Download Unit 1: Gas Exchange and Ventilation in Bony Fish

Topic 3: Gas Exchange
in Fish
By the end of the lesson you should be able to:
1) Label a diagram to show the gill lamellae and gill
filaments of a fish
2) Understand what counter-current flow is, and
why it is useful
Fish Anatomy
Mouth (Buccal
Cavity)
Operculum
Buccal Floor
Behind the Operculum…
…lie the gills – the fish’s
equivalent of our lungs
Gill Structure
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Gill arches
Gill filaments
Lamellae
Large surface area
Adaptations of the gills for efficient
gas exchange
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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
Counter Current Principle
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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
Concurrent flow =
blood and water in same direction
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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%.
Counter current flow = blood and
water flow in opposite directions
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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.
Counter Current Principle
Check your understanding
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Fish questions