Unit 1 Lecture 5 - Pancreas

Structure of the Pancreas

• The pancreas is a retroperitoneal organ, positioned posterior to the parietal peritoneum.

• It is located deep to the greater curvature of the stomach.

• The pancreas has a tadpole shape:

  • Tail: The left end of the gland.

  • Body: The middle region.

  • Head: The right end, encircled by the C-shaped duodenum and extending downward to form the uncinate process.

Functions of the Pancreas

• The pancreas serves two primary functions:

  • Exocrine Function: Secretion of pancreatic juice.

  • Endocrine Function: Production of hormones such as insulin and glucagon.

Exocrine Function

• Pancreatic juice is secreted into the duodenum via pancreatic ducts for digestion.

• Composition of Pancreatic Juice:

  • Contains various types of enzymes necessary for digestion of all food categories (carbohydrates, proteins, lipids).

  • Produced by clusters of secretory cells known as acini which contain zymogen granules.

  • Zymogen Granules: Store inactive digestive enzymes that get activated in the duodenum.

Digestive Enzymes in Pancreatic Juice

• Enzymes released:

  • Proteases (Inactive forms):

    • Trypsinogen: Activated to trypsin by enteropeptidase.

    • Chymotrypsinogen: Activated to chymotrypsin by trypsin.

    • Pro-carboxypeptidase: Activated to carboxypeptidase by trypsin.

  • Other Active Enzymes:

    • Amylase: Digests starches (carbohydrates).

    • Lipase: Breaks down lipids.

    • Nuclease: Digests nucleic acids.

• Summary of enzyme activation:

  • Active enzymes released directly from the pancreas (e.g., amylase, lipase, nuclease).

  • All inactive forms of proteases are activated in the duodenum:

    • $ext{Trypsinogen} <br>ightarrow ext{Trypsin}$

    • $ext{Chymotrypsinogen} <br>ightarrow ext{Chymotrypsin}$

    • $ext{Pro-carboxypeptidase} <br>ightarrow ext{Carboxypeptidase}$

Importance of Bicarbonate in Pancreatic Juice

• Watery solution: Contains bicarbonate ions produced by duct cells to neutralize acidic chyme from the stomach.

• Bicarbonate provides optimal conditions for pancreatic enzymes, working best at a neutral pH.

Endocrine Function

• Pancreatic islets (Islets of Langerhans) produce hormones:

  • Insulin: Lowers blood glucose.

  • Glucagon: Raises blood glucose.

• Hormones are transported through the bloodstream to their target sites.

Regulation of Pancreatic Secretion

• Regulation occurs via two mechanisms:

  • **Hormonal Regulation:

  1. Secretin:** Released in response to acidic chyme; stimulates bicarbonate-rich pancreatic juice.

  • Cholecystokinin (CCK): Released in response to fatty or protein-rich chyme; stimulates enzyme-rich pancreatic juice.

• Vagal Stimulation: Also triggers release of pancreatic juice similar to bile secretion regulation.

• Neural regulation involves activation of the vagus nerve during cephalic and gastric phases, leading to secretion of pancreatic juice.

• Upon entry of chyme into the duodenum:

  • Enteroendocrine cells release secretin for bicarbonate and cholecystokinin for enzymes into the bloodstream to act on the pancreas.

Key Points on Hormonal Actions

• Secretin

  • Secreted in response to acidic chyme.

  • Stimulates bicarbonate secretion from pancreatic duct cells.

• Cholecystokinin (CCK)

  • Stimulates enzyme-rich juice secretion from acini.

  • Induces gallbladder contraction and bile secretion.

Summary of Digestive Enzymes in the Digestive System

1. Salivary Glands

   • Secrete Amylase to digest carbohydrates.

2. Stomach

   • Secretes Pepsinogen (activated to pepsin) to digest proteins.

3. Pancreas

   • Releases both active and inactive forms of enzymes for digestion functions:

     • Proteins: Digested by proteases (trypsin, chymotrypsin, carboxypeptidase).

     • Carbohydrates: Digested by amylase.

     • Lipids: Digested by lipases.

     • Nucleic Acids: Digested by nucleases.

Table Reference

• Refer to Table 23.1 in the textbook for a detailed summary of the production sites, stimuli, target organs, and activities of hormones involved in digestion, such as CCK and secretin.

Common Misconceptions about CCK

• True Statements about CCK:

  • Allows entry of bile into the duodenum by relaxing the pancreatic duodenal sphincter.

  • Secretion is stimulated by fatty chyme.

  • Enhances secretion of pancreatic enzyme-rich juice.

  • Promotes gallbladder contraction.

• Incorrect statement: CCK is released from the pancreas.

  • Correct Information: CCK is released from the duodenum, not the pancreas.

Structure of the Pancreas

• The pancreas is an organ hidden behind your stomach.

• It has a funny tadpole shape:

  • Tail: The skinny left end.

  • Body: The middle section.

  • Head: The wide right end that fits into the curve of your small intestine (the duodenum).

Functions of the Pancreas

• The pancreas has two main jobs:

  • Exocrine Job: Making "digestive juice" to break down the food you eat.

  • Endocrine Job: Making "hormones" like insulin to control your blood sugar.

The Digestive Job (Exocrine)

• It sends juices into your small intestine to help you digest bread, meat, and fats.

• What is in this juice?

  • It has enzymes, which are like tiny chemical scissors that cut food into pieces your body can use.

  • It has a liquid that acts like baking soda (bicarbonate). This cancels out the strong acid from your stomach so it doesn't burn your intestines.

The Food-Breakers (Enzymes)

• Proteases (Protein Breakers): These start out "asleep" so they don't digest the pancreas itself! They "wake up" only when they reach the intestine.

  • $\text{Trypsinogen (Asleep)} \rightarrow \text{Trypsin (Awake)}$

  • $\text{Chymotrypsinogen (Asleep)} \rightarrow \text{Chymotrypsin (Awake)}$

• Amylase: This breaks down starches like potatoes or pasta.

• Lipase: This breaks down fats like butter or oil.

The Sugar Job (Endocrine)

• Special tiny spots called "islets" make signals that travel in your blood:

  • Insulin: This lowers your sugar when it's too high (like after eating candy).

  • Glucagon: This raises your sugar when it's too low (like when you are hungry).

How the Pancreas Knows to Work

• Your body uses chemical messengers to tell the pancreas to start:

  • Secretin: This tells the pancreas to send out the "baking soda" juice when it feels stomach acid.

  • CCK (Cholecystokinin): This tells the pancreas to send out the "food-breaking" juice when you eat fatty or protein-rich food.

• The Brain Signal: Your brain even sends a signal through the vagus nerve to tell the pancreas to get ready as soon as you smell or taste food!

Structure of the Pancreas

• The pancreas is a long, flat gland tucked behind your stomach (this position is called retroperitoneal).

• It is shaped a bit like a tadpole:

  • Head: The wide part that sits in the curve of the small intestine (the duodenum).

  • Body: The middle section.

  • Tail: The thin end pointing toward your left side.

The Two Main Jobs of the Pancreas

• The pancreas is unique because it performs two very different tasks:

  • Exocrine Job (Digestion): Making juices that help break down the food you eat.

  • Endocrine Job (Blood Sugar): Making hormones that control how your body uses sugar for energy.

Job 1: Helping with Digestion

• The pancreas makes "pancreatic juice" and sends it into the duodenum to help with the final stages of digestion.

• Bicarbonate: This juice contains a chemical called bicarbonate (similar to baking soda). Since stomach acid is very strong, bicarbonate neutralizes it so the acid doesn't burn your intestines.

• Digestive Enzymes: These are like chemical tools that "cut up" food into tiny pieces your body can absorb:

  • Amylase: Digests starches (like bread and pasta).

  • Lipase: Breaks down fats (like butter and oils).

  • Proteases (Protein Cutters): These start out "asleep" (inactive) so they don't accidentally digest the pancreas itself! They only wake up when they reach the intestine.

    • $\text{Trypsinogen (Asleep)} \rightarrow \text{Trypsin (Awake)}$

    • $\text{Chymotrypsinogen (Asleep)} \rightarrow \text{Chymotrypsin (Awake)}$

Job 2: Managing Blood Sugar

• Deep inside the pancreas are tiny "islands" of special cells called islets. They release hormones directly into your bloodstream:

  • Insulin: Acts like a key to let sugar into your cells, which lowers blood sugar levels.

  • Glucagon: Signals the body to release stored sugar, which raises blood sugar levels when you are hungry.

How the Pancreas Knows When to Work

• Your body uses signals to tell the pancreas when to start making juice:

  1. Secretin: This hormone is like an "acid alarm." It tells the pancreas to send out the bicarbonate juice to handle stomach acid.

  2. CCK (Cholecystokinin): This is like a "heavy meal alarm." It tells the pancreas to release plenty of enzymes when you eat fats or proteins.

  3. The Vagus Nerve: Your brain actually starts the process. When you smell or taste food, the vagus nerve tells the pancreas to get ready for a meal.