## Hormonal Regulation of Bile and Pancreatic Juice Secretion 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). ### 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.

1.12 Relation Of Secretion Of Bile And Pancreatic Juice With The Secretin Of Hormone | Digestive System Of Man | Biology 12