Liver Anatomy, Metabolism, and Liver Function Tests
Anatomy and Location
The liver is the largest internal organ and is functionally complex, resilient, and capable of regenerating cells after short-term illnesses or disease.
Located beneath the diaphragm in the upper right quadrant and protected by the rib cage.
Four lobes exist, but focus for this course is on the two main lobes: the right lobe and the left lobe.
The diaphragm sits above the liver; ribs overlap the liver and provide protection.
Blood Supply and Biliary System
Liver receives blood from two sources:
Hepatic artery: provides the oxygenated blood; supplies about 25% of the liver’s blood.
Portal vein: brings nutrient-rich blood (not primarily oxygenated).
Common bile duct: carries bile from the liver/gallbladder toward the small intestine; important for understanding jaundice types.
Liver Microstructure: Lobules and Cells
The functional units of the liver are lobules.
Blood enters lobules via branches of the portal vein and hepatic artery and flows through the sinusoids.
Blood exits the lobules via the central vein, which drains into the hepatic vein.
Lobules perform metabolic and excretory functions.
Two main liver cell types:
Hepatocytes: the primary metabolic and excretory cells.
Kupffer cells (referred to in the transcript as “comfort cells”): liver macrophages that phagocytose foreign and waste material.
Blood flow sequence in the microscopic view: hepatic artery (oxygenated) and portal vein (nutrient-rich) enter; bile duct and bile canaliculi are in the vicinity; blood flows through sinusoids to central vein; bile is formed and transported to ducts.
Bile canaliculi collect bile produced by hepatocytes and feed into the bile ducts.
Metabolic, Secretory, and Circulatory Roles
The liver is central to metabolism, secretion, digestion, detoxification, and storage.
It stores large amounts of blood and contributes to coagulation via production of certain clotting factors (hemostasis).
It participates in circulation and maintains vascular stability.
It produces bile, which is essential for digestion and emulsification of fats.
Metabolism: Carbohydrates and Storage
Carbohydrate metabolism is a major liver function:
Liver stores glucose as glycogen (glycogenesis) for energy reserves.
When needed, liver releases glucose via glycogenolysis.
In a depleted glycogen state, gluconeogenesis occurs: generating glucose from non-carbohydrate substrates.
Substrates for gluconeogenesis include pyruvate, lactate, and some amino acids.
Key glycolytic and gluconeogenic terms:
Glycogenolysis: breakdown of glycogen to glucose.
Glycogenesis: synthesis of glycogen from glucose.
Gluconeogenesis: production of glucose from non-carbohydrate substrates.
The liver also participates in hormone metabolism, including the RAS system via angiotensinogen.
Lipid Metabolism and Biosynthesis
The liver is central to lipid metabolism and biosynthesis:
It synthesizes fatty acids, cholesterol, and triglycerides.
It forms lipoproteins such as LDL and VLDL.
It can break down fatty acids to form triglycerides, phospholipids, and cholesterol.
Lipid-related products are important for hormone synthesis and membrane structure.
Bile Formation and Excretion
The liver synthesizes bile, which contains:
Bilirubin esters, bile salts, cholesterol, and phospholipids.
Bile is excreted into the common hepatic duct and stored in the gallbladder.
When needed, the gallbladder contracts to release bile back through the common bile duct into the small intestine along with pancreatic juices.
Drug Metabolism and Detoxification
A major liver function is drug metabolism: the liver acts as a gatekeeper for substances absorbed from the GI tract, regulating what enters systemic circulation.
Detoxification pathways are diverse; a key system is the cytochrome P450 family (CYP450).
The liver detoxifies many drugs and foreign substances before they reach systemic circulation.
Protein Synthesis and Storage
The liver synthesizes most body proteins, with one notable exception (the prompt invites you to recall a specific protein—if you’re unsure, email for clarification).
The liver is a key site for urea synthesis, converting ammonia (from amino acid deamination) to urea for renal excretion.
Storage functions include glycogen and certain vitamins and minerals:
Water-soluble vitamins: potential storage; fat-soluble vitamins A, D, E, K are stored to varying extents.
B vitamins: B12 mentioned as stored.
Minerals: iron and copper storage anticipated for later lectures.
Bilirubin, Heme Metabolism, and Excretion
Bilirubin metabolism overview:
Heme from hemoglobin is broken down to bilirubin.
The globin portion and iron are recycled; heme is converted to bilirubin.
Bilirubin initially binds to albumin as unconjugated (indirect) bilirubin and travels to the liver.
In the liver, bilirubin is conjugated by the enzyme glucuronosyltransferase (UGT; sometimes called glucuronotransferase in the lecture), producing conjugated (direct) bilirubin, which is water-soluble.
Conjugated bilirubin is excreted into the intestines via the bile duct and can be converted by bacteria to urobilinogen.
Some bilirubin can re-enter the bloodstream and be excreted via urine if conjugated; most bilirubin and urobilinogen are excreted in feces; some urobilinogen is excreted in urine.
Delta bilirubin: bilirubin irreversibly bound to albumin during hepatic obstruction; less common and not the main focus for most tests.
Important terminology:
Unconjugated bilirubin: water-insoluble; bound to albumin; can only be excreted after conjugation in the liver.
Conjugated bilirubin: water-soluble; excreted into the intestines.
Visual pathway of bilirubin metabolism:
Red cells are phagocytized by the reticuloendothelial system (spleen, bone marrow, liver).
Heme → bilirubin; bilirubin binds albumin → unconjugated bilirubin; liver conjugates bilirubin with the enzyme glucuronotransferase → conjugated bilirubin; conjugated bilirubin enters intestines and, with bacteria, forms urobilinogen; urobilinogen is excreted in feces or reabsorbed and excreted in urine.
Special notes:
High unconjugated bilirubin indicates impaired conjugation or excessive production (e.g., hemolysis); high conjugated bilirubin suggests biliary obstruction or hepatic excretion problems.
Delta bilirubin is a form bound to albumin and is clinically relevant in certain obstructive conditions.
Practical colorimetric detection:
Ehrlich's reaction detects urobilinogen in feces or urine by a color change.
Fecal urobilinogen may be decreased with clay-colored stools; urine and feces may show urobilinogen depending on disease state.
Urobilinogen measurement is part of liver function assessment and bilirubin interpretation.
Urobilinogen in urine and feces is discussed in relation to bilirubin breakdown and GI processing.
Immunology and Secretory Functions
The liver can secrete secretory IgA (sIgA), an important mucosal immunoglobulin.
Kupffer cells (liver macrophages) line the sinusoids and are responsible for phagocytosis within the liver.
Bilirubin Measurement and Analytical Methods
Liver function test panels (liver panel) typically include:
Bilirubin (total), alkaline phosphatase (ALP), alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma-glutamyl transferase (GGT), albumin, and total protein. Prothrombin time (PT) is often assessed to gauge coagulation.
Purpose of the panel: determine metabolic, detoxification, and excretory status; correlate specific tests with disease states.
Metabolic panel focuses on albumin as a marker of synthetic function; decreased albumin indicates diminished synthetic protein production, possibly due to liver disease or other conditions.
Prothrombin time (PT) assesses coagulation factor production; an increased PT suggests impaired liver synthesis of coagulation factors.
Bilirubin interpretation:
Total bilirubin = Direct (conjugated) bilirubin + Indirect (unconjugated)bilirubin.
Age considerations: adult bilirubin levels are typically lower than newborn bilirubin levels.
Measurement techniques for bilirubin:
Ehrlich's reaction is the basic method to measure bilirubin.
Expanded Ehrlich-based methods include Mallory–Alvin and the Gendras–Gorff variants (as noted in the transcript), with Gendras–Gorff (Gendrosgigalff) being commonly used; all are essentially improvements on Ehrlich's reaction and have different sensitivities to pH, hemoglobin, and protein content.
A slide-based method with three layers can be used in automated analyzers; sample is added to the middle layer where reagents are present; bilirubin is measured via optical reading (commonly at
).
BUBC method (unconjugated vs conjugated bilirubin): distinguishes conjugated and unconjugated bilirubin; unconjugated peak around , conjugated around .
Spectrophotometric measurements: based on Ehrlich’s reaction; used in newborn testing for total bilirubin; other substances can absorb at similar wavelengths, making it less reliable for some applications.
Enzymes as Markers of Liver Injury
The key liver enzymes assessed in a liver panel:
ALT (alanine aminotransferase): more specific to liver and often higher than AST in liver injury.
AST (aspartate aminotransferase): found in many tissues; elevation suggests liver injury but is less specific.
GGT (gamma-glutamyl transferase): elevated with liver damage, particularly alcoholic liver disease; also elevated with biliary obstruction/post-hepatic issues.
ALP (alkaline phosphatase): elevated in bone and liver diseases; helps differentiate hepatic vs bone disease when used with other tests.
LDH (lactate dehydrogenase): a general marker of tissue damage; various isoenzymes exist; a liver-specific isoenzyme exists and can point to liver involvement.
Interpreting patterns:
ALT usually higher than AST in liver injury and is more liver-specific.
AST is present in other tissues; mild elevations can occur with non-liver tissue damage.
In obstructive liver disease, ALT and AST may be low or only mildly elevated; ALP and GGT tend to rise more with biliary obstruction.
GGT elevations strongly suggest biliary or alcohol-related liver disease when accompanied by ALP elevations.
ALP elevations point to biliary tract issues or bone disease; correlation with bilirubin and imaging is needed.
LDH elevations indicate tissue damage but are not specific to the liver; liver-specific LDH isoenzyme can be informative.
Kinds of disease states discussed (in context): viral hepatitis, hepatic necrosis, hepatic ischemia, biliary obstruction, alcoholic liver disease, and other contexts that elevate these enzymes.
Acute vs. Chronic Disease Context
The lectures emphasize acute (short-term) liver diseases versus chronic conditions, with a focus on how anatomy and biochemistry relate to acute presentations (e.g., acute hepatitis, ischemia) and how they would appear on a liver panel.
Practical Implications and Clinical Reasoning
Bilirubin metabolism and excretion are central to understanding jaundice; distinguishing conjugated vs unconjugated bilirubin helps identify the underlying problem (conjugation defect vs excretion/obstruction).
Ammonia handling and hepatic encephalopathy: inability to convert ammonia to urea can lead to elevated ammonia, crossing the blood-brain barrier and causing confusion, coma, or other cognitive impairment.
Ammonia handling details:
Ammonia levels must be kept on ice when drawn because ambient exposure can alter levels.
Ammonia monitoring is clinically relevant in liver dysfunction and hepatic encephalopathy risk.
Urea cycle and liver function: the liver’s ability to convert ammonia to urea is essential for preventing neurotoxicity; impairment has serious consequences.
Summary of Key Takeaways
The liver is a multifunctional organ with critical roles in metabolism, detoxification, bile production, storage, and immune function.
Blood supply and bile drainage are organized through the hepatic artery, portal vein, sinusoids, central veins, and the biliary tree leading to the gallbladder and small intestine.
Bilirubin metabolism distinguishes unconjugated (indirect) and conjugated (direct) bilirubin; conjugation by glucuronosyltransferase enables bilirubin excretion via bile; urobilinogen is formed in the intestine and can appear in feces and urine.
Liver function tests (LFTs) include bilirubin, ALT, AST, ALP, GGT, LDH, albumin, total protein, and PT; patterns of elevation help differentiate hepatocellular injury vs cholestasis vs impaired synthetic function.
Alcohol use, biliary obstruction, and hepatocellular injury each produce characteristic enzyme patterns (e.g., GGT and ALP elevations with biliary disease; ALT > AST with hepatocellular injury).
Clinical interpretation requires looking at panels as a whole, not single markers in isolation.