1.12 Relation of Secretion of Bile and Pancreatic Juice With the Secretion of Hormone

The secretion of bile and pancreatic juice is regulated by hormones produced by the walls of the duodenum. The two main hormones involved are secretin and cholecystokinin (CCK). These hormones are released in response to acidic chyme, proteins, fats, and vitamins in the duodenum.

Definition

Secretin: Secretin is a hormone produced by the endocrine cells (S cells) in the walls of the duodenum. It is released in response to acidic chyme entering the duodenum. Secretin stimulates the liver and pancreas to secrete bicarbonate.

Cholecystokinin (CCK): Cholecystokinin is a hormone produced by the endocrine cells (I cells) in the walls of the duodenum. It is released in response to fatty acids and proteins in the duodenum. CCK stimulates the gallbladder to contract and release bile, and stimulates the pancreas to secrete enzymes.

Main Idea

• Secretin → liver (bicarbonate release) and pancreas (bicarbonate secretion)
• CCK → gallbladder (bile release) and pancreas (enzyme secretion)

Core Concept

• Acidic chyme in duodenum → secretin release → bicarbonate secretion
• Fatty acids in duodenum → CCK release → bile release from gallbladder
• Proteins and fats in duodenum → CCK release → pancreatic enzymes secretion
• Secretin → stimulates liver to release bicarbonate into bile
• Secretin → stimulates pancreas to secrete water and bicarbonate
• CCK → stimulates gallbladder contraction → bile release
• CCK → stimulates pancreas to secrete enzymes

Detailed Explanation

Secretion of Bile is Related to Secretin Hormone

• Acidic chyme in the lumen of the duodenum stimulates endocrine cells to release the hormone secretin.
• Secretin is produced by the duodenum.
• Secretin is carried through the circulatory system to the liver.
• Secretin stimulates the liver to release bicarbonate into the bile.

• Fatty acids in the lumen of the duodenum stimulate endocrine cells to release the hormone cholecystokinin (CCK).
• CCK stimulates contractions in the smooth muscle of the gall bladder.
• This allows bile release into the duodenum.

Summary of Bile Secretion:

Secretion of Pancreatic Juice is Related to Secretin Hormone

• Pancreatic juice secretion is regulated by the hormones secretin and cholecystokinin (CCK).
• These hormones are produced by the walls of the duodenum upon detection of acidic food, proteins, fats, and vitamins.
• Pancreatic secretion consists of:
• An aqueous bicarbonate component from the duct cells
• An enzymatic component from the acinar cells

• The predominant effect of secretin on the pancreas is to stimulate duct cells to secrete water and bicarbonate.
• As soon as this occurs, the enzymes secreted by the acinar cells are flushed out of the pancreas.
• They travel through the pancreatic duct into the duodenum.

• CCK stimulates acinar cells to secrete digestive enzymes.
• CCK also stimulates the gallbladder to contract and release bile.

Summary of Pancreatic Juice Secretion:

Hormones Regulating Bile and Pancreatic Secretion

Mechanism of Action

• Acidic chyme enters the duodenum.
• S cells in the duodenum detect the acid.
• S cells release secretin into the blood.
• Secretin travels to the pancreas and liver.
• In the pancreas, secretin binds to receptors on duct cells.
• Duct cells secrete water and bicarbonate.
• In the liver, secretin stimulates bicarbonate release into bile.
• Bicarbonate neutralizes acidic chyme.

• Fatty acids and proteins enter the duodenum.
• I cells in the duodenum detect these nutrients.
• I cells release CCK into the blood.
• CCK travels to the gallbladder and pancreas.
• In the gallbladder, CCK stimulates contraction.
• Bile is released into the duodenum.
• In the pancreas, CCK stimulates acinar cells.
• Acinar cells secrete digestive enzymes.

Intermediate Level

Comparison of Secretin and CCK

Regulation of Bile Secretion

Regulation of Pancreatic Secretion

Other Hormones Affecting Digestion

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Secretin and CCK are produced by the duodenum.
• 2. Secretin is released in response to acidic chyme.
• 3. CCK is released in response to fatty acids and proteins.
• 4. Secretin stimulates the liver to release bicarbonate into bile.
• 5. Secretin stimulates the pancreas to secrete water and bicarbonate.
• 6. CCK stimulates the gallbladder to contract and release bile.
• 7. CCK stimulates the pancreas to secrete digestive enzymes.
• 8. Bicarbonate neutralizes acidic chyme.
• 9. Bile emulsifies fats.
• 10. Pancreatic enzymes digest proteins, carbohydrates, and fats.
• 11. Secretin and CCK work together to regulate digestion.
• 12. Secretin primarily affects the aqueous component of pancreatic juice.
• 13. CCK primarily affects the enzymatic component of pancreatic juice.
• 14. Both hormones travel through the blood to their target organs.
• 15. The duodenum is the sensor for the presence of chyme.

Remember

• - Secretin = from duodenum; stimulates bicarbonate secretion
• - CCK = from duodenum; stimulates bile release and enzyme secretion
• - Acidic chyme → secretin → bicarbonate
• - Fatty acids → CCK → bile release
• - Proteins and fats → CCK → pancreatic enzymes
• - Secretin → liver → bicarbonate into bile
• - Secretin → pancreas → water and bicarbonate
• - CCK → gallbladder → contraction → bile release
• - CCK → pancreas → enzyme secretion
• - Bicarbonate = neutralizes acid
• - Bile = emulsifies fats
• - Enzymes = digest food

Common Mistakes

• 1. Confusing secretin and CCK — Secretin stimulates bicarbonate; CCK stimulates bile and enzymes.
• 2. Thinking secretin stimulates enzyme secretion — Secretin stimulates bicarbonate secretion.
• 3. Thinking CCK stimulates bicarbonate secretion — CCK stimulates enzyme secretion.
• 4. Forgetting the role of the duodenum — The duodenum is the sensor for chyme.
• 5. Mixing up the targets of secretin and CCK — Secretin targets liver and pancreas; CCK targets gallbladder and pancreas.
• 6. Believing bile contains enzymes — Bile contains no enzymes; it emulsifies fats.
• 7. Thinking pancreatic enzymes are secreted in active form — They are secreted as inactive zymogens.
• 8. Confusing the aqueous and enzymatic components — Secretin affects the aqueous component; CCK affects the enzymatic component.
• 9. Forgetting that both hormones travel through the blood — They are hormones, so they travel in the blood.
• 10. Thinking the stomach controls bile and pancreatic secretion — The duodenum controls these secretions.

Quick Revision

The secretion of bile and pancreatic juice is regulated by the hormones secretin and cholecystokinin (CCK), which are produced by the walls of the duodenum. Acidic chyme in the duodenum stimulates the release of secretin, which travels to the liver and pancreas. Secretin stimulates the liver to release bicarbonate into bile and the pancreas to secrete water and bicarbonate. Bicarbonate neutralizes acidic chyme. Fatty acids in the duodenum stimulate the release of CCK, which travels to the gallbladder and pancreas. CCK stimulates the gallbladder to contract and release bile, and the pancreas to secrete digestive enzymes. Bile emulsifies fats; enzymes digest proteins, carbohydrates, and fats. Proteins and fats in the duodenum also stimulate CCK release. Secretin and CCK work together to regulate digestion.

60-Second Revision

• Secretin = from duodenum; stimulates bicarbonate secretion
• CCK = from duodenum; stimulates bile release and enzyme secretion
• Acidic chyme → secretin → bicarbonate
• Fatty acids → CCK → bile release
• Proteins and fats → CCK → pancreatic enzymes
• Secretin → liver → bicarbonate into bile
• Secretin → pancreas → water and bicarbonate
• CCK → gallbladder → contraction → bile release
• CCK → pancreas → enzyme secretion
• Bicarbonate = neutralizes acid
• Bile = emulsifies fats
• Enzymes = digest food
• Warning: Secretin stimulates bicarbonate; CCK stimulates bile and enzymes

CHAPTER COMPLETED

• Chapter Completed:
• 1. Digestive System of Man

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• 2. Blood Circulatory System of Man

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