Human Body — Respiratory System
1. Introduction
The respiratory system helps us breathe and provides oxygen to every cell in the body.
Respiration is the process by which living organisms obtain oxygen from the environment and use it to release energy from food.
Every living cell needs energy for growth, repair, movement, and other life processes. Respiration provides this energy.
2. Organs of the Respiratory System
| Organ | Description | Function |
|---|---|---|
| Nose (nasal cavity) | Openings (nostrils), lined with hair and mucus | Filters, warms, and moistens inhaled air |
| Pharynx | Common passage for air and food | Connects nose to trachea |
| Larynx (voice box) | Contains vocal cords | Produces sound, allows air to pass |
| Trachea (windpipe) | Tube with C-shaped cartilage rings | Carries air to the lungs |
| Bronchi | Two branches of the trachea | Carry air to each lung |
| Bronchioles | Smaller branches in the lungs | Distribute air within the lungs |
| Alveoli | Tiny air sacs (millions) | Site of gas exchange (oxygen into blood, CO2 out) |
| Lungs | Spongy, elastic organs in the chest cavity | Main organs of respiration |
Worked Example: Trace the path of air from the nose to the alveoli.
Nose -> Pharynx -> Larynx -> Trachea -> Bronchi -> Bronchioles -> Alveoli.
Common Mistake: Thinking we breathe in only oxygen and breathe out only carbon dioxide. Inhaled air contains about 21% oxygen and 0.04% CO2. Exhaled air contains about 16% oxygen and 4% CO2. We do breathe out some oxygen.
3. The Mechanism of Breathing
Breathing is the physical process of taking in air (inhalation) and giving out air (exhalation).
Inhalation (Breathing In)
- The diaphragm contracts and flattens.
- The rib cage moves upward and outward.
- The chest cavity expands.
- Air rushes into the lungs.
Exhalation (Breathing Out)
- The diaphragm relaxes and moves up.
- The rib cage moves downward and inward.
- The chest cavity contracts.
- Air is pushed out of the lungs.
Exam Focus (2 marks): 'What is the role of the diaphragm in breathing?'
The diaphragm is a dome-shaped muscle below the lungs. When it contracts (flattens), the chest cavity expands, causing inhalation. When it relaxes, the chest cavity contracts, causing exhalation.
4. Breathing vs Respiration
| Feature | Breathing | Respiration |
|---|---|---|
| Definition | Mechanical process of air movement | Chemical process of energy release |
| Location | Lungs (external) | Every living cell (internal) |
| Type | Physical process | Chemical process |
| Involves | Inhalation and exhalation | Oxidation of glucose |
| Energy | No energy released | Energy (ATP) released |
| Waste product | CO2 and water vapour | CO2 and water |
Common Mistake: Using 'breathing' and 'respiration' interchangeably. Breathing is the physical movement of air; respiration is the chemical process inside cells that releases energy.
5. Types of Respiration
Aerobic Respiration (with oxygen)
Glucose + Oxygen -> Carbon dioxide + Water + Energy (36 ATP)
Occurs in: Most cells.
Products: CO2, water, large amount of energy.
Anaerobic Respiration (without oxygen)
Glucose -> Lactic acid + Energy (2 ATP)
OR
Glucose -> Alcohol + CO2 + Energy (2 ATP)
Occurs in: Muscle cells during intense exercise, yeast.
Products: Lactic acid (muscles) or alcohol (yeast), little energy.
Worked Example: Why do your muscles feel sore after heavy exercise?
During heavy exercise, the body cannot supply enough oxygen to the muscles quickly enough. Muscles resort to anaerobic respiration, producing lactic acid. The buildup of lactic acid causes muscle soreness.
6. Importance of Oxygen
- Energy release: Oxygen is essential for aerobic respiration, which releases maximum energy from food.
- Cell function: All cells need energy (from respiration) to perform their functions.
- Brain function: The brain is highly sensitive to oxygen shortage. Brain damage can occur within minutes without oxygen.
- Burning of waste: Oxygen helps break down waste substances in the body.
7. Comparison Table: Inhaled vs Exhaled Air
| Gas | Inhaled Air | Exhaled Air |
|---|---|---|
| Oxygen | 21% | 16% |
| Carbon dioxide | 0.04% | 4% |
| Nitrogen | 78% | 78% |
| Water vapour | Variable | Saturated |
| Temperature | Cool | Warm |
8. Self-Test
- List the organs of the respiratory system in order from nose to lungs.
- Differentiate between breathing and respiration.
- What is the function of the alveoli?
- Describe what happens during inhalation.
- Differentiate between aerobic and anaerobic respiration.
- Why does the air we breathe out contain less oxygen than the air we breathe in?
- Why do we breathe faster when we exercise?
9. Answers to Self-Test
- Nose -> Pharynx -> Larynx -> Trachea -> Bronchi -> Bronchioles -> Alveoli.
- Breathing: physical movement of air in/out. Respiration: chemical release of energy in cells.
- Alveoli are tiny air sacs in the lungs where oxygen passes into the blood and CO2 comes out (gas exchange).
- Diaphragm contracts and flattens, rib cage moves up and out, chest cavity expands, air rushes in.
- Aerobic: uses oxygen, produces lots of energy, CO2 and water. Anaerobic: no oxygen, produces less energy, lactic acid or alcohol.
- Some oxygen in the inhaled air is absorbed by the blood in the lungs and used for respiration in body cells.
- During exercise, muscles need more energy, so they need more oxygen. Faster breathing supplies more oxygen to the blood.
