In-Depth Notes on the Digestive System

Overview of the Digestive System

• Organs: Mouth, Esophagus, Stomach, Small Intestine, Large Intestine.
• General Functions:
  • Breakdown of food
  • Nutrient absorption
  • Elimination of waste

Digestive System Components

Accessory Digestive Organs/Structures

• Salivary Glands: Secrete saliva containing enzymes for carbohydrate breakdown.
• Liver: Produces bile for fat emulsification.
• Gallbladder: Stores bile and releases it into the small intestine.
• Pancreas: Produces pancreatic juice with enzymes and bicarbonate.

Alimentary Canal

• Mouth: Mechanical breakdown and initial carbohydrate digestion.
• Pharynx: Connects mouth with esophagus.
• Esophagus: Transports food via peristalsis to the stomach.
• Stomach: Acid and enzyme secretion; begins protein digestion.
• Small Intestine: Final enzymatic breakdown and nutrient absorption.
• Large Intestine: Absorbs water and forms feces.
• Rectum: Regulates feces elimination.
• Anus: Facilitates fecal expulsion.

Structure of the GI Tract Layers

• Layers vary based on function:
  1. Mucosa
  2. Submucosa
  3. Muscularis Externa
  4. Serosa (Visceral Peritoneum)
  • Note: Structure can differ in various GI regions (e.g., adventitia)

Digestive Processes

1. Ingestion: Intake of food.
2. Motility: Involves swallowing and peristalsis, which is a wave-like contraction that moves food through the GI tract.
3. Secretion: Release of substances such as enzymes, acids, bile, and mucus.
   • Total digestive secretions approximately 9,000 mL.
4. Digestion:
   • Mechanical Digestion: Involving chewing and segmentation (contractions push food forward then backward).
   • Chemical Digestion: Involves hydrolysis of food using digestive enzymes.
5. Absorption: Passage of nutrients, vitamins, and minerals into blood or lymph.
6. Elimination: expulsion of indigestible waste as feces.

Mouth and Salivary Glands

• Mechanics: Mechanical breakdown occurs through chewing; chemical digestion via saliva.
• Salivary Glands:
  • Types: Parotid, Sublingual, Submandibular.
  • Composition of saliva: 97% water, electrolytes, enzymes, mucin, and antimicrobial agents.

Pharynx

• Composed of 3 regions:
  • Nasopharynx
  • Oropharynx
  • Laryngopharynx

Esophagus

• Connects pharynx to stomach (about 10 inches long).
• Muscularis externa comprised of different muscle types:
  • Superior 1/3: Skeletal muscle
  • Middle: Mixed skeletal and smooth muscle
  • Inferior 1/3: Smooth muscle.
• Gastroesophageal Sphincter: Prevents acid reflux; dysfunction results in heartburn.

Stomach Functions

• Storage: Holds food.
• Mixing & Digestion: Breaks down food into chyme; absorbs some substances.
• Muscularis Externa Layers:
  1. Longitudinal layer
  2. Circular layer
  3. Oblique layer.

Stomach Mucosa

• Cell Types:
  • Surface Mucous Cells: Secrete alkaline fluid (mucin).
  • Mucous Neck Cells: Secrete acidic fluid (mucin).
  • Parietal Cells: Secrete hydrochloric acid and intrinsic factor.
  • Chief Cells: Produce pepsinogen and gastric lipase.
• Gastric Pits and Glands: Lead to secretion of digestive juices.

Hormonal Signaling in Digestion

• Signaling Types:
  • Autocrine: Cell affects itself.
  • Paracrine: Cell affects nearby cells.
  • Endocrine: Hormones travel through blood to distant target cells (e.g., gastrin from G-cells).