Digestion in the Small Intestine Notes

Small Intestine Digestion

Overview

• The small intestine is the primary site for the digestion of carbohydrates, lipids, and proteins.
• Food passes through the small intestine in approximately 3-5 hours.
• The small intestine has a diameter of 2.5 cm and is 7 meters long.

Sections of the Small Intestine

1. Duodenum (25-30 cm):
   • Most digestive enzymes are added here.
   • Acid is neutralized.
   • Digestion requires input from the pancreas, liver, and gallbladder.
2. Jejunum (6-7 m):
   • Digestion continues.
   • Absorption starts.
3. Ileum (1.5 m):
   • The majority of nutrients are absorbed here.

Chemical Digestion

• The pyloric sphincter regulates the movement of chyme into the small intestine.

Role of the Small Intestine

• When chyme enters the duodenum:
  • Cholecystokinin (CCK) is released (hormone).
  • Prosecretin is converted into its active form, secretin (hormone), and then released.

Cholecystokinin (CCK)

• Secreted from the duodenum lining and released into the bloodstream.
• Signals the pancreas to secrete various substances.
• Signals the stomach to slow down the speed of digestion, allowing the small intestine to effectively digest fats.

Secretin

• Secreted from the duodenum.
• Stimulates the liver to produce more bile.
• Stimulates the pancreas to release enzymes and bicarbonate ions (to protect the small intestine from stomach acids).
• Acts as a digestive rate regulator, preventing more food from entering the duodenum from the stomach until the current batch is digested.

Role of the Pancreas in Digestion

• The pancreas secretes several substances into the duodenum through the pancreatic duct:
  • Pancreatic amylase
  • Trypsinogen (inactive form) → trypsin (active form)
  • Lipase
  • Bicarbonate ions

Pancreatic Amylase

• Breaks down carbohydrates, continuing carbohydrate digestion that began in the mouth.

Trypsinogen

• An inactive form of trypsin.
• Once in the duodenum, it is converted into trypsin (active form) by the enzyme enterokinase.

Trypsin

• Breaks down proteins, continuing protein digestion that began in the stomach.

Lipase

• Breaks down lipids (fats) into fatty acid molecules.
• Fats in chyme are present as large globules. Lipases can only act on the surface of these globules, so the liver and its secretions (bile) are required to increase the surface area by emulsification.

Bicarbonate Ions

• Neutralize acidic chyme, raising the pH from approximately 2.5 to 9.0.
• Increasing the pH of chyme protects the small intestine.
• The change in pH inactivates pepsin, which requires an acidic environment to be activated.

Factors Affecting Enzyme Action

• Acidic environments stop the action of amylase but activate pepsinogen -> pepsin in the stomach.
• Pepsin is discontinued in the small intestine since it is only active in acidic conditions; protein digestion is then carried out by trypsin.

The Liver

• Produces and secretes bile.

Bile

• Emulsifies fats, breaking them into tiny droplets, giving lipases a much greater surface area on which to act, which increases the rate of lipid digestion.
• The liver also detoxifies blood (alcohol) and produces and stores glycogen and fat-soluble vitamins.

The Gall Bladder

• Stores bile.
• Lipids in the small intestine stimulate the gallbladder to contract, squeezing bile out and into the duodenum via the bile duct.

Organ Roles in Digestion/Enzymes

• Duodenum:
  • Releases CCK, signaling the pancreas to release substances and the stomach to slow down.
  • Releases Secretin, signaling the liver to make bile and the pancreas to release substances; also signals the stomach to slow down.
• Pancreas:
  • Pancreatic amylase: breaks down sugars.
  • Trypsinogen (inactive) → trypsin (active): breaks down proteins.
  • Lipase: breaks down fats.
  • Bicarbonate ions: neutralizes acid.
• Liver:
  • Produces bile, which emulsifies fats thus increase surface area for lipase action.
• Gall Bladder:
  • Stores and releases bile.

Digestive Enzymes