Digestive System Notes

Introduction

• Food is broken down through digestion to be absorbed and used by cells.
• Gastroenterology studies the upper digestive tract, proctology the lower.

Overview of the Digestive System

• GI Tract: Mouth, esophagus, stomach, small intestine, large intestine.
• Accessory Structures: Teeth, tongue, salivary glands, liver, gallbladder, pancreas.
• Digestion Processes:
  • Ingestion: Eating.
  • Secretion: Release of water, acid, buffers, and enzymes.
  • Mixing and propulsion: Contraction and relaxation of smooth muscles.
  • Digestion:
    • Mechanical: Physical movements.
    • Chemical: Hydrolysis reactions.
  • Absorption:Nutrient passage into blood or lymph.
  • Defecation: Elimination of waste.

Layers of the GI Tract

• Layers: Mucosa, submucosa, muscularis, serosa.
• Mucosa:
  • Epithelium: Protection/secretion.
  • Lamina propria: Connective tissue, blood/lymph vessels, nerves, sensors.
  • Muscularis mucosa: Increases surface area.
• Submucosa: Connective tissue, submucosal plexus, glands, lymphatic tissue.
• Muscularis:
  • Skeletal muscle: Mouth, pharynx, upper esophagus and external anal sphincter (voluntary).
  • Smooth muscle: Rest of tract (circular and longitudinal fibers).
• Serosa: Visceral peritoneum.

Neural Innervation of the GI Tract

• Enteric Nervous System (ENS):
  • Submucosal plexus: Mucosa movements, vasoconstriction, secretion.
  • Myenteric plexus: GI tract motility.
• Autonomic Nervous System (ANS):
  • Parasympathetic: Increases GI secretion and motility.
  • Sympathetic: Decreases GI secretion and motility.
• Gastrointestinal Reflex Pathways: Regulate GI activity based on stimuli.

Peritoneum

• Largest serous membrane in the body.
• Parietal peritoneum: Lines abdominal cavity wall.
• Visceral peritoneum: Covers organs.
• Peritoneal cavity: Space between layers, contains serous fluid.
• Retroperitoneal organs: Kidneys, pancreas (behind peritoneum).
• Peritoneal folds: Support organs, contain vessels/nerves.

Mouth

• Structures: Cheeks, palate, lips, tongue.
• Salivary Glands: Parotid, submandibular, sublingual.
  • Saliva: Lubricates, dissolves food, starts carbohydrate digestion.
• Tongue: Manipulates food, contains lingual lipase (triglyceride digestion).
• Teeth: Mechanical digestion.
  • Crown, root, neck; dentin, enamel, cementum.
  • Types: Incisors, cuspids (canines), premolars (bicuspids), molars.
• Mechanical Digestion: Mastication (chewing) forms bolus.
• Chemical Digestion: Salivary amylase converts starches to maltose.

Pharynx

• Funnel-shaped tube connecting internal nares to esophagus and larynx
• Oropharynx and laryngopharynx have digestive and respiratory functions

Esophagus

• Muscular tube connecting pharynx to stomach.
• Layers: Mucosa, submucosa, muscularis, adventitia.
• Upper and lower esophageal sphincters.
• Peristalsis: Propels bolus.

Deglutition

• Swallowing; moves bolus from mouth to stomach.
• Stages: Voluntary, pharyngeal (involuntary), esophageal (involuntary).
• Deglutition center in medulla and lower pons.

Stomach

• J-shaped organ for mixing and holding food; protein and triglyceride digestion.
• Anatomy: Cardia, fundus, body, pylorus, rugae (folds).
• Histology:
  • Mucous surface cells.
  • Gastric pits and glands.
  • Cells: Mucous neck, chief (pepsinogen, gastric lipase), parietal (HCl), enteroendocrine (gastrin).
• Mechanical Digestion: Mixing waves.
• Chemical Digestion:
  • Pepsin: Proteins to peptides.
  • Gastric lipase: Some fat digestion.
• Absorption: Water, electrolytes, some drugs, alcohol.
• Gastric emptying: Chyme release into duodenum (2-6 hours).

Pancreas

• Anatomy: Head, body, tail; pancreatic and accessory ducts.
• Histology: Islets (hormones), acini (pancreatic juice).
• Pancreatic Juice:
  • Enzymes: Pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase, pancreatic lipase, ribonuclease, deoxyribonuclease.
  • Sodium bicarbonate: Neutralizes stomach acid.

Liver and Gallbladder

• Liver Anatomy: Lobes (left, right, caudate, quadrate), falciform ligament.
• Gallbladder: Stores and concentrates bile.
• Liver Histology: Lobules, hepatocytes, sinusoids, Kupffer cells, central vein.
• Blood Supply: Hepatic artery, hepatic portal vein, hepatic vein.
• Functions:
  • Bile production (emulsifies triglycerides).
  • Metabolism (carbohydrate, lipid, protein).
  • Removes drugs/hormones.
  • Excretes bilirubin.
  • Synthesizes bile salts.
  • Stores vitamins/minerals.
  • Phagocytosis.
  • Vitamin D activation.

Small Intestine

• Major digestion and absorption site.
• Anatomy: Duodenum, jejunum, ileum; circular folds (plicae circularies).
• Histology:
  • Villi: Lacteals (fat absorption).
  • Epithelial cells: Absorptive, goblet, enteroendocrine, Paneth.
  • Microvilli (brush border): Increase surface area, contain enzymes.
  • Intestinal glands (crypts of Lieberkuhn).
  • Duodenal (Brunner’s) glands (duodenum): Alkaline mucus.
  • Peyer’s patches (ileum): Lymphatic nodules.
• Intestinal Juice: Absorption vehicle.
• Brush Border Enzymes: Intracellular digestion.
• Mechanical Digestion: Segmentation, peristalsis.
• Chemical Digestion:
  • Carbohydrates: Monosaccharides.
  • Proteins: Amino acids.
  • Lipids: Fatty acids, monoglycerides (emulsification by bile salts).
  • Nucleic Acids: Nucleotides.
• Absorption: Diffusion, facilitated diffusion, osmosis, active transport.
  • Monosaccharides, amino acids: Blood capillaries.
  • Lipids: Lacteals (chylomicrons).
  • Electrolytes: Active transport.
  • Vitamins: Simple diffusion.
  • Water: Osmosis.

Large Intestine

• Anatomy: Cecum, colon, rectum, anal canal, appendix.
• Colon Subdivisions: Ascending, transverse, descending, sigmoid.
• Histology: No villi or circular folds; goblet cells, taeniae coli, haustra.
• Mechanical Movements: Haustral churning, peristalsis, mass peristalsis.
• Chemical Digestion: Bacterial action.
• Absorption: Water, electrolytes, some vitamins.
• Feces: Undigested material, bacteria, etc.
• Defecation Reflex: Elimination of feces.

Phases of Digestion

• Cephalic: Stimulates gastric secretion and motility.
• Gastric: Neural/hormonal regulation.
• Intestinal: Neural/hormonal regulation (secretin, CCK).
• Other Hormones: Motilin, substance P, bombesin, vasoactive intestinal polypeptide, gastrin-releasing peptide, somatostatin.

Development of the Digestive System

• Endoderm: Epithelium and glands.
• Mesoderm: Smooth muscle and connective tissue.

Aging and the Digestive Tract

• Decreased secretion/motility, loss of muscle strength/tone, changes in feedback, diminished sensation.

Disorders: Homeostatic Imbalances

• Dental caries, periodontal diseases, peptic ulcers, diverticulitis, tumors, hepatitis, anorexia nervosa.