Gas exchange in fish

- Water is denser than air and contains much less oxygen per litre, so oxygen diffusion rates are much slower.

- Land gas exchange systems would not work in water, as lungs etc require too much energy.

- So in water/fish the gas exchange system is gills, where water only has to flow over them in one direction (ventilation fo the gills).

In bony fish:

- The pressure in the mouth is reduced by the mouth floor being lowered.

- The operculum is pressed against the body wall, thus water enters the mouth to equalise the pressure of the outside.

- The mouth closes increasing pressure of the mouth cavity, and a valve prevents water from leaving.

- This increased pressure causes the operculum to open, and so water leaves, passing over the gill filaments in doing so.

In cartilaginous fish:

- They must keep swimming to keep water flowing across the gills.

- Gill filaments are fragile and occur in large stacks.

- They need water to keep them apart and so to expose the large surface area needed for gas exchange.

-The Gill lamelae are the main site of gas exchange which have a large blood supply and give the filamnets their large surface area. They also have a short diffusion distance.

- For survival fish require a conbstant flow of water due to the slow diffusion of water.

- When a fish is out of water it cannot survive for very long because it's filaments all stick together, making the surface area not large enough for effective gas exchange.

- They have th usual features for efficient exchange: large SA, rich blood supply and short diffusion distance/thin walls.

However is also has:

• A countercurrent exchange system - the blood in the gill filaments and the water moving over the gills flow in different directions to ensure a steeper concentration gradient and so more exchange can take place (80% oxygen is extracted).
• Overlapping gill filaments - if water flows over filaments too quickly then it limits the amount of gas that can be exchanged, so by having overlaps resistance of water is increased which slows the flow of water to give more time for gas exchange to take place.