Digestive System
The liver is the largest internal organ (~1.5 kg), strategically located in the upper right abdomen. It’s not just a digestive gland — it’s your body’s biochemical laboratory, processing nutrients, detoxifying blood, and producing bile for fat digestion.
⚙️ 1️⃣ Structure and Blood Flow
- Dual blood supply: Hepatic artery (25%) → oxygenated blood; Portal vein (75%) → nutrient-rich blood from GIT.
- Blood passes through hepatic sinusoids, mixes, and drains into the central vein → hepatic vein → inferior vena cava.
Kupffer cells (macrophages) line the sinusoids and remove bacteria, old RBCs, and debris.
Hepatocytes are the main functional cells — they perform nearly 500 metabolic tasks!
💪 2️⃣ Major Functions of the Liver
A. Metabolic Functions
| Category | Key Functions |
|---|---|
| Carbohydrate metabolism | Glycogenesis, glycogenolysis, gluconeogenesis — maintains blood glucose. |
| Lipid metabolism | Synthesizes cholesterol, triglycerides, phospholipids; produces ketone bodies during fasting. |
| Protein metabolism | Synthesizes plasma proteins (albumin, clotting factors), deaminates amino acids, forms urea. |
Clinical note: ↓ Albumin → edema (↓ oncotic pressure); ↓ Clotting factors → prolonged bleeding (↑ PT/INR).
B. Detoxification and Storage
- Detoxifies drugs, alcohol, and toxins (via cytochrome P450 enzymes).
- Converts ammonia → urea (non-toxic).
- Stores glycogen, iron (as ferritin), copper, fat-soluble vitamins (A, D, E, K), and vitamin B₁₂.
Clinical pearl: Failure of detoxification → hepatic encephalopathy (↑ ammonia → brain dysfunction).
C. Immune and Endocrine Roles
- Kupffer cells remove microbes from portal blood.
- Inactivates hormones like estrogen, cortisol, insulin.
- Converts thyroxine (T₄) → active triiodothyronine (T₃).
💧 3️⃣ Bile Formation and Composition
The liver continuously secretes bile (≈ 600–1000 mL/day). It’s stored and concentrated in the gallbladder, then released into the duodenum during meals.
Composition of Bile
| Component | Function |
|---|---|
| Bile salts | Emulsify fats → form micelles for absorption |
| Bile pigments (bilirubin) | Waste from hemoglobin breakdown |
| Cholesterol & phospholipids | Solubilize lipids |
| Bicarbonate & water | Neutralize gastric acid |
🧠 Key bile salts
- Derived from cholesterol → cholic acid, chenodeoxycholic acid (conjugated with glycine or taurine).
- Reabsorbed in ileum and returned via enterohepatic circulation.
Clinical tip: Interruption of bile salt recycling (ileal disease or resection) → fat malabsorption and steatorrhea.
🩸 4️⃣ Bilirubin Metabolism (High-Yield)
| Step | Process | Location |
|---|---|---|
| 1️⃣ | RBC breakdown → hemoglobin → unconjugated bilirubin | Spleen |
| 2️⃣ | Transported bound to albumin | Plasma |
| 3️⃣ | Conjugation with glucuronic acid (via UDP-glucuronyl transferase) → conjugated bilirubin | Liver |
| 4️⃣ | Excreted in bile → intestine → converted by bacteria → urobilinogen → excreted in feces (as stercobilin) & urine (as urobilin) | Gut & kidneys |
Clinical pearls: Unconjugated (indirect) hyperbilirubinemia: hemolysis, neonatal jaundice. Conjugated (direct) hyperbilirubinemia: obstruction, hepatitis.
Jaundice = total bilirubin > 2 mg/dL
🧩 5️⃣ Enterohepatic Circulation
- 95% of bile salts are reabsorbed in the ileum and returned to the liver via the portal vein.
- This recycling reduces the need for continuous bile synthesis.
- Interrupted in diarrhea or ileal resection → ↓ fat absorption → steatorrhea.
⚠️ 6️⃣ Clinical Correlations
| Condition | Problem | Effect |
|---|---|---|
| Hepatitis | Inflammation of liver | ↑ AST, ALT, jaundice |
| Cirrhosis | Fibrosis replacing hepatocytes | Portal hypertension, ascites |
| Gallstones (cholelithiasis) | Crystallized cholesterol or bilirubin | Biliary colic, jaundice |
| Obstructive jaundice | Blocked bile flow | Pale stools, dark urine, pruritus |
🧠 Summary Table
| Function | Example / Significance |
|---|---|
| Metabolic | Glucose regulation, lipid & protein synthesis |
| Storage | Glycogen, vitamins, iron |
| Excretory | Bilirubin via bile |
| Synthetic | Albumin, clotting factors |
| Protective | Detoxification, phagocytosis |
| Digestive | Bile production for fat emulsification |