📘 CHAPTER 18: DIGESTIVE SYSTEM – MASTER REVIEW

🔹 I. FUNCTIONS OF THE DIGESTIVE SYSTEM

Main goals:

1. Ingestion – taking in food

2. Digestion – mechanical (chewing) + chemical (enzymes)

3. Absorption – nutrients move into blood/lymph

4. Elimination – removal of undigested food as feces

🔹 II. STRUCTURAL OVERVIEW

🔸 Alimentary Canal (GI tract)

Continuous tube from mouth → anus
Includes:

• Mouth

• Pharynx

• Esophagus

• Stomach

• Small intestine (duodenum, jejunum, ileum)

• Large intestine (colon, rectum, anal canal)

🔸 Accessory Organs

Aid digestion, but food does not pass through them
Includes:

• Teeth & tongue

• Salivary glands

• Liver

• Gallbladder

• Pancreas

🔹 III. WALL OF THE DIGESTIVE TRACT

From inner to outer:

1. Mucosa – secretes mucus, enzymes, absorbs

2. Submucosa – blood vessels, nerves

3. Muscularis – movement (peristalsis, segmentation)

4. Serosa – outer covering (aka visceral peritoneum)

🔹 IV. MOUTH & PHARYNX

🦷 Teeth

• Incisors = cut

• Canines (cuspids) = tear

• Molars/premolars = grind

😛 Tongue

• Helps position food (bolus), aids swallowing

💦 Salivary Glands

• Parotid (largest)

• Submandibular

• Sublingual

Saliva contains:

• Salivary amylase → begins carb digestion

🔹 V. ESOPHAGUS & SWALLOWING

• Muscular tube → transports food from pharynx → stomach

• Upper esophageal sphincter (UES) = blocks air

• Lower esophageal sphincter (LES) = blocks reflux

⚠ Clinical:

• GERD = LES fails → acid reflux

• Hiatal hernia = stomach protrudes through diaphragm

• Barrett esophagus = precancerous from chronic GERD

🔹 VI. STOMACH

🫙 Regions:

• Cardia – near LES

• Fundus

• Body

• Pylorus – ends at pyloric sphincter

🔬 Functions:

• Churns food → chyme

• Begins protein digestion

🔥 Secretions:

• Pepsin (activated by HCl) → proteins

• Intrinsic factor → vitamin B₁₂ absorption

⚠ Clinical:

• Gastritis = inflammation

• Gastric ulcer = H. pylori or NSAIDs

• Pyloric stenosis = infant vomiting

🔹 VII. SMALL INTESTINE (main absorption site)

📏 Segments:

1. Duodenum – receives enzymes & bile

2. Jejunum – major absorption

3. Ileum – vitamin B12 + bile salts

🧬 Structures for absorption:

• Plica – folds

• Villi – finger-like projections

• Microvilli (brush border) – on epithelial cells

🧪 Enzymes:

• Amylase → carbs

• Lipase → fats

• Peptidase/trypsin → proteins

• Sucrase/lactase → sugars

⚠ Clinical:

• Enteritis = inflammation

• Gastroenteritis = stomach + small intestine

• Malabsorption syndrome

• Maldigestion = enzyme/bile deficiency

🔹 VIII. LIVER, GALLBLADDER, PANCREAS

🧬 Liver

• Makes bile

• Detoxifies blood

• Processes nutrients

• Stores glycogen & vitamins

💼 Gallbladder

• Stores & concentrates bile

• Bile released into duodenum via common bile duct

• Triggered by CCK hormone

🔬 Pancreas

• Exocrine: enzymes for carbs, fats, proteins

• Endocrine: insulin & glucagon (covered in Ch. 12)

⚠ Clinical:

• Gallstones (cholelithiasis)

• Choledocholithiasis = stones in bile duct

• Pancreatitis

• Pancreatic cancer

🔹 IX. LARGE INTESTINE

🧻 Segments:

1. Cecum (→ appendix)

2. Ascending colon

3. Transverse colon

4. Descending colon

5. Sigmoid colon

6. Rectum

7. Anal canal → anus

Functions:

• Absorbs water, salts, vitamin K

• Houses gut microbiome (flora)

⚠ Conditions:

• Diarrhea = fast motility

• Constipation = slow motility

• Diverticulosis → diverticulitis

• Colitis, Crohn’s, ulcerative colitis

• Colorectal cancer

🧬 Appendix

• Immune function

• Can become inflamed (appendicitis) → peritonitis if ruptured

🔹 X. PERITONEUM

• Serous membrane around abdominal organs

  • Visceral = on organs

  • Parietal = on cavity walls

Extensions:

• Mesentery – holds intestines in place

• Greater omentum – fatty "lace apron"

⚠ Clinical:

• Peritonitis – often from ruptured appendix

• Ascites – fluid in peritoneal cavity (seen in cirrhosis, cancer)

🔹 XI. END PRODUCTS OF DIGESTION

🔹 XII. ABSORPTION – How Nutrients Enter Body

• Monosaccharides, amino acids, minerals, vitamins, water → blood capillaries

• Fatty acids + glycerol → lacteals (lymph)

Processes:

• Active transport

• Osmosis

• Diffusion

🧠 FINAL KEY THINGS TO REMEMBER

✅ Digestion starts in mouth (amylase)
✅ Protein digestion begins in stomach (pepsin)
✅ Most absorption happens in small intestine
✅ Bile is made in liver, stored in gallbladder
✅ Villi + microvilli = absorption boost
✅ Large intestine absorbs water + makes vitamin K
✅ LES problem = GERD
✅ CCK triggers bile release
✅ Pepsin = proteins, Lipase = fats, Amylase = carbs
✅ Final products:

• Carbs → glucose

• Protein → amino acids

• Fat → fatty acids + glycerol