Seven

Question 1

List some functions of the liver.

Answer 1

The liver serves multiple critical functions for the body. The liver is intimately involved in blood conditioning (I prefer this work over filtration) and detoxification, glucose homeostasis, protein synthesis, lipid/cholesterol metabolism, bile acid synthesis/bile formation, vitamin absorption, and hormone metabolism just to name a few of its many jobs.

Question 2

Describe the hemodynamics of the liver. Where does its blood come from? Where does it go?

Answer 2

The liver receives its blood flow from two sources. The majority (75-80%) comes from the portal vein. The portal vein forms from the confluence of the splenic vein and
the mesenteric veins that drain the intestines. The remaining 20 to 25% of blood flow to the liver is from the hepatic artery (a branch of the celiac artery). Whereas the vascular

resistances in the intestine and the spleen primarily determine portal vein blood flow, the

intrahepatic vascular resistance determines hepatic arterial blood flow. Hepatic arterial

blood flow typically changes reciprocally with portal blood flow in an attempt to

maintain a constant total liver perfusion and oxygen supply. The liver is a bit pressed for

oxygen since the organ is metabolically very active, and the majority of the blood flow to

the liver is from venous sources in which oxygen tension is less than arterial sources.

The hepatic artery is therefore an important (although not always essential) source of oxygen to the liver. The hepatic artery is also important as a source of oxygen to the

biliary tree in the area of the porta hepatis.

Altogether, the blood flow to the liver is about 1500 to 1800 ml/min; that’s about

one fourth of the cardiac output to you and me. Once in the liver, the blood percolates

though the parenchyma of the organ and finally is returned to the systemic circulation

through the right and left hepatic veins that drain into the inferior vena cava. The

pressure gradient between the portal veins and hepatic veins is remarkably low, about 5

mm Hg. The diagram below outlines the splanchnic circulation and portal flow.

Question 3

List 5 cells of the liver and describe their function.

Answer 3

HEPATOCYTES. Hepatocytes are the workhorses of the liver; they are responsible for

much of the liver’s metabolic abilities that will be discussed in this lecture. They account

for 60% of the cells in the liver.

ENDOTHELIAL CELLS. Endothelial cells line the sinusoids. They are self-
proliferating cells that account for 10% of the cells in the liver. While these cells perform

a barrier function between blood and hepatocytes, open fenestrations form a “sieve plate”

that allows nutrients, waste products, hormones, and other substances in blood to come

into contact with hepatocytes. Cellular receptors mediate uptake of selected proteins.

Endothelial cells also produce nitrous oxide and other effector molecules that contribute

to the mediation of hepatic blood flow.

STELLATE CELLS (Ito cells). These are fat-storing cells that are also responsible for

the synthesis of extracellular matrix. These cells respond to vasoactive substances and

thereby provide the cellular machinery that regulates sinusoidal blood flow. In the event

of severe or persistent liver injury these cells can become dangerously activated; the cells

actually transform into myofibroblasts that cause fibrogenesis in the liver. This can then

lead to irreversible liver damage.

KUPFFER CELLS. These are the resident macrophages of the liver. They can be self-
proliferating but also can be recruited from extrahepatic sources. They are located

predominantly in the periportal region (towards the sinusoidal inflow) and their main

function is phagocytosis of particulate matter (cellular debris, parasites, viruses, bacteria)

and receptor-mediated endocytosis of unwanted molecules (e.g., immune complexes,

bacterial endotoxins). Like stellate cells, activated Kupffer cells can contribute to liver

injury.

CHOLANGIOCYTES. These are columnar epithelial cells that line the bile ducts.

Question 4

Describe the role of the liver as the glucostat. Define glycolysis. Define gluconeogenesis. What are the substrates for it in the liver? Where do they come from? Define oxidative phosphorylation. Define glycogenolysis.

Answer 4

Brain cells, erythrocytes, myocytes, and cells of the renal cortices are “picky eaters”. They vastly prefer and require glucose as their primary energy source. The liver plays the central role as a “glucostat”. It removes glucose, if it is in excess, via glycogen synthesis, glycolysis, and lipogenesis and liberates glucose if needed via glycogenolysis and gluconeogenesis. Glucose moves into and out of the liver cell by way of the Glut-2 transporter. Some glucose, fructose, and galactose metabolism takes place in the liver, but not much. For the most part, glucose cycles to muscle and other body tissues where lactate, pyruvate, and alanine are generated by glycolysis. These trioses are then returned to the liver and converted into carbohydrate via gluconeogenesis. In summary, glucose
homeostasis is maintained in the hepatocyte by interdependent cyclic pathways that serve as branch points for the metabolism of simple sugars. More on this later…

A few terms need to be defined here. Glycolysis is the pathway by which glucose

is phosphorylated and broken down anaerobically to make ATP. This pathway plays a

minor role in the liver. Oxidative phosphorylation is the utilization of pyruvate (formed

from glycolysis) to make ATP. Gluconeogenesis is the production of glucose from lactate, pyruvate, and amino acids cycled to the liver from muscles via a pathway

commonly referred to as the “hexose shunt”. Glycogenolysis is the release of glucose

stores from glycogen.

Question 5

Describe the daily rhythm of liver metabolism in a fed state, short term and long term fasting state.

Answer 5

During a typical carbohydrate rich meal, glucose is absorbed at a rate of more

than 40 g/h. Over half of this is taken up by the liver and converted to glycogen, or

converted to fatty acids; only a small amount is oxidized in the liver. During a short term

fast, the liver releases glucose at a rate sufficient to cover the body’s glucose needs (about

7. 5 g/h), two thirds of which is derived from glycogenolysis and the remainder from
   gluconeogenesis. After about 24 hours of fasting, glycogen stores become depleted. In

this case, cerebral metabolism adapts and glucose requirements fall to 2 g/h, whereas

ketone bodies (from fatty acids) are more heavily utilized as an energy fuel. Also, if

starvation continues, the liver enters a catabolic mode by which muscle can be broken

down, with amino acids (especially alanine and glutamine) delivered to the liver for use

as substrates in gluconeogenesis.

Question 6

What are plasma lipoproteins made up of? What do the apoproteins do? List the 5 lipoproteins. What other enzymes does the liver synthesize that are important to FA metabolism?

Answer 6

The liver has a pivotal role in lipoprotein metabolism. Plasma lipoproteins are

complex particles with a surface made up of cholesterol, phospholipids, and specific

apoproteins. These particles have a central hydrophobic core of cholesterol esters and

triglycerides (3 fatty acids attached to one lonely glycerin base). Different lipoprotein

classes differ in the relative amount of their apoproteins and are directed with specific

functions. The apoproteins are not only essential structural (solubilizing/emulsifying)

components of lipoprotein particles, but also act as recognition sites for receptor

mediated endocytosis of lipoproteins and as activators of enzymes involved in lipid

metabolism. In addition to the synthesis and secretion of lipoproteins, the liver also

secretes lipid-metabolizing enzymes, such as lecithin:cholesterol acyltransferase (LCAT)

and hepatic triglyceride lipase.

There are 5 major lipoproteins. They are HDL (high density lipoproteins), IDL

(intermediate density lipoproteins), LDL (low density lipoproteins), VLDL (would you

believe very low density lipoproteins), and CM (chylomicrons-translates kind of into

‘small blobs of fat’).

Question 7

Describe chylomicrons including their life cycle, what they consist of, which apoproteins they have, etc.

Answer 7

Chylomicrons are the largest particles. They consist largely (more than 90%) of

triglycerides and are secreted by the intestinal mucosa into the intestinal lymph, from

where they enter the systemic circulation. They support exogenous, dietary triglycerides

and cholesterol and are processed in the capillaries of skeletal muscle and adipose tissue.

Here, chylomicrons bind to the endothelial surface and their triglycerides are digested by

membrane bound lipoprotein lipase, an enzyme that is activated by apoprotein C-II, a

constituent of chylomicrons. The hydrolytic products (fatty acids and monoglycerides)

are taken up by adipocytes and skeletal muscle cells where they are deposited as fat or

oxidized as fuel. The remaining lipoprotein particles (conveniently called chylomicron

remnants), are rich in cholesterol, apo B-48, and apo-E. The liver takes up these

remnants via the chylomicron remnant receptor. Thus, dietary triglycerides are delivered

to muscle and fat, whereas dietary cholesterol is targeted to the liver.

Question 8

Describe the life cycle of VLDL, IDL, and LDL. What are they composed of? Which apoproteins do they contain?

Answer 8

During carbohydrate-rich feeding, the liver converts carbohydrates into fatty acids that after esterification are released into the bloodstream in the form of apo-E, apo C-II, and apo B-100 rich VLDL. VLDL is processed in the skeletal muscle and adipose tissue

in a similar way to chylomicrons. Their remnants are termed IDL and contain apoB-100

and apo-E. IDL are taken up in part by the liver via the LDL receptor or undergo further

processing in plasma (loss of apo-E) to yield cholesterol rich LDL. LDL functions to

supply extrahepatic tissues with cholesterol, where uptake occurs via LDL-receptor

mediated endocytosis. Like many peripheral tissues, the liver parenchyma expresses the

LDL receptor. Cholesterol is retaken by the liver and used for bile acid synthesis or

excreted as it is into bile. Kupffer cells also serve as a scavenger to remove especially

high concentrations of cholesterol.

Question 9

What does HDL do/contain? What are they converted to? What happens with excess LDLs, especially if the liver has deficient LDL receptors? How do statins work?

Answer 9

HDL is formed from unesterified cholesterol that is released during cellular

turnover. Esterification of cholesterol in HDL occurs by the action of LCAT, an enzyme

synthesized and released by the liver into circulation. These cholesterylesters are then

found as IDL and may eventually appear in LDL (as shown in the figure below). Apo B-

100 is recognized by the LDL receptor; when LDL binds to its receptor it triggers LDL

internalization. In the event of LDL receptor deficiency or deficit, LDL uptake occurs

via scavenger cells and cholesterol esters are deposited in macrophages. Over time this

leads to the formation of foam cells, xanthomas, and atherosclerotic plaques.

The LDL receptor density on the hepatocyte cell membranes is regulated by the

hepatocellular cholesterol content; low cellular cholesterol upregulates the receptor and

simultaneously activates HMG-CoA reductase, the rate-controlling enzyme in cholesterol

synthesis. Inhibition of this enzyme further increases LDL receptor density and helps

with serum cholesterol reduction (this is the mechanism of action of the most commonly

prescribed cholesterol lowering medicines—WOW! This stuff really does matter!).

Question 10

Which proteins are synth and secreted in the liver? Which types of proteins are they? What are some exceptions? Due to this protein synth, what happens in acute liver failure? Describe the process of liver protein synth. What things decrease liver protein synth? Increase?

Answer 10

Except for immunoglobulins, most circulating plasma proteins are synthesized in

the liver. They are produced by hepatocytes, except for von Willebrand factor, which is

produced by endothelial cells. Except for albumin and C-reactive protein, all proteins

secreted by the liver are glycoproteins. A list of the most important secreted hepatic

proteins is on the next page:

The half-lives of plasma proteins are highly variable and range from hours

(clotting factors) up to 20 days (albumin). Accordingly, acute liver failure is often first

manifest as a blood coagulation defect. The capacity of the liver to synthesize albumin is

high. Under normal conditions about 12 gm of albumin are produced and exported from

the liver every day. This can increase up to 4 fold in the setting of albumin losses.

Hepatocytes are constitutively protein-secreting cells. They continuously

synthesize and secrete proteins; there are no stores of newly synthesized proteins that can

be released upon demand. Thus, the hepatocellular protein secretory rate is primarily

influenced by alterations in the rate of protein synthesis (controlled at transcriptional and

translational levels). More rapidly secreted proteins (e.g. albumin) are transported from

endoplasmic reticulum for Golgi apparatus quickly. On the other hand, more slowly

secreted proteins (e.g. fibrinogen and transferrin) move through the cellular protein

synthesis and secretion machinery more slowly.

Protein synthesis in the liver decreases upon amino acid deprivation (starvation)

and decreases under the influence of glucagon. These conditions simultaneously

stimulate proteolysis and RNA breakdown, whereas RNA synthesis is inhibited.

Conversely, high amino acids concentrations, and insulin stimulate protein synthesis and

inhibit proteolysis. Thyroid and growth hormone also stimulate hepatic synthesis of

plasma proteins.

Question 11

When does the liver release amino acids? How does this occur? When is this release inhibited? Which amino acids have the highest rate of extraction? Which AAs does the liver not degrade? Where are they degraded? Describe what happens with AAs in the liver and muscle in starvation. Where does the ammonia in the portal system and liver come from? What does the liver do with it?

Answer 11

The liver acts as a metabolic buffer in the control of plasma amino acid

concentrations. In the setting of lower amino acid levels, the liver releases amino acids.

The amino acids are derived from hepatic proteolysis and this process is stimulated by glucagon, and amino acid deprivation; this same process is inhibited by plentiful amino

acids and insulin. Conversely, in the setting of the postprandial absorptive state, amino

acids are delivered to the liver in high concentrations and are efficiently extracted to be

metabolized by this magnificent organ.

The highest rate of amino acid extraction is observed for the gluconeogenic amino acids alanine, serine, and threonine. In addition, the liver degrades most amino acids.

Exceptions are the branched chain amino acids, leucine, valine, and isoleucine—these are

only used for protein synthesis in the liver and are not catabolized (these are broken down

primarily in the skeletal muscle).

In starvation, the muscle becomes a major site of amino acid release. A glucose-
alanine cycle between the liver and muscle exists in this situation; alanine released from

the muscle is taken up by the liver and used for gluconeogenesis; glucose is delivered to

the muscle and enters the glycolytic pathway. The pyruvate generated from glycolysis in

the muscle is transaminated to alanine that flows back to the liver. Look how the words

the muscle line up four times in a row—except for the fourth I didn’t plan that!

The portal venous blood contains high amounts of ammonia that is generated by

the action of intestinal bacteria on amino acids, especially glutamine. Ammonia is also

produced in the liver during the catabolism of amino acids. Ammonia is detoxified in the

liver by both the liver specific pathway of urea synthesis and by glutamine synthesis.

Question 12

What role does the liver have with hormones? Give specific examples.

Answer 12

Many peptide hormones are substantially cleared during a single liver passage.

These same hormones when present can markedly impact on the control of hepatocyte

function. Because of these interactions, hormone gradients along the liver acinus

contribute to the expression and maintenance of zonal hepatocyte heterogeneity.

On the flip side, the liver is also the site of hormone transformation from inactive

to active forms. Examples of this are vitamin D and thyroid hormone. In the liver,

vitamin D is hydroxylated in position 25 (with hydroxylation at position 1 performed in

the kidney). These hydroxylation reactions are required for the conversion of vitamin D

to its active form (1,25-dihydroxycholecalciferol). The liver is the site of 70% of total-
body T4 (prohormone) to T3 conversion via deiodination. The liver secretes T3 and this 

is responsible for most thyroid hormone effect.

The figure below tabulates the hepatic role in the processing of several hormones

and mediators.

The inability to metabolize many of these hormones/neurotransmitters may be in

large part responsible for portal hypertensive encephalopathy—cerebral dysfunction that

results as a consequence of liver disease.

Question 13

What are the two ways in which the liver can detoxify substances leading to excretion?

Answer 13

Many potentially harmful endo- and xenobiotics are hydrophobic and therefore

lipid soluble. Still these lipid soluble molecules are small and are easily filtered by the

renal glomeruli. Once in the renal tubules, lipid soluble molecules readily cross cell

membranes and are reabsorbed (so not efficiently excreted in the tinkle). These

molecules can often be taken up by the liver, for detoxification and excretion.

Detoxification consists of either (i) transformation of the molecule into a water-soluble

one that can be excreted by the kidney (via filtration or tubular secretion), or (ii)

modification and then secretion into bile that is able to accommodate more hydrophobic

molecules.

Question 14

Describe phase I and phase II reactions. How do these occur with acetaminophen?

Answer 14

There are two reaction types that are most important. They are simple names

phase I reactions and would you believe phase II reactions. Phase I reactions are

mediated by a family of oxidative enzymes collectively referred to as the cytochrome P-

450 system. Phase I reactions can both toxify or detoxify a drug or other substance. As a

rule, phase I reactions are rate limiting steps in the metabolism of a drug. They are often

affected by other drugs, age, and genetic variations. Phase II reactions are conjugation

reactions through which a metabolite or xenobiotic is conjugated to a solubilizing

molecule such as glucoronide, glutathione, or a sulfate (just to name a few of the more

important conjugates). Conjugation reactions are among the most powerful reactions

known in biology. Phase II reactions are fast, well preserved (even in liver failure), and

provided that enough conjugate is around work every time.

An example of such molecular processing is the role of the liver in “clearance” (or

“riddance”) of a favorite student drug, acetaminophen (Tylenol by trademark). Most

acetaminophen is conjugated by a phase II reaction to glucuronic acid or sulfate—the

resulting conjugate is excreted in the urine. The remainder goes through a phase I reaction mediated mostly by cytochrome P-450 2E1; the result is a toxic molecule

(NAPQI). Usually, NAPQI is quickly detoxified by conjugation (phase II again) to

glutathione (labeled GSH in the diagram below).

Question 15

What are the components of bile?

Answer 15

Bile is among the most important of the liver’s products. It’s a golden-green fluid

that is produced in the liver and transported into the intestine; in health, it resembles fresh

motor oil. The principal contents of bile are bile acids, phospholipids (including

lecithin), cholesterol, and bile pigments. Bile also contains water, electrolytes (especially

the cation sodium, and the anions chloride and bicarbonate), some proteins and some

metabolites.

Question 16

Describe how the structure of bile acids leads to their function. What is their life cycle like?

Answer 16

Bile acids are the major organic constituents of bile and account for 50 percent of

the solid components of bile. Let’s keep things simple, bile acids are carboxylic acids

with a cyclopentanoperhydrophenathrene nucleus and a branched side chain of three to

nine carbon atoms that ends in a carboxyl group. Now, I know what you thinking – HO

HUM! Bernstein, stop being so chemically simplistic! But forgive me as I state more of

the obvious about this chemistry. This structure makes bile acids amphipathic in the

sense that they have a hydrophobic nucleus and a hydrophilic end. The chemical

properties of bile are further defined by their concentration in bile. At low concentration,

bile acids do not interact with one another. But if you crowd bile acids together and

achieve a critical micellar concentration, the hydrophobic regions interact with one

another and the hydrophilic regions interact with one another to form micelles. Micelles

can transport water insoluble fats in the hydrophobic core and can remain water soluble

because of a hydophilic shell. An example of a micelle is shown on the top of the next

page.

Bile acids are the metabolites of cholesterol. Primary bile acids (cholic and

chendeoxycholic acids) are synthesized exclusively in the liver and undergo

enterohepatic circulation that will be described below—briefly though most bile acids in

the intestine are reabsorbed in the terminal ileum, transported back to the liver via and

portal circulation, and recycled. Primary bile acids are metabolized by intestinal bacteria

to form secondary bile acids (deoxycholic acid and lithocholic acid). Bile acids are

conjugated in the liver (forming bile salts) to decrease toxicity and to facilitate excretion.

Question 17

Describe phospholipids and their interaction with bile acids.

Answer 17

Phospholipids are the second most abundant organic compounds in bile.

Phospholipids are also amphipathic with a hydrophobic fatty acid chain and a hydrophilic

phosphatidylcholine grouping; alone they are not water-soluble. In an aqueous

environment, phospholipids will form crystals. Fortunately, bile salts are potent

solubizers of phospholipids. Micelles then form that contain phospholipids and bile acids; this combination is remarkably capable of solubilizing other lipids and especially

cholesterol.

Question 18

Describe the free cholesterol in bile. How can it lead to cholesterol crystals or gall stones?

Answer 18

A relatively small amount of free cholesterol is present in bile; it accounts for

only four percent of the total solids in bile. The presence of cholesterol in bile is

important for two reasons. First, bile cholesterol can be excreted and therefore it

regulates body stores of cholesterol. In bile, most cholesterol is nonesterified and is

therefore water insoluble. In the presence of appropriate concentrations of bile salts and

phospholipids, cholesterol is kept safe in the interior or soluble micelles. In setting of

excess bile cholesterol relative to concentration of bile salts and phospholipids,

cholesterol crystals and gallstones can form.

Question 19

Describe bilirubin in bile. Describe its complete life cycle.

Answer 19

Bilirubin is the last of the important organic solids in bile. Bilirubin is the most

significant of a group of bile pigments. These pigments account for only 2 percent of the

solids in bile. They are chemically related to porphyrins like hemoglobin, from which

they are derived. Bilirubin alone is insoluble in water; fortunately, in the body bilirubin

is almost always conjugated to glucoronic acid and thereby rendered soluble. Bilirubin

and the other bile pigments are powerfully colored; in fact, stercobilinogen is responsible

to the brown color of feces (a.k.a. stool, doodie, or ?*!^). Bilirubin metabolism is

illustrated in the diagram below.

Hemoglobin—>bilirubin—>into liver, glucoronidated—>into intestine—->can be excreted, converted to bilirubin again, converted to urobilinogen, to kidney, excreted.

Question 20

Describe the life cycle of bile (synth, secretion, transport, modifications at the different steps of transport,

Answer 20

To review biliary functional anatomy, bile starts off as the secretion of

hepatocytes into microscopically small bile canalicula (see page 3). Canalicular bile is

the primary secretion product of the hepatocytes. Bile’s ionic composition is modified by

the epithelial cells (cholangiocytes) in the bile ductules.

The bile flows into progressively larger ducts and then into two ducts, simply

termed the right and left hepatic ducts (the name of the duct correlates with the particular

hepatic lobe from which it drains). Along the way, the bile is further modified by the

secretory and absorptive functions of the cholangiocytes.

On the canalicular level, osmotic filtration is the major determinant of bile

formation. In this regard, osmotically active solutes are transported from the hepatocytes

to the biliary canalicula and are concentrated there by energy dependent transport processes. Some of filtration is bile salt dependent and some functions independently of

the transport of bile salts. Water then flows into the canalicula to dissipate osmotic

gradients and this promotes forward flow of the fluid. These general principles are

shown in the diagrams below.

Question 21

Describe the extrahepatic flow of bile including the function of the gallbladder. How is this process regulated?

Answer 21

Returning to a more macroscopic level, the right and left hepatic ducts merge

outside the liver to form a common hepatic duct. The common duct, termed the common

hepatic duct proximal to the cystic duct takeoff and termed the common bile duct below

that landmark drains into the intestine through a muscular sphincter, the sphincter of Oddi

(named after that dog that Garfield always picks on). If the sphincter is closed, some bile

will be stored and concentrated in the gallbladder. It is a vestigial organ from our

caveman days when we didn’t eat but every three or four days. Then we would kill a

wooly mammoth or the like and gorge ourselves 15,000 calories at a time…much of it

pure fat. Fortunately, we stored enough bile stored in the gallbladder to help with fat

emulsification. Now that we have Schnuck’s, we really don’t need gallbladders; they’re

probably more trouble than they’re worth. Discharge of bile from the gallbladder into the

duodenum by a short “endocrine reflex”; it is mediated by cholecystokinin (CCK).

Released by secretory cells of the duodenum, in response to fat, CCK causes contraction

of gallbladder smooth muscle.

Question 22

What is the function of bile?

Answer 22

The function of bile resides not only in the excretion of xenobiotics and

endobiotics (such as aged proteins, bilirubin, cholesterol, and porphyrins) but also serves

digestive functions. This digestive function largely is based upon the detergent effect of

bile to emulsify/solubilize fats, and providing the ingredients to for micelles for fat

absorption.

Question 23

Describe enterohepatic circulation as it pertains to bile. What happens when it’s not functioning as it should?

Answer 23

Once bile is excreted into the intestine it performs many of its digestive functions.

About 95% of the bile salts are then reabsorbed in the ileum and returned to the

hepatocytes via the portal circulation. The bile acid pool recirculates 5 to 15 times per

day (twice a meal). In the setting of ileal dysfunction, diarrhea can result from the irritant

activity bile acids delivered into the colon. Alternatively, reduced reabsorption of bile

acids by the ileum can deplete bile stores and cause fat malabsorption. The following

figure illustrated the enterohepatic circulation.