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The Digestive System II (Chapter 25)
Lecture # 13: The Digestive System 2
Anatomy of Liver and Pancreas.
Chemical Digestion and Absorption.
Objectives
1- To describe the gross and
microscopic anatomy of the liver and
the pancreas.
2- To describe how each major class of
nutrient is chemically digested, and
name the enzymes involved.
3- To describe how each nutrient is
absorbed by the small intestine.
Gross Anatomy of Liver
Liver
The liver is a reddish brown gland located
immediately inferior to the diaphragm.
It is the body’s largest gland weighing about 1.4 kg
(3 pounds).
It has a variety of functions, but only the secretion
of bile contributes to digestion.
Caudate lobe
Right lobe
Inferior vena cava
Left lobe
Falciform
ligament
Round
ligament
(a) Anterior view
Quadrate lobe
Gallbladder
(b) Posterior view
Microscopic Anatomy of Liver
To the
hepatic vein
Hepatic
Lobule
To the inferior
vena cava
Central vein
Hepatic Triad:
Branch of hepatic
portal vein
Branch of proper
hepatic artery
Bile ductule
Blood from
the intestine and
stomach
Hepatocytes
Hepatic
sinusoid
Bile
canaliculum
To the right and
left hepatic ducts
Hepatic sinusoids:
They are blood-filled channels that fill
spaces between the plates of hepatocytes.
Hepatic sinusoids are lined by a fenestrated
endothelium that separates hepatocytes from
blood cells and allows plasma into the space
between the hepatocytes and endothelium.
Hepatocytes have brush border of microvilli that
project into this space.
Hepatic macrophages (Kupffer cells) are
phagocytic cells in the sinusoids that remove
bacteria and debris from the blood.
Functions of the Hepatocytes:
After a meal, the hepatocytes absorb from the blood: glucose, amino acids, iron, vitamins,
and other nutrients for metabolism or storage.
They remove and degrades hormones, toxins, bile pigments, and drugs.
They secrete into the blood: albumin, lipoproteins, clotting factors, angiotensinogen, and
other products
Between meals, they breaks down stored glycogen and releases glucose into the blood.
Bile canaliculum
Bile ductule
Right hepatic ducts
Left hepatic ducts
Common hepatic duct
Cystic duct
Bile duct
Gallbladder
It stores and
concentrates bile
Pancreatic duct
Accessory
pancreatic duct
Pancreas
Duodenum
Minor duodenal papilla
Hepatopancreatic sphincter
Major duodenal papilla
Jejunum
Hepatopancreatic ampulla
Pancreas
It is a spongy retroperitoneal gland posterior to the greater curvature of the stomach.
The head of the pancreas is encircled by the duodenum.
It is both an endocrine and exocrine gland.
The endocrine portion consists of the pancreatic islets that secrete insulin and glucagon.
The exocrine portion is about 99% of pancreas and secretes 1200 to 1500 mL of
pancreatic juice per day.
Accessory
pancreatic duct
Pancreatic duct
Tail
Body
Head
Endocrine pancreas
(produces hormones)
Islets of Langerhans
1- Beta cells: Insulin
2- Alpha cells: Glucagon
3- Delta cells: Somatostatin
4- F cells: Pancreatic polypeptide
Exocrine pancreas
(produces enzymes)
Pancreatic acini
They secrete large quantities of an
alkaline, enzyme rich fluid.
Digestion
All digestion reactions consists of
hydrolysis reactions:
Enzyme
Starch (Polymer of Glucose)
9
H2O
Glucose
Enzyme
Protein (Polymer of Aminoacids)
9
H2O
Aminoacids
Carbohydrate Digestion
Starch is the most digestible
carbohydrate.
Cellulose and chitin are indigestible.
Monosaccharides
Disaccharides
Polysaccharides
1
Salivary
amylase
hydrolyzes
starch into oligosaccharides (up to 8
glucose residues long). It works best
at pH of 6.8 – 7.0 of oral cavity.
2 When reaching the small intestine,
pancreatic amylase converts the
remaining starch to oligosaccharides
and maltose within 10 minutes.
Oligosaccharides
Salivary
amylase
Amylase quickly
denatured on contact
with stomach acid and
digested by pepsin.
Starch
Pancreatic
amylase
3 Oligosaccharides and maltose
contacts brush border enzymes
(dextrinase,
glucoamylase,
maltase, sucrase, and lactase).
Brush border
of microvilli
4 Monosaccharides
are
absorbed
immediately.
Dextrinase, glucoamylase,
maltase, sucrase, and
lactase
Carbohydrate Digestion in the Small Intestine
1In the oral cavity, 50% of
dietary starch is digested.
Salivary
amylase
stops
working in stomach at pH less
than 4.5
2
3
4
2 When reaching the small
intestine, pancreatic amylase
converts starch to oligosaccharides
and
maltose
within 10 minutes.
3 Oligosaccharides and maltose contacts brush border enzymes (dextrinase,
glucoamylase, maltase, sucrase, and lactase) act upon oligosaccharides,
maltose, sucrose, lactose, and fructose to glucose.
4
Monosaccharides are absorbed immediately.
Protein Digestion
The amino acids absorbed by the small intestine come from three sources:
1- Dietary proteins
2- Digestive enzymes digested by each other
3- Sloughed epithelial cells digested by enzymes
Enzymes that digest proteins are called proteases or peptidases
Mouth
Peptidases are absent from the saliva.
No chemical digestion of proteins
occurs in the oral cavity.
Polypeptides
Pepsin ( ) hydrolyzes certain
peptide bonds, breaking
protein down into smaller
polypeptides.
Stomach
Digestion of proteins continues in the small intestine because pepsin is
inactivated when it passes into the duodenum and mixes with the alkaline
pancreatic juice (pH 8).
Pancreatic enzymes trypsin and chymotrypsin take over the process
hydrolyzing polypeptides into even shorter oligopeptides.
Pancreatic carboxypeptidase removes amino acids from –COOH end of the
chain.
Small intestine
Actions of
pancreatic enzymes
Trypsin ( ) and
chymotrypsin ( )
hydrolyze other peptide
bonds, breaking
polypeptides down into
smaller oligopeptides.
Carboxypeptidase ( )
removes one amino acid
at a time from the carboxyl
(–COOH) end of an
oligopeptide.
Brush border enzymes (contact digestion) also remove one aminoacid
at a time.
Carboxypeptidase removes amino acids from –COOH end of the
chain.
Aminopeptidase removes them from the –NH2 end.
Dipeptidases split dipeptides in the middle and release two free
amino acids.
Actions of brush
border enzymes
Carboxypeptidase ( ) of the brush
border continues to remove amino
acids from the carboxyl (–COOH)
end.
Aminopeptidase ( ) of the brush
border removes one amino acid at a
time from the amino (–NH2) end.
Dipeptidase ( ) splits dipeptides (
into separate amino acids ( ).
)
Lipid Digestion and Absorption
Hydrophobic quality of lipids makes their digestion and absorption more
complicated that carbohydrates and proteins.
Enzymes that digest lipids (fats) are called lipases.
The lingual lipase secreted by the intrinsic salivary glands of the tongue is
active in mouth, but more active in stomach along with gastric lipase. 10-15%
of lipids digested before reaches duodenum.
Pancreatic lipase in the small intestine digest most of the fats.
Lipid Digestion
and
Absorption
1- Emulsification
1- Emulsification 2- Fat Hydrolysis 3- Lipid uptake by micelles
4- Chylomicron Formation
5- Chylomicron Exocytosis
Components of the bile
Fat globule is broken up
and coated by lecithin and
bile acids.
Emulsification
droplets
2- Fat Hydrolysis
Pancreatic lipase acts on triglycerides. It removes the first and third fatty acids from
glycerol backbone and leaves the middle one.
The product of lipase action are two free fatty acids (FFAs) and a mono-glyceride.
Emulsification droplets are
acted upon by pancreatic
lipase, which hydrolyzes
the first and third fatty
acids from triglycerides
usually leaving the middle
fatty acid.
Pancreatic lipase
3- Lipid uptake by micelles
Micelles are made in the liver and they are very small droplets in the bile.
They consist of 20 to 40 bile acid molecules aggregated with their hydrophilic side groups
facing outward and their hydrophobic steroid rings facing inward.
Micelles
Micelles in the bile pass
to the small intestine and
pick up several types of
dietary and semidigested
lipids.
4- Chylomicron Formation
Intestinal cells absorb lipids from micelles, resynthesize triglycerides, and
package triglycerides, cholesterol, and phospholipids into protein-coated
chylomicrons.
Lipoprotein
5- Chylomicron Exocytosis
Golgi complex packages chylomicrons into secretory vesicles. They are
released from basal cell membrane by exocytosis and enter the lacteal
(lymphatic capillary) of the villus.
They enter the bloodstream when lymphatic fluid enters the subclavian vein via
the thoracic duct.