids and a mixture of gases (dimethyl
sulfide, H2, N2, CH4, and CO2)
• Dimethyl sulfide is quite odorous
• About 500 ml of gas is produced each day
• Synthesize Vitamin B complex vitamins and Vitamin K
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• Most bacteria exist peacefully with their host in the large
intestine – but an elegant system keeps them from
breaching the mucosal barrier
• The epithelial cells of the gut mucosa respond to specific
bacterial components by releasing chemicals that recruit
immune cells, particularly dendritic cells into the mucosa.
• The dendritic cells pry open the tight junctions between
the epithelial cells and send extensions into the lumen of
sample the microbial antigens
• They then migrate to the nearby lymphoid follicles
(MALT) where they present antigens to T cells.
• An IgA antibody response restricted to the gut lumen is
triggered that prevents the bacteria from straying into
tissues deep to the mucosa.
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Motility of the Large Intestine
• Haustral contractions (occur every 30 minutes
or so)
• Slow segmenting movements that occur
mainly in the transverse and descending colon
• Haustra sequentially contract in response to
distension
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Mass Movements
• Long slow-moving but powerful contractions
that move over large areas of the colon- three
to four times a day and force the contents
towards the rectum.
• Typically, they occur during or just after
eating, which indicates the presence of food in
the stomach activates the gastrocolic reflex in
the colon.
• Of the 500 cc of fluid entering the cecum only
about 150cc becomes feces.
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Motility of the Large Intestine
• Gastrocolic reflex
• Initiated by presence of food in the stomach
• Activates three to four slow powerful peristaltic
waves per day in the colon (mass movements)
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Defecation
• Mass movements force feces into rectum
• Distension initiates spinal defecation reflex
• Parasympathetic signals
• Stimulate contraction of the sigmoid colon and
rectum
• Relax the internal anal sphincter
• Conscious control allows relaxation of
external anal sphincter
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Impulses from
cerebral cortex
(conscious
control)
1
Sensory
nerve fibers
Distension, or stretch, of the
rectal walls due to movement
of feces into the rectum
stimulates stretch receptors
there. The receptors transmit
signals along afferent fibers to
spinal cord neurons.
2
Voluntary motor
nerve to external
anal sphincter
Sigmoid
colon
A spinal reflex is initiated in
which parasympathetic motor
(efferent) fibers stimulate
contraction of the rectal walls
and relaxation of the internal
anal sphincter.
Stretch receptors in wall
Rectum
External anal
sphincter
(skeletal muscle)
Involuntary motor nerve
(parasympathetic division)
Internal anal sphincter
(smooth muscle)
3
If it is convenient to defecate, voluntary motor
neurons are inhibited, allowing the external anal
sphincter to relax so that feces may pass.
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Figure 23.31
Chemical Digestion
• Catabolic
• Enzymatic
• Hydrolysis
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Chemical Digestion and Absorption of
Carbohydrates
• Digestive enzymes
• Salivary amylase, pancreatic amylase, and
brush border enzymes (dextrinase,
glucoamylase, lactase, maltase, and sucrase)
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Bonding Carbohydrate monomers together
• Monosaccharides bond together by the
removal of a water molecule (dehydration
synthesis) to form a covalent bond between
the two monosaccharides known as a
“glycosidic bond”
• When bond two monosaccharides together
termed a disaccharide, when join 3 – 10
together termed an Oligosaccharide – more
than 10 together – termed a polysaccharide
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Common Disaccharides
• Sucrose – table sugar (glucose alpha 1,2 to
fructose)
• Maltose – in beer (glucose alpha 1,4 to glucose)
• Lactose – in milk (galactose beta 1, 4 to glucose)
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Amylose
Alpha 1,4 Linkages (Linear)
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Amylopectin
Alpha 1,4 and Alpha 1,6 (Branching)
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Alpha 1,4 and many Alpha 1,6
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Humans Cannot Break the Beta Bond in
Cellulose (Fiber) so it adds bulk to the diet
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Chemical Digestion and Absorption of Carbohydrates
• Digestive enzymes
• Salivary amylase, pancreatic amylase, and brush border
enzymes (dextrinase, glucoamylase, lactase, maltase, and
sucrase)
• The amylases (salivary, pancreatic and brush border type)
break alpha 1, 4 linkages
• The dextrinase brush border enzyme breaks alpha 1,6
linkages
• The lactase breaks the Beta 1,4 between glucose and
galactose
• Maltase breaks the alpha 1,4 between glucose and glucose
• Sucrase breaks the alpha 1,2 bond between glucose and
fructose
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Chemical Digestion and Absorption of
Carbohydrates
• Absorption
• Secondary active transport (cotransport) with
Na+
• Facilitated diffusion of some monosaccharides
• Enter the capillary beds in the villi
• Transported to the liver via the hepatic portal
vein
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Carbohydrate digestion
Foodstuff
Enzyme(s)
and source
Site of
action
Starch and disaccharides
Oligosaccharides
and disaccharides
Lactose Maltose Sucrose
Galactose Glucose Fructose
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Salivary
amylase
Pancreatic
amylase
Brush border
enzymes in
small intestine
(dextrinase, glucoamylase, lactase,
maltase, and sucrase)
Mouth
Small
intestine
Small
intestine
Path of absorption
• Glucose and galactose
are absorbed via
cotransport with
sodium ions.
• Fructose passes via
facilitated diffusion.
• All monosaccharides
leave the epithelial
cells via facilitated
diffusion, enter the
capillary blood in the
villi, and are
transported to the liver
via the hepatic portal
vein.
Figure 23.32 (1 of 4)
Protein Digestion and Absorption
• Dietary Proteins (125 grams a day)
• Enzyme Proteins in Gut (15 – 25 grams)
• Proteins in sloughed mucosal cells (15 – 25
grams)
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R group still free
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Chemical Digestion and Absorption of
Proteins
• Enzymes: pepsin in the stomach
• Pancreatic proteases
• Trypsin, chymotrypsin, and carboxypeptidase
• Brush border enzymes
• Aminopeptidases, carboxypeptidases, and
dipeptidases
• Absorption of amino acids is coupled to active
transport of Na+ and in the cases of
Dipeptides and Tripeptide – coupled to H+
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Peptidases (enzymes that break the peptide bond)
• Exopeptidases – break the end amino acids off at the Nterminal (Aminopeptidase) or C- Terminal
(Carboxypeptidase- from pancreas and brush border)
• Endopeptidases – break peptide bonds within protein
• Pepsin – from chief cells in stomach breaks peptide
bonds between tyrosine and phenylalanine
• Trypsin (from pancreas) cleaves peptide chains mainly at
the carboxyl side of the amino acids lysine or arginine,
except when either is followed by proline
• Chymotrypsin (from pancreas) - preferentially cleaves
peptide amide bonds where the carboxyl side of amide
bond (the S1 position) is a tyrosine, tryptophan, or
phenylalanine
• Dipeptidase – from brush border splits dipeptides
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Amino acids of protein fragments
Brush border enzymes
Apical membrane (microvilli)
Lumen of
intestine
Pancreatic
proteases
1 Proteins and protein fragments
are digested to amino acids by
pancreatic proteases (trypsin,
chymotrypsin, and carboxypeptidase), and by brush border
enzymes (carboxypeptidase,
aminopeptidase, and dipeptidase)
of mucosal cells.
Na+
Na+
Absorptive
epithelial
cell
2 The amino acids are then
absorbed by active transport into
the absorptive cells, and move to
their opposite side (transcytosis).
Amino
acid
carrier
3 The amino acids leave the
Active transport
Passive transport
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Capillary
villus epithelial cell by facilitated
diffusion and enter the capillary
via intercellular clefts.
Figure 23.33
Protein digestion
Foodstuff
Protein
Large polypeptides
Small polypeptides,
small peptides
Amino acids
(some dipeptides
and tripeptides)
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Enzyme(s)
and source
Pepsin
(stomach glands)
in presence
of HCl
Pancreatic
enzymes
(trypsin, chymotrypsin,
carboxypeptidase)
Brush border
enzymes
(aminopeptidase,
carboxypeptidase,
and dipeptidase)
Site of
action
Path of absorption
• Amino acids are absorbed
by cotransport with
Stomach
sodium ions.
• Some dipeptides and
tripeptides are absorbed
via cotransport with H++
Small
and hydrolyzed to amino
intestine
acids within the cells.
• Amino acids leave the
epithelial cells by
Small
facilitated diffusion, enter
intestine
the capillary blood in the
villi, and are transported
to the liver via the hepatic
portal vein.
Figure 23.32 (2 of 4)
Chemical Digestion and Absorption of
Lipids
• Pre-treatment—emulsification by bile salts
• Enzymes—pancreatic lipase
• Absorption of glycerol and short chain fatty
acids
• Absorbed into the capillary blood in villi
• Transported via the hepatic portal vein
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Chemical Digestion and Absorption of
Lipids
• Absorption of monoglycerides and fatty acids
• Cluster with bile salts and lecithin to form
micelles
• Released by micelles to diffuse into epithelial
cells
• Combine with proteins to form chylomicrons
• Enter lacteals and are transported to systemic
circulation
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Fat globule
1 Large fat globules are emulsified
(physically broken up into smaller fat
droplets) by bile salts in the duodenum.
Bile salts
Fat droplets
coated with
bile salts
2 Digestion of fat by the pancreatic
enzyme lipase yields free fatty acids and
monoglycerides. These then associate
with bile salts to form micelles which
“ferry” them to the intestinal mucosa.
Micelles made up of fatty
acids, monoglycerides,
and bile salts
3 Fatty acids and monoglycerides leave
micelles and diffuse into epithelial cells.
There they are recombined and packaged
with other lipoid substances and proteins
to form chylomicrons.
4 Chylomicrons are extruded from the
Epithelial
cells of
small
intestine
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Lacteal
epithelial cells by exocytosis. The
chylomicrons enter lacteals. They are
carried away from the intestine by lymph.
Figure 23.34
Short Chain Fatty Acids
• Passage of short chain fatty acids is quite
different from what we have described. These
fat breakdown products do no depend on the
presence of bile salts or micelles, are not
recombined to form triglycerides within the
intestinal lumen cells, and simply diffuse into
the portal blood for distribution.
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Fat digestion
Foodstuff
Enzyme(s)
and source
Unemulsified
fats
Emulsification by
the detergent
action of bile
salts ducted
in from the liver
Pancreatic
lipases
Monoglycerides Glycerol
and fatty acids
and
fatty acids
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Site of
action
Path of absorption
• Fatty acids and monoglycerides
enter the intestinal cells via
diffusion.
Small
intestine • Fatty acids and monoglycerides
are recombined to form
triglycerides and then
combined with other lipids and
proteins within the cells, and
the resulting chylomicrons are
Small
extruded by exocytosis.
intestine
• The chylomicrons enter the
lacteals of the villi and are
transported to the systemic
circulation via the lymph in the
thoracic duct.
• Some short-chain fatty acids
are absorbed, move into the
capillary blood in the villi by
diffusion, and are transported
to the liver via the hepatic
portal vein.
Figure 23.32 (3 of 4)
Chemical Digestion and Absorption of
Nucleic Acids
• Enzymes
• Pancreatic ribonuclease and
deoxyribonuclease – break into nucleotides.
• Intestinal brush border enzymes
(nucleosidases and phosphatases)
• Absorption
• Active transport
• Transported to liver via hepatic portal vein
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Nitrogenous Bases
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Nucleic acid digestion
Foodstuff
Enzyme(s)
and source
Nucleic acids
Pentose sugars,
N-containing bases,
phosphate ions
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Pancreatic ribonuclease and
deoxyribonuclease
Brush border
enzymes
(nucleosidases
and phosphatases)
Site of
action
Path of absorption
• Units enter intestinal cells
by active transport via
Small
intestine membrane carriers.
• Units are absorbed into
capillary blood in the villi
Small
and transported to the
intestine
liver via the hepatic portal
vein.
Figure 23.32 (4 of 4)
Vitamin Absorption
• In small intestine
• Fat-soluble vitamins (A, D, E, and K) are carried
by micelles and then diffuse into absorptive cells
– Thus to get maximal absorption fat soluble
vitamins – need to eat some fat containing food
• Water-soluble vitamins (vitamin C and B
vitamins) are absorbed by diffusion or by
passive or active transporters.
• Vitamin B12 binds with intrinsic factor, and is
absorbed by endocytosis in the terminal ileum
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Vitamin Absorption
• In large intestine
• Vitamin K and B vitamins from bacterial
metabolism are absorbed
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Electrolyte Absorption
• Mostly along the length of small intestine
• Iron and calcium are absorbed mainly in
duodenum
• Na+ is coupled with absorption of glucose and
amino acids
• Ionic iron is stored in mucosal cells with ferritin
• K+ diffuses in response to osmotic gradients
• Ca2+ absorption is regulated by vitamin D and
parathyroid hormone (PTH)
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• Na+/K+ pump in basal membrane of mucosal epithelial cells
sets up gradient
• Na+ helps monosaccharides get absorbed (glucose and
galactose) and the amino acids
• The anions generally follow Na+
• Chloride is actively transported out of the lumen – and
particularly by a HCO3- exchange transporter in the terminal
intestine
• K+ follows behind water – as follows leaves the intestinal
lumen it creates a high concentration gradient for K+ - so K+
is then pulled by the osmotic gradient – thus if for some
reason water is not passively absorbed properly – K+ is loss
from the body and some is even pulled in the lumen from the
interstitial space
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Iron Absorption 1
• Iron is brought into the cell through an active
transport process involving the protein DMT-1
(divalent metal transporter-1), which is
expressed on the apical surface of enterocytes in
the initial part of the duodenum. DMT-1 is not
specific to iron, and can transport other metal ions
such as zinc, copper, cobalt, manganese, cadmium
or lead.
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Iron Absorption 2
• Once inside the enterocyte, there are two
fates for iron:
(1) It may leave the enterocyte and enter the
body via the basolateral transporter known
as ferroportin.
(2) It can be bound to ferritin, an intracellular
iron-binding protein. For the most part, iron
bound to ferritin in the enterocyte will remain
there. This iron will be lost from the body
when the enterocyte dies and is sloughed off
from the tip of the villus.
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Iron 3
• Iron that enters the body from the basolateral
surface of the enterocyte is rapidly bound to
transferrin, an iron-binding protein of the
blood. Transferrin delivers iron to red blood cell
precursors, that take up iron bound to transferrin
via receptor-mediated endocytosis.
• Normally, the capacity of transferrin to bind iron
in the plasma greatly exceeds the amount of
circulating iron. The transferrin saturation
(percent of transferrin occupied by iron) is
measured to determine if an individual has an
excessive load of iron in the body. The normal
transferrin saturation is in the range of 20-45%.
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Water Absorption
• 95% is absorbed in the small intestine by
osmosis
• Net osmosis occurs whenever a concentration
gradient is established by active transport of
solutes
• Water uptake is coupled with solute uptake
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Malabsorption of Nutrients
• Causes
• Anything that interferes with delivery of bile or
pancreatic juice
• Damaged intestinal mucosa (e.g., bacterial
infection)
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Malabsorption of Nutrients
• Gluten-sensitive enteropathy (celiac disease)
• Gluten damages the intestinal villi and brush
border
• Treated by eliminating gluten from the diet (all
grains but rice and corn)
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Developmental Aspects
• In the third week
• Endoderm has folded and foregut and hindgut
have formed
• Midgut is open and continuous with the yolk
sac
• Mouth and anal openings are nearly formed
• In the eighth week
• Accessory organs are budding from endoderm
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Lung bud
Brain
Oral
membrane
Heart
Yolk sac
Cloacal
membrane
Body
stalk
Stomodeum
Foregut
Stomach
Liver
Site of
liver
development
Midgut
Spinal cord
Bile
duct
Gallbladder
Hindgut
Cystic duct
Ventral pancreatic bud
Dorsal
pancreatic
bud
Duodenum
Proctodeum
Endoderm
(a)
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(b)
Figure 23.35
Developmental Aspects
• Fetal nutrition is via the placenta, but the GI
tract is stimulated to mature by amniotic fluid
swallowed in utero
• The newborn’s rooting reflex helps the infant
find the nipple; the sucking reflex aids in
swallowing
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Developmental Aspects
• During old age
• GI tract activity declines, absorption is less
efficient, and peristalsis is slowed
• Diverticulosis, fecal incontinence, and cancer
of the GI tract
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Cancer
• Stomach and colon cancers rarely have early
signs or symptoms
• Metastasized colon cancers frequently cause
secondary liver cancer
• Prevention
• Regular dental and medical examination
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