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🫁 Physiology of the Gastrointestinal (GI) Tract - Complete Competitive Exam Guide

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OVERVIEW: The GI Tract at a Glance

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SECTION 1: MOUTH AND SALIVARY GLANDS

Salivary Secretion (~1000-1500 mL/day)

• Parotid glands - serous secretion (amylase-rich)
• Submandibular glands - mixed serous + mucous
• Sublingual glands - mainly mucous

• Water (99.5%), electrolytes (Na+, K+, Cl-, HCO3-)
• pH: 6.0-7.0 (slightly acidic to neutral)
• Ptyalin (salivary amylase) and lingual lipase

Salivary Enzymes & Their Roles

Key Exam Facts - Saliva:

• Saliva is hypotonic compared to plasma
• Aldosterone increases Na+ reabsorption and K+ secretion in salivary ducts
• Parasympathetic stimulation (VII, IX CN) = profuse watery saliva
• Sympathetic stimulation = scant, thick mucous saliva
• Salivary amylase is inactivated by stomach acid (pH < 4)
• Lingual lipase survives in the stomach and digests ~30% of fat

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SECTION 2: ESOPHAGUS

Functions:

1. Transport of food from pharynx to stomach (peristalsis)
2. Prevention of reflux (Lower Esophageal Sphincter - LES)
3. Mucous secretion for lubrication and protection

Esophageal Peristalsis:

• Primary peristalsis - continuation of swallowing reflex (mediated by vagus nerve and enteric nervous system)
• Secondary peristalsis - initiated by distention of esophagus alone (no swallowing needed; removes remaining food)

Lower Esophageal Sphincter (LES):

• Normally contracted at 15-35 mmHg (prevents GERD)
• Relaxes via VIP (Vasoactive Intestinal Peptide) and NO (Nitric Oxide)
• Stimulated to contract by: gastrin, motilin, substance P
• Inhibited (relaxed) by: secretin, CCK, GIP, glucagon

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SECTION 3: STOMACH

Anatomy of Gastric Secretory Regions:

• Oxyntic (Gastric) glands - Fundus and Body (proximal 80%) - secrete HCl, pepsinogen, intrinsic factor, mucus
• Pyloric glands - Antrum (distal 20%) - secrete mucus and gastrin

Cell Types in Oxyntic Glands:

HCl Secretion by Parietal Cells

• Located on the canalicular membrane of parietal cells
• For every H+ pumped into lumen, one K+ is pumped into cell
• H+ concentration in gastric juice: ~160 mmol/L (pH ~0.8)
• This is 3 million times higher than blood H+ concentration
• Requires >1500 cal of energy per liter of gastric juice

1. Acetylcholine (ACh) - from vagus nerve - acts on M3 receptors
2. Gastrin - from G cells - acts on CCK-B receptors
3. Histamine - from ECL cells - acts on H2 receptors (most potent)

Exam Tip: Omeprazole (PPI) blocks H+/K+ ATPase. Ranitidine blocks H2 receptors.

Phases of Gastric Secretion:

Pepsin (from Pepsinogen):

• Chief cells secrete pepsinogen (inactive)
• Pepsinogen activated to pepsin by:
  • HCl (pH < 2) - initial activation
  • Pepsin itself (autocatalysis)
• Pepsin digests proteins at pH 1.8-3.5
• Inactivated at pH > 5
• Pepsin cleaves peptide bonds adjacent to aromatic amino acids (Phe, Tyr, Trp)

Intrinsic Factor:

• Secreted by parietal cells
• Required for absorption of Vitamin B12 in terminal ileum
• Loss of parietal cells → pernicious anemia (megaloblastic)

Gastric Motility - Stomach Emptying:

• Pyloric pump - antral peristalsis propels chyme through pylorus
• Gastric emptying regulated by enterogastric reflexes from duodenum
• Inhibited by: acidic chyme (pH <3.5-4), fat products, hyper/hypotonic fluids, distension
• Rate of emptying depends on: volume, consistency, fat content of meal
• Liquids empty faster than solids; carbohydrates > protein > fat

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SECTION 4: SMALL INTESTINE

Three Segments:

• Duodenum (25 cm) - digestion zone; receives bile + pancreatic juice
• Jejunum (2.5 m) - major absorption zone
• Ileum (3.5 m) - absorbs bile acids, B12; leads to ileocecal valve

Secretions of the Small Intestine (~1800 mL/day):

• Formed by enterocytes of Crypts of Lieberkühn
• pH: 7.5-8.0 (slightly alkaline)
• Brunner's glands (in duodenum) - secrete mucus rich in HCO3- (protect duodenum from stomach acid)

Brush Border Enzymes (Membrane-bound):

Key Exam Point: Enterokinase is the KEY enzyme - it activates trypsinogen to trypsin, which then activates all other pancreatic proteases (cascade activation).

Small Intestinal Motility:

• Mixing (segmentation) contractions - stationary, rhythmic; mix chyme with secretions
• Propulsive (peristaltic) contractions - move chyme aborally
• Migrating Motor Complex (MMC) - occurs between meals; sweeps residue toward colon every 90 min; "intestinal housekeeper"
• Basal electrical rhythm (BER/slow waves): duodenum = 12/min, ileum = 8-9/min

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SECTION 5: PANCREAS

Pancreatic Secretion (~1500 mL/day):

• Acinar cells - secrete digestive enzymes
• Ductal cells - secrete HCO3- (pH 8.0; up to 145 mmol/L bicarbonate)

Pancreatic Enzymes and Their Roles:

Note: Pancreatic lipase requires colipase (co-factor) to function in the presence of bile salts.

• RNase - digests RNA
• DNase - digests DNA

Regulation of Pancreatic Secretion:

Phases of Pancreatic Secretion:

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SECTION 6: LIVER AND BILE

Bile Secretion (~600-1000 mL/day):

• Produced continuously by hepatocytes
• Stored and concentrated in gallbladder (up to 10x concentration)
• Released by CCK stimulation (contraction of gallbladder + relaxation of Sphincter of Oddi)

Composition of Bile:

Bile Salts - Key Facts:

• Primary bile acids: cholic acid and chenodeoxycholic acid (synthesized from cholesterol in liver)
• Conjugated with glycine/taurine → bile salts (more water soluble)
• Secondary bile acids: deoxycholic acid, lithocholic acid (formed by gut bacteria)
• Enterohepatic circulation - 95% of bile salts reabsorbed in terminal ileum, return via portal vein to liver (recycled 2-3 times per meal)

Functions of Bile:

1. Emulsification - bile salts break fat globules into tiny droplets (increased surface area for lipase)
2. Micelle formation - solubilize fat digestion products for absorption
3. Vitamin absorption - fat-soluble vitamins A, D, E, K depend on bile
4. Cholesterol excretion - main route for cholesterol elimination
5. Excretion of bilirubin, drugs, toxins

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SECTION 7: LARGE INTESTINE

Functions:

1. Absorption of water and electrolytes (converts liquid chyme to solid stool)
2. Storage of waste
3. Mucus secretion (no villi, no digestive enzymes - only mucus-secreting cells)
4. Bacterial fermentation of undigested carbohydrates

Large Intestinal Motility:

• Haustral contractions - slow mixing movements (every 8-15 min)
• Mass movements - propulsive contractions; occur 1-3x/day; triggered by gastrocolic reflex (food entering stomach)
• Transit time: 18-72 hours

Fluid Handled by Large Intestine:

• ~1500 mL enters from ileocecal valve daily
• ~100-200 mL excreted in stool (maximum absorption capacity ~5-8 L/day)

Defecation Reflex:

• Intrinsic reflex - via enteric nervous system (weak)
• Parasympathetic defecation reflex - via pelvic nerves (S2-S4) - much more powerful
• Voluntary control via external anal sphincter (somatic, pudendal nerve)

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SECTION 8: GI HORMONES - MASTER TABLE

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SECTION 9: DIGESTION AND ABSORPTION SUMMARY

Carbohydrate Digestion & Absorption:

• Glucose + Galactose: absorbed via SGLT-1 (Na+-dependent, active transport)
• Fructose: absorbed via GLUT-5 (facilitated diffusion)
• All monosaccharides exit enterocyte via GLUT-2 into portal blood

Protein Digestion & Absorption:

• Absorbed as amino acids via Na+-dependent cotransporters
• Di- and tripeptides absorbed via PepT1 (H+-cotransporter) - more efficient than amino acid absorption

Fat Digestion & Absorption:

• Long-chain fatty acids → re-esterified to triglycerides → chylomicrons → lymphatics (lacteals) → thoracic duct → blood
• Medium/short-chain fatty acids → portal blood directly

Fat-Soluble Vitamin Absorption:

• Vitamins A, D, E, K - require bile salts and micelles; absorbed with fat
• Vitamin B12 - requires intrinsic factor (IF); absorbed at terminal ileum via cubam receptor
• Folate - absorbed in proximal jejunum
• Iron - absorbed as Fe2+ (ferrous) in duodenum; facilitated by vitamin C and HCl

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SECTION 10: INTESTINAL IMMUNE FUNCTION

• Peyer's patches - lymphoid aggregates in ileum; contain M cells that sample antigens
• IgA - secreted into gut lumen; first line of immune defense
• Tight junctions between enterocytes form a mucosal barrier

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QUICK EXAM MNEMONICS

• Solubilize fats (micelle formation)
• Lipolysis assistance
• Absorption of fat-soluble vitamins
• Bilirubin + cholesterol excretion

• Proenzyme (zymogen) secreted
• Enterokinase activates trypsinogen → trypsin
• Protease cascade - trypsin activates all others

• Acetylcholine (vagus, M3 receptor)
• CCK-B (gastrin-like receptor)
• ECL histamine (H2 receptor)

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HIGH-YIELD COMPETITIVE EXAM POINTS

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🏥 Physiology of the Gastrointestinal (GI) Tract

For INC Nursing Students | Competitive Exam Edition

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WHAT IS THE GI TRACT?

5 Basic Functions (Remember: "M-S-D-A-E")

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ORGANS OF THE GI TRACT (Sequence to Remember)

Mouth → Pharynx → Esophagus → Stomach → Small Intestine
→ Large Intestine → Rectum → Anus

• Salivary glands
• Liver
• Gallbladder
• Pancreas

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SECTION 1: MOUTH (ORAL CAVITY)

What happens here?

• Mechanical digestion - teeth cut, tear, grind food (mastication)
• Chemical digestion - saliva begins breakdown of starch
• Bolus formation - tongue shapes chewed food into a soft ball (bolus)

Salivary Glands (3 pairs):

Composition of Saliva:

• 99.5% water
• Electrolytes (Na+, K+, HCO3-)
• Ptyalin (salivary amylase) - digests starch
• Lingual lipase - digests some fats
• Mucus - lubricates food
• IgA - immune protection
• Lysozyme - kills bacteria

Salivary Enzymes:

Control of Saliva:

• Parasympathetic stimulation (seeing, smelling food) → lots of watery saliva
• Sympathetic stimulation (stress, fear) → dry mouth, thick mucus

🩺 Nursing Application:

• Patients on anticholinergic drugs (atropine) get dry mouth (xerostomia)
• Nil per oral (NPO) patients need oral care to prevent drying and infection
• Parotid gland swelling = Mumps

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SECTION 2: PHARYNX AND ESOPHAGUS

Swallowing (Deglutition) - 3 Phases:

Esophagus:

• Length: ~25 cm
• No digestive enzymes produced here
• Function: Transport food from throat to stomach

Peristalsis:

• Primary peristalsis - follows the swallowing reflex
• Secondary peristalsis - triggered if food remains; clears the esophagus

Lower Esophageal Sphincter (LES) / Cardiac Sphincter:

• Normally kept closed (prevents acid reflux)
• Opens when food arrives
• Weakness of LES → GERD (Gastroesophageal Reflux Disease)

🩺 Nursing Application:

• Keep head end of bed elevated 30-45° after feeding (prevents aspiration and GERD)
• Dysphagia (difficulty swallowing) - assess swallowing before oral feeds
• Achalasia - LES fails to relax; patient has difficulty swallowing

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SECTION 3: STOMACH

Parts of the Stomach:

• Cardia - where esophagus meets stomach
• Fundus - upper dome-shaped part
• Body - main part
• Antrum/Pylorus - lower part; controls emptying

What the Stomach Does:

1. Stores food (capacity ~1.5 liters, up to 4 liters when full)
2. Churns food with gastric juice → chyme (semi-liquid paste)
3. Begins protein digestion
4. Secretes intrinsic factor (needed for B12 absorption)
5. Slowly releases chyme into small intestine

Cells and Their Secretions:

Gastric Juice:

• Volume: 2000-3000 mL/day
• pH: 1.5 - 3.5 (very acidic)
• Contains: HCl, pepsin, mucus, intrinsic factor, water

Gastric Enzymes:

Key Point: Pepsin is a protease - it starts protein digestion. It ONLY works in acid (pH 1.8-3.5). Antacids that raise pH above 5 will inactivate pepsin.

How HCl is Made (Parietal Cell):

• Uses H+/K+ ATPase pump (the proton pump)
• This pump is blocked by Proton Pump Inhibitors (PPIs) like omeprazole, pantoprazole
• Histamine (H2) blockers like ranitidine also reduce acid

3 Phases of Gastric Secretion (Very Common Exam Topic):

Stomach Emptying:

• Liquid meals empty faster than solid meals
• Fat slows gastric emptying the most
• Controlled by enterogastric reflex - if duodenum is full or acidic, it sends signals to slow the stomach
• Hormones that SLOW emptying: Secretin, CCK, GIP

🩺 Nursing Application:

• Peptic ulcer - H. pylori destroys mucus layer; treat with triple therapy (PPI + 2 antibiotics)
• Pernicious anemia - parietal cells destroyed → no intrinsic factor → no B12 absorption → megaloblastic anemia; treat with IM B12 injections
• Patients on NSAIDs are at risk for gastric ulcers (NSAIDs reduce prostaglandins that protect mucosa)
• Post-gastrectomy patients need B12 injections for life
• Dumping syndrome - after gastric surgery; rapid emptying of hypertonic food into small intestine → hyperosmotic diarrhea, hypoglycemia; advise small, frequent meals, avoid liquids with meals

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SECTION 4: SMALL INTESTINE

Most Important Organ for Digestion and Absorption

Special Features Increasing Absorption Surface Area:

1. Plicae circulares (circular folds) - permanent folds of mucosa
2. Villi - finger-like projections (0.5-1 mm tall); each contains a lacteal (lymph capillary) for fat absorption
3. Microvilli (brush border) - tiny hair-like projections on each villus cell

Together these increase surface area from ~3,300 cm² to ~2,000,000 cm² (600-fold increase!)

Intestinal Secretions (~1800 mL/day):

• Produced by Crypts of Lieberkühn (between villi)
• pH: 7.5 - 8.0
• Contains watery fluid + brush border enzymes
• Brunner's glands (duodenum only) secrete alkaline mucus to protect against stomach acid

Brush Border Enzymes (Attached to Microvilli):

Most important enzyme to remember for exams: ENTEROKINASE - it starts the whole protein digestion cascade

Intestinal Motility:

🩺 Nursing Application:

• Lactose intolerance = deficiency of lactase → bloating, diarrhea after milk; advise lactose-free diet or lactase supplements
• Celiac disease = gluten destroys villi → malabsorption; gluten-free diet
• Short bowel syndrome - surgical removal of large part of small intestine → severe malabsorption; may need TPN (Total Parenteral Nutrition)
• Auscultate bowel sounds before starting oral feeds post-surgery; absent sounds = ileus
• Normal bowel sounds: 5-35 sounds/minute

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SECTION 5: PANCREAS

Pancreatic Secretion (~1500 mL/day)

• pH: 8.0-8.3 (very alkaline - neutralizes stomach acid in duodenum)
• Two types of cells:
  • Acinar cells - secrete digestive enzymes
  • Ductal cells - secrete sodium bicarbonate (NaHCO3)

Pancreatic Enzymes (MOST IMPORTANT for exams):

Protein-Digesting Enzymes (all secreted as INACTIVE zymogens):

Why inactive? To PREVENT the pancreas from digesting itself. In acute pancreatitis, enzymes activate inside the pancreas → autodigestion → severe abdominal pain, raised serum amylase and lipase.

Fat-Digesting Enzymes:

Carbohydrate-Digesting Enzyme:

Nucleic Acid-Digesting:

• RNase - digests RNA
• DNase - digests DNA

Control of Pancreatic Secretion:

Phases of Pancreatic Secretion:

🩺 Nursing Application:

• Acute pancreatitis - raised serum amylase (>3x normal) and lipase (more specific); keep patient NPO, IV fluids, pain management
• Chronic pancreatitis - enzyme deficiency → steatorrhea (fatty, foul-smelling stools); give pancreatic enzyme supplements (pancrelipase) with meals
• Cystic fibrosis - thick mucus blocks pancreatic ducts → enzyme deficiency in children
• Serum amylase also raised in: salivary gland disorders, intestinal obstruction

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SECTION 6: LIVER AND BILE

Bile (~600-1000 mL/day):

• Made in hepatocytes (liver cells) continuously
• Stored and concentrated in gallbladder (up to 10x concentrated)
• Released into duodenum via common bile duct through Sphincter of Oddi

Composition of Bile:

Role of Bile in Fat Digestion:

1. Emulsification - bile salts break large fat globules into tiny droplets → increases surface area for lipase to work
2. Micelle formation - bile salts surround fat digestion products and carry them to the intestinal wall for absorption
3. Without bile: fat passes into stool undigested = steatorrhea

Enterohepatic Circulation of Bile Salts:

• 95% of bile salts are reabsorbed at terminal ileum → return to liver via portal blood → re-secreted in bile
• Only 5% lost in stool (replaced by fresh synthesis from cholesterol)
• Recycled 2-3 times per meal

🩺 Nursing Application:

• Jaundice - bilirubin builds up in blood; skin/eyes turn yellow; 3 types:
  • Pre-hepatic (hemolytic) - too much RBC breakdown
  • Hepatic (liver disease) - liver can't process bilirubin
  • Post-hepatic (obstructive) - bile duct blocked (e.g., gallstones)
• Gallstones (Cholelithiasis) - cholesterol precipitates in gallbladder; patient has right upper quadrant pain after fatty meals
• Fat-soluble vitamins (A, D, E, K) deficiency occurs when bile is absent (obstructive jaundice, bile duct ligation)
• Vitamin K deficiency → bleeding tendency; give Vitamin K before surgery in jaundiced patients

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SECTION 7: LARGE INTESTINE

Parts (Remember: "CARTSR"):

Length: ~1.5 meters

Functions:

1. Water and electrolyte absorption (the main job)
2. Storage of feces
3. Mucus secretion (no digestive enzymes here)
4. Bacterial fermentation of undigested carbs → Vitamin K, Vitamin B12, short-chain fatty acids, gases (CO2, H2, methane)

Key Point: The large intestine has NO villi and produces NO digestive enzymes - only mucus.

Fluid Balance:

• Receives ~1500 mL/day from small intestine
• Absorbs ~1300-1400 mL
• Excretes only 100-200 mL in stool
• Maximum absorption capacity: 5-8 liters/day (important in diarrhea management)

Movements of Large Intestine:

Gastrocolic Reflex:

• Food entering the stomach triggers mass movements in the colon
• That's why many people feel the urge to defecate after meals

Defecation Reflex:

1. Feces fills rectum → rectal wall stretches
2. Signals travel to sacral spinal cord (S2-S4)
3. Internal anal sphincter (involuntary, smooth muscle) relaxes
4. External anal sphincter (voluntary, skeletal muscle, pudendal nerve) - person consciously relaxes this

Loss of voluntary control = fecal incontinence (common after spinal cord injury, perineal injury)

🩺 Nursing Application:

• Constipation - encourage high-fiber diet, adequate fluids (1.5-2 L/day), physical activity, regular toilet routine
• Diarrhea - monitor fluid and electrolyte status; oral rehydration solution (ORS) composition: glucose + NaCl + KCl + sodium citrate
• Paralytic ileus - no bowel sounds, no flatus post-surgery; NPO, nasogastric decompression
• Hirschsprung's disease - congenital absence of ganglion cells → no peristalsis → intestinal obstruction in newborns; presents as failure to pass meconium within 48 hours

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SECTION 8: GI HORMONES - SIMPLIFIED TABLE

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SECTION 9: DIGESTION AND ABSORPTION - SIMPLIFIED

Where Each Nutrient is Digested:

How Nutrients Enter the Blood:

• Glucose, amino acids, water-soluble vitamins → absorbed into blood capillaries → portal vein → liver
• Fats (long-chain) → packaged as chylomicrons → absorbed into lacteals (lymph) → thoracic duct → blood (bypasses liver)

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SECTION 10: DAILY FLUID VOLUMES - HIGH YIELD TABLE

Exam Fact: Only 100-200 mL of the ~9 liters entering the GI per day is actually lost in stool. Diarrhea can rapidly cause dehydration and electrolyte imbalance!

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NURSING MNEMONICS - QUICK REVISION

• Cephalic (30%) - thinking/smelling food
• Gastric (60%) - food in stomach
• Intestinal (10%) - chyme in duodenum

• Secretin
• Inhibitory GIP
• Glucagon
• Somatostatin

• D, E, K, A - all need bile for absorption

• B12 (intrinsic factor-bound)
• Bile salts (enterohepatic circulation)
• It's the Box (terminal ileum)

• Chymotrypsinogen → chymotrypsin
• Elastase (proelastase → elastase)
• Carboxypeptidase (proenyzme → active)
• All activated by trypsin (itself activated by enterokinase)

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MOST ASKED INC EXAM QUESTIONS - QUICK ANSWERS

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