Digestive System

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Digestive System — Key Facts for MDCAT

Alimentary Canal (Gastrointestinal Tract) — Complete Pathway: Mouth → Pharynx → Oesophagus → Stomach → Small intestine → Large intestine → Rectum → Anus

Mouth:

• Mechanical digestion: Chewing (mastication) by teeth — breaks food into smaller pieces
• Chemical digestion: Salivary amylase (ptyalin) begins starch digestion → maltose
• Salivary glands: Parotid (largest), submandibular, sublingual
• Teeth: Incisors (cutting), canines (tearing), premolars (grinding), molars (grinding)
• Adult has 32 teeth: 8 incisors, 4 canines, 8 premolars, 12 molars

Oesophagus:

• About 25 cm long
• Peristalsis (rhythmic wave-like contractions) moves food toward stomach
• gastro-oesophageal sphincter (LES): Prevents reflux of stomach contents into oesophagus
• J-shaped stomach stores and mixes food

Stomach:

• Regions: Cardiac, fundus, body, pyloric
• Gastric juice: HCl (kills bacteria; activates pepsinogen) + pepsin (protein digestion) + intrinsic factor (for B12 absorption)
• Temporary storage: Food stays 2–5 hours
• pH: 1.5–2 (highly acidic)
• Mucus: Protects stomach wall from HCl (mucus bicarbonate barrier)

Small Intestine (~6 metres long): Three parts: Duodenum (first 25 cm) → Jejunum (~2.5 m) → Ileum (~3.5 m)

• Villi: Finger-like projections (1 mm) that increase surface area ~600×
• Microvilli: Further increase surface area (brush border)
• Lacteals: Lymphatic vessels in villi for fat absorption

Large Intestine (~1.5 metres):

• Functions: Water absorption, formation of faeces, vitamin K and B synthesis (by bacteria)
• Regions: Caecum → Ascending colon → Transverse colon → Descending colon → Sigmoid colon → Rectum → Anus
• Appendix: Vestigial organ attached to caecum; has immunological function

⚡ Exam tip: The liver produces BILE (stored in gallbladder), which emulsifies fats (increases surface area for lipase action) but does NOT contain digestive enzymes. Bile is NOT a digestive enzyme — it’s a emulsifying agent. The pancreas produces pancreatic juice containing trypsin (protein digestion), pancreatic amylase (starch), and lipase (fats).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students who want genuine understanding.

Digestive System — Complete Study Guide

Accessory Digestive Organs:

Organ	Secretion	Function
Salivary glands	Salivary amylase, maltase	Starch → maltose
Liver	Bile (no enzymes)	Emulsifies fats
Gallbladder	Stores and concentrates bile	Releases bile into duodenum
Pancreas	Pancreatic juice (amylase, lipase, trypsinogen)	Digests carbs, fats, proteins
Intestinal glands (Crypts of Lieberkühn)	Intestinal juice (maltase, sucrase, lactase, peptidases)	Final digestion

Digestive Enzymes and Their Actions:

Enzyme	Source	Substrate	Product
Salivary amylase	Salivary glands	Starch	Maltose
Pepsin	Stomach (chief cells)	Proteins	Polypeptides
Gastric lipase	Stomach	Fats	Fatty acids + glycerol
Pancreatic amylase	Pancreas	Starch	Maltose
Trypsin	Pancreas (activated by enterokinase)	Proteins	Polypeptides
Chymotrypsin	Pancreas	Proteins	Polypeptides
Lipase	Pancreas	Fats	Fatty acids + glycerol
Maltase	Intestinal brush border	Maltose	Glucose
Sucrase	Intestinal brush border	Sucrose	Glucose + Fructose
Lactase	Intestinal brush border	Lactose	Glucose + Galactose
Peptidases	Intestinal brush border	Polypeptides	Amino acids

Carbohydrate Digestion Summary: Starch → (salivary amylase) → Maltose → (maltase) → Glucose

Fat Digestion: Fats → (bile emulsification) → Fat droplets → (pancreatic lipase) → Fatty acids + Glycerol → Micelles → absorbed by intestinal cells → reassembled into triglycerides → enter lymphatic system (lacteals) → transported as chylomicrons

Mineral and Vitamin Absorption:

Substance	Site of Absorption	Mechanism
Iron	Duodenum	Active transport; increased by vitamin C
Calcium	Duodenum	Requires vitamin D; active transport
Vitamin B12	Ileum	Requires intrinsic factor (from stomach)
Folate	Jejunum	Passive diffusion
Vitamin A, D, E, K	Ileum	Requires bile salts; passive diffusion
Sodium	Jejunum	Active transport
Water	Small and large intestine	Osmosis

⚡ Common mistakes: Forgetting that bile is produced by the liver (not stored there — that’s the gallbladder’s job). Confusing the function of the gallbladder (stores bile) with the liver (produces bile). Thinking fat digestion produces glycerol directly — lipase acts on triglycerides to produce fatty acids and monoglycerides, which then combine to form triglycerides within intestinal cells.

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🔴 Extended — Deep Study (3mo+)

Comprehensive documentation for students on a longer study timeline.

Digestive System — Advanced Notes

Neural and Hormonal Control of Digestion:

Hormone	Source	Stimulus	Effect
Gastrin	Stomach (G cells)	Food in stomach	Stimulates HCl secretion; gastric motility
Secretin	Duodenum	Acid in duodenum	Stimulates pancreatic bicarbonate secretion
Cholecystokinin (CCK)	Duodenum	Fats and proteins in duodenum	Stimulates bile release; pancreatic enzyme secretion; satiety
Gastric Inhibitory Peptide (GIP)	Duodenum	Glucose and fats in duodenum	Inhibits gastric secretion and motility
Motilin	Duodenum	Fasting state	Stimulates fasting motility pattern

Gastrointestinal Reflexes:

• Ileogastric reflex: Inhibits gastric motility when ileum is distended
• Enterogastric reflex: Inhibits gastric secretion when fat is in duodenum
• Defaecation reflex: Distension of rectum → internal anal sphincter relaxes (involuntary) → external sphincter relaxation (voluntary if appropriate)

Absorption Mechanisms:

1. Passive diffusion: No energy required; follows concentration gradient (water, small lipids, vitamins)
2. Facilitated diffusion: Carrier-mediated; follows gradient (fructose)
3. Active transport: Against gradient; requires ATP (glucose, amino acids, iron, calcium)
4. Co-transport (secondary active): Na⁺ gradient drives glucose uptake (SGLT1 transporter)
5. Endocytosis: For large molecules (some antigens, bacteria)

Gut Microbiota:

• ~100 trillion microorganisms in large intestine (gut microbiome)
• Mainly Bacteroidetes and Firmicutes
• Functions: Ferment undigested carbohydrates → short-chain fatty acids; synthesize vitamin K and B vitamins; immune system development; pathogen exclusion
• Dysbiosis: Imbalance associated with obesity, inflammatory bowel disease, diabetes

Common Digestive Disorders:

Disorder	Description	Symptoms
Gastroesophageal reflux disease (GERD)	Chronic acid reflux	Heartburn, regurgitation
Peptic ulcer	Erosion of stomach/duodenal lining (H. pylori or NSAIDs)	Pain, bleeding
Lactose intolerance	Deficiency of lactase enzyme	Bloating, diarrhoea after dairy
Coeliac disease	Autoimmune reaction to gluten	Malabsorption, diarrhoea, weight loss
Constipation	Reduced peristalsis	Infrequent, hard stools
Diarrhoea	Increased peristalsis	Frequent, watery stools
Haemorrhoids	Swollen rectal veins	Pain, bleeding
Appendicitis	Inflammation of appendix	Abdominal pain, fever

Hepatitis:

• Hepatitis A: Fecal-oral; acute; no chronic carrier state
• Hepatitis B: Blood/body fluids; can become chronic (→ liver cancer); vaccine available
• Hepatitis C: Blood; most become chronic; leading cause of liver transplant
• Hepatitis D: Requires HBV co-infection; more severe
• Hepatitis E: Fecal-oral; especially dangerous in pregnancy

MDCAT Question Patterns: MDCAT Pakistan digestive system questions frequently test: (1) sequence of digestive tract, (2) functions of digestive enzymes and their sites of action, (3) role of bile (emulsification, not digestion), (4) absorption sites for different nutrients, (5) accessory organs (liver, gallbladder, pancreas), (6) hormones controlling digestion, (7) digestive disorders. 2–3 questions per paper. Enzyme-substrate relationships and bile function are very high-yield.

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