Digestive System – Key Points

Digestive System Overview

• Purpose: extract & absorb nutrients for cellular energy
• Utilizes chemical (enzymatic) and mechanical (physical) processes

Chemical vs Mechanical Digestion

• Chemical: enzymatic breakdown of large, non-absorbable molecules → smaller absorbable units
• Mechanical: physical reduction & propulsion of food (chewing, swallowing, peristalsis, defecation)

Mechanisms of Digestion

• Ingestion: food entry into oral cavity
• Digestion: mechanical & chemical breakdown
• Motility: muscular movement (e.g., peristalsis, segmentation)
• Secretion: release of enzymes, acids, bile, etc.
• Absorption: transfer of nutrients across GI mucosa to internal environment
• Elimination: expulsion of residue
• Regulation: neural & hormonal coordination of above steps

Oral Cavity & Teeth

• Lined by mucous membrane; hard & soft palates, uvula (blocks nasopharynx), tongue (anchored by frenulum)
• Teeth: 20 deciduous → 32 permanent
  • Incisors (cut), canines (tear), premolars & molars (crush)
• Mastication = chewing (initial mechanical digestion)

Salivary Glands

• Produce ≈ 1 L saliva/day (serous with amylase, mucous for lubrication, or mixed)
  1. Parotid – largest, serous
  2. Submandibular – mixed
  3. Sublingual – smallest, mucous

Esophagus & Stomach

• Esophagus: 25\,cm (≈10 in), mucus-lined muscular tube
  • Upper esophageal sphincter (UES): blocks air entry
  • Lower esophageal sphincter (LES): prevents gastric reflux
• Stomach: muscular pouch beneath diaphragm; receives bolus from esophagus, delivers chyme to small intestine

Small Intestine

• Length ≈ 7\,m (≈20 ft); small diameter enhances motility
  1. Duodenum – first 25\,cm (≈10 in); receives bile & pancreatic juice; ulcer-prone
  2. Jejunum – next 2.4\,m (≈8 ft); thick, vascular
  3. Ileum – final 3.6\,m (≈12 ft); thinner walls, Peyer’s patches

Large Intestine

• Length ≈ 1.5\,m (≈5 ft); larger diameter
• Receives chyme via ileocecal valve; lacks villi → minimal nutrient absorption (mainly Na^+)
• Normal transit: 3–5 days

Accessory Organs

• Liver (largest gland, 4 lobes)
  • Stores/releases glucose, produces bile, processes vitamins/minerals, detoxifies, recycles RBCs
• Gallbladder
  • Stores & concentrates bile; connects to liver & pancreas via common bile duct
• Pancreas
  • Endocrine: insulin & glucagon; Exocrine: pancreatic juice to duodenum
• Vermiform Appendix
  • 3\,cm worm-shaped extension of cecum; reservoir for normal gut microbiota