1.12 Bile, Pancreatic Juice and Secretin Hormone | Digestive System Of Man | Biology 12

This section details how hormones released by the duodenum, specifically secretin and cholecystokinin (CCK), regulate the release of digestive secretions from the liver, gallbladder, and pancreas in response to the presence of food.

Regulation of Bile Secretion

Bile, produced by the liver and stored in the gallbladder, is crucial for fat digestion. Its release is controlled by two main hormones.

Cholecystokinin (CCK)
- Stimulus: The presence of fatty acids in the chyme entering the duodenum stimulates endocrine cells in the duodenal wall to release CCK.
- Action: CCK travels through the bloodstream and stimulates the smooth muscles of the gallbladder to contract.
- Result: The contraction forces bile out of the gallbladder, through the bile duct, and into the duodenum to emulsify fats.
Secretin
- Stimulus: The presence of acidic chyme (low pH) from the stomach in the duodenum stimulates other endocrine cells to release secretin.
- Action: Secretin travels via the circulatory system to the liver.
- Result: It stimulates the liver to produce and secrete bicarbonate ions into the bile, making the bile more alkaline. This helps neutralize the stomach acid in the duodenum.

Regulation of Pancreatic Juice Secretion

Pancreatic juice contains digestive enzymes and bicarbonate. Its secretion is also regulated by secretin and CCK. The pancreas functions as both an endocrine and exocrine gland.

Pancreatic Juice Components:
- Aqueous Bicarbonate Component: Secreted by the duct cells of the pancreas. Its primary role is to neutralize stomach acid.
- Enzymatic Component: Secreted by the acinar cells of the pancreas. Contains enzymes that digest proteins, carbohydrates, and fats.
Secretin
- Stimulus: Acidic chyme in the duodenum.
- Action: The predominant effect of secretin on the pancreas is to stimulate the duct cells to secrete large amounts of water and bicarbonate.
- Result: This alkaline fluid neutralizes the acidic chyme in the duodenum, creating an optimal pH for pancreatic enzymes to function. It also helps flush the enzymes from the pancreas into the small intestine.
Cholecystokinin (CCK)
- Stimulus: The presence of proteins and fats in the chyme.
- Action: CCK stimulates the acinar cells to release their digestive enzymes into the pancreatic duct.

Diagram: Action of Enzymes

The following diagram shows the overall flow of enzymatic action in the gastrointestinal tract (GIT).

Action of enzymes in GIT — Figure 1.16: Flow chart showing action of enzymes in GIT from mouth to small intestine

Possible Questions/Answers

Q: What is the primary stimulus for the release of Cholecystokinin (CCK)? A: The presence of fatty acids and proteins in the chyme within the duodenum.
Q: What are the two main effects of the hormone secretin? A: 1. It stimulates the liver to release bicarbonate into the bile. 2. It stimulates the pancreatic duct cells to secrete a watery fluid rich in bicarbonate.
Q: Why is the secretion of bicarbonate from the pancreas and liver important? A: Bicarbonate is an alkaline substance that neutralizes the acidic chyme coming from the stomach. This protects the intestinal wall from acid damage and creates the optimal pH environment required for pancreatic and intestinal digestive enzymes to function effectively.

Summary

The presence of chyme in the duodenum triggers the release of two key hormones: secretin and cholecystokinin (CCK).
Acidic chyme is the stimulus for secretin release. Secretin's main job is to cause the secretion of bicarbonate from both the pancreas and the liver.
Fats and proteins in the chyme are the stimulus for CCK release. CCK's main jobs are to cause the gallbladder to contract (releasing bile) and stimulate the pancreas to release digestive enzymes.
This hormonal feedback system ensures that digestive juices are secreted only when food is present, optimizing the efficiency of digestion and nutrient absorption.

Biological Significance: This process is a classic example of endocrine regulation in the digestive system. It demonstrates how the body uses chemical messengers (hormones) to coordinate the functions of different organs (stomach, duodenum, liver, pancreas) to achieve a complex physiological task-the complete digestion of food.

Regulation of Bile Secretion

Bile, produced by the liver and stored in the gallbladder, is crucial for fat digestion. Its release is controlled by two main hormones.

Cholecystokinin (CCK)
- Stimulus: The presence of fatty acids in the chyme entering the duodenum stimulates endocrine cells in the duodenal wall to release CCK.
- Action: CCK travels through the bloodstream and stimulates the smooth muscles of the gallbladder to contract.
- Result: The contraction forces bile out of the gallbladder, through the bile duct, and into the duodenum to emulsify fats.
Secretin
- Stimulus: The presence of acidic chyme (low pH) from the stomach in the duodenum stimulates other endocrine cells to release secretin.
- Action: Secretin travels via the circulatory system to the liver.
- Result: It stimulates the liver to produce and secrete bicarbonate ions into the bile, making the bile more alkaline. This helps neutralize the stomach acid in the duodenum.

Regulation of Pancreatic Juice Secretion

Pancreatic juice contains digestive enzymes and bicarbonate. Its secretion is also regulated by secretin and CCK. The pancreas functions as both an endocrine and exocrine gland.

Pancreatic Juice Components:
- Aqueous Bicarbonate Component: Secreted by the duct cells of the pancreas. Its primary role is to neutralize stomach acid.
- Enzymatic Component: Secreted by the acinar cells of the pancreas. Contains enzymes that digest proteins, carbohydrates, and fats.
Secretin
- Stimulus: Acidic chyme in the duodenum.
- Action: The predominant effect of secretin on the pancreas is to stimulate the duct cells to secrete large amounts of water and bicarbonate.
- Result: This alkaline fluid neutralizes the acidic chyme in the duodenum, creating an optimal pH for pancreatic enzymes to function. It also helps flush the enzymes from the pancreas into the small intestine.
Cholecystokinin (CCK)
- Stimulus: The presence of proteins and fats in the chyme.
- Action: CCK stimulates the acinar cells to release their digestive enzymes into the pancreatic duct.

Diagram: Action of Enzymes

The following diagram shows the overall flow of enzymatic action in the gastrointestinal tract (GIT).

Possible Questions/Answers

Q: What is the primary stimulus for the release of Cholecystokinin (CCK)? A: The presence of fatty acids and proteins in the chyme within the duodenum.
Q: What are the two main effects of the hormone secretin? A: 1. It stimulates the liver to release bicarbonate into the bile. 2. It stimulates the pancreatic duct cells to secrete a watery fluid rich in bicarbonate.
Q: Why is the secretion of bicarbonate from the pancreas and liver important? A: Bicarbonate is an alkaline substance that neutralizes the acidic chyme coming from the stomach. This protects the intestinal wall from acid damage and creates the optimal pH environment required for pancreatic and intestinal digestive enzymes to function effectively.

Summary

The presence of chyme in the duodenum triggers the release of two key hormones: secretin and cholecystokinin (CCK).
Acidic chyme is the stimulus for secretin release. Secretin's main job is to cause the secretion of bicarbonate from both the pancreas and the liver.
Fats and proteins in the chyme are the stimulus for CCK release. CCK's main jobs are to cause the gallbladder to contract (releasing bile) and stimulate the pancreas to release digestive enzymes.
This hormonal feedback system ensures that digestive juices are secreted only when food is present, optimizing the efficiency of digestion and nutrient absorption.