Gas Exchange Surfaces
Principles of Gas Exchange
All gas exchange surfaces share common features that maximise the rate of diffusion, as described by Fick's law:
Rate of diffusion ∝ (surface area × concentration difference) / diffusion distance
To maximise gas exchange, surfaces are adapted to:
- Provide a large surface area
- Maintain a steep concentration gradient (usually by ventilation and blood flow)
- Have a short diffusion distance (thin barriers)
- Be moist — gases dissolve before diffusing across cell membranes
Gas Exchange in Mammals: The Lungs
Structure of the Gas Exchange System
Air enters through the nose/mouth → trachea → bronchi (one to each lung) → bronchioles → terminal bronchioles → alveoli.
Trachea and bronchi:
- Walls contain C-shaped rings of cartilage that hold the airway open and prevent collapse during inhalation (the gap in the C faces posteriorly, allowing the oesophagus to expand during swallowing)
- Lined with ciliated epithelium and goblet cells — goblet cells secrete mucus that traps inhaled particles and pathogens; cilia beat in a coordinated wave to move mucus up toward the throat (the mucociliary escalator)
- Smooth muscle in the walls can contract to narrow the airway (bronchoconstriction, e.g. in asthma)
Bronchioles:
- Smaller airways with no cartilage (walls are thin with smooth muscle and elastic fibres)
- Smooth muscle controls airway diameter — important in regulating airflow to different regions of the lung
- Terminal bronchioles lead to alveolar ducts and alveoli
Alveoli: The Gas Exchange Surface
The lungs contain approximately 480 million alveoli, providing a total surface area of around 70 m² (roughly the size of a tennis court).
Adaptations for efficient gas exchange:
- Large surface area — the enormous number of alveoli
- Thin walls — each alveolus is lined by a single layer of type I pneumocytes (squamous epithelial cells), approximately 0.1-0.5 μm thick
- Dense capillary network — each alveolus is wrapped in capillaries; the total diffusion barrier (alveolar epithelium + capillary endothelium + shared basement membrane) is only ~0.5 μm
- Steep concentration gradient maintained by:
- Ventilation (breathing) — constantly refreshes air in the alveoli
- Blood flow — deoxygenated blood arriving in pulmonary capillaries has low pO₂ and high pCO₂; after gas exchange, oxygenated blood is carried away, maintaining the gradient
- Moist lining — a thin film of moisture on the alveolar surface allows gases to dissolve before diffusing. Type II pneumocytes secrete surfactant, a phospholipid mixture that reduces surface tension and prevents alveoli from collapsing during exhalation
Ventilation: The Mechanism of Breathing
Ventilation is the mechanical process that moves air in and out of the lungs, maintaining the concentration gradient for gas exchange.
Inhalation (active process):
1. External intercostal muscles contract, pulling the rib cage up and out
2. Diaphragm contracts and flattens (moves downward)
3. Thoracic volume increases
4. Intrapulmonary pressure decreases (drops below atmospheric pressure)
5. Air flows into the lungs down the pressure gradient
Exhalation (passive at rest; active during exercise):
1. External intercostal muscles relax; ribs move down and in under gravity and elastic recoil
2. Diaphragm relaxes and returns to its dome shape
3. Thoracic volume decreases
4. Intrapulmonary pressure increases (rises above atmospheric pressure)
5. Air flows out of the lungs
6. During forced exhalation, internal intercostal muscles contract and abdominal muscles contract to further reduce thoracic volume
Lung Volumes
Lung volumes can be measured using a spirometer:
| Volume | Definition | Typical value |
|---|---|---|
| Tidal volume (TV) | Volume breathed in or out in a normal breath | ~0.5 L |
| Vital capacity (VC) | Maximum volume that can be breathed in after maximum exhalation | ~4.6 L |
| Residual volume | Volume remaining in the lungs after maximum exhalation (prevents alveoli from collapsing) | ~1.2 L |
| Total lung capacity | Vital capacity + residual volume | ~5.8 L |
Ventilation rate = tidal volume × breathing rate
Gas Exchange in Other Organisms
Fish: Gills
Fish use gills for gas exchange with water. Each gill arch bears many gill filaments, and each filament carries many lamellae (thin plates) richly supplied with blood capillaries.
Key adaptation: countercurrent flow — water flows over the lamellae in the opposite direction to blood flow within the capillaries. This maintains a concentration gradient along the entire length of the lamella, achieving up to 80% oxygen extraction (compared to ~25% with parallel flow).
Insects: Tracheal System
Insects have a system of tracheae (air-filled tubes) that branch into fine tracheoles penetrating directly to individual cells. Air enters through spiracles on the body surface. Gas exchange occurs at the ends of tracheoles, where oxygen dissolves in a thin film of fluid. This is a direct system — the circulatory system does not carry respiratory gases.
Plants: Stomata and Mesophyll
Gas exchange in plants occurs through stomata (pores in leaf epidermis, controlled by guard cells) and across the surfaces of spongy mesophyll cells. Air spaces between mesophyll cells provide a large internal surface area. CO₂ diffuses in for photosynthesis; O₂ diffuses out. At night, the pattern reverses as respiration predominates.
Exam Tips
- AQA frequently asks you to apply Fick's law to explain adaptations — always mention surface area, concentration gradient, AND diffusion distance
- Spirometer trace interpretation is a common question — be able to identify tidal volume, vital capacity, and calculate ventilation rate from the trace
- When comparing gas exchange in different organisms, focus on how each meets the requirements of Fick's law in its particular environment
- Remember that countercurrent flow in fish gills maintains a gradient along the ENTIRE length — parallel flow would reach equilibrium partway along