6.3 Gas exchange in fish

Question 1

Describe the structure of a fish.

Answer 1

• they have a waterproof and gas-tight outer covering

- they have a small surface area to volume ratio so their body surface cannot supply and remove their respiratory gases.

Question 2

Describe the structure of the gills.

Answer 2

• made up of gill fillaments
• at right angles to these are gill lamellae which increase the SA
• water is taken in through the mout and forced over the gills and out through an opening on the side of the body.

Question 3

What is counter-current flow?

Answer 3

the flow of water over the gill lamellae and the flow of blood within them are in opposite directions.

Question 4

How does counter-current flow work?

Answer 4

• blood that is well loaded with oxygen meets water that has max. concentration of oxygen so that diffusion happens.
• blood with little oxygen meets water with a low conc of oxygen so diffusion takes place.

Question 5

What are the advantages of counter current flow over parallel flow?

Answer 5

diffusion gradient is maintained across the entire gill lamellae and around 80% oxygen is absorbed. with parallel flow this is only around 50%