Gas exchange in fish

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  • Created by: zoolouise
  • Created on: 02-05-16 14:35

Gas exchange in fish

Fish are very active hence why they need a good supply of oxygen. Gas exchange takes place across the girls and gills have the following:

  • One-way current of water - kept flowing by a ventilation mechanism 
  • Many folds - increases the surface area for gas exchange
  • Large surface area - density of water prevents gills from collapsing on top of each other

There are two main types of fish and they both ventilate their gills in different ways. These two types of fish include:

  • Cartilaginous fish - skeleton of cartilage. e.g. sharks
  • Bony fish - skeleton of bone. e.g. salmon, cod

Cartilaginous fish

These types of fish include sharks. They have gills in five spaces on each side and they're called gill pouches, these open at gill slits. The ventilation mechanism is less efficient compared to bony fish. This is because:

  • There is no mechanism to force water over their gills so they must swim for ventilation to take place
  • The blood travels in the same direction as the water - parallel flow. Diffusion can only take place until 50% oxygen concentration is reached in the blood. This is when equilibrium is reached and so no more diffusion takes place.
  • Gas exchange in parallel flow doesn't take place across the whole gill lamellae, it stops when the oxygen concentration in the blood and water area are equal.


Bony fish

Bony fish have a skeleton made of bone. Their gills are covered by a flap called the operculum, they are therefore protected. They don't open directly on the side of the fish like they do in cartilaginous fish. They live in both freshwater and seawater and are the most numerous of aquatic organisms.


Ventilation is needed to maintain a continuous, unidirectional


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