Gas exchange in fish
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of the principles of diffusion. You can test your knowledge on this below.
How do insects reduce water loss while maintaining efficient gas exchange?
Insects have spiracles that can close to prevent water loss. Their exoskeleton is waterproof, and the spiracles may be surrounded with hairs to trap water vapour and reduce water loss.
How does the tracheal system in insects ensure oxygen reaches all cells efficiently?
The tracheae branch into tiny tracheoles that extend directly to respiring cells. The tracheoles have thin walls, providing a short diffusion distance and as they are highly branched, they provide a large surface area for oxygen uptake.
How does ventilation occur in active insects?
Larger insects use abdominal pumping to pump air in and out of the tracheal system, maintaining a steep concentration gradient for rapid diffusion of oxygen and carbon dioxide.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains gas exchange in fish or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Intro to Gas Exchange in Fish
Fish use gills for gas exchange, allowing them to extract oxygen from water efficiently. Since water is denser than air and contains less oxygen, fish have evolved specialised adaptations to maximise oxygen uptake.
How Fish Ventilate Their Gills
Fish actively ventilate their gills:
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Inhalation (Water Intake):
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The fish lowers the floor of its mouth (buccal cavity), increasing the volume and decreasing the pressure, causing water to flow in.
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The operculum (gill cover) remains closed to ensure water enters.
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Exhalation (Water Flow Over Gills):
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The fish raises the floor of the buccal cavity, decreasing the volume and increasing the pressure, forcing water over the gills.
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The operculum opens, allowing water to leave, ensuring continuous flow over the gills for gas exchange.
How the Gills Are Adapted for Efficient Gas Exchange
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Large Surface Area
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Gills are made of gill arches covered in many gill filaments, which are further covered in gill lamellae.
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The highly folded structure increases the surface area for diffusion.
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Short Diffusion Pathway
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The gill lamellae have a single layer of thin epithelial cells, minimising the distance between the water and the blood capillaries, ensuring rapid diffusion of gases.
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Steep Concentration Gradient via the Counter-Current Mechanism
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Blood flows in the opposite direction to the water, ensuring that water with higher oxygen concentration is always next to blood with lower oxygen concentration.
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This maintains a steep diffusion gradient along the entire length of the gill, maximising oxygen uptake.
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If blood and water flowed in the same direction, equilibrium would be reached, and diffusion would stop.
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Many capillaries in the gill lamellae, minimising the distance between the water and the blood capillaries, ensuring rapid diffusion of gases.
Key Terms
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Gill Filaments: Thin structures in fish gills that increase the surface area for gas exchange.
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Gill Lamellae: Small projections on the gill filaments that further increase surface area and contain capillaries for diffusion.
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Counter-Current Exchange: The mechanism in which blood and water flow in opposite directions to maintain a steep oxygen gradient.
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Operculum: The bony flap covering the gills that controls water flow.
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Buccal Cavity: The mouth cavity of the fish involved in ventilation.
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Exam Tip
When explaining the counter-current system, clearly state how it maintains a steep oxygen concentration gradient along the entire gill surface, rather than just mentioning opposite flows.
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Explain how the gill structures ensure efficient gas exchange in fish. (4 marks)
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Gill filaments and lamellae provide a large surface area for rapid diffusion.
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Thin epithelium of the lamellae ensures a short diffusion distance for rapid diffusion.
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Counter-current flow meaning water flows in the opposite direction to blood in the capillaries
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This maintains a steep concentration gradient across the entire length of the gill, maximising oxygen uptake by diffusion.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!