Pancreatic Secretion
1. Functional Anatomy of Pancreas
● Pancreas is a mixed gland with both exocrine and endocrine parts.
● Exocrine pancreas:
○ Composed of acini (grape-like clusters of secretory cells).
○ Each acinus drains into small ducts → interlobular ducts → main pancreatic duct →
opens into duodenum along with bile duct at ampulla of Vater.
● Endocrine pancreas:
○ Islets of Langerhans (1–2% of gland).
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○ Secretes hormones like insulin, glucagon, somatostatin, pancreatic polypeptide.
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● Location: Retroperitoneal, lies transversely behind stomach.
2. Composition and Function of Pancreatic Juice
● Daily secretion: ~1.5 L/day.
● Composition:
○ Water and electrolytes (Na⁺, K⁺, HCO₃⁻, Cl⁻).
○ Enzymes:
◆ Proteolytic: trypsinogen, chymotrypsinogen, procarboxypeptidase.
◆ Amylolytic: pancreatic amylase.
◆ Lipolytic: lipase, phospholipase, cholesterol esterase.
◆ Nucleases: ribonuclease, deoxyribonuclease.
● Functions:
○ Neutralization of gastric acid (HCO₃⁻).
○ Digestion of proteins, carbohydrates, fats, nucleic acids.
○ Provides optimum pH (7.8–8.4) for action of enzymes.
3. Mechanism of Pancreatic Secretion
● Two components:
. Acinar secretion → mainly enzymes.
. Ductal secretion → mainly bicarbonate-rich watery secretion.
● Enzyme secretion occurs by exocytosis from zymogen granules in acinar cells.
● Ductal cells secrete bicarbonate through Cl⁻/HCO₃⁻ exchanger and CFTR channel (Cl⁻
recycling).
● Secretion depends on active transport, osmotic gradient, and water movement.
4. Regulation of Pancreatic Secretion
● Nervous control:
○ Parasympathetic (vagus) → stimulates secretion (via acetylcholine).
○ Sympathetic → inhibits secretion.
● Hormonal control:
○ Secretin (from duodenum): stimulates bicarbonate secretion from ductal cells.
○ Cholecystokinin (CCK) (from duodenum/jejunum): stimulates enzyme secretion from
acinar cells.
○ Gastrin: mild stimulatory effect on enzyme secretion.
● Phases of secretion (similar to gastric secretion):
○ Cephalic phase – vagal stimulation by sight/smell of food.
○ Gastric phase – gastric distension and gastrin.
○ Intestinal phase – secretin and CCK (major phase).
5. Collection of Pancreatic Juice (Experimental Physiology)
● Pavlov’s animal experiments:
○ Cannulation of pancreatic duct to collect juice.
○ Juice analyzed for volume, composition, enzyme activity.
● In humans: In humans, a multilumen tube is passed into the intestine near the ampulla.
Balloons inflate to enlarge the area, and pancreatic juice is aspirated through the middle
lumen.
6. Applied Physiology
● Pancreatitis: Inflammation of pancreas; activation of enzymes within pancreas causes
autodigestion.
● Pancreatic insufficiency: Inadequate enzyme secretion → maldigestion, steatorrhea.
● Cystic fibrosis: Defective CFTR channel → reduced bicarbonate secretion, thick secretions,
malabsorption.
● Zollinger–Ellison syndrome: Gastrinoma causes excess acid → inactivates pancreatic
enzymes → malabsorption.
● Diabetes mellitus: Endocrine pancreatic dysfunction (insulin deficiency).
