Transcript Functional Anatomy of the Liver

Liver: Transport and Metabolic
Functions I
Cindy McKinney, Ph.D.
Cell Biology and Physiology
Block 5
Gastroenterology and Endocrinology
Lecture Objectives
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Describe Functional anatomy of the Liver
Describe Blood Supply to The Liver
Define three arrangements of hepatocyte organization
Describe portal ancinus (zonal) organization
Describe zonal heterogenity of liver
Describe the mechanisms that are involved in the biotransformation
of compounds by the liver
Define the processes involved in the synthesis of bile acids
Describe bile composition
Describe bile flow dynamics
Describe metabolic functions of the liver
Define ammonia handling by the liver
Describe synthesis and storage of fat soluble vitamins
Reading
Required Reading:
Guyton and Hall Textbook of Medical Physiology, John. E Hall, 12 th edition
•Secretion of Bile by the Liver; Functions of the Biliary Tree, pp 783-786
•The Liver as an Organ, pp 837-842
Overview
• Metabolically active—receives
approximately 28% total body blood flow
• Highly aerobic organ-extracts
approximately 20% of O2 used by body
• Directs synthesis and degradation:
a) Carbohydrates
b) Proteins
c) Lipids
Functional Anatomy of the Liver
Functional unit of the liver is a “lobule” containing a branch
of the hepatic vein at its center with cells ordered around it.
Portal Triad at each corner of the
lobule hexagon containing branches
of:
hepatic artery
portal vein
bile duct
Note:
Space of Disse—extracelluar gap
PORTAL TRIAD
Functional Anatomy of Liver
• Hepatocytes (yellow) occupy 80%
of the paraenchymal volume
• Form a one cell thick epithelium
that forms a functional barrier between
two fluid compartments with distinct ionic
compositions
tiny canalicular lumen ---bile
sinusoidal blood space---blood
• Hepatocytes alter the composition of these
fluid spaces by vectorial transport of solutes
across their membranes
• Apical and basolateral hepatocyte
membranes ---many distinct characteristics
Functional Anatomy of Liver
Space of Disse: perisinusoidal space = an
extracellular “gap” between endothelial cells
that line the sinusoids and the basolateral
membranes of the hepatocytes
Hepatocytes have microvilli on basolateral side
(in the Space of Disse) that project into space.
This facilitates contact with solutes transported in
sinusoidal blood.
Single cell thickness of hepatocytes -- tight junctions and desmosomes between cells
• apical membrane faces canalicular lumen
• basolateral membrane faces the Space of Disse
Bile Canaliculi
Two adjacent hepatocytes juxtapose their groove-like apical membranes along their
common face --- forming tiny canaliculus (1 μm diameter)
Organization of the canaliculi: “chicken-wire” appearance
Functional Anatomy of the Liver
Other cell types in the liver—comprise 6% of cells
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Endothelial cells (2.8%) ---lining of vascular sinusoids
forming fenestrated structures allowing free movement of
plasma solutes (not RBCs) into the Space of Disse
• Kupffer cells--- fixed macrophages in sinusoidal vascular
space ---remove particulates from circulation
• Stellate Cells---located in Space of Disse; contain large fat
droplets in their cytoplasm may be involved in pathogenesis
of cirrhosis. Central role in Vitamin A storage. May be
capable of transforming into proliferative, fibrogenic, and
contractile myofibroblasts.
Describe Blood Flow of Liver
Hepatic artery=25% total circulation
Portal vein=75% total circulation
Blood from portal venules and hepatic
arterioles drain into the network of hepatic
sinusoids
Blood from the sinusoids central veins
hepatic vein
Portal Triad (Bile duct, hepatic artery, portal vein) + Lymphatics + Nerves= “PORTAL TRACT”
Describe Blood Flow of Liver
• Hepatic artery forms peribiliary plexus
where there is bi-directional exchange of solutes
(electrolytes and bile acids etc) between the bile
and the blood in the portal tract
• Peribiliary plexus portal circulation
absorbed substances from biliary ductportal
vein
• Portal vein drains back to systemic circulation
(substances are reprocessed by hepatocytes)
Describe Blood Flow of Liver
Hepatocyte Organization
Three Alternative Arrangements
1. Classic Hepatic Lobule (hexagon): central vein=core and lobule includes all
hepatocytes drained by and single central vein. Boundaries =portal triads
2. Portal Lobule (Triangle): portal triad=core of hepatic lobule contains all the
hepatocytes drained by a single bile duct. Boundaries = three central veins
3. Portal Acinus (Rhomboid): group of hepatocytes supplied by a single source of
arterial blood. Small 3D mass of hepatocytes that are irregular in size and shape
---one axis is a line between two triads ( high pO2) and another axis formed by a
line between the central vein (low pO2)
Three Arrangements of Hepatocyte
Organization
Lowest O2, solutes modified by
other hepatocytes
Hepatocytes in intermediate layer
Highest O2 and solutes
Proposed by Rappaport---defined three zones of hepatocytes
Zone I: hepatocytes perfused first (high pO2 and solutes) most resistant to effects of
circulatory compromise or nutritional deficiency or other forms of cell injury, first to
Regenerate
Zone II: hepatocytes in intermediate layers
Zone III: Most distal section of pericentral hepatocytes ---located near terminal central vein
Exposed to progressively lesser concentrations of O2 and nutrients, receive solutes that
May have been modified by hepatocytes in zone I and zone II
Exact functional zones of hepatocytes in this organization are difficult to define
Zonal Heterogeniety of Liver
The functional zones defined by the portal acinus model allow for the presence of
specialized microenvironments (provided by microcirculation of the zone) surrounding the
hepatocytes. Consequently, some enzymes are selectively expressed in hepatocytes of one
zone or another.
NOTE: Reversing the blood flow, reverses the zones. Thus, the predominant enzyme activity
is controlled by the microenvironment created by hepatic microcirculation at physical cell
location.
Zone I: Enzymes involved in oxidative energy metabolism with β-oxidation, AA metabolism,
Ureagensis, gluconeogenesis, cholesterol synthesis and bile formation
Zone II: Enzymes overlap Zones I and III
Zone III: Enzymes involved in glycogen synthesis, glycolysis, liponeogensis, ketogenesis,
xenobiotic metabolism (CYP 450) and glutamine formation.
1) detoxification mechanisms + involved in biotransformation of drugs
2) drug induced toxicity  cell necrosis located in Zone III
(example: Effects of Acetaminophen)
Mechanisms of
Biotransformation in Liver
Liver detoxifies and metabolizes many endogenous AND exogenous compounds
Reactions are divided in two phases:
Phase I: oxidation and reduction reactions mediated via cytochrome P450 oxidases
Phase II: conjugation of phase I metabolites with glucuronate, sulphate or glutathione
RH
Phase I
Cyp 450s
ROH
Phase II
RO-conjugate
Phase I:
a) Cytochrome P450 (heme protein) named because they absorb light at 450 nm when bound
to CO—function to insert O2 into substrate
a) Reside in ER (microsome fraction)----multi-gene family (150 isoforms known)
b) Typically catalyze hydroxylation reactions
c) Responsible for drug /chemical carcinogen metabolism, bile acid synthesis, activation
and inactivation of vitamins
d) Some products may be directly secreted if they are water soluble
e) Usually require phase II biotransformation (conjugation) for excretion
Phase II: Bilirubin Metabolism and Excretion
Bilirubin=heme degradation
RBCs (aged or damaged) are
removed by macrophages of
reticuloendothelial system
heme biliverdin bilirubin
bilirubin+ serum albumin
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Conjugated to glucuronic acid(s)
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Secreted by liver carried by bile to
small intestine
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Bacterial in ileum and colon
deconjugate urobilinogen (colorless)
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Some absorbed by enterocytes/plasma
In colon converted to sterocobilin
main pigment of feces
Kidney filters plasma; oxidized to
Urobilin (yellow)
Mechanisms of
Biotransformation in Liver
Phase II:
a) Conjugation reactions make compounds more water soluble (hydrophillic) leading to
excretion in blood or bile
b) Common products--- glucuronides, sulfates, and mercapturic acids
c) Critical step in detoxification
d) Three major mechanisms
Conjugation to Glucuronate (UGT)
Conjugation to Sulfate (-SO4)
Conjugation to Glutathione (GSH)
e) Other contributing mechanisms
Methylation (examples catechols and thiols)
Gray Baby Syndrome
Acetylation (examples amines and hydrazines)
Conjugation to glycine, taurine or glutamine ( bile acids)
Clinical Note: Defects in conjugation enzymes (genetic or enzyme saturation) can be fatal
Example: Gray Baby Syndrome
Decrease in glucuronidation capacity in infants+ administration of chloramphenicol
Results in lethargy, ashen gray appearance and circulatory collapse- COMA
Phase II Biotransformation:
Conjugation to Glucuronate
Enzymes: Uridine Diphosphate Glucuronosyl Transferases (UGTs)
• Two families based on substrate specificity
• Located in ER of hepatocytes (microsomes)
UGT Family 1:
 Four known members. Genes encoded on chromosome 2
 Catalyze the conjugation of glucuronic acid with phenols or bilirubin
UGT Family 2:
 At least five UGTs known. Genes encoded on chromosome 4.
 Catalyze the glucuronidation of steroids or bile acids.
Clinical Note: UGT Family 1 essential for conjugation of
bilirubin to form excreted in bile. Congenital deficit of
UGT activity results in jaundice at birth and bilirubin
encephalophy seen in patients with
Crigler-Najjar Type I Syndrome
(autosomal recessive –rare—incidence 1/106 live births
Phase II Biotransformation:
Conjugation to Glucuronate
Clinical Note: Crigler-Najjar Syndrome Type II
Can be treated with phenobarbitol; found in Amish, potential for
neurologic damage (kernicturous-brain damage) by accumulation of
bilirubin in brain
Phase II Biotransformation:
Conjugation to Sulfonate
Enzyme= Sulfotransferases
• Located in cytosol (NOT ER)
• Catalyze sulfation of steroids, catechols and foreign compounds
(alcohol, metabolites of carcinogenic hydrocarbons)
• Location in cytosol suggests these sulfotransferases act cooperatively with UGTs.
• Sulfate products are non-toxic and readily eliminated
-exception: sulfate esters of carcinogens (these are recirculated)
SO4 + 2 ATP
3’ Phosphoadenosine-5’-phosphate (PAPS)
HNSO3H
NH2
+ PAPS
sulfotransferases
+ PAP
Phase II Biotransformation:
Conjugation to Glutathione (GSH)
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Enzyme=Glutathione-S-Transferases (GST)
Located in cytosol
Hepatocytes conjugate reduced GSH
Conjugation at the cysteine residue in GSH
Liver has the highest [GSH] (approximately 5 mM) ---90% in cytosol and 10% in mitochondria
Substrates for GST modification:
• Electrophillic metabolites from lipophillic
compounds
• Products of lipid peroxidation
• Alkyl and aryl halides
Excreted in bile and further modified by removal
of glutamyl residue from glutatthione by
ϒ-glutamyl transpeptidase located on surface of
bile duct epithelium.
Glutathione conjugates can be secreted in plasma where a ϒ-glutamyl transpeptidase
(brush border of proximal tubule in kidney) removes the glutamyl reside from
glutathione). Dipeptidase removes glycine residue to produce cysteine-S- conjugate. Final
conjugate is excreted in urine or acteylated in the liver (mercapturic derivative) and excreted.
Bile Formation
Bile from hepatocytes
Small canaliculi
Small terminal ductules (canals of Herring)
Perilobular bile ducts
Interlobular bile ducts
Septal ducts
Cystic duct (gallbladder)
Lobar ducts
Biliary Tree --Canaliculi form an extensive
meshwork surrounding and draining
hepatocytes
Left and right hepatic ducts
Common hepatic bile duct
COMMON BILE DUCT
Enters into doudenal lumen at
Sphincter of Oddi
Pancreatic Duct (Ampulla of Vater)
Bile Formation (Choleresis)
Three steps to bile formation:
1) Active secretion of bile from hepatocytes into liver canaliculi
2) Bile transported with water rich fluid secreted from intrahepatic and extrahepatic
ducts
a) approximately 900 ml/day from steps 1-2
3) Between meals, approximately 50% of hepatic bile (estimate 450 ml/day) diverted
to gallbladder
GALLBLADDER ---stores and concentrates bile and remaining solutes (10-20 fold)
a) isosmotically removes salts and water
b) concentrates remaining solutes: bile salts, bilirubin, cholesterol, lecithin
c) stores concentrated bile
Approximately 500 ml bile /day reaches the duodenum via the Ampulla of Vater
Mixture transferred to duodenum = “dilute” hepatic bile + “concentrated” gallbladder bile
Bile Formation (Choleresis)
Notes on bile formation:
• Bile secretion =energy dependent process
hydrostatic pressure in the hepatic canaliculi > sinusoidal perfusion pressure
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Bile formation requires:
-an active, energy dependent secretion of inorganic and organic solutes
-passive movement of water follows solutes
Passive movement of water through interhepatic tight junctions carries other solutes
along termed “Solvent Drag”
Canalicular bile is an isosmotic fluid: passage of water carries other small ions
Further down the biliary tree (ducts and gallbladder)---pore size between cells is
significantly smaller----consequently the role of solvent drag in bile production
Bile Composition
• Hepatic bile and gallbladder bile are iso-osmotic with plasma
(~300 mosmole/kg)
Composition: water + inorganic electrolytes (Na+, Cl- , andHCO3-) + organic solutes
(bilirubin, cholesterol, fatty acids, phospholipids)
Functionally important solutes:
1) Micelle forming bile acids/salts:
lipid digestion/ toxic substance excretion
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2) Phospholipids:
solubilize cholesterol protect hepatocyte from
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cytotoxicity of bile acids
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3) Immunoglobulin A:
inhibits bacterial growth in bile
Excretory products in bile: cholesterol, bile pigments, plants sterols, trace minerals,
Lipophillic drugs/metabolites, oxidized glutathione, antigen-antibody complexes
Bile Synthesis
Cholesterol converted to “primary bile acids” (cholic and chenodeoxycholic acids)
Rate limiting enzyme= 7α-dehydroxylase (P450 family in Smooth ER)
- feedback inhibition by bile acids
Principle route of cholesterol excretion and catabolism
-essential factor in total body cholesterol balance
Hepatocytes conjugate primary bile acids to glycine/taurine= bile saltssecreted into bile
Secondary bile acids (deoxycholic and lithocholic acids) = breakdown products from
bacterial action in the ileum and colon---recirculated to liver via enterohepatic circulation
Will also be conjugated to glycine and taurine=bile salts
Bile Flow
Choliangiocyte secretions
Constant flow
Linear Δ w/bile acid secretions
[Glutathione] generated osmotic flow
Total bile flow sum of:
• Canalicular flow from hepatocytes
• Ductular flow from secretions of
cholangiocytes lining bile ducts
Example: ursodeoxycholic bile acid increases bile flow by direct
stimulation of biliary excretion
Canalicular flow: increases linearly following rate of bile acid secretion
Two components:
1) bile acid independent: constant flow independent of bile acid secretion
a) secretion of ORGANIC compounds drives the independent flow
(example: High [glutathione] in bile generates a potent osmotic driving force
for canalicular flow)
2) bile acid dependent: rising component that changes linearly with bile acid secretion
a) micellar form of bile acids are predominant osmotic driving force for
water movement in bile acid dependent flow
b) bile acids increase electrolyte and water flow by stimulating the
Na+-coupled co-transport mechanisms or modulating other solute transporters
Metabolic Functions of Liver
Synthesis and degradation of:
1. Carbohydrates
-gluconeogensis
-storage of glucose as glycogen
-glycogenolysis
2. Proteins
-synthesizes nonessential AA
-synthesizes plasma proteins
- urea formation from AA
3. Lipids
Provides ENERGY to other systems by exporting
-participates in FA oxidation
GLUCOSE
-synthesizes lipoproteins,
KETO ACIDS (acetoacetate etc)
cholesterol and phospholipids
Critical for process of oxidation in peripheral tissues
Liver: Carbohydrate Metabolism
Between meals ----[Glucose]  (Fasting state Insulin  Glucagon )
Liver metabolism becomes a source of plasma glucose for other tissues
a) de novo synthesis (gluconeogenesis) is one of liver’s important major functions
1) essential for maintaining normal plasma glucose concentrations
b) Glycogenolysis also delivers glucose to plasma---breakdown of glycogen
1) stored liver glycogen may account for 7-10% total liver organ weight
Note: Glycogenolysis in liver yields glucose as major product
Glycogenolysis in muscle produces lactic acid as major product
In Fed State: Liver serves as a reserve for glucose (Insulin Glucagon  )
Used to synthesize glycogen
Liver captures glucose from portal blood
broken down to pyruvate
Oxidized to H20 and CO2 (TCA Cycle)
aerobic phase
Glycolysis
anaerobic phase
Liver also converts glucose to glycogen—carbohydrate not stored as glycogen or oxidized for
Energy is usually utilized to metabolize fat
Liver Protein Synthesis
Liver produces a wide variety of proteins that are exported to blood plasma
Major PROTEIN products include:
• major plasma proteins ---maintenance of colloidal osmotic pressure in plasma
-maximum rate: 15-50 g/day synthesized
• factors for hemostasis (blood clotting) and fibrinolysis (blood clot degradation)
• Carrier proteins (bind and transport hormones and other substances in blood)
• Pro-hormones and lipoproteins
Examples: proteins synthesized by Liver:
1) Albumin -25% of all protein production in liver
2) a1-globulins- HDL and VHDL, cholesterol, glycoproteins, haptoglobins and mucoproteins
3) a2-globulins-ceruloplasmin, glycoproteins, macroglobulin, plasminogen, prothrombin
4) b-globulins-LDL, VLDL lipoproteins, transferrin
5) Blood clotting factors-Factors I, II, V, VII, VIII, IX, and X (vitamin K is needed for the
synthesis of some factors)
Liver Amino Acid Metabolism
and Urea Formation
Liver captures dietary amino acids ---absorbed by GI tract and carried to liver in the
portal blood circulation
Two mechanisms for hepatocyte capture located on both basolateral and apical
hepatocyte surfaces:
1) Na+ dependent transporters (12 highly specific types) similar to small GI
2) Na+ independent transporters
AAs in liver are used immediately for de novo synthesis of proteins OR degraded for energy
AA degradation:
deamination produces NH4 (ammonia) and α-keto acids enters TCA cycle
enters the urea cycle
ATP production
Exits hepatocyte via urea channel (aquaporin 9)
Travels via blood to kidneys and excreted
Urea Cycle: Amino Acids
AA Transporter
Circulated in blood
Removed by kidneys
Water soluble product
AA Transporter
Ammonia Handling and the
Urea Cycle
• Ammonia small, neutral molecule derived from protein catabolism/ bacterial activity
- derived from bacteria urease activity in colon (50%)
-NH3 passively crosses colonic epithelium- travels via portal circulation to liver
-approx 40-50% of NH3 comes from kidney
• Like bilirubin and ammonia is toxic to CNS
• Highly membrane permeable---no transporter required
• Liver critical to prevention of ammonia accumulation in blood
-Liver is only organ that can convert ammonia to urea via UREA CYCLE
• Hepatocytes efficiently extract ammonia from portal and systemic circulation 
where it enters the urea cycle urea subsequently transported back to systemic circulation
• Urea (small neutral molecule) filtered at the kidney glomerulus
• Approx 50% of filtered urea is excreted in urine
Clinical Notes:
• Metabolic activity of liver acutely compromised—coma and death can result
• Metabolic activity of liver chronically compromised:
--gradual decline in mental acuity reflecting cumulative action of [ammonia] +
 [other toxins] that are not cleared via liver and kidney
=Hepatic Encephalopathy
Synthesis and Secretion of Glutathione (GSH)
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Critical molecule in conjugations reactions  detoxification
GSH protective role against oxidative stress in a number of tissues
RBCs have low [GSH]  susceptible to oxidative stress induced hemolysis
- protection comes from reducing capacity GSHGSSH
• GSH release into blood via OATP-1 transporter ---exchanges GSH for organic solutes
• GSH can also move into bile by moving across the canalicular membrane
-mediated by MRP2 transporter and possibly another unknown one
• 90% of GSH in circulation is synthesized in the liver
GST
Liver Failure: Ammonia Handling/Encephalopathy
Urea cycle impaired:
[NH3] in circulation and tissues
Hepatic Encephalopathy:
1) Readily permeable NH3 crosses blood- brain barrier
2) Ammonia absorbed and metabolized by astrocytes---astrocytes swell
3) Increased activity () of inhibitory activity of γ-aminobutyric acid system (GABA)
4) Energy supply to other brain cells 
 signs of confusion, dementia and coma
However, ammonia levels are not always a direct correlate with the severity of
encephalopathy
Liver: Synthesis and Storage of Fat Soluble Vitamins D and K
• Vitamin D synthesized by skin cells under the
influence of UV light
• Dietary Vitamin D comes from animal (D3) and plant
(D2) sources
• First step in activation is a cytochrome P450 mediated
25-hydoxylation of vitamin in the liver
• 1-hydroxylation in the kidney completes activation
to full biological activity
full biological activity: 1,25 hydroxyvitamin D
• Degradation is also mediated in liver with hydroxlation
at carbon 24 by another cytochrome P450
• Vitamin D deficiency: Osteoporosis and rickets
• Vitamin K—fat soluble obtained by action of intestinal bacteria
• Essential for ϒ carboxylation of glutamate residues in coagulation factors II,VII, IX and X
• Intestinal absorption of two forms K1 and K2 similar other fat soluble vitamins
• Vitamin K deficiency (chronic) results in blood clotting abnormalities
Synthesis and Storage of Fat Soluble Vitamins D and K
Extrahepatic or intrahepatic cholestasis, fat
malabsorption, biliary fistulas, dietary deficiency
particularly in association with antibiotic therapy
may result in Vitamin K deficiency.