The Digestive System

Overview

• Purpose: Take in, break down nutrients (physically & chemically) for absorption & energy.
• Components: GI tract (mouth to anus) & accessory organs/glands (fluids into the GI tract).

Digestive Tract Anatomy

• GI Tract: Oral cavity, pharynx, esophagus, stomach, small & large intestines (sigmoid colon, rectum, anus).
• Accessory Organs/Glands: Tongue, teeth, salivary glands, liver, gallbladder, pancreas.

Process Overview

• Breaking Down Nutrients:
  • Proteins: Into amino acids.
  • Carbohydrates: Into monosaccharides.
  • Fats: Into fatty acids.
• Absorption: Different for proteins, carbs, and fats.
• Elimination: Non-absorbed materials.

Functions of the Digestive System

1. Ingestion: Taking food into the mouth.
2. Mastication: Chewing; physical breakdown by teeth.
3. Propulsion:
   • Swallowing.
   • Peristalsis: Moving material through GI tract using muscle contractions.
   • Mixing: Combining digestive enzymes & secretions with food.
4. Digestion:
   • Chemical: Enzymes & stomach acid break down food.
   • Mechanical: Chewing, stomach churning.
5. Secretion:
   • Mucus: Lubrication & protection from acids/enzymes.
   • Enzymes: Specific for proteins, carbs, fats.
6. Absorption: Nutrients into cells, then bloodstream/lymphatics for distribution.
7. Elimination: Removing undigested material & waste.

Peristalsis Explained

• Mechanism: Relaxation of muscle in front, contraction behind bolus to push food forward.
• Continuous Process: Moves food along GI tract.
• Esophagus: Moves food to stomach in about 10 seconds.

GI Tract Histology

• Four Layers:
  1. Mucosa: Innermost, epithelial cells, lamina propria, some smooth muscle.
  2. Submucosa: Connective tissue, nerves, blood/lymphatic vessels, glands.
  3. Muscularis: Circular & longitudinal muscle layers, myenteric plexus (nerves).
  4. Serosa (adventitia): Connective tissue, outermost layer.

Regulation of Digestive System

• Nervous Regulation:
  • Enteric Nervous System: Coordinates peristalsis & local reflexes.
  • Parasympathetic: Increases secretion & motility.
  • Sympathetic: Inhibits contraction & secretion.
• Chemical Regulation: Neurotransmitters (e.g., serotonin), hormones (e.g., gastrin).

Mesenteries & Peritoneum

• Mesenteries: Double layer of connective tissue, supports organs.
• Peritoneum: Visceral (covers organs) & parietal (lines abdominal cavity).

Oral Cavity

• Functions: Initial digestion (physical & chemical), chewing, saliva production.
• Salivary Glands: Parotid, submandibular, sublingual. Produce saliva with enzymes like amylase & lipase.

Swallowing Process

1. Voluntary Phase: Moving bolus by tongue into pharynx.
2. Pharyngeal Phase: Reflexive, coordinated by medulla, ensures food enters esophagus.
3. Esophageal Phase: Peristalsis moves food to stomach.