Gas exchange in fish

What are the two types of fish? Define them

1. Cartilaginous fish- includes sharks and rays- no ventilation mechanism- have to keep swimming in order for oxygenated water to flow over their gills.

2. Bony fish- most other fish- use a ventilation mechanism.

Describe the ventilation mechanism in bony fish.

- Water is taken in through the mouth and forced over the gills and out through an opening behind the gills.

- Water is kept moving over the gills as the movement of the mouth floor and operculum are coordinated to produce a stream of water, in through the mouth, over the gills and out the operculum.

- The gills are ventilated as oxygen dissolved in the water is absorbed by the gills.

Describe the structure of gills.

- Gill arches support the gills and are associated with blood vessels.

- Gill filaments increase the surface area of the gills

- Gill lamellae contain capillaries and are at right angles to gill filaments to further increase the surface area and are the site gas exchange

What is countercurrent flow?

Water and blood flow in opposite directions (bony fish).

What is parallel flow?

Water and blood flow in the same direction (cartilaginous fish).

Which flow is more efficient?

Countercurrent

Why is countercurrent flow more efficient?

- Water and blood flow in opposite directions so blood is always meeting water with a higher oxygen concentration.

- This means a concentration gradient/diffusion gradient is maintained across the whole length of the gill lamellae so diffusion of oxygen into the blood can occur across the whole length of the gill.

- This ensures maximum possible gas exchange is achieved.

Why is parallel flow less efficient?

- Water and blood flow in the same direction so equilibrium is reached halfway across the gill.

- This means a net diffusion of oxygen is no longer diffusing into the blood.