Gas Exchange & Breathing

**1. Introduction & Core Concept**

Karachi's air quality ranks among the worst in South Asia — PM2.5 particles, vehicle exhaust, factory smoke. Every breath you take in Karachi delivers not just oxygen to your blood, but also pollutants that your lungs must deal with. Understanding how your lungs work gives you insight into why air quality is a health issue, not just an inconvenience.

Your lungs perform gas exchange: getting O₂ into your blood and CO₂ out. This happens across millions of tiny air sacs called alveoli — a surface area of approximately 70 m² (the size of half a badminton court) folded inside your chest.

**2. Core Theory**

2.1 — The Respiratory System

Air pathway: Nose/Mouth → Trachea → Bronchi → Bronchioles → Alveoli

| Structure | Function |

|---------|---------|

| Nose | Warms, moistens, filters air (hairs + mucus) |

| Trachea | Rings of cartilage keep airway open; lined with cilia and mucus |

| Bronchi | Two branches (left and right lung) |

| Bronchioles | Smaller branches; no cartilage; smooth muscle controls diameter |

| Alveoli | Site of gas exchange; grape-like air sacs |

2.2 — Alveoli: Adaptations for Gas Exchange

• Huge number → enormous total surface area (~70 m²)
• One cell thick (one cell's diffusion distance)
• Moist lining (gases dissolve to cross)
• Rich capillary supply (maintains concentration gradient)
• Good blood flow keeps gradient steep

Gas exchange by diffusion:

• O₂: alveolus (high) → blood capillary (low)
• CO₂: blood capillary (high) → alveolus (low)

2.3 — Breathing Mechanism (Ventilation)

*Inhalation (breathing in):*

• Diaphragm contracts (flattens)
• External intercostal muscles contract (ribs move up and out)
• Volume of chest cavity increases → pressure decreases → air rushes IN

*Exhalation (breathing out):*

• Diaphragm relaxes (domes up)
• Intercostal muscles relax (ribs fall in and down)
• Volume decreases → pressure increases → air pushed OUT

2.4 — Smoking and Lung Disease

Smoking damages the respiratory system:

• Tar: coats alveoli, reduces gas exchange surface area.
• Carbon monoxide: binds to haemoglobin instead of O₂ (250× stronger affinity) — reduces oxygen carrying capacity.
• Nicotine: addictive; constricts blood vessels; raises blood pressure.
• Carcinogens in tar: cause mutations → lung cancer.

Diseases linked to smoking:

• Chronic bronchitis: inflamed bronchi, excess mucus production.
• Emphysema: alveoli walls break down → fewer, larger sacs → reduced surface area → breathlessness.
• Lung cancer: uncontrolled cell division of lung tissue.

**3. Worked Examples**

Example 1: Explain why alveoli have a moist lining.

*Answer:* Oxygen and carbon dioxide must dissolve in the moisture before they can diffuse across the alveolar wall. A dry alveolus could not facilitate gas exchange.

Example 2: Compare gas exchange in a healthy lung and a smoker's lung.

*Answer:* In a healthy lung, millions of alveoli provide a large surface area for rapid diffusion. In a smoker with emphysema, alveolar walls have broken down, producing fewer, larger sacs. This drastically reduces the total surface area and thus the rate of gas exchange, causing breathlessness.

Example 3: During a PE lesson, a student's breathing rate increases from 15 to 30 breaths/min. Explain why.

*Answer:* Exercise increases the rate of aerobic respiration in muscles, producing more CO₂. Rising CO₂ in the blood is detected by the brain (medulla oblongata), which sends nerve signals to increase breathing rate to expel CO₂ and take in more O₂.

**4. Pakistan Angle**

Karachi's air quality regularly records AQI (Air Quality Index) above 150 (unhealthy) due to vehicle emissions, factory smoke, and open burning of waste. The fine particles (PM2.5) penetrate deep into the alveoli, causing inflammation and reducing gas exchange efficiency. A 2022 study found that Karachi residents have measurably lower lung capacity than people in cleaner-air cities — direct evidence of impaired alveolar function.

Pakistan has one of the highest rates of smoking among men in South Asia (~32% of males). Combined with air pollution, this creates a severe burden of COPD (Chronic Obstructive Pulmonary Disease) on Pakistan's healthcare system. The biology of alveoli is at the centre of this public health crisis.

**5. Exam Strategy**

• Breathing vs Respiration: breathing = ventilation (physical movement of air). Respiration = chemical energy release in cells. Never confuse these.
• Alveoli adaptations: always give the reason for each adaptation, not just the feature (e.g., "one cell thick — so diffusion distance is short").
• For smoking questions, link each chemical (tar, CO, nicotine) to its specific effect — do not mix them up.
• Concentration gradient: gas exchange only continues by diffusion if the gradient is maintained by blood flow removing O₂ and delivering CO₂. Mention blood flow in your answers.
• Emphysema: reduced surface area → reduced rate of diffusion → less O₂ in blood → breathlessness.