Gaseous exchange in fish

  • Created by: Grace Bu
  • Created on: 26-04-21 18:40

Why do fish need a specialised ventilation mechani

Why do fish need a specialised ventilation mechanism?

- Fish are active organisms with a high metabolic rate.

Gas exchange takes place across their gills.

- Unidirectional current of water over gills.

- Many folds in gills

- Large surace area.

Specialised ventilation system.

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Inspiration in fish

Inspiration in fish

1) Mouth opens

2) Operculum closes

3) Buccal floor lowers

4) Volume of buccal cavity increases

5) Pressure inside buccal cavity decreases

6) Water flows into mouth from high rpessure to low pressure.

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Expiration in fish

Expiration in fish

1) Mouth closes

2) Operculum closes

3) Buccal floor raised

4) Volume inside buccal cavity decreases

5) Pressure inside buccal cavitiy increases.

6) Water flows out over gillls from high pressure to low pressure

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Structure of gills

Structure of gills related to their structure:

- The gills are made up of fillaments

- Each fillament is divided into lamellae

- The lamellae help increase the surface area

- The llamellae are thin and ensure that the diffusion distance between the blood in the lamellae and the water is short.

- The gills are bright red due to the large amount of blood vessels.

Oxygen & Carbon Dioxide: Gas Exchange and Transport in Animals | Organismal  Biology (

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Counter current flow

Counter current flow

The counter current system maintains a diffusion gradient along the whole length of the gill plate.

There is only a counter current flow in bony fish.

- Specialised mechanism for ventilation ensures blood and water flow in opposite directions.

- Water always has a higher oxygen concentration than blood in capillaries.

- Oxygen diffuses into gill capillaries across entire length of lamellae.

- Gill absorbs about 80% of oxygen from water across gills. 

As distance across lamellae increases:

- % oxygen in blood increases

- % oxygen in water decreases

- Maintain concentration gradient across whole gill lamellae

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Parallel flow

Parallel flow

- Blood travels through gill capillaries in the same direction as water flow across gills.

- Oxygen diffuses from water into blood across gills.

- Diffusion can only occur until concentrations are equal

- Diffusion therefore only occurs across part of the gill lamellae

- Because the oxygen concentration in gill capillaries increases, only about 50% of oxygen is absorbed from water.

As distance along gill lamellae increases:

- % oxygen in blood increases

- % oxygen in water decreases

- Until both reach 50% oxygen

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