6.3 Gas exchange in fish

Question 1

Why do fishes have inadequate body surfaces to supply and remove respiratory gases?

Answer 1

1. They have a waterproof and therefore gas-tight outer covering.
2. Being relatively large they also have small surface area to volume ratio.

Question 2

Describe/explain the structure of gills

Answer 2

• gills are located within the body of the fish, behind the head
• gills are made up of gill filaments
• gill filaments are stacked up in a pile
• at right angles to gill filaments are gill lamellae which increase the surface area of gills

Question 3

How do fish begin to intake oxygen?

Answer 3

Water is taken in through the mouth and forced over the gills and out through an opening on each side of the body.

Question 4

What is countercurrent flow?

Answer 4

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

This is important for ensuring that the maximum possible gas exchange is achieved.

Question 5

How does countercurrent flow work?

Answer 5

As water over lamellae and blood within lamellae flow in opposite directions , :

1. blood that is already well loaded with oxygen meets water, which has its maximum concentration of oxygen. Therefore DIFFUSION of oxygen from he water to the blood takes place.
2. Blood with little oxygen in it meets water which has had most, but not all, of its oxygen removed. Again, diffusion of oxygen from the water to blood takes place.

As a result, a diffusion gradient for oxygen uptake is maintained across the entire width of the lamellae. In this way, 80% of the oxygen available in the water is absorbed into the blood of the fish.