Respiratory System
Introduction
Respiration is the process by which living organisms obtain energy from food. ICSE Class 9 covers the human respiratory system, the mechanism of breathing, and the comparison between aerobic and anaerobic respiration.
Need for Respiration
All living cells require energy for various activities. This energy comes from the breakdown of glucose through respiration.
Importance of respiration:
- Provides energy for cellular activities
- Removes carbon dioxide (a waste product)
- Maintains body temperature
Organs of Respiration in Humans
Nose and Nasal Cavity
- Air enters through nostrils
- Nasal hairs filter dust particles
- Mucus moistens and warms the air
- Olfactory receptors detect smell
Pharynx (Throat)
- Common passage for air and food
- Epiglottis: Flap that closes the windpipe during swallowing
Larynx (Voice Box)
- Contains vocal cords
- Produces sound when air passes through
Trachea (Windpipe)
- Tube supported by C-shaped cartilage rings
- Prevents collapse during breathing
- Lined with ciliated epithelium and mucus
Bronchi and Bronchioles
- Trachea divides into two bronchi (one to each lung)
- Bronchi further divide into smaller bronchioles
- Bronchioles end in alveoli
Lungs
- Two large, spongy, elastic organs in the thoracic cavity
- Right lung: 3 lobes; Left lung: 2 lobes (to accommodate heart)
- Alveoli: Tiny air sacs where gaseous exchange occurs
- Millions of alveoli provide a large surface area
- Surrounded by capillaries
Alveoli: Site of Gaseous Exchange
Features:
- Thin walls (one cell thick)
- Rich blood supply (capillaries)
- Large surface area
Exchange: Oxygen moves from alveoli to blood; CO₂ moves from blood to alveoli.
Mechanism of Breathing
Inhalation (Breathing In)
- Diaphragm contracts and flattens
- Intercostal muscles contract (ribs move upward and outward)
- Volume of thoracic cavity increases
- Pressure inside lungs decreases (below atmospheric)
- Air rushes into the lungs
Exhalation (Breathing Out)
- Diaphragm relaxes and becomes dome-shaped
- Intercostal muscles relax (ribs move downward and inward)
- Volume of thoracic cavity decreases
- Pressure inside lungs increases (above atmospheric)
- Air moves out of the lungs
| Feature | Inhalation | Exhalation |
|---|---|---|
| Diaphragm | Contracts, moves down | Relaxes, moves up |
| Rib cage | Moves up and out | Moves down and in |
| Chest volume | Increases | Decreases |
| Air pressure in lungs | Decreases | Increases |
| Air movement | Into lungs | Out of lungs |
Aerobic vs Anaerobic Respiration
Aerobic Respiration
Occurs in the presence of oxygen.
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (36-38 ATP)
Site: Mitochondria Products: CO₂, H₂O, large amount of energy
Anaerobic Respiration
Occurs in the absence of oxygen.
In muscles (during vigorous exercise): C₆H₁₂O₆ → 2C₃H₆O₃ + Energy (2 ATP) (Lactic acid accumulates, causing muscle fatigue)
In yeast (fermentation): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy (2 ATP) (Ethanol is produced)
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen required | Yes | No |
| Energy produced | 36-38 ATP | 2 ATP |
| End products | CO₂ + H₂O | Lactic acid OR ethanol + CO₂ |
| Site | Mitochondria | Cytoplasm |
| Example | Most organisms | Muscles, yeast, bacteria |
Respiratory Volumes
| Volume | Description | Average (adult) |
|---|---|---|
| Tidal volume | Air inhaled/exhaled in normal breath | ~500 mL |
| Inspiratory reserve volume | Extra air inhaled forcibly after normal breath | ~3000 mL |
| Expiratory reserve volume | Extra air exhaled forcibly after normal breath | ~1100 mL |
| Vital capacity | Maximum air exhaled after deepest inhalation | ~4600 mL |
| Residual volume | Air remaining in lungs after maximum exhalation | ~1200 mL |
Common Mistakes With Fixes
| Mistake | Correction |
|---|---|
| Breathing and respiration are the same | Breathing is physical (air movement); respiration is chemical (energy release) |
| Lactic acid is good for muscles | Lactic acid causes muscle fatigue and cramp |
| Anaerobic respiration occurs only in bacteria | Anaerobic respiration also occurs in yeast and human muscles during exercise |
| Oxygen enters blood directly through lung walls | Oxygen enters through ALVEOLI (not the whole lung) |
ICSE Exam Focus
| Topic | Marks (approx.) | Frequency |
|---|---|---|
| Mechanism of breathing | 4-5 marks | Very common |
| Aerobic vs anaerobic respiration | 4-5 marks | Very common |
| Human respiratory system (labelled diagram) | 5-6 marks | Very common |
| Gaseous exchange in alveoli | 3-4 marks | Frequently asked |
Self-Test
Q1: Draw a labelled diagram of the human respiratory system.
Q2: Explain the mechanism of inhalation and exhalation.
Q3: Differentiate between aerobic and anaerobic respiration.
Q4: What is the role of the diaphragm in breathing?
Q5: Write the chemical equation for aerobic respiration.
