Human Body — Digestive System
1. Introduction
The digestive system is responsible for breaking down food into simpler substances that the body can absorb and use.
Digestion is the process by which complex food is broken down into simpler, soluble substances.
Why do we need digestion? The food we eat (rice, bread, fruits, meat) is made of large molecules that cannot be used directly by our body. Digestion breaks them down into small molecules that can be absorbed into the blood.
2. Organs of the Digestive System
The digestive system consists of the alimentary canal (a long tube from mouth to anus) and associated digestive glands.
| Organ | Description | Function |
|---|---|---|
| Mouth | Teeth, tongue, salivary glands | Chewing, mixing with saliva |
| Oesophagus | Muscular tube | Carries food from mouth to stomach |
| Stomach | J-shaped muscular bag | Mixes food with gastric juices |
| Small intestine | Long coiled tube (about 6 m) | Complete digestion and absorption |
| Large intestine | Wider tube (about 1.5 m) | Absorbs water and minerals |
| Rectum | Last part of large intestine | Stores undigested waste |
| Anus | Opening at the end | Eliminates waste (egestion) |
Associated Glands
| Gland | Secretion | Function |
|---|---|---|
| Salivary glands | Saliva | Moistens food, starts starch digestion |
| Liver | Bile | Breaks down fats |
| Pancreas | Pancreatic juice | Digests proteins, fats, carbohydrates |
Common Mistake: Thinking the liver produces digestive enzymes. The liver produces bile, which helps in fat digestion but does NOT contain enzymes. Enzymes are produced by the pancreas and salivary glands.
3. The Process of Digestion
Step 1: Mouth
- Teeth break food into smaller pieces (mechanical digestion).
- Saliva contains an enzyme (amylase) that breaks down starch into sugar.
- The tongue helps mix food with saliva and pushes it into the oesophagus.
Step 2: Oesophagus
- Muscles contract in a wave-like motion called peristalsis.
- Peristalsis pushes food down into the stomach.
Step 3: Stomach
- Gastric juices contain hydrochloric acid (HCl) and enzymes.
- HCl kills bacteria and creates an acidic medium.
- Enzymes (pepsin) break down proteins.
- The food becomes a semi-liquid paste called chyme.
Step 4: Small Intestine
- Duodenum (first part): Bile from the liver and pancreatic juice enter here.
- Bile breaks down fats.
- Pancreatic juice breaks down proteins, carbohydrates, and fats.
- The intestinal wall absorbs nutrients into the blood through tiny finger-like projections called villi.
- Villi increase the surface area for absorption.
Exam Focus (3 marks): 'What are villi? Why are they important?'
Villi are tiny, finger-like projections on the inner wall of the small intestine. They increase the surface area for absorption of digested food into the bloodstream.
Step 5: Large Intestine
- Absorbs water and minerals from the undigested food.
- The remaining solid waste (faeces) is pushed to the rectum.
Step 6: Rectum and Anus
- Faeces is stored in the rectum and eliminated through the anus.
4. Balanced Diet
A balanced diet contains all the essential nutrients in the right proportions.
Nutrients
| Nutrient | Function | Sources |
|---|---|---|
| Carbohydrates | Provide energy | Rice, wheat, bread, potatoes |
| Proteins | Growth and repair | Milk, eggs, meat, pulses |
| Fats | Store energy, insulate | Butter, oil, nuts, ghee |
| Vitamins | Protect from diseases | Fruits, vegetables |
| Minerals | Various body functions | Milk (calcium), spinach (iron) |
| Fibre | Helps digestion | Vegetables, whole grains |
| Water | All life processes | Water, juices, fruits |
Common Mistake: Thinking all fats are bad. Fats are essential for storing energy and absorbing certain vitamins. Only excess fat consumption is harmful.
5. Comparison Table: Digestive Organs
| Organ | Secretion | What It Digests |
|---|---|---|
| Mouth (saliva) | Salivary amylase | Starch |
| Stomach | Gastric juice (HCl + pepsin) | Proteins |
| Small intestine (pancreatic juice) | Trypsin, amylase, lipase | Proteins, starch, fats |
| Small intestine (bile) | Bile salts | Fats (emulsification) |
| Small intestine (intestinal juice) | Various enzymes | All food groups |
6. Self-Test
- List the main organs of the digestive system in order from mouth to anus.
- What is peristalsis?
- Name the enzyme in saliva and what it digests.
- What is the role of hydrochloric acid in the stomach?
- What are villi? Where are they found?
- Differentiate between the small intestine and large intestine.
- Name three nutrients and their sources.
7. Answers to Self-Test
- Mouth -> Oesophagus -> Stomach -> Small intestine -> Large intestine -> Rectum -> Anus.
- Wave-like muscle contractions that push food through the oesophagus and digestive tract.
- Salivary amylase. It digests starch into sugar.
- Kills bacteria in food and creates an acidic medium for enzymes to work.
- Finger-like projections in the small intestine that absorb digested food.
- Small intestine: long, narrow, absorbs nutrients. Large intestine: shorter, wider, absorbs water.
- Carbohydrates (rice, wheat) — energy. Proteins (milk, eggs) — growth. Vitamins (fruits) — protection from diseases.
