Digestive System Lecture Notes

Overview

• Today's lesson focuses on the digestive system.
• Previous knowledge from earlier years will be expanded with more details.
• The digestive system is a long tube that starts from the mouth and ends at the rectum.

Key Organs in Digestion

Introduction of New Organs

• Liver and Pancreas are introduced as important organs.
• Both organs are not part of the elementary canal but secrete juices to aid in digestion.

Functions of the Liver

• Bile: A yellow-greenish liquid secreted by the liver.
  • Function: Emulsifies fats (breaks down fats into smaller droplets).
  • Process:
    • Bile is stored in the gallbladder before being secreted into the small intestine.
    • Emulsification increases the surface area for fat digestion by an enzyme called lipase.

Functions of the Pancreas

• Pancreatic Juices: Contains enzymes such as amylase and trypsin.
  • Amylase: Breaks down starch into maltose.
  • Trypsin: Breaks down proteins into polypeptides.
  • Both enzymes are secreted into the small intestine for digestion.

Digestion Process in the Elementary Canal

Entry of Food

1. Mouth:

   • Food is chewed (physical digestion).
   • Salivary amylase starts breaking down starch into maltose.
   • Resulting mixture is called a bolus.

2. Esophagus:

   • Muscular tube that moves bolus to the stomach through peristalsis (wave-like muscle contractions).

Stomach Digestion

• Gastric Juice: Composed of hydrochloric acid and digestive enzymes.

  • Physical Digestion: Churns food into a liquid mixture called chyme.
  • Chemical Digestion:
    • Pepsin (a protease) breaks down proteins into polypeptides.
    • Hydrochloric acid helps kill bacteria and provides an acidic environment for pepsin to function.

• Digestion time in the stomach takes around 2 to 6 hours.

Small Intestine

• Majority of digestion occurs here due to:

  • Bile from the liver.
  • Pancreatic juices from the pancreas.
  • Intestinal juices from the small intestine itself.

• Both physical and chemical digestion occurs:

  • Emulsification by bile breaks down fat into fat globules, increasing surface area for lipase action.
  • Enzymes (amylase, protease, etc.) break down carbohydrates, proteins, and fats into simpler molecules (glucose, amino acids, glycerol, and fatty acids).

Absorption

• Small molecules are absorbed into the bloodstream in the small intestine.
• Undigested food moves to the large intestine.

Large Intestine

• Absorbs excess water and mineral salts from undigested food.
• Remaining material is formed into feces and stored before being eliminated from the body.