GI 02: Anatomy

🌸 Lecture 2 Overview: Anatomy & Physiology of the GI System

🧭 Learning Objectives

You’ll need to:

1. Know the major parts of the GI tract (mouth → anus)

2. Understand liver, gallbladder, and pancreas functions

3. Know where and how nutrients are absorbed

4. Recognize what goes wrong in disorders like diarrhea, CF, pancreatitis

🍞 Mouth (Oral Cavity)

🧱 Structure

Includes:

• Cheeks, lips, teeth, tongue

• Salivary glands

• Oral mucosa

• Palate

⚙ Function

• Breaks down, moistens, and softens food

• Starts digestion (amylase + lipase)

• Swallowing: pharynx coordinates movement to the esophagus and prevents food from entering the nose or lungs.

💧 Salivary Glands

🏗 Structure & Secretions

There are 3 major glands:

Saliva contents:
Water, ions (Na⁺, K⁺, Cl⁻, HCO₃⁻), mucin, lysozyme, albumin, amylase, lingual lipase

💡 Function

• Keeps mouth moist

• Lubricates food

• Helps with taste (dissolves molecules)

• Cleans teeth

• Slightly alkaline → protects against acid & reflux

🧬 Salivary Gland Cell Types

• Serous acinar cells: secrete amylase (protein enzymes)

• Mucous acinar cells: secrete watery mucus

🚫 Xerostomia (Dry Mouth)

• Causes: Sjögren’s syndrome, medications, head/neck radiation

• Problems: ↓ saliva → ↓ pH → tooth decay, swallowing issues, oral infections

👅 Tongue

🧠 Structure & Innervation

👅 Taste Receptors

• GPCRs → sweet, bitter, umami

• Ion channels → sour, salty

• Each taste bud = 50–100 cells, each tuned to one taste

🧵 Esophagus

🧱 Structure

• Upper: skeletal muscle

• Middle: skeletal + smooth

• Lower: smooth muscle

→ Skeletal part = under conscious control (start of swallowing)

⚙ Function

• Moves food → stomach via peristalsis

• Two sphincters:

  • Upper: opens to let food in

  • Lower: keeps stomach acid out (prevents reflux)

⚠ Disorders

Dysphagia = difficulty swallowing

• Common! (~30–40% have tablet-swallowing issues)

• Big tablets (≥11 mm) or capsules (size #00) can worsen it

• Switching formulations can change drug absorption/metabolism

🍲 Stomach

⚙ Function

• Mechanically churns food (lower ⅔)

• Pacemaker: ~3 contractions/min

• Controls gastric emptying: liquids leave faster than solids

• Reservoir: stretches (via vagovagal reflex) to hold food

• Empties through pyloric sphincter to duodenum

🧪 Secretions

4 cell types:

🧠 Gastric Regulation

• Gastrin stimulates HCl + pepsinogen release

• Somatostatin (SST) inhibits gastrin → stops acid when no longer needed

🕓 Gastric Emptying

• Protein & fat slow emptying (especially lipids)

• Hyperosmotic food (too concentrated) also clears slower

🌀 Migrating Motor Complex

• “Housekeeping contractions” after emptying to clear debris or foreign objects

💧 Substances absorbed in stomach:

• Water

• Na⁺, K⁺

• Weak acids (ASA)

• Ethanol (~20%)

🩸 Peritoneum & Peritoneal Cavity

• Peritoneum: thin serous membrane

  • Parietal → lines abdominal wall

  • Visceral → covers organs

• Important sites for:

  • Peritonitis (infection if GI tract ruptures)

  • Peritoneal dialysis

💚 Liver

🧠 Function

• Produces bile salts from cholesterol

• Bile → emulsifies fats → helps absorption

• Contains bilirubin (waste from hemoglobin) → gives bile its color

• Bile stored in the gallbladder

💛 Gallbladder & Pancreas

🏗 Structure

• Common bile duct = hepatic duct + cystic duct

• Opens into duodenum via sphincter of Oddi

💡 Gallbladder Function

• Concentrates bile (reabsorbs Cl⁻, HCO₃⁻, water)

• Fat/protein → trigger CCK release → gallbladder contracts → bile released

• Bile contents:

  • 80% bile acids (form micelles for fat solubility)

  • 16% lecithin

  • 4% cholesterol

• FGF-19 feedback suppresses new bile synthesis

🪨 Gallstones (Cholelithiasis)

• Cause: high cholesterol secretion, pregnancy, rapid weight loss

• Problem: stones block bile → intense pain after eating (biliary colic)

🧠 Pancreatic Secretion Control

Stimulated by:

• CCK + vagus nerve

• Secretin (from duodenum, triggered by acid)

Pancreas secretes alkaline, enzyme-rich fluid (20 enzymes total):

• Amylase → carbs

• Lipase → fats

• Trypsin/chymotrypsin → proteins

• Ribonuclease → nucleic acids

⚙ Pancreas Function

• Duct cells: secrete HCO₃⁻ (via CFTR Cl⁻ channel) → neutralize stomach acid

• Acinar cells: secrete digestive enzymes

⚠ Pancreatic Diseases

🧬 Cystic Fibrosis

• CFTR mutation → can’t secrete HCO₃⁻ → enzymes can’t flush → get trapped → digestion fails

• Treatment: enzyme replacement + antacids

🔥 Acute Pancreatitis

• Causes: gallstones, alcohol, high triglycerides

• Symptoms: epigastric pain radiating to back, fever, tachycardia, low BP

💔 Chronic Pancreatitis

• Irreversible; often alcohol/smoking related

• Causes pain, glucose intolerance, malabsorption, delayed transit

🦠 Small Intestine

🏗 Structure

• Receives chyme from stomach

• Mixes with digestive juices

• Has villi & microvilli → increase absorption surface

• Layers (inner → outer): mucosa → submucosa → muscularis → serosa

⚙ Functions

💧 Water & Electrolytes

• 9 L fluid daily; ~99% reabsorbed

• Water follows Na⁺ osmotically

• Failure → diarrhea

⚡ Sodium

• Absorbed with glucose (via SGLT-1)

• Regulated by aldosterone

• Colon uses ENaC channels

🌊 Chloride

• Absorbed in exchange for HCO₃⁻

• Secreted via CFTR channel (regulated by cAMP/cGMP)

🍞 Carbohydrates

• Only absorbed as monosaccharides

• Amylase → starch → glucose

• Lactase → lactose → glucose + galactose

• Sucrose → glucose + fructose

• Glucose uses SGLT-1 in, GLUT2 out

🍗 Proteins

• Broken into amino acids + small peptides

• Absorbed by specific transporters and PEPT1

🧈 Lipids

• Triglycerides → free fatty acids + micelles (via bile)

• Absorbed passively in SI; active reabsorption in terminal ileum

⚠ Diarrhea

• Too much Cl⁻ secretion → too much water loss

• Causes:

  • Cholera toxin ↑ cAMP

  • E. coli toxin ↑ cGMP

  • C. difficile ↑ Cl⁻ secretion

• Consequence: severe dehydration if untreated

• CF does the opposite (↓ Cl⁻ secretion → thick mucus → obstruction)

💩 Large Intestine

🧱 Structure

• Cecum (appendix attached)

• Colon (no villi; bacterial fermentation → short-chain fatty acids)

• Rectum + anal canal → store & expel waste

• Ileocecal valve prevents reflux

⚙ Function

• Absorbs water, Na⁺, minerals

• Gastrocolic reflex: stomach stretch → colon contractions → urge to poop

• Defecation reflex: internal sphincter (autonomic) + external sphincter (voluntary)

⚠ Diseases