51 60. Pancreatic juice – composition and functions. Regulation of pancreatic secretion. Formation and secretion of bile. Composition and function of bile. Regulation of bile secretion. Functions of the liver.

Pancreatic Juice – Composition & Functions

Pancreatic Juice – Composition & Functions

• Volume: ~700 mL/day

• pH: 7–8 (alkaline)

Main Components:

1. Water

2. Bicarbonate ions (HCO₃⁻) – neutralize acidic chyme from the stomach

3. Digestive enzymes:

   • Proteins: trypsin, chymotrypsin, carboxypeptidase (secreted in inactive forms to prevent autodigestion)

   • Carbohydrates: pancreatic α-amylase

   • Fats: pancreatic lipase

   • Nucleic acids: ribonuclease, deoxyribonuclease

Functions:

• Neutralizes acidic chyme

• Provides digestive enzymes to break down:

  • Proteins → peptides and amino acids

  • Carbohydrates → simple sugars

  • Lipids → fatty acids and monoglycerides

  • Nucleic acids → nucleotides

phases of pancreatic secretion

1. Cephalic Phase (~10–15%)

• Triggered by: Sight, smell, taste, thought of food, chewing

• Mediated by: Vagus nerve (parasympathetic)

• Mechanism: ACh acts on M3 receptors → stimulates pancreatic acinar cells → small amount of enzyme-rich juice

• Note: This phase prepares the pancreas for digestion before food enters the stomach.

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2. Gastric Phase (~10%)

• Triggered by: Stomach distension → vagovagal reflex

• Mechanism: Gastrin (from G-cells) and vagal stimulation can cause a minor increase in enzyme secretion

• Note: Plays a supporting role in stimulating the pancreas, mainly via neural pathways.

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3. Intestinal Phase (~70–80%)

• Most important phase

• Triggered by: Acidic, fatty, or protein-rich chyme entering duodenum

• Hormonal control:

  • Secretin → stimulates bicarbonate secretion (neutralizes acid)

  • CCK → stimulates secretion of digestive enzymes (from acinar cells)

• Nervous input: Parasympathetic stimulation (via vagus) reinforces secretion

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Summary:

While the phases are similar to gastric secretion, the targets are different:

• Gastric secretion → mostly HCl, pepsinogen

• Pancreatic secretion → enzymes and bicarbonate

regulation

Main stimuli (nervous + hormonal):

1. Acetylcholine (ACh) – from vagus nerve; stimulates enzyme secretion

2. Cholecystokinin (CCK) – from duodenum in response to fats/proteins; stimulates pancreastic juice secreiton

3. Secretin – from duodenum in response to acid; stimulates bicarbonate-rich juice

bile prodiction + composition

Production

• Site: Made by hepatocytes in the liver

• Volume: ~0.8–1 L/day

• pH: 7.5–8.5

• Secretion stages:

  1. Hepatocytes produce bile (contains bile acids, cholesterol, etc.) → stored and concentrated in gallbladder

  2. On stimulation, bile is released into the duodenum via the common bile duct

Composition of Bile

• Water

• Bile salts (derived from cholesterol)

• Bile pigments (e.g., bilirubin)

• Cholesterol

• Phospholipids (e.g., lecithin)

• Electrolytes

Functions of Bile

• Emulsifies fats → increases surface area for pancreatic lipase

• Aids absorption of fatty acids, monoglycerides, cholesterol, and fat-soluble vitamins (A, D, E, K)

• Excretion route for:

  • Bilirubin (from hemoglobin breakdown)

  • Excess cholesterol, drugs, and toxins

Regulation of Bile Secretion

Nervous Control

• Parasympathetic (vagal) stimulation → increases bile production in the liver

Hormonal Control

• Secretin → stimulates bicarbonate-rich bile secretion by liver

• Cholecystokinin (CCK) → stimulates:

  • Gallbladder contraction (releases stored bile)

  • Relaxation of sphincter of Oddi (bile enters duodenum)

Functions of the Liver

• Regulates blood glucose (via glycogenesis, glycogenolysis, gluconeogenesis)

• Stores triglycerides and metabolizes fatty acids

• Synthesizes plasma proteins (e.g., albumin, clotting factors)

• Detoxifies drugs, alcohol, hormones, ammonia → urea

• Produces and excretes bile

• Synthesizes bile salts from cholesterol

• Stores vitamins (A, D, E, K, B12) and minerals (iron, copper)

• Performs phagocytosis (via Kupffer cells)

• Activates vitamin D (in cooperation with kidneys)

• Filters and stores blood (important in fetal life and adult metabolism)

R SSSS FAP (TWO STORES, TWO SYNTHESIZES)