1.11 Structure and Function of Pancreas

The pancreas is a complex glandular organ that performs both endocrine and exocrine functions. As an exocrine gland, it secretes pancreatic juice containing digestive enzymes and bicarbonate into the small intestine. As an endocrine gland, it secretes hormones (insulin and glucagon) into the blood.

Definition

Pancreas: The pancreas is a soft, elongated, lobulated gland located behind the stomach and within the curve of the duodenum. It is composed of both endocrine tissue (islets of Langerhans) and exocrine tissue (acinar cells). The exocrine portion secretes pancreatic juice, which contains digestive enzymes and bicarbonate.

Pancreatic Juice: Pancreatic juice is the exocrine secretion of the pancreas. It is slightly alkaline (pH ~8) and contains an aqueous component (bicarbonate) and an enzymatic component (digestive enzymes).

Main Idea

• Neutralizes acidic chyme — through bicarbonate
• Digests all major classes of food — through enzymes (proteins, carbohydrates, fats, nucleic acids)

The pancreas is a mixed gland because it has both endocrine and exocrine parts.

Core Concept

• Pancreas = both endocrine and exocrine gland
• Exocrine part = acinar cells → pancreatic juice
• Endocrine part = islets of Langerhans → insulin and glucagon
• Pancreatic juice = alkaline (pH 8); contains bicarbonate + enzymes
• Enzymes = trypsin, chymotrypsin, carboxypeptidase, amylase, lipase, nucleases
• Enzymes secreted in inactive forms = trypsinogen, chymotrypsinogen, procarboxypeptidase
• Activation = enterokinase activates trypsinogen to trypsin
• Bicarbonate = neutralizes acidic chyme from stomach

Detailed Explanation

Structure of the Pancreas

• Located behind the stomach.
• Within the curvature of the duodenum.
• Extends to the spleen.

Parts of the Pancreas:

• The pancreatic duct runs through the pancreas.
• It joins the common bile duct.
• It empties into the duodenum at the ampulla of Vater.
• Pancreatic juice flows through this duct into the duodenum.

Pancreas as an Exocrine Gland

The exocrine secretion of the pancreas is called pancreatic juice. It has two major components:

• Aqueous component — bicarbonate
• Enzymatic component — digestive enzymes

• Bicarbonate neutralizes the acidic chyme that enters the small intestine from the stomach.
• It raises the pH to about 8, which is optimal for pancreatic enzymes.
• It also protects the intestinal lining from acid damage.

Enzymatic Component: The enzymatic component of pancreatic juice is important for the digestion of all major classes of food.

Enzyme Precursors (Zymogens)

• Trypsinogen — inactive form of trypsin
• Chymotrypsinogen — inactive form of chymotrypsin
• Procarboxypeptidase — inactive form of carboxypeptidase

• If these were produced in their active forms, they would digest the tissues producing them.
• Secretion in inactive form protects the pancreas from self-digestion.

Activation of Enzymes:

• Trypsinogen is activated by the proteolytic enzyme enterokinase (secreted by intestinal glands) into trypsin.
• Trypsin then activates more trypsinogen.
• Trypsin also activates chymotrypsinogen and procarboxypeptidase.
• This cascade ensures that enzymes are activated only after they reach the small intestine.

Functions of Pancreatic Enzymes

• Trypsin — digests proteins into polypeptides
• Chymotrypsin — digests proteins into polypeptides
• Carboxypeptidase — digests polypeptides into amino acids

• Pancreatic amylase — continues the polysaccharide digestion that was initiated in the oral cavity
• Digests starch into maltose and glucose

• Pancreatic lipase — breaks down lipids into free fatty acids, glycerides, and cholesterol
• It is the principal enzyme for fat digestion

• Deoxyribonuclease — reduces DNA to its component nucleotides
• Ribonuclease — reduces RNA to its component nucleotides

Pancreas as an Endocrine Gland

The endocrine part of the pancreas consists of pancreatic islets (islets of Langerhans).

• Scattered throughout the pancreas.
• Contain different types of cells:

• Secreted by beta cells.
• Lowers blood glucose.
• Promotes glucose uptake by cells.
• Promotes glycogenesis.

• Secreted by alpha cells.
• Raises blood glucose.
• Promotes glycogenolysis.
• Promotes gluconeogenesis.

Intermediate Level

Exocrine vs Endocrine Pancreas

Pancreatic Juice

Activation Cascade

Regulation of Pancreatic Secretion

• Released by duodenal endocrine cells.
• Stimulates duct cells to secrete water and bicarbonate.
• Flushes enzymes out of the pancreas.

• Released by duodenal endocrine cells.
• Stimulates acinar cells to secrete enzymes.
• Stimulates gallbladder contraction.

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Pancreas is both an endocrine and exocrine gland.
• 2. Exocrine part = acinar cells → pancreatic juice.
• 3. Endocrine part = islets of Langerhans → insulin and glucagon.
• 4. Pancreatic juice is alkaline (pH 8).
• 5. Bicarbonate neutralizes acidic chyme.
• 6. Pancreatic juice contains enzymes for all major nutrients.
• 7. Proteolytic enzymes are secreted in inactive forms.
• 8. Trypsinogen is activated by enterokinase.
• 9. Trypsin activates more trypsinogen and other zymogens.
• 10. Pancreatic amylase digests starch.
• 11. Pancreatic lipase digests fats.
• 12. Nucleases digest DNA and RNA.
• 13. Secretin stimulates bicarbonate secretion.
• 14. CCK stimulates enzyme secretion.
• 15. Pancreas has head, neck, body, and tail.

Remember

• - Pancreas = both endocrine and exocrine
• - Exocrine = acinar cells → pancreatic juice
• - Endocrine = islets of Langerhans → insulin, glucagon
• - Pancreatic juice = alkaline (pH 8)
• - Bicarbonate = neutralizes acidic chyme
• - Enzymes = trypsin, chymotrypsin, carboxypeptidase, amylase, lipase, nucleases
• - Zymogens = trypsinogen, chymotrypsinogen, procarboxypeptidase
• - Enterokinase = activates trypsinogen to trypsin
• - Trypsin = activates more trypsinogen and other zymogens
• - Secretin = stimulates bicarbonate secretion
• - CCK = stimulates enzyme secretion
• - Parts = head, neck, body, tail

Common Mistakes

• 1. Thinking the pancreas is only an endocrine gland — It is both endocrine and exocrine.
• 2. Believing pancreatic enzymes are secreted in active form — They are secreted as inactive zymogens.
• 3. Confusing enterokinase and trypsin — Enterokinase activates trypsinogen; trypsin activates more trypsinogen and other zymogens.
• 4. Mixing up secretin and CCK — Secretin stimulates bicarbonate; CCK stimulates enzymes.
• 5. Thinking pancreatic juice is acidic — Pancreatic juice is alkaline (pH 8).
• 6. Forgetting the role of bicarbonate — Bicarbonate neutralizes acidic chyme.
• 7. Confusing alpha and beta cells — Alpha cells secrete glucagon; beta cells secrete insulin.
• 8. Believing the pancreas only digests proteins — It digests all major nutrients.
• 9. Mixing up the parts of the pancreas — Head, neck, body, tail.
• 10. Thinking pancreatic enzymes work in the stomach — They work in the small intestine.

Quick Revision

The pancreas is both an endocrine and exocrine gland. The exocrine part (acinar cells) secretes pancreatic juice, which is alkaline (pH 8) and contains bicarbonate and digestive enzymes. Bicarbonate neutralizes acidic chyme from the stomach. Enzymes digest all major nutrients: trypsin, chymotrypsin, and carboxypeptidase digest proteins; pancreatic amylase digests starch; pancreatic lipase digests fats; nucleases digest DNA and RNA. Proteolytic enzymes are secreted as inactive zymogens (trypsinogen, chymotrypsinogen, procarboxypeptidase) to prevent self-digestion. Trypsinogen is activated by enterokinase to trypsin; trypsin then activates more trypsinogen and other zymogens. Secretin stimulates bicarbonate secretion; CCK stimulates enzyme secretion. The pancreas has a head, neck, body, and tail.

60-Second Revision

• Pancreas = both endocrine and exocrine
• Exocrine = acinar cells → pancreatic juice
• Endocrine = islets of Langerhans → insulin, glucagon
• Pancreatic juice = alkaline (pH 8); bicarbonate + enzymes
• Enzymes = trypsin, chymotrypsin, carboxypeptidase, amylase, lipase, nucleases
• Zymogens = trypsinogen, chymotrypsinogen, procarboxypeptidase
• Enterokinase = activates trypsinogen to trypsin
• Trypsin = activates more trypsinogen and other zymogens
• Secretin = bicarbonate secretion
• CCK = enzyme secretion
• Warning: Enzymes are secreted in inactive form to prevent self-digestion

NEXT TOPIC

• Chapter:
• 1. Digestive System of Man

• Topic:
• 1.12 RELATION OF SECRETION OF BILE AND PANCREATIC JUICE WITH THE SECRETION OF HORMONE

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