Exocrine pancreatic secretion !
- Mechanism
- Exam priority: medium
The pancreas sends bicarbonate to the duodenum to neutralize acid, and enzymes that digest protein, fat and starch.
Updated 2026-11-01
Also called: Pancreatic juice, Pancreatic enzymes
See it in 3D: Pancreas · Pancreatic duct · Duodenum · Ampulla of Vater
What it is
The exocrine pancreas secretes ≈1–1.5 L/day of juice with two parts. Acinar cells make the digestive enzymes, many as inactive precursors (zymogens), and duct cells secrete a bicarbonate-rich fluid. CCK and acetylcholine mainly drive enzymes; secretin drives bicarbonate. Enzyme activation should happen only in the duodenum.
How it works, step by step
- Food stimulates the vagus (cephalic phase), and gastric distension adds to it: some enzymes arrive.
- Acid reaching the duodenum releases secretin; fat and amino acids release CCK.
- In the ducts, carbonic anhydrase makes HCO₃⁻; it exits into the lumen in exchange for Cl⁻ that CFTR recycles, and water follows.
- In the acini, CCK (partly through vagal reflexes) and acetylcholine raise Ca²⁺ and discharge zymogen granules.
- The alkaline juice raises duodenal pH to ≈6–7, where pancreatic enzymes work best and bile salts form micelles.
- In the duodenum, enteropeptidase cleaves trypsinogen; trypsin activates chymotrypsinogen, proelastase, procarboxypeptidases and procolipase.
- Duodenal negative feedback (the CCK-releasing peptides are degraded when trypsin is plentiful) turns secretion down as digestion ends.
Enzymes of pancreatic juice
| Enzyme | Form secreted | Substrate and products |
|---|---|---|
| Trypsin, chymotrypsin, elastase | Zymogens (trypsinogen…) | Proteins → peptides (endopeptidases) |
| Carboxypeptidases A and B | Procarboxypeptidases | C-terminal end of peptides → amino acids |
| Pancreatic amylase | Active | Starch → maltose, maltotriose and α-limit dextrins |
| Lipase (with colipase) | Active (colipase as procolipase) | Triglycerides → 2-monoglyceride + fatty acids |
| Phospholipase A2 | Prophospholipase | Phospholipids → lysophospholipids + fatty acid |
| Cholesterol esterase | Active | Cholesterol esters → free cholesterol |
Clinical notes
- Acute pancreatitis: premature trypsin activation inside the gland (gallstones and alcohol are the main causes); serum lipase > 3 times the upper limit supports the diagnosis.
- Cystic fibrosis: defective CFTR → juice poor in water and bicarbonate, thick, which plugs the ducts and destroys the gland.
- Chronic pancreatitis: exocrine insufficiency with steatorrhea and, later, diabetes from loss of islets.
- Amylase can also rise from the salivary glands (mumps); lipase is more specific to the pancreas.
Test yourself
Think of the answer, then tap the question to see it.
Why is enteropeptidase the key to all pancreatic protein digestion?
It activates trypsinogen to trypsin in the duodenum, and trypsin activates the rest of the zymogens. Without it the pancreatic proteases stay inactive.
Which hormone controls pancreatic bicarbonate, and how does bicarbonate reach the duct lumen?
Secretin. It raises cAMP in duct cells, opens CFTR, and the Cl⁻ that leaves is exchanged for HCO₃⁻; water follows by osmosis.
Why does cystic fibrosis damage the exocrine pancreas?
Without working CFTR, ducts secrete little water and bicarbonate; the juice is thick, proteins precipitate and plug the ducts, and enzymes injure the gland.
What normally stops the pancreas from digesting itself?
Proteases are secreted as zymogens, the trypsin inhibitor SPINK1 neutralizes any trypsin activated by mistake, and ductal flow washes enzymes quickly into the duodenum.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Gastrointestinal hormones
- Fat digestion and absorption
- Protein digestion and absorption
- Carbohydrate digestion and absorption
- Patterns of malabsorption
- Bile and the enterohepatic circulation
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 23.6 Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder (OpenStax (Rice University), 2022); Acute Pancreatitis (StatPearls (NCBI Bookshelf), 2024); Chronic Pancreatitis (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 65). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.