Two molecules made in the liver, packaged into bile, and sent out to the gut — but for entirely different reasons.
A walk-through of how cholesterol becomes the body's most efficient fat detergent — from hepatocyte to ileum and back again.
Hepatocytes take cholesterol and convert it into primary bile acids. The liver then conjugates these acids with glycine or taurine to form bile salts — better detergents. Bile salts are secreted into the small intestine to emulsify dietary fat. In the terminal ileum, ~95% are reabsorbed and shipped back to the liver via the portal vein. The 5% that escapes into the colon gets modified by bacteria into secondary bile acids. This whole loop is called enterohepatic circulation, and the small amount lost in feces is replaced by new synthesis from cholesterol.
Synthesis happens in the smooth ER and mitochondria of hepatocytes. The pathway has many enzymatic steps, but only a few are worth memorizing as an M1.
Before secretion, bile acids are conjugated to an amino acid via an amide bond at the carboxyl group (C-24). This converts the weakly amphipathic acid into a strongly amphipathic, ionized salt — a far better detergent.
In humans, glycine conjugates outnumber taurine conjugates roughly 3:1.
Both primary bile acids get conjugated — not just cholic acid. So you actually have four conjugated primary bile acids in human bile. The same conjugation can happen to secondary bile acids after they return to the liver from the colon.
| Category | Members |
|---|---|
| Unconjugated primary made by liver from cholesterol |
Cholic acid (CA) Chenodeoxycholic acid (CDCA) |
| Conjugated primary primary + glycine or taurine |
From cholic acid: glycocholic acid, taurocholic acid From CDCA: glycochenodeoxycholic acid, taurochenodeoxycholic acid |
| Secondary (unconjugated) made by gut bacteria via 7α-dehydroxylation |
Deoxycholic acid (from cholic) Lithocholic acid (from CDCA) |
| Secondary (re-conjugated) colon → reabsorbed → liver re-conjugates |
Glycodeoxycholic, taurodeoxycholic, etc. (beyond most M1 exams) |
The unconjugated bile acid has a pKa around 6, meaning it's only partially ionized at duodenal pH. After conjugation:
At intestinal pH (~6–8), both are fully ionized — they exist as anions paired with Na⁺ or K⁺, hence the name "bile salts." Being ionized keeps them in the lumen instead of diffusing back across the gut wall too early, and makes them excellent at forming micelles.
Effective micelle formation requires strong amphipathic character at intestinal pH. That essentially means one thing: conjugated primary bile acids.
| Form | Effective at micelle formation? | Why |
|---|---|---|
| Conjugated primary | YES ✓ | Fully ionized at duodenal pH → strongly amphipathic → forms efficient micelles. This is the workhorse. |
| Unconjugated primary | Weakly | pKa ~6 → only partially ionized at intestinal pH → weaker detergent. Also tends to passively diffuse out of the lumen too early. |
| Secondary (unconjugated) | No | Lost a hydroxyl group via bacterial 7α-dehydroxylation → more lipophilic, less amphipathic. Also located in the colon, where there's no fat to absorb anyway. |
Bile salts are actively pumped from hepatocytes into the bile canaliculi by the BSEP transporter (Bile Salt Export Pump). They join cholesterol, phospholipids, and conjugated bilirubin to form bile, which travels:
Canaliculi → bile ductules → hepatic ducts → common bile duct → (stored & concentrated in gallbladder) → duodenum via sphincter of Oddi
The gallbladder concentrates bile 5–20× by reabsorbing water. After a fatty meal, CCK (released by I-cells of the duodenum) causes gallbladder contraction and sphincter of Oddi relaxation, dumping bile into the duodenum.
In the gut lumen, bile salts perform two key functions:
The ~5% of bile salts that escape ileal reabsorption reach the colon, where bacteria — particularly Clostridium species — perform two key modifications:
Some secondary bile acids are passively reabsorbed in the colon and returned to the liver, where they can be re-conjugated and re-secreted.
Synthesis is metabolically expensive, so the body recycles bile salts aggressively. Roughly 95% are reabsorbed — almost entirely at the terminal ileum — and shuttled back to the liver via the portal vein.
The body's total bile acid pool is only ~3–5 g, but each pool member recycles ~6 times per day, giving the gut roughly 20–30 g of bile salts available daily for fat absorption. New synthesis from cholesterol replaces only the small fraction lost in feces (~500 mg/day).
Bile acid synthesis is tightly regulated by feedback from bile acids themselves. When bile acids return to the liver via the portal vein, they bind a nuclear receptor called FXR (Farnesoid X Receptor) in hepatocytes. FXR activation does three important things:
Loss of ASBT-mediated reabsorption → bile salt pool depletes faster than the liver can replace it → fat malabsorption, steatorrhea, fat-soluble vitamin (A, D, E, K) deficiency. Unabsorbed bile salts irritate the colon and cause bile acid diarrhea.
Bile is supersaturated with cholesterol relative to bile salts and phospholipids. When the bile salt:cholesterol ratio falls (e.g., obesity, rapid weight loss, estrogen, Native American genetics), cholesterol precipitates out. Cholic acid and ursodeoxycholic acid can be used therapeutically to dissolve small cholesterol stones.
Common bile duct blocked (stone, tumor) → bile can't reach gut. Result: jaundice (↑ conjugated bilirubin), pale stools (no stercobilin), steatorrhea (no bile salts in gut), and pruritus from bile acid accumulation in skin.
Cholestyramine, colesevelam, colestipol bind bile salts in the gut lumen → prevent reabsorption → liver consumes more cholesterol making new bile acids → ↓ LDL. Side effects: constipation, fat-soluble vitamin malabsorption, interference with absorption of other drugs.
| Concept | The one-liner |
|---|---|
| Starting material | Cholesterol — bile acid synthesis is the main route of cholesterol excretion. |
| Rate-limiting enzyme | 7α-hydroxylase (CYP7A1) in hepatocyte smooth ER. |
| Primary bile acids | Cholic acid (trihydroxy) & chenodeoxycholic acid (dihydroxy). |
| Conjugation | With glycine or taurine → bile salts (lower pKa, better detergent). Both cholic and CDCA get conjugated → four conjugated primary bile acids total. |
| Secondary bile acids | Made by gut bacteria via 7α-dehydroxylation → deoxycholic & lithocholic. |
| What forms micelles | Conjugated primary bile acids — they're the only ones that are strongly amphipathic at intestinal pH. |
| Reabsorption site | Terminal ileum (ASBT transporter). ~95% efficiency. |
| Feedback regulator | FXR — bile salts inhibit their own synthesis. |
| Key hormone | CCK — empties the gallbladder after a fatty meal. |
| Daily loss | ~500 mg in feces, replaced by new synthesis from cholesterol. |
How the body disposes of old red blood cells — a waste-handling pathway that turns toxic, water-insoluble heme into the pigments of bile, stool, and urine.
Old red blood cells are phagocytosed by macrophages in the spleen and liver. Their heme is broken down into biliverdin, then reduced to unconjugated (indirect) bilirubin — a toxic, water-insoluble pigment. It travels through blood bound to albumin, gets taken up by hepatocytes, and is conjugated with glucuronic acid by the enzyme UGT1A1 to form conjugated (direct) bilirubin — now water-soluble. The liver secretes it into bile, which carries it to the gut. Bacteria reduce it to urobilinogen, most of which becomes stercobilin (brown stool) — with small amounts recycled to the liver or filtered to urine as urobilin (yellow).
Synthesis happens in the reticuloendothelial system (RES) — primarily splenic macrophages, but also Kupffer cells in the liver and macrophages in bone marrow. RBCs live ~120 days; when senescent, they're engulfed and broken down.
Unconjugated bilirubin is fat-soluble and water-insoluble. It cannot travel freely in plasma. Instead, it binds albumin in a 1:1 ratio and is shuttled to the liver.
Newborns have immature UGT1A1 (low activity in first week of life). Combined with high RBC turnover and breakdown of fetal hemoglobin, unconjugated bilirubin builds up. If it exceeds albumin binding capacity, free bilirubin can cross the BBB and deposit in basal ganglia → kernicterus (irreversible neurologic damage). Phototherapy converts bilirubin to a water-soluble photoisomer that can be excreted without conjugation.
At the hepatocyte sinusoidal membrane, unconjugated bilirubin dissociates from albumin and is taken up into the cell by transporters (OATPs). Inside the hepatocyte, three steps occur:
↓ TRAVELS TO LIVER BOUND TO ALBUMIN ↓
UGT1A1 catalyzes the conjugation in the hepatocyte smooth ER.
Conjugated bilirubin is secreted in bile and travels:
Canaliculi → bile ductules → common bile duct → duodenum → travels through small bowel → reaches colon
Unlike bile salts, conjugated bilirubin is not absorbed in the small intestine. It must reach the colon, where bacteria do two things:
Urobilinogen is the branch point — from here, the molecule has three fates.
| Type | Mechanism | Bilirubin Pattern |
|---|---|---|
| Prehepatic | Too much bilirubin produced (hemolysis) — overwhelms liver's conjugation capacity | ↑ Unconjugated (indirect) |
| Intrahepatic | Hepatocyte dysfunction — impaired uptake, conjugation, or secretion (hepatitis, cirrhosis) | ↑ Both, often mixed |
| Posthepatic | Obstruction of bile flow (stones, tumors) — conjugated bilirubin backs up into blood | ↑ Conjugated (direct) |
Normal in first week of life. Caused by immature UGT1A1 + ↑ RBC turnover. Unconjugated hyperbilirubinemia. Treated with phototherapy (converts bilirubin to water-soluble lumirubin). Resolves spontaneously.
Mild UGT1A1 deficiency (~30% normal activity). Mild unconjugated hyperbilirubinemia worsened by stress, fasting, illness. Benign — no treatment needed. Affects ~5% of population. Patients notice mild jaundice when sick or fasting.
Severe UGT1A1 deficiency. Type I: near-complete absence → fatal kernicterus without liver transplant. Type II: partial deficiency → responds to phenobarbital (induces residual UGT1A1 activity).
Defects in secretion of conjugated bilirubin into bile (MRP2 defect in Dubin-Johnson). Cause mild conjugated hyperbilirubinemia. Both are benign. Dubin-Johnson classically causes a black liver on biopsy (epinephrine metabolite accumulation).
Gallstone in CBD, pancreatic head tumor, cholangiocarcinoma. Conjugated bilirubin backs up into blood. Classic triad: jaundice + dark urine (conj. bilirubin filtered) + pale stools (no stercobilin in gut). Also: pruritus from bile acids and steatorrhea from no bile salts in gut.
| Concept | The one-liner |
|---|---|
| Starting material | Heme from senescent RBCs (~120 day lifespan). |
| Site of synthesis | Reticuloendothelial system — splenic macrophages mainly. |
| Rate-limiting enzyme | Heme oxygenase (releases Fe²⁺ + CO + biliverdin). |
| Plasma transport | Unconjugated bilirubin bound to albumin (not water-soluble). |
| Hepatic enzyme | UGT1A1 — adds glucuronic acid → conjugated (water-soluble). |
| Canalicular transporter | MRP2 — pumps conjugated bilirubin into bile (rate-limiting step overall). |
| Gut transformation | Bacteria → urobilinogen (colorless intermediate). |
| End product (stool) | Stercobilin → brown color. |
| End product (urine) | Urobilin → yellow color. |
| Toxic form | Unconjugated → crosses BBB → kernicterus in neonates. |