Organisms Exchange Substances with their Environment: Gas Exchange - Fish

Question 1

Why can’t fish use their bodies as an exchange surface?

Answer 1

• They have a waterproof, impermeable outer membrane and a small surface area to volume ratio

Question 2

Describe the structure of a fish’s gas exchange system

Answer 2

• Water enters fish through mouth and passes out through gills
• Gills are located inside a fish’s head underneath gill slits called the operculum
• Gills are made of many thin plates called gill filaments
• Lamellae are positioned at right angles to gill filaments

Question 3

How are gills adapted to increase the efficiency of a fish’s gas exchange system?

Answer 3

• Gills have many thin gill filaments
• This provides a large surface area
• Increases rate of diffusion

Question 4

Describe how a fish’s circulatory system is involved in gas exchange

Answer 4

• In gills, blood flows through lamellae in one direction and water flows in opposite direction - this is called counter-current flow
• Water with a high oxygen concentration always flows next to blood with a lower oxygen concentration
• A steep concentration gradient is maintained over the whole length of the gill
• Maximises amount of oxygen diffusing into blood from water

Question 5

How are lamellae adapted to increase the efficiency of a fish’s gas exchange system?

Answer 5

• Lamellae provide a large surface area
- This increases rate of diffusion

• Lamellae are at right angles to gill filaments so blood flows in opposite directions to water
- This ensures counter-current flow

• Lamellae have good blood supply
- Maintains concentration gradient for faster rates of diffusion

• Thin surface of cells
- Shortens diffusion pathway, which increases rate of diffusion

Question 6

Describe how ventilation occurs in a fish

Answer 6

• Mouth opens and operculum is closed
• Buccal cavity floor is lowered
• This increases volume and lowers pressure
• Water rushes into mouth, down a pressure gradient
• Buccal cavity floor raises
• Pressure in buccal cavity is higher than opercular cavity
• Water moves from buccal cavity, over gills, into operculum cavity
• Mouth closed and operculum opens
• Water rushes out of operculum