Ch 23- Digestive System, Part 2
• Small intestine
• Pancreas
• Liver
-Hepatic portal system
• Gall bladder
-Bile
• Large Intestine
• Chemical digestion and absorption
Junction of stomach and small
intestine
Small Intestine
• Major function- responsible for 90% of
absorption and also important for digestion
• Divided into three regions
– Duodenum- ‘mixing bowl’
– Jejunum- chemical
digestion/absorption
– Ileum- protection
from large intestine
Layers of the small intestine
Plica
Unique to the small intestine is the presence of plica
(plicae circulares).
Plica are permanent folds of the mucosa and submucosa.
One plica
One villus
On the plica, you will see villi-Present only in small intestine
-Crypts spaced between them
-Do not confuse with ‘mircovilli’
Plica and villi
Each plica supports a forest of villi. Each villus is covered with
simple columnar epithelium. These epithelial cells have microvilli.
Phew!
Villus and enzymes of the brush
microvilli
border cells
Brush
border cell
The lamina propria of each
villus contains a lacteal,
nerves and blood vessels
Regional Features of the Small Intestine
• Duodenum- site of chyme entry, where it is
blasted with HCO3- (and other things)
– Along duodenum, pH of chyme changes from
1-2 to 7-8
– Mucus glands of mucosa
– Also, Brunner’s glands (submucosal glands)
• Jejunum- first half has numerous plicae and
villi, but they decrease in number en route to
ileum
– Brush border enzymes
• Ileum - lack plicae,
contain masses of
lymphoid tissue..why??
Duodenum
Villi
Crypts
Plica
Brunner’s Glands
(submucosal glands)
Jejunum- Panorama
Ileum
Goblet Cells
Crypts
Peyer’s Patches
What organs secrete into the
small intestine?
Liver
Gall bladder
Anatomy of the pancreas
-An accessory structure of the small intestine
Pancreas
• Where have you seen the pancreas before?
• Primarily an exocrine gland (99% of cells)
• Composed of a series of ductules that are
surrounded by secretory cells, acinar cells
Acinar Cells
• Cuboidal epithelial cells that
form pancreatic acini
• Pancreatic juice produced by
acinar cells
• Alkaline mixture of enzymes,
water, and ions
• Over 1200 ml of ‘juice’
produced each day!
• Travels in pancreatic duct,
joins the common bile duct
Enzymes produced by the
pancreas
•
•
•
•
Pancreatic alpha-amylase
Pancreatic lipase
Nucleases
Four inactive proteolytic enzymes
– Not activated until they reach the small
intestine
Proteolytic Enzymes
• Inactive trypsinogen converted to active
trypsin by enteropeptidase (enterokinase)
• Trypsin then converts other proenzymes
into their active forms
– Chymotrypsin
– Carboxypeptidase
Inactive 1
proteases
released
from acinar
cells
2
3
6
trypsinogen
4 enteropeptidase
Brush border cells
5
Inactive protease  active protease
Trypsin is now active, and
will activate other proteases:
Chymotrypsin
Carboxypeptidase
Liver
• Largest visceral organ, 3 lb
• Surrounded by fibrous capsule
• Has numerous lobes, with numerous lobules
– Cells are called hepatocyes
• Receives ~25% of total cardiac output!!
• Blood flow to and from the liver is unique
Hepatic Portal System
• Liver receives 1/3 of
blood via hepatic artery
(not shown)
• 2/3 from hepatic
portal vein
• This major vein drains
capillaries from:
esophagus, stomach,
small and large intestine
and THEN heads to
liver
•Venous blood from
liver dumps directly into
inferior vena cava
Any guesses as to a major
function of the liver?
Hepatic Portal System
-The portal venous blood contains all of the products of digestion
absorbed from the GI tract.
-Thus all nutrients are processed/viewed in the liver (by
hepatocytes) before being released back into the central veins.
Liver
• Each lobe divided into lobules
• Cells of the liver are hepatocytes
– Arranged as a single layer of cells, stacked
– Divided by sinusoids
– Blood flows through sinusoids, past Kupffer
cells (resident macrophage)
Kupffer
bacteria
RBCs
Liver lobules
Blood is
flowing
towards
center
Hepatocytes arranged into ‘walls’. Walls radiate outward from a
central point. The central area contains a single, central vein.
Liver lobules- Portal Triads
1 mm
Portal
Triad
Central
Vein
Major functions of the liver
• Metabolic regulation
• Hematological regulation
• Bile production
(its only true
digestive function)
Metabolic Regulation
• Hepatocytes regulate the
composition of the blood
and blood nutrients
• They do this by
selectively secreting and
absorbing molecules
from the blood
• What molecules?
Carbs, lipids, amino
acids, waste products,
vitamins, minerals and
drugs
Portal triad
Metabolic regulation
• Hepatocytes adjust the contents of the blood by
selective absorption and secretion
– Remove/store excess nutrients
– Correct deficiencies
• Hepatocytes are able to take in and/or release large
molecules with ease, partly because the nearby
capillaries are so leaky!
hepatocytes
A big ol’ leaky capillary, a
sinusoidal capillary
What role does the liver play in
amino acid metabolism?
• Removes excess amino acids from the
bloodstream or releases stored amino acids that
are deficient
• Besides maintenance/growth, what might these
amino acids be used for?
Alanine, valine,
leucine, proline,
etc, etc.
What role does the liver play in lipid
metabolism?
• Regulates circulating levels of triglycerides,
fatty acids, and cholesterol
• If levels decline, lipid stores are broken
down and released
• ALSO, we will return to Bile in 3 minutes,
an important player in lipid breakdown
What role does the liver play in
carbohydrate metabolism?
• Blood glucose levels are kept
at or near 90 mg/dl
• If blood glucose levels rise,
glucose is stored in liver
as ______?
• If glucose levels drop,
glycogen reserves in the liver
are broken down and glucose
is released into blood
Other metabolic functions of the liver
• Removal of waste products
– Ammonia is converted to urea
• Vitamin storage
– The fat soluble ones!
• Mineral storage
• Drug inactivation
Hematological regulation
• Synthesis of plasma proteins (think albumins, etc.)
• Removal of hormones (like a giant sponge, the
liver takes NE, epi, thyroid hormones, steroid
hormones, etc. ‘out of the game’)
• Removal of antibodies
• Removal (or storage) of toxins
• Phagocytosis (Kupffer cells chomp up RBCs and
are APCs!)
• Production of bile (1 liter/day)
Kupffer
RBCs
Hepatocytes produce bile
• Bile is made and then
secreted into
channels called bile
canaliculi
• Canaliculi merge to
join the bile ducts at
each of the triads
triad
What is bile?
• Bile is an alkaline solution
containing: water, electrolytes,
bilirubin, phospholipids,
triglycerides, cholesterol, and lipids
known as bile salts
• Function of bile (bile salts):
emulsify fats for digestion
– Bile salts are not enzymes, they merely
help to bust up large fat droplets into
smaller fat droplets.
– Why is this important?
Gall Bladder
• Major functions:
–Bile storage
–Bile concentration, modification
Gall bladder
receives bile from
liver via the cystic
duct
What regulates release of bile?
• When chyme enters the small intestine, CCK is
released
• CCK relaxes sphincter (sphincter of Oddi) and
triggers muscle contractions that squeeze the gall
bladder and send bile out, into duodenum
• How clever that CCK secretion increases when
chyme is high in fats!
• Interesting note: CCK also slows stomach
movement (release of food to duodenum)
– What does this tell you about a high fat meal?
Review of Digestion at Small Intestine
• Chyme enters duodenum from stomach
• Gets blasted with HCO3-, enzymes and bile
– Which enzymes? Where from? Are all enzymes active
upon arrival? Which ones are? Which ones are not?
• Segmentation provides mechanical digestion
• Bile salts surround fats, increasing lipase
effectiveness
• ABSORBPTION of nutrients!!
• Once most absorption done, two waves of
peristalsis push left-overs into large intestines.
Each wave takes ~2 hours.
Animation-Recap
The last part..large intestine
• General functions
– Reabsorption of water
– Absorption of vitamins made by bacteria
– Compaction and storage of fecal material
Anatomy of the large intestine
The large intestine is
comprised of sections:
1. Cecum and appendix
2. Colon (ascending,
transverse, descending,
sigmoid)
3. Rectum, leads to
anal canal and anal canal
NoteLarge diameter
Thin walls
Histology of the Large Intestine
• Colon- Simple columnar with goblet cells
• Rectum- the anal canal is stratified squamous
• So, as you move from colon to out: epithelium changes
from columnar (with goblet cells) stratified squamous
 keratinized stratified squamous.
• Mucosa does not produce enzymes
• No villi
Site ofVitamins, bile, and some H2O
absorption
A word about bacterial flora
Most bacteria are DOA (dead on arrival) at the large intestine, but
some survive and comprise the bacterial flora
Over 700 species!
Functions?
Chemical Digestion and Absorption
• Virtually all nutrients from the diet are
absorbed into blood (or lymph vessels)
across the mucosa of the small intestine.
• Much of this absorption happens via cotransport or diffusion across the apical
membrane of intestinal epithelial cells (the
brush border cells), aka eneterocytes
Chemical digestion and absorption
Part 1 & 2
First steps
of
mechanical
and
chemical
digestion
Part 3
Final steps
of
chemical
digestion
and
absorption
Recall polymers and monomers
Hydrolysis
• Hydrolysis is breaking of a bond ‘with water’
or the addition of water
• Digestive enzymes hydrolyze one specific
substrate
ABCDase
A-B-C-D + H2O
A-B-H + OH-C-D
Absorption
• Carbohydrates- simple, hexose sugars will
be absorbed
• Proteins- di or tripeptides are typically
absorbed, also amino acids
• Fats- Fatty acids and monoglycerides are
absorbed (and then converted immediately
back to triglycerides)
Carbohydrates
• Sucrase catalyzes the cleavage of the bond
between (glucose-fructose) in sucrose
• Lactase catalyzes the cleavage of the bond
between (glucose-galactose) in lactose
• Maltase catalyzes the cleavage of the bond
between (glucose-glucose) in maltose
Where do we find these three enzymes?
• Enzymes are transmembrane (or at least
membrane-associated) proteins
• Function at the microvilli of enterocytes
(brush border cells)
• Apical surface
Sucrase
Maltase
Lactase
brush border cells
Disaccharides in intestinal lumen interact
with brush border enzymes, and the
enzymatic products are absorbed
Sucrose
Hexose
sugars
Sucrase
Maltose
Hexose
sugars
Maltase
Lactose
Lactase
brush border cells
Hexose
sugars
When brush border
epithelial cells fail
to make lactase….
Lactose is not digested or
absorbed.
Bacteria ferment lactose
Gas, diarrhea, etc.
Sucrase
Maltase
Lactase
Absorption of Carbs
• Monosaccharides (glucose and galactose)
are into epithelial cell membranes via
cotransport (along with Na+)
• Fructose moves in via
facilitated diffusion
• All monosacs. pass
through basal membrane of cell
(toward capillary) by
facilitated diffusion
Co-transport
• A special type of
passive transport
• Moves more than one
molecule at same time,
with molecules moving in
same direction
• Doesn’t directly require ATP, but cell expends
ATP to maintain homeostasis
• Cotransport can function despite an opposing
concentration gradient
– Example- glucose/Na+ co-transporter
Absorbed nutrients can enter a
capillary or a lacteal
Sucrose
Maltose
Hexose
sugars
L
U
M
E
N
Animation-absorption
Carbs and
peptides enter
capillaries.
Fats enter
lacteals.
Capillary
Lacteal
Protein Digestion
• Effects of mastication (mechanical
digestion)
• Effects of pH on protein structure
• What enzymes are first involved?
• Where are they found?
• What enzymes are at brush border?
• At brush border, trypsinogen is converted to
trypsin by enterokinase
• Trypsin then activates: chymotrypsin, elastase
and carboxypepsidase
• Dipeptidases at brush border!
trypsinogen
trypsin
enterokinase
Activates
pancreatic
peptidases.
Trypsin activates
pancreatic
proteases.
Dipeptidases are
membrane
enzymes.
Dipeptidases!
Absorption of proteins
• Amino acids are absorbed by
– Facilitated diffusion
– Cotransport
• Once inside the cell they
transported out via:
– Facilitated diffusion
diffusion
Hepatic portal!
Digestion of lipids
• What enzymes are involved?
• Where are they found?
Lipid Digestion and Absorption
• First with lipases
– Break down triglycerides to free fatty acids or
monoglycerides
• Form micelles with bile salts
• Micelles diffuse across the columnar cells
• Inside cells, free fatty acids are re-attached to
monogylcerides, forming triglycerides (TGs) again
• TGs coated with phospholipids
and proteins form chylomicrons
Chylomicrons
• Transported out of the columnar cells via
exocytosis
• Enter lacteal…..so, when and where will
chylomicrons actually reach the blood?
diffusion
chylomicron
Lacteal
Vitamin absorption
• The small intestine absorbs dietary vitamins, and
the large intestine absorbs some of the K and B
vitamins made by resident bacteria
• Fat soluble (A, D, E, and K)
– Enter mixed with micelles
• Water soluble (B vitamins and vit. C)
– All by diffusion except Vit. B12, which requires
intrinic factor from stomach to be absorbed
Ion Absorption
•
•
•
•
Most transported along conc. gradients
Na+ the more eaten, the more absorbed
Calcium regulated by vitamin D and PTH
Most anions move by diffusion
Ion and Vitamin Absorption
Water Absorption
• Cells cannot actively absorb or
secrete H2O
• All water moves via passive
transport, down its concentration
gradient
• As intestinal cells absorb nutrients
and ions, their concentration falls in
the lumen, and increases in cells,
thus water moves into cells.
•The small intestine and large
intestine are key in water
reabsorption