Physiology of Liver, Liver Function Tests and Pathophysiology of Jaundice

Introduction

The liver is a central metabolic organ that processes nutrients and detoxifies substances. It converts bilirubin into a conjugated form for excretion, and disturbances in its production, conjugation, or bile flow result in distinct types of jaundice.

Physiology Of Liver

• The liver is a vital organ that regulates metabolism, detoxification, and nutrient processing. It weighs approximately 1.5 kilograms in adults and is enclosed by a protective capsule.
• Liver dysfunction leads to significant systemic abnormalities.

Functional Anatomy

• A key function is bile production, essential for digestion and absorption of lipids.
• Bile is secreted continuously by hepatocytes into small ducts within the liver. These ducts merge to form the hepatic ducts and then the common hepatic duct.
• Bile is stored and concentrated in the gallbladder between meals.
• During digestion, bile is released into the duodenum through the common bile duct.
• Release of bile is stimulated by the presence of chyme in the intestine. This coordinated system ensures efficient digestion and elimination of waste products.

Blood Supply

The liver has a dual blood supply from the portal vein and hepatic artery.

Portal Vein

• The portal vein is formed by the union of the superior mesenteric and splenic veins.
• It carries nutrient-rich but relatively oxygen-poor blood from the gastrointestinal tract, spleen, and pancreas.

Hepatic Artery

The hepatic artery supplies oxygen-rich arterial blood to the liver tissue.

Portal Circulation

• Branches of the portal vein and hepatic artery enter the liver and form sinusoids.
• Blood from both sources mixes within the sinusoids, allowing efficient exchange.
• Hepatocytes absorb nutrients and oxygen, and release metabolic products into the blood.
• Sinusoids drain into the central vein of each lobule.
• Central veins merge to form hepatic veins, which empty into the inferior vena cava.

Bile Secretion by Liver Cells

Hepatocytes also produce bile, which is secreted into bile canaliculi and then into hepatic ducts.

Histology of Liver

• The liver is composed of numerous lobules, which are the structural units.
• Each lobule is typically polygonal with a central vein at its center.
• Hepatocytes are arranged in plates radiating outward from the central vein. These plates are usually one cell thick and are separated by sinusoids.
• Sinusoids facilitate exchange between blood and liver cells.
• Specialized cells within sinusoids contribute to filtration and immune functions.
• The structural organization supports efficient metabolism, detoxification, and bile formation.
• Hepatocytes possess a strong capacity for regeneration, which is important in liver injury.

Sinusoids and Bile Canaliculi

• In each liver lobule, sinusoids are vascular channels located between plates of hepatocytes.
• Each sinusoid receives mixed blood from branches of the portal vein and hepatic artery.
• Blood flows through sinusoids and ultimately drains into the central vein.
• The sinusoidal wall is primarily formed by endothelial cells, which allow exchange of substances.
• Specialized macrophages called Kupffer cells are present along the sinusoidal lining. These cells play a role in phagocytosis and immune defense.
• The space of Disse lies between hepatocytes and sinusoidal endothelium. It permits transfer of nutrients, metabolites, and plasma proteins between blood and liver cells.
• Bile canaliculi are small channels formed between adjacent hepatocytes.
• Hepatocytes synthesize bile and secrete it into these canaliculi.
• Canaliculi transport bile toward interlobular bile ducts for further drainage.

Portal triad

• The portal triad is located at the corners of liver lobules. It consists of a branch of the portal vein, a branch of the hepatic artery, and a bile ductule.
• The connective tissue region containing these structures is called the portal canal.
• The space of Mall, near the portal canal, is the initial site of lymph formation in the liver.
• Close anatomical association between sinusoids and bile canaliculi ensures efficient exchange and secretion.

Functions of Liver

• The liver is a central organ for metabolism and maintenance of internal homeostasis. It performs multiple vital roles essential for survival and physiological balance.
• Secretory function: The liver synthesizes and secretes bile, which aids in fat digestion and absorption.
• Bile also facilitates excretion of waste products such as bilirubin and excess cholesterol.
• Metabolic functions: The liver regulates metabolism of carbohydrates, lipids, and proteins.
• It maintains blood glucose by glycogenesis, glycogenolysis, and gluconeogenesis.
• It is involved in metabolism of amino acids, urea formation, iron handling, and alcohol breakdown.
• Synthetic functions: The liver synthesizes essential plasma proteins such as albumin and clotting factors.
• It also produces lipoproteins, acute phase proteins, and bile salts required for digestion.
• Storage functions: The liver stores glycogen, vitamins such as A, D, B12, and minerals like iron. These reserves are mobilized during fasting or increased physiological demand.
• Detoxification: The liver neutralizes toxins, drugs, and harmful metabolic byproducts.
• It converts lipid-soluble substances into water-soluble forms for excretion.
• Excretory function: The liver excretes bilirubin, cholesterol, and certain heavy metals through bile.
• Immune function: Kupffer cells act as macrophages that remove bacteria and debris from blood.
• Endocrine functions: The liver converts vitamin D to its active form and thyroxine (T4) to triiodothyronine (T3).
• It produces growth mediators such as insulin-like growth factor and degrades several hormones.

Liver Function Tests

• Liver function cannot be assessed by a single test because of its diverse roles.
• Therefore, a panel of liver function tests is used for diagnosis, prognosis, and monitoring therapy.
• Tests for bile formation and excretion:
  • Serum bilirubin estimation evaluates hepatic uptake, conjugation, and excretion.

• Serum enzyme assays:
  • Alkaline phosphatase and gamma glutamyl transpeptidase indicate cholestasis or biliary obstruction.
  • Aminotransferases such as aspartate aminotransferase and alanine aminotransferase reflect hepatocellular injury.
  • Other enzymes like 5 prime nucleotidase, lactate dehydrogenase, and cholinesterase provide supportive information.

• Tests of metabolic functions:
  • Serum proteins and clotting factors assess synthetic capacity of the liver.
  • Serum ammonia reflects impaired detoxification.
  • Evaluation of lipid, lipoprotein, and carbohydrate metabolism provides additional functional insight.

• Immunologic tests:
  • Detection of antibodies helps identify infectious or autoimmune liver disorders.

• Ancillary investigations:
  • Imaging such as ultrasonography and procedures like liver biopsy help confirm structural abnormalities and diagnosis.

Pathophysiology Of Jaundice

• Jaundice is characterized by yellow discoloration of the skin, sclera, and mucous membranes due to elevated bilirubin in blood.
• Bilirubin is formed from the breakdown of hemoglobin released during red cell destruction.

Red Cell Breakdown

• Red blood cells have an average lifespan of about 120 days.
• Aged or damaged red cells are removed by macrophages of the mononuclear phagocyte system. These macrophages are located mainly in the spleen, liver, bone marrow, and lymph nodes.
• Hemoglobin is degraded into heme and globin within these cells.
• Heme is further converted into bilirubin, which enters circulation for hepatic processing.
• Excessive red cell destruction increases bilirubin production and may lead to jaundice.

Table 41.1: Liver function tests and their significances.

Category	Test	Clinical Significance
Bile Formation & Excretion	Serum bilirubin (0.2–0.8 mg/dL)	Elevated in hepatocellular injury, biliary obstruction, and hemolysis.
	Urine bilirubin	Presence indicates conjugated hyperbilirubinemia, commonly in hepatitis.
	Fecal bilirubin/urobilinogen	Reduced or absent in obstructive jaundice.
	Urobilinogen	Increased in hemolysis and hepatocellular disease; absent in obstruction.
	Serum bile acids	Elevated in cholestasis and impaired bile flow.
	Bromsulphalein clearance	Detects impaired hepatic excretory function.
Enzyme Markers	Alkaline phosphatase	Markedly increased in biliary obstruction; also rises in bone disorders.
	Gamma glutamyl transpeptidase	Confirms hepatobiliary origin of elevated alkaline phosphatase.
	Aspartate aminotransferase	Increased in liver injury and also in cardiac or muscle damage.
	Alanine aminotransferase	More specific indicator of hepatocellular damage.
	5 prime nucleotidase	Elevated in liver disease and supports cholestatic origin.
	Lactate dehydrogenase	Increased in hepatic tumors or extensive tissue injury.
	Cholinesterase	Reduced in liver failure and malnutrition.
Metabolic Function Tests	Serum proteins and albumin	Low albumin indicates impaired synthetic function.
	Clotting factors	Prolonged clotting time reflects reduced protein synthesis.
	Serum ammonia	Increased in hepatic encephalopathy and severe liver failure.
	Lipid profile	Elevated in cholestasis; decreased in chronic liver disease.
	Blood glucose	Reduced in severe hepatic damage due to impaired glucose regulation.
Immunologic Tests	Autoantibodies	Help identify autoimmune liver diseases.
	Viral antibodies	Diagnose infections such as hepatitis B.
Imaging & Tissue Diagnosis	Ultrasonography	Detects structural abnormalities and bile duct obstruction.
	Liver biopsy	Confirms diagnosis in unclear or chronic liver disorders.

• A combination of tests provides a comprehensive assessment of hepatic function, injury, and etiology.

Hemoglobin Catabolism

• Hemoglobin from aged red blood cells is degraded by macrophages into heme and globin.
• Globin is broken into amino acids and reused in protein synthesis.
• Heme is converted by enzymatic oxidation to release iron, which is recycled. The remaining heme structure is converted into biliverdin.
• Biliverdin is reduced to bilirubin, which is lipid-soluble and not water-soluble.
• Unconjugated bilirubin binds to albumin in plasma for transport to the liver.

Bilirubin Metabolism

• This complex cannot be filtered by kidneys, so bilirubin is not normally present in urine.
• In hepatocytes, bilirubin metabolism occurs in three stages: uptake, conjugation, and excretion.
• Uptake: Bilirubin enters liver cells and binds to intracellular proteins, preventing its return to blood.
• Conjugation: Bilirubin combines with glucuronic acid to form bilirubin glucuronide, a water-soluble compound. This reaction occurs in the endoplasmic reticulum and requires specific transferase enzymes.
• Excretion: Conjugated bilirubin is actively secreted into bile canaliculi. This step is rate-limiting and essential for normal bilirubin clearance.
• Proper metabolism ensures elimination of bilirubin through bile and prevents its accumulation in blood.

Fate of Conjugated Bilirubin

• Conjugated bilirubin is secreted into bile and delivered to the small intestine.
• In the large intestine, intestinal bacteria convert it into stercobilinogen after removing glucuronic acid.
• About 80 percent of stercobilinogen is oxidized to stercobilin.
• Stercobilin is excreted in feces and gives stool its characteristic brown color.
• Around 20 percent of stercobilinogen is reabsorbed into the portal circulation.
• Most of this is taken up again by the liver and re-excreted into bile, forming enterohepatic circulation.
• A small fraction enters systemic circulation and is excreted by kidneys as urobilinogen.
• Urobilinogen contributes to the normal yellow color of urine after oxidation.
• Conjugated bilirubin itself is not reabsorbed, but its metabolites are partly reabsorbed.

Normal Plasma Bilirubin Level:

• Normal plasma bilirubin ranges from 0.2 to 0.8 milligrams per deciliter.
• Clinical jaundice becomes apparent when bilirubin exceeds about 2 milligrams per deciliter.

Table 41.2: Differences between conjugated and unconjugated bilirubin.

Feature	Unconjugated Bilirubin	Conjugated Bilirubin
Solubility	Lipid-soluble, not water-soluble	Water-soluble
Albumin binding	Strong binding	Weak binding
Renal excretion	Not excreted in urine	Excreted in urine
Reaction type	Indirect reaction	Direct reaction
Tissue affinity	Can cross blood–brain barrier	Does not cross barrier
Clinical note	Risk of kernicterus	Indicates hepatobiliary excretion

Types of Jaundice

• Jaundice becomes clinically evident when serum bilirubin exceeds 2 milligrams per deciliter.
• Levels between 0.8 and 2 milligrams per deciliter indicate subclinical jaundice.
• Yellow discoloration first appears in the sclera due to bilirubin affinity for elastin.
• Jaundice is broadly classified based on mechanism into increased production or decreased excretion of bilirubin.

Increased Production of Bilirubin

• Excessive red cell destruction leads to hemolytic jaundice. This condition occurs in various hemolytic disorders.
• Increased bilirubin load exceeds hepatic uptake and conjugation capacity.

Decreased Excretion of Bilirubin

• Impaired hepatic processing results in hepatic jaundice. It commonly occurs in conditions such as viral hepatitis.
• Defective conjugation or transport of bilirubin contributes to accumulation.
• Obstruction to bile flow produces obstructive jaundice.
• Common causes include bile duct stones and strictures.
• Conjugated bilirubin cannot reach the intestine and accumulates in blood.
• Hepatic and obstructive types often overlap in practice.
• In hepatic disease, intrahepatic cholestasis adds an obstructive component.
• In obstruction, prolonged bile stasis damages hepatocytes, adding a hepatocellular component.

Table 41.3: Differences between different types of jaundice.

Feature	Hemolytic	Hepatic	Obstructive
Bilirubin level	Mild rise	Moderate rise	Marked elevation
Urine bilirubin	Absent	Present	Present
Urobilinogen	Increased	Reduced	Absent
Van den Bergh reaction	Indirect	Biphasic	Direct
Liver function	Normal	Impaired	Variable

Laboratory Diagnosis of Jaundice

Hemolytic Jaundice

• Hemolytic jaundice results from excessive breakdown of red blood cells and increased bilirubin production. The liver conjugates a larger quantity of bilirubin than normal.
• More conjugated bilirubin reaches the intestine via bile.
• Intestinal bacteria convert this to increased stercobilinogen.
• Consequently, fecal stercobilinogen and urinary urobilinogen levels are elevated.
• Unconjugated bilirubin remains bound to albumin in plasma. This complex is not filtered by kidneys; therefore, urine bilirubin is absent. Hence, this condition is termed acholuric jaundice.
• Liver function tests are usually within normal limits.

Hepatic Jaundice

• Hepatic jaundice occurs due to impaired hepatocellular function.
• All stages of bilirubin handling, including uptake, conjugation, and excretion, are affected.
• The most significant defect is in excretion of conjugated bilirubin.
• Conjugated bilirubin accumulates in hepatocytes and leaks into blood.
• Both unconjugated and conjugated bilirubin levels increase in plasma.
• Conjugated bilirubin is water-soluble and appears in urine, causing dark coloration.
• Reduced bile flow leads to decreased formation of stercobilinogen. Therefore, fecal stercobilinogen and urinary urobilinogen levels are reduced.
• Liver synthetic function is impaired, resulting in decreased albumin levels.
• Plasma globulins, especially gamma globulins, are often elevated.
• The albumin to globulin ratio is therefore altered.
• In neonates, hepatic immaturity may impair bilirubin conjugation, causing neonatal jaundice.

Obstructive Jaundice

• Obstructive jaundice occurs due to blockage of bile flow into the intestine.
• Absence of bile in the intestine prevents formation of stercobilinogen.
• Stools become pale or clay-colored due to lack of stercobilin.
• Urinary urobilinogen is absent because intestinal conversion is reduced.
• Reduced bile salts impair fat digestion, leading to steatorrhea.
• Conjugated bilirubin accumulates proximal to the obstruction. It regurgitates into blood, increasing conjugated bilirubin levels.
• Being water-soluble, it is excreted in urine, producing dark yellow urine.
• Bile salts also enter circulation and may appear in urine.
• Early stages may show near-normal liver function.
• Prolonged obstruction causes hepatocellular damage and impaired liver function.

van den Bergh test:

• The Van den Bergh test distinguishes conjugated and unconjugated bilirubin. It is based on color formation with a diazo reagent.
• Immediate color change indicates a direct positive reaction, due to conjugated bilirubin.
• Delayed color after alcohol addition indicates an indirect reaction, due to unconjugated bilirubin.
• Hemolytic jaundice shows an indirect reaction.
• Obstructive jaundice shows a direct reaction.
• Hepatic jaundice often shows a biphasic response, reflecting mixed bilirubin forms.