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The gas exchange organs of fish are called gills. Fish possess several gills located between their mouth cavity (buccal cavity) and a chamber at the sides of their mouth called the operculum. Figure 1 below shows the main elements that are involved in gas exchange in fish.

Oxygen passes from the water into the blood at the gills. Removal of carbon dioxide also occurs, as the blood containing high concentrations of the waste gas goes to the gills, and the carbon dioxide diffuses out into the water down a diffusion gradient (external water has lower concentrations of carbon dioxide than levels in the blood, so this sets up a diffusion gradient.)

Figure 1:
Gas exchange in fish

Each gill is composed of many filaments that are each covered in many lamellae. The lamellae contain blood capillaries, which have blood flowing in the opposite direction to the water. These structures are shown in figures 2,3,4 and 5, below.

Figure 2
Gas exchange in fish

Figure 3
Gas exchange in fish

Figure 4
Gas exchange in fish

Figure 5
Gas exchange in fish

In figures 2, 3, 4 and 5 above the following features can be seen

Figure 2. Each gill is composed of many filaments increasing its surface area.

Figure 3. Each filament is covered in lamellae. These further increase the surface area, and because they are thin, ensure that the diffusion distance between the blood, in the lamellae, and the water is small.

Figure 4. The blood flows through the lamellae in the opposite direction to the water. This is a countercurrent system. It ensures the maximum exchange possible occurs. The details of the exchange are shown in Figure 5.

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