Digestive System in Humans

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Digestion is the breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream. The digestive system comprises the alimentary canal and associated glands.

Alimentary Canal (GIT): Mouth → Pharynx → Oesophagus → Stomach → Small intestine (duodenum + jejunum + ileum) → Large intestine (caecum, colon, rectum) → Anus

Four Main Processes:

1. Ingestion: Taking food into the mouth
2. Digestion: Mechanical and chemical breakdown of food
3. Absorption: Movement of digested nutrients into blood/lymph
4. Egestion/Defaecation: Elimination of faeces

Teeth — Mechanical Digestion:

• Incisors: Cutting (front, 4 upper + 4 lower)
• Canines: Tearing (pointed, 2 upper + 2 lower)
• Premolars: Grinding (8 total)
• Molars: Grinding (12 total including wisdom teeth)
• Dental formula (adult): 2, 1, 2, 3 (top and bottom on one side) = 32 teeth

Enzymes and Their Functions:

Enzyme	Site of Action	Substrate	Products
Salivary amylase	Mouth	Starch	Maltose
Pepsin	Stomach	Proteins	Peptides
Trypsin	Duodenum	Proteins	Peptides
Pancreatic amylase	Duodenum	Starch	Maltose
Maltase	Ileum	Maltose	Glucose
Lipase	Duodenum	Fats	Fatty acids + Glycerol

⚡ WAEC Tip: Remember the substrate for each enzyme. A common question is “Name the enzyme that digests蛋白质 (proteins) in the stomach” — answer is pepsin. The inactive form (pepsinogen) is activated by HCl.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of human digestion.

Peristalsis: The muscular walls of the alimentary canal contract rhythmically to push food along:

1. Circular muscles contract behind food → bolus pushed forward
2. Longitudinal muscles contract ahead of food → canal shortens
3. Wave of contraction moves along the canal This is involuntary and continues even if you are upside down!

The Stomach:

• 容量: About 1.5 litres
• Gastric glands secrete:
  • HCl: Creates acidic pH (≈2) — kills bacteria, activates pepsinogen
  • Pepsinogen: Inactive enzyme activated to pepsin by HCl
  • Mucus: Protects stomach lining from HCl
  • Intrinsic factor: Required for vitamin B₁₂ absorption
• Food停留: 2-6 hours; proteins denatured by HCl
• Chyme: Semi-liquid food mixture released into duodenum

The Small Intestine:

• Duodenum (≈25 cm): Receives chyme from stomach + pancreatic juice + bile
• Jejunum (≈2.5 m): Main absorption site
• Ileum (≈3.5 m): Absorbs vitamin B₁₂ and bile salts

Villi and Absorption: The inner surface of the small intestine has circular folds → covered in villi (finger-like projections) → each villus has microvilli (brush border). This increases surface area by 600 times.

Absorption mechanisms:

• Simple diffusion: Small non-polar molecules (water, lipids, CO₂)
• Facilitated diffusion: Glucose, amino acids (via carrier proteins)
• Active transport: Glucose, amino acids, ions (requires ATP, against concentration gradient)
• Pinocytosis: Large molecules (some proteins)

Each Villus Has:

• Capillary network → absorbs glucose, amino acids
• Lacteal (lymphatic vessel) → absorbs fatty acids and glycerol (fatty acids combine with bile salts to form micelles for absorption)

The Large Intestine:

• Caecum and appendix: Small pouch at start of large intestine
• Colon: Absorbs water and mineral salts (1.5 L of water absorbed per day)
• Rectum: Stores faeces before egestion
• Faeces composition: Water (75%), bacteria (dead), undigested fibre, bile pigments (bilirubin = brown colour), epithelial cells

Pancreas:

• Exocrine function: Produces pancreatic juice containing:
  • Trypsin (protease)
  • Pancreatic amylase
  • Lipase
  • Sodium bicarbonate (neutralises stomach acid)
• Endocrine function: Produces insulin and glucagon (hormones regulating blood sugar)

Liver:

• Produces bile (stored in gallbladder)
• Bile contains bile salts (emulsify fats — break large globules into small droplets)
• Bile does NOT contain enzymes
• Metabolic functions: Stores glycogen, deaminates amino acids, synthesises plasma proteins, detoxifies alcohol and drugs

⚡ Common Student Mistakes: Students often say bile “digests fats” — WRONG. Bile only emulsifies fats (increases surface area for lipase). Digestion of fats is done by lipase. Also confuse where different enzymes work — pepsin in the stomach, trypsin and pancreatic enzymes in the duodenum.

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for thorough preparation.

Chemical Digestion in Detail:

Carbohydrate Digestion: Starch → (salivary amylase in mouth) → Maltose → (pancreatic amylase in duodenum) → Maltose → (maltase in ileum) → Glucose

Protein Digestion: Protein → (pepsin in stomach) → Polypeptides → (trypsin in duodenum) → Peptides → (peptidases in ileum) → Amino acids

Fat Digestion: Fat globules → (bile emulsification) → Small fat droplets → (lipase in duodenum) → Fatty acids + Glycerol → (in micelles) → absorbed into villi → reassembled into triglycerides → into lacteals → lymphatic system → blood

Mineral and Vitamin Absorption:

Nutrient	Absorption Site	Notes
Iron	Duodenum	Stored in ferritin; needs vitamin C for absorption
Calcium	Duodenum	Needs vitamin D for absorption
Vitamin B₁₂	Ileum	Needs intrinsic factor from stomach
Folate	Jejunum
Vitamin A, D, E, K	Ileum	Fat-soluble vitamins
Vitamin C, B complex	Jejunum	Water-soluble

Deficiency Diseases:

• Vitamin A deficiency: Night blindness, xerophthalmia, dry skin
• Vitamin C deficiency: Scurvy (bleeding gums, poor wound healing)
• Vitamin D deficiency: Rickets (children), osteomalacia (adults)
• Iron deficiency: Anaemia (fatigue, pale skin)
• Protein deficiency: Kwashiorkor (oedema, fatty liver), marasmus (wasting)

Digestive Disorders:

• Peptic ulcer: Erosion of stomach/duodenal lining by HCl and pepsin; caused by Helicobacter pylori bacteria
• Appendicitis: Inflammation of appendix
• Constipation: Dry, hard faeces due to excess water reabsorption in colon
• Diarrhoea: Too little water reabsorption (infections, food poisoning)
• Gastroenteritis: Inflammation of stomach and intestines (vomiting, diarrhoea)

Nutrition and Energy Requirements:

Basal Metabolic Rate (BMR): Energy used at complete rest ≈ 7,500 kJ/day for average adult male.

Recommended Daily Allowance (RDA) for Nigerian adult male:

• Energy: 10,000 kJ
• Protein: 60 g (must contain essential amino acids)
• Iron: 15 mg (higher for menstruating females)
• Vitamin A: 750 μg

Balanced Diet:

• Carbohydrates: 50-60% of diet — main energy source (yam, rice, cassava, maize)
• Proteins: 10-15% — growth and repair (beans, fish, meat, eggs)
• Fats: 20-30% — energy storage, insulation, vitamin absorption
• Vitamins and minerals: Trace amounts — regulate body processes
• Fibre (roughage): 30 g/day — peristalsis, prevents constipation

Food Tests:

Test	Substance	Reagent	Positive Result
Iodine test	Starch	Iodine solution	Blue-black
Benedict’s test	Reducing sugars	Benedict’s reagent → heat	Orange-red precipitate
Biuret test	Proteins	NaOH + CuSO₄	Violet/purple
Emulsion test	Fats	Ethanol + water	White emulsion

⚡ WAEC Examination Patterns: Draw and label the digestive system. Explain peristalsis. Name the enzymes and their functions. Describe what happens to food in each part of the GIT. Explain the absorption of nutrients across the villus. Be prepared for a “trace the journey of a [sandwich] through the digestive system” type question.