1. Topic 3: Gas Exchange in Fish
By the end of the lesson you should be able to:
Label a diagram to show the gill lamellae and gill filaments of a fish
Understand what counter-current flow is, and why it is useful

2. Fish Anatomy
Mouth (Buccal Cavity)
Operculum
Buccal Floor

3. …lie the gills – the fish’s equivalent of our lungs
Behind the Operculum…
…lie the gills – the fish’s equivalent of our lungs

4. Gill Structure
Gill arches
Gill filaments
Lamellae
Large surface area

6. Adaptations of the gills for efficient gas exchange
Large surface area : volume - allows more diffusion
Permeable membranes - allows gases to diffuse
Thin (flattened cells) - short diffusion distance
Good vascular (blood) supply - maintains concentration gradients

7. Counter Current Principle
Oxygen content of water is much lower than air, so fish have to be especially well adapted to take oxygen into their blood
Fish use a system called counter current flow where by the water and the blood flow in opposite directions over the gills

10. Concurrent flow = blood and water in same direction
When the blood first comes close to the water, the water is fully saturated with oxygen and the blood has very little.
There is therefore a very large concentration gradient and oxygen diffuses out of the water and into the blood.
As you move along the lamella, the water is slightly less saturated and blood slightly more but the water still has more oxygen in it so it diffuses from water to blood.
This continues until the water and the blood have reached equal saturation.
After this the blood can pick up no more oxygen from the water because there is no more concentration gradient. The maximum saturation of the water is 100% so the maximum saturation of the blood is 50%.

11. Counter current flow = blood and water flow in opposite directions
As the blood flows in the opposite direction to the water, it always flows next to water that has given up less of its oxygen.
This way, the blood is absorbing more and more oxygen as it moves along. Even as the blood reaches the end of the lamella and is 80% or so saturated with oxygen, it is flowing past water which is at the beginning of the lamella and is 90 or 100% saturated.
Therefore, even when the blood is highly saturated, having flowed past most of the length of the lamellae, there is still a concentration gradient and it can continue to absorb oxygen from the water.

12. Counter Current Principle

13. Check your understanding
Fish questions