Human Digestive System

Syllabus: General Science (UPSC Prelims )
Source: NCERT

The human digestive system converts complex food into simple, absorbable forms. Large biomolecules like carbohydrates, proteins, and fats cannot be used by the body directly, so digestion breaks them into glucose, amino acids, fatty acids, and glycerol.

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1. Alimentary Canal (Digestive Tract)

The alimentary canal is a long tube through which food passes. It includes:
Mouth → Pharynx → Oesophagus → Stomach → Small Intestine → Large Intestine → Rectum → Anus

Activities of the digestive tract are controlled by nerves and hormones for proper coordination.

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2. Mouth and Buccal Cavity

Main functions

• Ingestion (taking in food)
• Chewing¹ with teeth (32 permanent teeth)
• Mixing food with saliva
• Formation of bolus

Saliva

• Secreted by salivary glands (parotid, submaxillary, sublingual)
• Contains:
  • Salivary amylase → breaks starch → maltose
  • Lysozyme → antibacterial
• Optimal pH: 6.8

Tongue

• Helps mix food, swallowing, tasting (taste buds on papillae)

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3. Oesophagus (Food Pipe)

• Connects mouth to stomach
• Swallowing is aided by mucus
• Food moves through peristalsis
• Epiglottis prevents entry of food into windpipe
• Food enters stomach through gastro-oesophageal sphincter

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4. Stomach

Functions

• Stores food for 4–5 hours
• Mixes food with gastric juice to form chyme

Secretions

• HCl → kills bacteria + provides acidic pH
• Pepsinogen → Pepsin (digests proteins)
• Mucus → protects stomach lining
• In infants: Rennin digests milk

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5. Small Intestine

Made of duodenum → jejunum → ileum

Key functions

• Major site of digestion and absorption of food.
• Receives:
  • Bile from liver
  • Pancreatic juice
• Inner surface has villi → increase absorption area

Enzyme activity in small intestine

Pancreatic enzymes

• Trypsinogen → Trypsin (activated by enterokinase)
• Breaks proteins, fats, carbohydrates, nucleic acids

Bile

• No enzymes
• Helps emulsify fats (breaks into small droplets)

Intestinal juice

• Contains disaccharidases, lipases, dipeptidases
• Alkaline medium (pH 7.8)

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6. Absorption of Digested Food

Absorption occurs through intestinal mucosa into blood or lymph.

Modes of absorption

• Simple diffusion → glucose, amino acids, electrolytes
• Facilitated transport → glucose, amino acids
• Active transport → nutrients against concentration gradient
• Fats:
  • Converted to micelles → chylomicrons
  • Absorbed into lacteals (lymph vessels)

Sites of absorption

• Mouth → some drugs
• Stomach → water, alcohol
• Small intestine → maximum absorption
• Large intestine → water, minerals

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7. Large Intestine

Includes caecum, colon, rectum

Functions

• Absorbs water, salts
• Forms faeces
• Stores faeces in rectum
• Removes waste through egestion

Contains appendix, a vestigial organ.

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8. Digestive Glands

Salivary glands

• Produce saliva → carbohydrate digestion

Liver

• Largest gland
• Produces bile (stored in gall bladder)
• Bile helps digest fats

Pancreas

• Exocrine: pancreatic enzymes
• Endocrine: insulin, glucagon

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9. Disorders of Digestive System

Common disorders

• Jaundice → liver affected; yellow eyes/skin
• Vomiting → reflex controlled by medulla
• Diarrhoea → watery stools, poor absorption
• Constipation → irregular bowel movement
• Indigestion → incomplete digestion due to overeating, low enzymes, anxiety
• Parasitic infections → tapeworm, roundworm, pinworm

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10. Key Steps of Digestion

Ingestion → Digestion → Absorption → Assimilation → Egestion

1. Question Reference
   (HPAS PYQ 2025) ↩︎