CHAPTER 25 – LIVER FUNCTION

INTRODUCTION

♦Liver

Functions Metabolism of carbohydrates, lipids,

proteins, and bilirubin Detoxification of harmful substances Storage of essential compounds Excretion of substances to prevent

harm Can regenerate cells that have been

destroyed by some short-term injury or disease or have been removed

If the liver becomes nonfunctional, death will occur within approximately 24 hours due to hypoglycemia

ANATOMY

Gross Anatomy

Reddish brown in color Largest organ inside the body Divided into two lobes

Right side is usually 6 times larger Divided by falciform ligament

Weights approximately 1.2 to 1.5 kg Held in place by ligamentous attachments Around 1500 mL of blood circulates in the

liver per minute Highly vascularized

Has two blood supplies

♦Hepatic Artery

Branch of the aorta Supplies oxygen-rich blood from the heart to

the liver About 25% of total blood supply

♦Portal Vein

Supplies nutrient-rich blood from the digestive tract

About 75% of total blood supply

*Note

The two blood supplies eventually merge into the hepatic sinusoid, which is lined with hepatocytes capable of removing potentially toxic substances from the blood

The liver is drained by a collecting system of veins that empties into the hepatic veins and ultimately into the inferior vena cava

♦Excretory system

Excretory system of the liver begins at the bile canaliculi

Small spaces between hepatocytes Forms the intrahepatic ducts where

excretory products of the cell can drain

Intrahepatic ducts Join to form right and left hepatic

ducts which drains the secretion from the liver

Common hepatic duct Formed by the right and left hepatic

ducts

Eventually joined with the cystic duct of the gallbladder to form the common bile duct

Combined digestive secretions are then expelled into the duodenum

Microscopic Anatomy

♦Lobules

Functional units of the liver Responsible for all the metabolic and

excretory functions Roughly six-sided structure with a centrally

located vein (central vein) with portal triads at each of the corners

Each portal triad contains a hepatic artery, a portal vein, and a bile duct surrounded by connective tissue

♦Two major cell types

◘Hepatocytes

Making up approximately 80% of the volume of the organ

Radiate outward from the central vein in plates to the periphery of the lobule

Perform major functions associated with the liver and are responsible for the regenerative properties of the liver

◘Kupffer cells

Macrophages that line the sinusoids of the liver

Act as active phagocytes capable of engulfing bacteria, debris, toxins, and other substances

3 Systems of the Liver

♦Hepatocyte System

Involves metabolic reactions Aerobic and anaerobic glucose

metabolism Glycogenesis Glycogenolysis Gluconeogenesis Lipoprotein synthesis Hormone production Xenobiotic metabolism (drug)

Macromolecular synthesis

♦Biliary System

Excretory Metabolism of bilirubin and bile salts

♦Reticuloendothelial System

Immune system and production of heme and globin metabolites

BIOCHEMICAL FUNCTIONS

Excretory and Secretory

One of the most important functions of the liver is processing and excretion of endogenous and exogenous substances into the bile or urine such as the major heme waste product, bilirubin

Only organ that can rid the body of heme waste products

♦Bile

Made up of bile acids or salts, bile pigments, cholesterol, and other substances extracted from the blood

Body produces approximately 3 L of bile per day and excretes 1 L

*NOTES:

Primary bile acids – cholic acid, chenodeoxycholic acid

Excretion is done via carrier mediated active transport

♦Bilirubin

Principal pigment in bile Derived from breakdown of RBCs

NOTES:

Porphyrin is reduced by heme oxygenase Biliverdin is reduced by biliverdin reductase B1

Prehepatic Hemobilirubin Indirect Prebilirubin

B2 Direct Hepatic Bilirubindiglucoronide Conjugated cholebilirubin

Step by step process

After 126 days, RBCs are phagocytized and hemoglobin is released

Hemoglobin is degraded into heme, globin, and iron

Iron is bound to transferrin and is stored in the liver or bone marrow for reuse

Globin is degraded into its constituent amino acids which are reused

Heme is converted to bilirubin in 2 to 3 hours

Bilirubin is bound by albumin and transported to the liver

This form is called unconjugated or indirect bilirubin

Insoluble in water and cannot be removed from the body until it is conjugated by the liver

Unconjugated bilirubin flows into the sinusoidal spaces and is released from albumin so it can be picked up by a carrier protein called ligandin

Ligandin Located in hepatocyte Responsible for transporting

unconjugated bilirubin to the endoplasmic reticulum (where it may be rapidly conjugated)

Conjugation (esterification) of bilirubin occurs in the presence of the enzyme uridyldiphosphate glucuronyl transferase (UDPGT), which transfers a glucuronic acid molecule to each of the two propionic acid side chains of bilirubin to form bilirubin diglucuronide (conjugated bilirubin)

This form of bilirubin is water soluble Able to be secreted from the

hepatocyte into the bile canaliculi Once in the hepatic duct, it combines with

secretions from the gallbladder through the cystic duct and is expelled through the common bile duct to the intestines

Intestinal bacteria work on conjugated bilirubin to produce mesobilirubin, which is reduced to form mesobilirubinogen and then urobilinogen (colorless)

Most of the urobilinogen formed (about 80%) is oxidized to an orange-colored product called urobilin (stercobilin) and is excreted in the feces

Urobilin gives the stool its brown color

The majority of the remaining 20% will be absorbed by extrahepatic circulation to be recycled

The other part will enter systemic circulation and be excreted in the urine

Approximately 200 to 300 mg of bilirubin is produced per day, and it takes a normally functioning liver to process the bilirubin and eliminate it from the body

The healthy adult has very low levels of total bilirubin (0.2 to 1.0 mg/dL)

Metabolism

♦Carbohydrates

Metabolism of carbohydrates is one of the most important functions of the liver

Liver can do three things to carbohydrates Use the glucose for its own cellular

energy requirements Circulate the glucose for use at the

peripheral tissues Store glucose as glycogen

♦Lipids

Metabolized in the liver under normal circumstances when nutrition is adequate and the demand for glucose is being met

Approximately 70% of the daily production of cholesterol (roughly 1.5 to 2.0 g)

♦Proteins

Almost all proteins are synthesized by the liver except for the immunoglobulins and adult hemoglobin

One of the most important proteins synthesized in the liver is albumin

Synthesis of positive and negative acute-phase reactants and coagulation proteins

♦Vitamins

Fat soluble A, D, E, K

Water soluble B12

Detoxification and Drug Metabolism

Every substance that is absorbed in the gastrointestinal tract must first pass through the liver (referred to as first pass)

The liver can serve as a barrier to prevent toxic or harmful substances from reaching systemic circulation

It may either bind the material reversibly so as to inactivate the compound or it may chemically modify the compound so it can be excreted

The drug-metabolizing system of the liver is responsible for the detoxification of many drugs through oxidation, reduction, hydrolysis, hydroxylation, carboxylation, and demethylation

Storage

Glycogen Lipids Amino acids and proteins

LIVER FUNCTION ALTERATIONS DURING DISEASE

Drug and Alcohol-Related Disorders

♦Drugs

Liver is a primary target organ for adverse drug reactions because it plays a central role in drug metabolism

Drugs can cause liver damage by a variety of mechanisms, but the most common is via an immune-mediated injury to the hepatocytes

Drug induces an adverse immune response directed against the liver itself and results in hepatic and/or cholestatic disease

Other drugs Some antibiotics Antineoplastic agents Tranquilizers Lipid-lowering medication Anti-inflammatory drugs

♦Alcohol

Ethanol In very small amounts, causes very

mild, transient, and unnoticed injury to the liver

Can lead to alcoholic cirrhosis Alcohol-induced liver injury may be

categorized into three stages Alcoholic fatty liver Alcoholic hepatitis Alcoholic cirrhosis

*Alcoholic fatty liver

Represents the mildest category where very few changes in liver function are measurable

Characterized by slight elevations in aspartate aminotransferase (AST), alanine

aminotransferase (ALT), and Gamma glutamyltransferase (GGT)

Tends to affect young to middle-aged people with history of moderate alcohol consumption

*Alcoholic hepatitis

Fever, ascites, proximal muscle loss Moderately elevated AST, ALT, GGT, and

alkaline phosphatase (ALP) Bilirubin greater than 5 mg/dL

*Alcoholic cirrhosis

Dependent on the nature and severity of associated conditions such as a gastrointestinal bleeding or ascites

Increased liver function tests, decreased albumin, and prolonged prothrombin time

Liver biopsy is the only method by which a definitive diagnosis may be made

The risk of development of cirrhosis increases proportionally with the consumption of more than 30 g of alcohol per day

Amount of alcohol and effects >1000 mg/L – under influence 3000 mg/L – CNS impairment/coma 4000 mg/L – death

Reye Syndrome

Group of disorders caused by infectious, metabolic, toxic, or drug-induced disease found almost exclusively in children

Idiopathic Often preceded by a viral syndrome such as

varicella, gastroenteritis, or an upper respiratory tract infection

Strong epidemiologic association between ingestion of aspirin during a viral syndrome and subsequent development of the disease

Acute illness characterized by non-inflammatory encephalopathy and fatty degeneration of the liver, profuse vomiting, and varying degrees of neurological impairment

Degeneration of the liver is characterized by a mild hyperbilirubinemia and threefold

increases in ammonia and aminotransferases

Tumors

Primary liver cancer Begins in liver cells

Metastatic cancer When tumors from other parts of the

body spread to the liver Much more common Colon, lung, and breast cancer

commonly spread to the liver Previous infection with hepatitis Tumors of the liver may also be classified

as benign or malignant

♦Benign

Hepatocellular adenoma A condition occurring almost

exclusively in females of child-bearing age

Hemangiomas Masses of blood vessels with no

known etiology

♦Malignant

Hepatocellular carcinoma Also known as hepatocarcinoma and

hepatoma Most common

Bile duct carcinoma Hepatoblastoma

Uncommon hepatic malignancy of children

Hepatitis

Injury to the liver characterized by the presence of inflammation in the liver tissue

Infectious causes Bacterial Viral Parasitic infections

Non-infectious causes Radiation Drugs Chemicals Autoimmune diseases Toxins

Viral infections account for the majority of hepatitis cases

Infections with these viruses can lead to the onset of acute disease with symptoms, including jaundice, dark urine, fatigue, nausea, vomiting, and abdominal pain

♦Types

A RNA virus (picorna virus) Fecal-oral route

B Hepadna virus (DNA)

C Flavivirus

D Delta agent Needs coinfection with heap B

surface antigen E

Hepevirus Fecal-oral route

F Toga virus

G G-B virus

Transfusion-transmitted virus SEN virus Alcoholic Hepatitis Drug

Ketoconazole Methyl dopa Nitrofurantoine Isoniazid

Cirrhosis

Comes from the Greek word for yellow Irreversible scarring of liver 2 cardinal features

Fibrosis – prevents regeneration

Nodules – macronodules or micronodules

Scar tissue replaces normal, healthy liver tissue

Blocks the flow of blood through the organ Signs and symptoms

Fatigue Nausea Unintended weight loss Jaundice Bleeding from the gastrointestinal

tract Intense itching Swelling in legs and abdomen

May lead to portal hypertension portal vein is blocked

Splenomegaly Esophageal varices Fatal hemorrhage

Synthetic function is affected Hypoalbuminemia Low clotting factors

Ascitic fluid accumulates in abdomen Autoantibodies that lead to cirrhosis

Anti-P62 Anti-centromere

Jaundice or Icterus

French Jaune which means yellow

Yellowish discoloration of the skin and sclera

Hyperbilirubinemia

>1 mg/dL up to 2-3 mg/dL Associated with kernicterus

Classifications of Jaundice

Pre-hepatic Hemolytic hyperbilirubinemia

Hepatic Hepatocellular hyperbilirubinemia

Post-hepatic Obstructive hyperbilirubinemia

♦Pre-hepatic

Occurs when the problem causing jaundice occurs prior to liver metabolism

Most commonly caused by an increased amount of bilirubin being presented to the liver

RBC destruction Excessive amount of bilirubin is presented

to the liver for metabolism Characterized by unconjugated

hyperbilirubinemia Malaria and hemolytic anemia Unconjugated bilirubin is not seen in urine

because it is not water soluble and bound to albumin

♦Hepatic

Occurs when the primary problem resides in the liver

Can be due to disorders of bilirubin metabolism and transport defects or due to diseases resulting in hepatocellular injury or destruction

Due to impaired cellular uptake, defective conjugation or abnormal secretion of bilirubin by the liver cell

Increased B1 and B2 Viral hepatitis, liver cirrhosis, Fasciola

hepatica

*Gilbert Syndrome

Gilbert-Muelengracht syndrome Benign autosomal recessive hereditary

disorder Results from a genetic mutation in the gene

UGT1A1 Impaired cellular uptake of bilirubin

Asymptomatic; only mild icterus Increased B1 Characterized by intermittent unconjugated

hyperbilirubinemia

*Crigler-Najjar Syndrome

Syndrome of chronic non-hemolytic unconjugated hyperbilirubinemia

Deficiency of the enzyme UDPGT Type I

Complete absence of enzymatic bilirubin conjugation

No B2 is formed Bile is colorless

Type II Mutation causing a severe

deficiency of the enzyme responsible for bilirubin conjugation

*Lucey Driscoll

Increased B1 (defect in UDPGT, congenital, maternal steroids)

*Dubin-Johnson Syndrome

Deficiency of the canalicular multidrug resistance/multispecific organic anionic transporter protein (MDR2/cMOAT)

Liver’s ability to uptake and conjugate bilirubin is functional; however the removal of conjugated bilirubin from the liver cell and the excretion into the bile are defective

Obstructive in nature Delta bilirubin – conjugated bilirubin bound

to albumin

*Rotor Syndrome

Defect causing RS is not known Idiopathic Conjugated hyperbilirubinemia due to

defective excretion

♦Post-hepatic

Impaired excretion of bilirubin caused by mechanical obstruction of the flow of bile into the intestines

Increased B1 and B2 Stool loses its source of normal

pigmentation – clay colored Gallstones (cholecystolithiasis), stones in

the common bile duct (choledocholithiasis), constriction of bile ducts/strictures, parasitism

Results from biliary obstructive disease

LABORATORY TESTS

Direct Measurement of Natural Color

Used in the development of the Icterus Index

♦Icterus Index

Muelengracht Method Dilute serum with NSS and compare

with 0/01% potassium dichromate Newberger Method

Sodium citrate is used

♦Number of times the serum is diluted

Carotene, xanthophyll II, hemoglobin affects the result

Diazo-Colorimetric Procedure

♦Evelyn and Malloy (1937)

*Reagents

Diazotized sulphalinic acid (DSA) Sulphanilic acid Sodium nitrite

50% methanol Sodium acetate Ascorbic acid

♦Jendrassik and Grof (1938)

*Reagents

Diazotized sulphalinic acid (DSA) Sulphanilic acid Sodium nitrite

Caffeine sodium benzoate Sodium acetate ascorbic acid

*Advantages of JG over EM

Insensitive to sample pH changes Insensitive to a 50-fold variation in protein

concentration Adequate optical sensitivity even for low

bilirubin concentration Minimal turbidity and with constant serum

blank Not affected by hemoglobin up to 750

mg/dL

*Note

Avoid hemolyzed sample when measuring bilirubin

Specimen considerations for Bilirubin Measurement

Specimen: fasting, non-hemolyzed, non-lipemic

Source of errors: lipochrome, hemolysis, lipemia, exposure to light

Urine Urobilinogen – Ehrlich’s Method

*Reagents

p-dimethyl aminobenzaldehyde (Ehrlich’s reagent)

Ascorbic acid Reducing agent to maintain

urobilinogen in reduced state Sodium acetate

Stops the reaction and minimizes the combination of other chromogens

Product – red colored compound Ehrlich’s unit of measurement is used

1 Ehrlich unit = 1 mg urobilinogen Reference range

0.1-1.0 Ehrlich units/2 hours 0.5-4.0 Ehrlich units/day (0.868

mmol/day)

Fecal Urobilinogen (Ehrlich’s aldehyde/Watson’s Reaction)

Aqueous extract of fresh feces + Fe(OH)2 + Ehrlich’s reagent red colored compound

Fe(OH)2 reduces urobilin to urobilinogen

Reference values 75-275 Ehrlich’s unit/100g or 75-400

Ehrlich units/24 hours

*Clinical Significance of Urobilinogen

Increased Hemolytic disease Defective liver cell function

(hepatitis) Absence

Biliary obstruction

Hepatocellular disease

Test for Bile Salts

♦Hay Test

Sprinkle sulphur powder to 5 mL of urine Interpretation

Sulphur powder floats at the top – bile salts absent

Sulphur powder sinks to the bottom – bile salts present (obstructive jaundice)

Unreliable

Test for Synthetic Function

Measure the products of liver Total protein Albumin Α-globulin Prothrombin time

Prolonged – inadequate clotting factors

Albumin-Globulin ratio is also measured

Test for Excretory Function

Excretion of Foreign Dyes Measure % retention (serum

sample) Dyes are injected IV

Bromsulphalein Test Rose Bengal Test

♦Bromsulphalein Test

%retention-%excretion

BSP is toxic

♦Rose Bengal Test

Uses tagged I131

Radioactivity of the dye

Urine Bilirubin

♦Foam Test

Yellow foam indicates positive result

♦Urine color inspection

Brown to amber indicates positive result

♦Fouchet’s Test or Harrison Spot Test

Urine + BaCl2 filter Barium chloride acts as precipitant

Precipitate + FeCl3 in TCA green colored precipitate

♦Diazotization Test

Tablet test/Ictotest Strip test

Detoxification Function Test

Quick’s Test or Hippuric Acid Test Patient is given a sodium benzoate

meal 40% is excreted within 60 minutes

Sodium benzoate Benzoic Acid Conjuagted with glycine Hippuric acid in urine Titrate with standard alkali

Enzymes

AST – aspartate aminotransferase ALT – alanine aminotransferase ALP – alkaline phosphatase GGT – gamma glutamyl transferase 5’-nucleotidase LD – lactate dehydrogenase

[Flying Cockroach]