Digestive System Overview A Quick Summary

Digestive System Overview a Quick Notes

1. Organs of the Digestive System & Their Functions

A. Oral Cavity

• Teeth – Mechanical digestion (mastication); breaks food into smaller pieces.
• Salivary glands (paired):
  • Submandibular (75–80% of saliva)
  • Parotid (15–20%)
  • Sublingual (2–3%)
• Saliva functions
  • Moistens & forms bolus
  • Begins chemical digestion of carbohydrates via salivary amylase
  • Produces ~1.5 L/day

B. Esophagus

• Transports bolus to stomach, no digestion
• Muscle composition:
  • Upper ⅓ – skeletal (voluntary)
  • Middle – mixed
  • Lower ⅓ – smooth (involuntary)
• Peristalsis starts here

C. Layers of the GI Tract

From inner → outer:

1. Mucosa
   • Epithelium (simple columnar, except stratified in esophagus)
   • Lamina propria (capillaries, lymphatics)
   • Muscularis mucosa
2. Submucosa
   • Vessels, nerves
   • Contains Meissner's plexus (submucosal plexus)
3. Muscularis externa
   • Inner circular + outer longitudinal layers
   • Contains Auerbach’s plexus (myenteric plexus)
   • Responsible for segmentation & peristalsis
4. Serosa / visceral peritoneum

D. Stomach

Functions:

• Churns food → chyme
• Begins protein digestion

Cell types:

• Goblet cells – mucus
• Parietal cells – HCl + intrinsic factor (Vit B12 absorption)
• Chief cells – pepsinogen → pepsin (protein digestion)

Regions: Cardia, fundus, body, pylorus, pyloric sphincter

Rugae allow expansion up to ~1.5 L

E. Small Intestine (7 m long)

Primary site of chemical digestion & nutrient absorption

1. Duodenum

• Four parts (1st intraperitoneal; others retroperitoneal)
• Features:
  • Brunner’s glands (alkaline mucus)
  • Major duodenal papilla (bile + pancreatic enzymes enter)
• Location of:
  • Bile emulsification
  • Pancreatic enzyme activity

2. Jejunum

• Many circular folds (plicae)
• Upper left quadrant
• Major nutrient absorption

3. Ileum

• Fewer circular folds
• Peyer’s patches
• Ends at ileocecal valve

F. Large Intestine

Functions:

• Absorbs water, electrolytes, vitamins from flora
• Forms & stores feces

Features:

• Teniae coli – longitudinal muscle bands
• Haustra – sacculations
• Epiploic appendages – fat tags

Regions: Cecum → ascending → transverse → descending → sigmoid → rectum → anus

Appendix attached to cecum (McBurney’s point surface landmark)

Rectum = retroperitoneal
Anus:

• Internal sphincter: smooth (involuntary)
• External sphincter: skeletal (voluntary)

2. Accessory Digestive Organs

Liver

• Upper right quadrant
• Functions:
  • Produces bile
  • Detoxification
  • Produces albumin & clotting factors
  • Metabolism (carbs, lipids, proteins)
• Lobes: right, left, quadrate, caudate
• Portal triad: Proper hepatic artery, portal vein, common bile duct

Gallbladder

• Stores & concentrates bile
• Releases bile when CCK signals contraction
• Cystic duct + hepatic duct = common bile duct

Pancreas

• Retroperitoneal (tail touches spleen)
• Exocrine
  • Pancreatic duct → duodenum
  • Enzymes:
    • Amylase (carb digestion)
    • Proteases
    • Lipase (fat digestion)
• Endocrine
  • Islets of Langerhans
  • Insulin & glucagon

Spleen

(Not digestive but shares blood supply)

• Filters blood
• Recycles old RBCs
• Produces immune cells

3. Vascular Supply

Digestive organs are grouped by embryologic foregut, midgut, hindgut.

A. Foregut

Organs:

• Stomach
• Liver, gallbladder
• Pancreas
• Spleen
• 1st & 2nd part of duodenum

Arterial supply:
✔ Celiac trunk

Venous drainage:
✔ Gastric, splenic veins → portal vein

B. Midgut

Organs:

• Distal duodenum
• Jejunum, ileum
• Cecum, appendix
• Ascending colon
• Proximal 2/3 transverse colon

Arterial supply:
✔ Superior mesenteric artery (SMA)

Venous drainage:
✔ Superior mesenteric vein → portal vein

C. Hindgut

Organs:

• Distal 1/3 transverse colon
• Descending colon
• Sigmoid
• Rectum

Arterial supply:
✔ Inferior mesenteric artery (IMA)

Venous drainage:
✔ Inferior mesenteric vein → splenic vein → portal vein

Key Anastomoses

• Foregut ↔ Midgut: Pancreaticoduodenal arteries
• Midgut ↔ Hindgut: Marginal artery of Drummond

4. Innervation of the GI Tract

Sympathetic (inhibits digestion)

• Foregut: Greater splanchnic (T5–T9) → celiac ganglion
• Midgut: Lesser splanchnic (T10–T11) → SMA ganglion
• Hindgut: Lumbar splanchnic (L1–L2) → IMA ganglion

Effects:

• Decreased peristalsis
• Decreased secretion
• Increased sphincter tone

Parasympathetic (stimulates digestion)

• Foregut & midgut: Vagus nerve
• Hindgut: Pelvic splanchnic nerves (S2–S4)

Effects:

• Increased peristalsis
• Increased secretions
• Decreased sphincter tone

Digestive System in One Sentence

Food is mechanically and chemically processed in the mouth → broken down in the stomach → digested and absorbed in the small intestine → water reclaimed in the large intestine → waste removed through rectum and anus, all supported by liver, gallbladder, and pancreas.

Are you amazed or what about the mystery of this human GI system? That is the notes from class the other day.

Pal Ronnie