Liver physiology for a dr

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

Finding Sources

Now I have enough material from multiple authoritative sources to deliver a comprehensive, clinician-oriented answer. Let me compile it.

Liver Physiology

Anatomy & Vascular Supply

The liver is the largest internal organ, weighing 1,200-1,500 g (2-5% of adult body weight). Its dual blood supply is physiologically unique:

Portal vein (~75%): venous blood from the stomach, small and large intestines, pancreas, and spleen. This makes the liver the first-pass filter for all absorbed nutrients, drugs, and toxins.
Hepatic artery (~25%): oxygenated systemic blood.

Both vessels enter at the porta hepatis and branch into the portal triads (portal venule, hepatic arteriole, bile ductule). Blood flows through sinusoids toward the central hepatic vein, then to the hepatic veins and IVC.

Medical Physiology, p. 1400

Microanatomy: The Functional Unit

Classic Lobule vs. Acinus

Classic lobule: hexagonal, central vein at center, portal triads at 6 corners.
Hepatic acinus (Rappaport): the preferred functional unit. Three zones based on distance from the portal triad:
- Zone 1 (periportal): highest O2 and nutrient delivery; most resistant to ischemia; site of gluconeogenesis, oxidative metabolism, and fatty acid oxidation.
- Zone 2: intermediate.
- Zone 3 (centrilobular/perivenous): lowest O2; most vulnerable to ischemia and toxic injury (e.g., acetaminophen, alcohol); site of glycolysis and lipogenesis.

Sinusoids and Space of Disse

Hepatocytes form a one-cell-thick epithelium separating two fluid compartments:

Sinusoidal lumen (blood-side): lined by fenestrated endothelium - no basement membrane, allowing free passage of macromolecules.
Bile canalicular lumen (apical side): ~1 μm diameter channels formed by the apical membranes of adjacent hepatocytes.
Space of Disse: the perisinusoidal extracellular space between the endothelium and hepatocyte basolateral membrane, which bears extensive microvilli accounting for ~85% of hepatocyte surface area.

The vectorial transport (basolateral uptake → transcellular processing → apical secretion into bile) underpins most hepatic secretory functions.

Medical Physiology, p. 1400-1401

Non-Parenchymal Cells

Kupffer cells: fixed macrophages lining the sinusoids; constitute 80-90% of the fixed macrophages of the entire reticuloendothelial system. They phagocytose bacteria, endotoxins, parasites, aging RBCs, and particulate matter from the portal blood.
Hepatic stellate cells (Ito cells): perisinusoidal; store vitamin A; activated to myofibroblasts in fibrosis.
Sinusoidal endothelial cells: fenestrated, no basement membrane, facilitate exchange.

Metabolic Functions

Carbohydrate Metabolism

The liver is the principal organ maintaining euglycemia:

State	Hepatic Action
Fasting	Glycogenolysis (maintains glucose for 24-36 h), then gluconeogenesis from lactate, alanine, glycerol
Postprandial	Glycogen synthesis, glycolysis, lipogenesis to clear portal glucose load

Gluconeogenic substrates arrive via the Cori cycle (lactate from muscle/RBC), glucose-alanine cycle (alanine from muscle proteolysis), and glycerol from adipose lipolysis. Hepatic glycogen stores ~70-100 g; sufficient for ~24-36 hours.

Schwartz's Surgery, p. 1379; Medical Physiology

Lipid Metabolism

Synthesizes cholesterol, fatty acids, triglycerides, phospholipids.
Packages VLDL for peripheral delivery.
Oxidizes fatty acids (preferentially in zone 1); generates ketone bodies (acetoacetate, β-hydroxybutyrate) during starvation - these supply ~50% of CNS energy needs and spare glucose.
Converts cholesterol into bile acids (primary: cholic acid, chenodeoxycholic acid).
Schwartz's Surgery, p. 1379

Protein and Amino Acid Metabolism

Synthesizes all plasma proteins except immunoglobulins and von Willebrand factor, including:
- Albumin (~10-15 g/day; half-life 15-20 days; oncotic pressure maintenance)
- Clotting factors I, II, V, VII, VIII (partially), IX, X, XI, XII - vitamin K-dependent factors (II, VII, IX, X) require hepatic γ-carboxylation
- Fibrinogen, antithrombin, protein C, protein S
- Transferrin, ceruloplasmin, haptoglobin, acute-phase reactants
Converts ammonia (from amino acid deamination and colonic bacterial metabolism) to urea via the urea cycle - critical detox function.
Synthesizes non-essential amino acids; catabolizes excess amino acids by transamination/deamination.

Clinical consequence of hepatic failure: hypoalbuminemia (edema, ascites), coagulopathy (prolonged PT/INR), hyperammonemia (encephalopathy).

Costanzo Physiology, p. 394; Schwartz's Surgery

Bilirubin Metabolism

A key function and a major clinical indicator of hepatic health:

Production: Senescent RBCs phagocytosed by the RES (spleen, liver). Hemoglobin → heme → biliverdin (green) → unconjugated bilirubin (yellow). Also from ineffective erythropoiesis and myoglobin.
Hepatic uptake: Unconjugated bilirubin (lipid-soluble, non-water-soluble) bound to albumin in plasma; dissociates at the sinusoidal membrane and enters hepatocytes via OATPs.
Conjugation: In the hepatocyte smooth ER, UDP-glucuronyl transferase conjugates bilirubin with glucuronic acid → conjugated (direct) bilirubin - now water-soluble. (Note: UDP-glucuronyl transferase matures slowly after birth → physiological neonatal jaundice.)
Biliary excretion: Conjugated bilirubin secreted into bile canaliculi via the MRP2 transporter; enters the intestine.
Intestinal processing: Bacteria in the colon deconjugate bilirubin → urobilinogen (colorless). Some urobilinogen is reabsorbed, returned to the liver (enterohepatic circulation), and a small fraction spills into urine (urobilinuria). The remainder is oxidized to stercobilin (brown - stool color) and urobilin (yellow - urine color).

Jaundice Patterns - Clinical Correlates

Type	Mechanism	Bilirubin	Urine	Stool
Pre-hepatic (hemolytic)	Excess unconjugated production	Unconjugated ↑	Urobilinogen ↑	Normal/dark
Hepatocellular	Conjugation + excretion failure	Both ↑	Dark (conjugated)	Pale
Obstructive (cholestatic)	Bile duct blocked	Conjugated ↑	Dark (conjugated)	Clay-colored (no stercobilin)

Costanzo Physiology; Schwartz's Surgery, p. 1379-1380

Bile Formation and the Enterohepatic Circulation

Bile composition: water, electrolytes, bile salts, phospholipids (lecithin), cholesterol, bilirubin. The liver produces ~1 L/day.

Bile acids:

Primary: cholic acid and chenodeoxycholic acid (CDCA) - synthesized de novo from cholesterol in hepatocytes; rate-limiting enzyme is 7α-hydroxylase (CYP7A1).
Conjugated with glycine or taurine before secretion (forming bile salts).
Intestinal bacteria dehydroxylate primary bile acids → secondary bile acids (deoxycholic acid from cholic acid; lithocholic acid from CDCA).

Functions of bile:

Emulsification and absorption of dietary fats and fat-soluble vitamins (A, D, E, K) via micelle formation.
Excretion of cholesterol and waste products (bilirubin, xenobiotics).

Enterohepatic circulation:

~90-95% of secreted bile salts are actively reabsorbed in the terminal ileum via the ASBT (apical sodium-dependent bile acid transporter).
Returned to the liver via the portal vein; re-extracted by hepatocytes via NTCP (Na+/taurocholate cotransporting polypeptide) and OATPs.
The total bile acid pool (~3.5 g) recycles ~2x per meal, ~6-10x/day.
Only 5-10% lost in stool daily, replaced by new hepatic synthesis.
Bile acid secretion is the primary driving force for bile-acid-dependent bile flow.
Schwartz's Surgery, p. 1380; Costanzo Physiology; Medical Physiology

Drug Metabolism and Detoxification

The liver processes lipophilic xenobiotics into water-soluble products for excretion via a two-phase system:

Phase I reactions (cytochrome P450 system - CYPs, mainly CYP3A4, CYP2D6, CYP2C9):

Oxidation, reduction, hydrolysis
Occur in the smooth ER (centrilobular, zone 3)
Products may be active, inactive, or toxic (e.g., NAPQI from acetaminophen)

Phase II reactions (conjugation):

Glucuronidation (UGT enzymes), sulfation, glutathione conjugation (GST), acetylation, methylation
Produce polar, water-soluble metabolites for biliary or renal excretion

Classic example - Acetaminophen (APAP) toxicity:

At therapeutic doses: ~90% goes through safe glucuronidation/sulfation.
Overdose: CYP2E1 generates excess NAPQI; depletes glutathione; NAPQI binds covalently to hepatocyte proteins → centrilobular necrosis (zone 3).
Treatment: N-acetylcysteine (NAC) replenishes glutathione.
Schwartz's Surgery, p. 1381

Endocrine/Paracrine Functions

Vitamin D activation: 25-hydroxylation of cholecalciferol → 25(OH)D (calcidiol); second hydroxylation occurs in the kidney.
Thyroid hormone conversion: Deiodination of T4 → active T3.
IGF-1 synthesis: Liver is the primary source of IGF-1 in response to GH.
Hormone inactivation: Estrogens, androgens, aldosterone, glucocorticoids - all inactivated in the liver.

Liver Function Tests - Physiology Basis

Test	Reflects
ALT, AST	Hepatocyte cytoplasmic integrity (released on cell death)
ALP, GGT	Biliary/canalicular membrane function; cholestasis
Total/direct bilirubin	Conjugation and excretion capacity
Albumin	Synthetic function (chronic; 15-20 day half-life)
PT/INR	Synthetic function, especially vitamin K-dependent factors (acute; factor VII has 6-h half-life)
Ammonia	Urea cycle and hepatic mass

Schwartz's Surgery, p. 1382

Summary: Functions at a Glance

Domain	Key Function
Vascular	First-pass extraction; filtering via Kupffer cells
Metabolic - Carbohydrate	Glycogen storage, glycogenolysis, gluconeogenesis
Metabolic - Lipid	Bile acid synthesis, VLDL, ketogenesis, cholesterol
Metabolic - Protein	Albumin, clotting factors, acute-phase proteins; urea synthesis
Excretory	Bilirubin conjugation and biliary excretion
Secretory	Bile formation (1 L/day)
Detoxification	Phase I/II drug metabolism; ammonia to urea
Endocrine	25-OH vitamin D, T4→T3, IGF-1 production
Immune	Kupffer cell phagocytosis (80-90% of fixed RES macrophages)

Sources: Costanzo Physiology 7th Ed. | Medical Physiology (Boron & Boulpaep) | Schwartz's Principles of Surgery 11th Ed. | Sleisenger & Fordtran's GI and Liver Disease

This is a shared conversation. Sign in to Orris to start your own chat.