Accessory Organs of Digestions
High-Yield Summary
- Pancreas, liver, gallbladder: food never passes through them directly, but they secrete enzymes/bile into the tract.
- Pancreas exocrine function (acinar cells): bicarbonate (neutralizes acid) + digestive enzymes for all 3 macronutrients (amylase, lipase, proteases). Proteases secreted as zymogens, activated in the duodenum (enteropeptidase → trypsin → cascade).
- Secretin (triggered by acid) → pancreatic bicarbonate. CCK (triggered by fats/amino acids) → pancreatic enzymes + gallbladder contraction.
- Liver: produces bile continuously (not an enzyme — emulsifies fat), processes bilirubin (disrupted → jaundice), regulates blood glucose (glycogenesis/glycogenolysis/gluconeogenesis), synthesizes plasma proteins (albumin, clotting factors), converts ammonia → urea, detoxifies.
- Gallbladder stores/concentrates bile — does NOT make bile or produce enzymes. CCK triggers contraction/release in response to dietary fat.
- All 3 macronutrients finish digestion in the small intestine, but start in different places (carbs: mouth; proteins: stomach; lipids: small intestine, most dependent on accessory organs).
Key Terms
- Acinar cells
- Pancreatic exocrine cells that produce pancreatic juice (bicarbonate + digestive enzymes).
- Zymogens
- Inactive enzyme precursors (e.g., trypsinogen) that prevent an organ from digesting itself.
- Bile salts
- Components of bile that emulsify fat — mechanically break large fat droplets into smaller ones, not via enzymatic bond-breaking.
- Bilirubin
- Waste product from RBC breakdown, processed by the liver and excreted in bile; buildup causes jaundice.
- Urea
- Non-toxic molecule the liver converts ammonia into, for safe excretion by the kidneys.
Liver vs. Gallbladder (Bile)
| Liver | Gallbladder |
|---|---|
| Produces bile continuously | Stores and concentrates bile (does NOT make it) |
| Processes bilirubin, regulates glucose, synthesizes plasma proteins, detoxifies | Releases bile into duodenum when CCK signals contraction (triggered by dietary fat) |
| Central metabolic/detox hub | Purely a reservoir — no enzyme production |
Secretin vs. CCK (Pancreatic Regulation)
| Secretin | CCK |
|---|---|
| Triggered by acidic chyme | Triggered by fats/amino acids |
| Stimulates pancreatic bicarbonate secretion | Stimulates pancreatic enzyme secretion + gallbladder contraction |
Common MCAT Trap
- Bile is NOT an enzyme — it never breaks chemical bonds. It only emulsifies fat (mechanical action), increasing surface area for pancreatic lipase.
- The gallbladder does not PRODUCE bile — only the liver does. The gallbladder is a storage/concentration reservoir only.
- Pancreatic proteases are secreted as zymogens and activated by enteropeptidase (a brush-border enzyme) — NOT by the pancreas itself. This spatial separation prevents pancreatic self-digestion.
Quick Recall
What are the 2 major components of pancreatic juice?
Does the gallbladder produce bile?
What causes jaundice?
How does bile aid fat digestion if it's not an enzyme?