3.1.5 Adaptations of Gas Exchange Surfaces

Gas Exchange in Fish:

Fish have a small surface area to volume ratio and an impermeable skin, so gases cannot diffuse directly through their body. As a result, fish rely on specialised structures for efficient gas exchange.

1. Structure of Gills:

• Fish have four pairs of gills, each supported by a bony arch.
• Each gill is made up of numerous gill filaments, which increase the surface area for gas exchange.
• Lamellae are thin projections on the gill filaments, further increasing the surface area.

2. Counter-Current Flow:

• Blood flows in the opposite direction to water over the lamellae.
• This maintains a steep diffusion gradient across the entire gill surface.
• It ensures that oxygen continuously diffuses from water (high concentration) into the blood (low concentration), maximising oxygen uptake.

3. Adaptations for Efficiency:

• Thin membranes: Reduce the diffusion distance for gases.
• Large surface area: Provided by filaments and lamellae for maximum gas exchange.
• Good blood supply: Capillaries in the lamellae transport oxygenated blood efficiently.

4. Water Dependency:

• The projections are held apart by water flow, ensuring maximum exposure for gas exchange.
• In the absence of water, the gill filaments stick together, preventing efficient gas exchange. This is why fish cannot survive long out of water.

Gas Exchange in Insects:

Insects rely on a tracheal system for gas exchange rather than blood.

5. Structure:

• Insects have spiracles (tiny openings) on their body surface.
• Spiracles lead to a network of tracheae, which branch into smaller tracheoles that deliver oxygen directly to tissues.

6. Adaptations:

• Tracheoles have thin walls, reducing the diffusion distance.
• The large surface area of the tracheal network ensures sufficient oxygen delivery.
• Abdominal movements help ventilate the tracheal system, maintaining a steep diffusion gradient.

Gas Exchange in Amphibians:

Amphibians, like frogs, use both their skin and lungs for gas exchange.

7. Skin:

• Moist and permeable, allowing diffusion of oxygen and carbon dioxide.
• Blood vessels close to the skin surface maintain the diffusion gradient.

8. Lungs:

• Used for gas exchange when the animal is active or out of water.
• Provide a larger surface area compared to skin alone.

Gas Exchange in Mammals:

Mammals have a highly efficient system adapted for their high metabolic demands.

9. Lungs:

• Alveoli provide a large surface area for gas exchange.
• Thin walls (one cell thick) reduce diffusion distance.
• Surrounded by a dense capillary network, maintaining a steep diffusion gradient.

10. Ventilation:

• Breathing movements maintain the flow of air, replenishing oxygen and removing carbon dioxide.
• Blood flow through capillaries ensures gases are constantly transported to and from the alveoli.

Summary of Adaptations for Gas Exchange:

Organism	Adaptation	Reason
Fish	Gills with filaments and lamellae, counter-current flow, and good blood supply.	Maximise oxygen uptake in water.
Insects	Tracheal system with spiracles, tracheae, and tracheoles.	Direct delivery of oxygen to tissues.
Amphibians	Moist skin and lungs.	Use skin for diffusion and lungs for activity.
Mammals	Alveoli with thin walls, large surface area, and ventilation system.	Meet high oxygen demand and metabolic rate.