### The Respiratory System
By the end of this section, you should be able to:
- Explain the importance of breathing.
- Describe the structure and function of the respiratory system.
- Understand gas exchange, factors affecting breathing, and the effects of smoking.
- Demonstrate knowledge of breathing hygiene and artificial respiration.

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### 1. Importance of Breathing
- Purpose: Supplies oxygen (O₂) to cells and removes carbon dioxide (CO₂), a waste product of cellular respiration.
- Diffusion limitation: Humans cannot rely on simple diffusion (unlike single-celled organisms) due to large body size and high cell count.

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### 2. Structure of the Human Respiratory System
#### Key Components:
- Nasal passages:
- Filter dust/pollen via hairs and mucus.
- Warm and moisten inhaled air.
- Trachea:
- Lined with cilia and mucus to trap particles.
- Supported by C-shaped cartilage rings (prevents collapse during swallowing).
- Larynx (voice box): Produces sound via vocal cords.
- Bronchi & Bronchioles: Branching tubes leading to alveoli.
- Alveoli:
- Tiny air sacs with a large surface area (~500 million; equivalent to a tennis court).
- Site of gas exchange; surrounded by capillaries.
- Diaphragm & Intercostal muscles: Control breathing movements.

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### 3. Mechanism of Breathing
#### Inhalation:
- Process:
- Diaphragm contracts (flattens).
- External intercostal muscles contract, lifting ribs upward/outward.
- Thoracic volume ↑ → pressure ↓ → air rushes in.
#### Exhalation:
- Process:
- Diaphragm relaxes (domes upward).
- Ribs drop due to gravity/internal intercostal muscle contraction.
- Thoracic volume ↓ → pressure ↑ → air forced out.
- Ventilation: Movement of air in/out of lungs.

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### 4. Gas Exchange in Alveoli
- Key Requirements:
- Large surface area, moist surfaces, short diffusion distance (0.001 mm), rich blood supply.
- Process:
- O₂ diffuses from alveoli into blood (high to low concentration).
- CO₂ diffuses from blood into alveoli.
- Composition of Air:

| Gas | Inhaled Air | Exhaled Air |
|--------------|-------------|-------------|
| Oxygen | 21% | 16% |
| CO₂ | 0.04% | 4% |
| Nitrogen | ~80% | ~80% |

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### 5. Factors Affecting Breathing Rate
1. Exercise: ↑ O₂ demand → ↑ breathing rate/depth.
2. Anxiety: Triggers "fight or flight" response → rapid breathing.
3. Drugs:
- Stimulants (e.g., cocaine) ↑ breathing rate.
- Depressants (e.g., opioids) ↓ breathing rate.
4. Altitude: Low O₂ at high elevations → ↑ breathing rate (compensatory mechanism).
5. Weight: Obesity restricts diaphragm movement → breathlessness.
6. Smoking: Damages lungs → chronic diseases (see below).

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### 6. Effects of Smoking
- Chemicals in Smoke:
- Nicotine: Addictive.
- Carbon Monoxide (CO): Binds to hemoglobin, reducing O₂ transport.
- Tar: Carcinogen; paralyzes cilia, leading to infections.
- Diseases:
- Bronchitis: Inflammation of bronchi.
- COPD (Chronic Obstructive Pulmonary Disease): Alveoli destruction → ↓ surface area.
- Lung cancer: 90% linked to smoking.
- Economic Impact: Loss of income (illness/death), increased healthcare costs.

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### 7. Breathing Hygiene
- Practices:
- Cover mouth/nose when coughing/sneezing.
- Maintain oral hygiene (prevents bad breath/infection).
- Avoid smoking/shisha.
- Artificial Respiration (Mouth-to-Mouth):
1. Check airway for obstructions.
2. Tilt head back, lift chin.
3. Pinch nostrils, seal mouth, and blow air into lungs (12 breaths/minute).

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