Gas Exchange Surfaces
Efficient exchange surfaces follow Fick's law: rate ∝ (surface area × concentration difference) ÷ thickness. Alveoli: huge surface area, one-cell-thick squamous epithelium next to capillary endothelium, steep gradient kept by ventilation and blood flow, elastic fibres for recoil. Airways: cartilage (trachea, bronchi) keeps them open; goblet cells make mucus; cilia sweep it away; smooth muscle controls diameter. Ventilation: inspiration (diaphragm and external intercostals contract, volume ↑, pressure ↓); expiration largely passive. Pulmonary ventilation = tidal volume × breathing rate. Fish gills use countercurrent flow to keep a gradient along the whole lamella; insects use tracheae and spiracles; leaves use stomata and air spaces.
Learning objectives
- Describe the structure and function of airway tissues.
- Explain how alveoli are adapted.
- Describe ventilation and calculate pulmonary ventilation.
- Compare gas exchange in fish, insects and plants.
Why do fish die quickly out of water even though air has more oxygen?
Core idea
Your alveoli have about 70 m² of surface, packed into lungs that fit in your ribcage.
Gas exchange surfaces maximise area and gradient and minimise distance, as Fick's law predicts.
Each organism solves the same problem.
Tidal volume 0.50 dm³; 12 breaths per minute.
- Goal
- PV.
- Strategy
- TV × BR.
- Multiply.
- Units.
Summary
- Big, thin, steep.
- Airway tissues.
- Countercurrent in gills.
- Tidal volume
- Volume of air per normal breath.
- Countercurrent
- Two fluids flowing in opposite directions.
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