Digestive System Overview

Introduction

• Digestive system, also known as the gastrointestinal (GI) system.
• Consists of organs involved in digestion and accessory organs aiding the process.
• Alimentary canal organs are those food passes through.
• Accessory organs assist in digestion through secretions.

Main Organs and Functions

Oral Cavity

• Contains accessory organs like:
  • Salivary glands: Secrete saliva to form bolus.
  • Teeth and Tongue: Mix food into a bolus.
• Food is formed into a ball called bolus for swallowing.

Esophagus

• Tube (~12 inches long) transporting bolus to the stomach.

Stomach

• Mechanically digests food by churning.
• Prepares food into chyme, acidic substance for further digestion.

Small Intestine

• Divided into three parts:
  • Duodenum: First 12 inches, initial digestion.
  • Jejunum: Primary site for nutrient absorption.
  • Ileum: Final part connecting to the large intestine.

Large Intestine (Colon)

• Composed of:
  • Ascending, transverse, descending, and sigmoid colons.
  • Ends at rectum leading to the anus.
• Functions in water reabsorption and feces formation.

Accessory Organs and Their Functions

Pancreas

• Produces digestive enzymes.
• Releases enzymes into the duodenum.

Liver

• Secretes bile to emulsify fats.
• Bile stored in the gallbladder.

Gallbladder

• Concentrates and stores bile.

Oral Cavity Details

Structure of a Tooth

• Crown: Above gum line.
• Neck: At gum line.
• Root: Below gum line.
• Three mineralized structures:
  • Enamel: Hardest, 100% mineralized.
  • Dentin: Intermediate hardness, 70% mineralized.
  • Cementum: Covers root, 50% mineralized.

Salivary Glands

• Parotid Gland: Near the ear.
• Submandibular Gland: Below the mandible.
• Sublingual Gland: Under the tongue.
• Controlled by cranial nerves 7 (facial) and 9 (glossopharyngeal).

Stages of Digestion

Cephalic Phase

• Triggered by sight, smell, thought of food.
• Vagus nerve stimulates gastric acid production.

Gastric Phase

• Initiated by the presence of food in the stomach.
• Chief cells release pepsinogen; activated to pepsin by stomach acid.

Intestinal Phase

• Triggered by acidic chyme entering the duodenum.
• Duodenal cells release secretin and CCK:
  • Secretin: Inhibits further acid production, stimulates pancreas for bicarbonate.
  • CCK: Stimulates enzyme release from pancreas and bile from gallbladder.

Absorption and Enzymes

Carbohydrates

• Broken down by amylase into oligosaccharides.
• Brush border enzymes convert them into monosaccharides for absorption.

Proteins

• Pepsin initiates digestion into polypeptides.
• Proteases from the pancreas break them down further.
• Brush border enzymes finish digestion to amino acids.

Fats

• Emulsified by bile; lipase breaks them into fatty acids and glycerol.
• Absorbed into lacteals in the small intestine.

Colon Functions

• Water Reabsorption: Maintains hydration and electrolyte balance.
• Microbial Flora: Aid in digestion and vitamin synthesis, immune modulation.

Conclusion

• Understanding the digestive system includes recognizing the roles of various organs and processes from ingestion to excretion.
• Digestion involves mechanical and chemical processes supported by enzymes and regulated by hormones and nervous system interactions.