Why do bony fish require specialised respiratory systems?

  1. Water is more dense than air, so the rate of oxygen diffusion is slower
  2. Water has less oxygen than air
  3. Bony fish are active so have high oxygen demands
  4. Bony fish usually have a low surface area to volume ratio
  5. Fish scales prevent surface gas exchange

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What are the organs of gas exchange in bony fish?

Gills are the respiratory organs of bony fish.

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Describe the structure and arrangement of fish gills.

Gills are composed of stacks of gill filaments.


These gill filaments have multiple gill lamellae to maximise surface area for gas exchange.


The gills are covered by an operculum flap.

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What are the key features of gills that maximise gas exchange?

  1. Large surface area of stacked filaments and lamellae
  2. Thin lamellae membranes minimise diffusion distance
  3. Good blood supply and countercurrent flow maintain steep diffusion concentration gradients
  4. Overlapping gill filament tips increase resistance to water and slow water flow, allowing more time for diffusion

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How does countercurrent exchange in fish gills allow them to maximise the oxygen they can extract from water?

Blood and water flow over the gills in opposite directions.


This maintains the diffusion gradient over the entire gill surface, so more oxygen can be extracted from water even when the oxygen concentration in the water is low.

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Where are the gills located in bony fish?

The gills are located under a flap called the operculum.

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