Lesson 54 - Gas exchange in fish

Question 1

What are the 2 types of fish and how does their gas exchange system differ?

Answer 1

• Cartilaginous fish - include sharks and rays - no ventilation mechanism - have to keep swimming in order for oxygenated water to flow over their gills
• Bony fish - most others - use a ventilation mechanism

Question 2

What is the gill structure?

Answer 2

Gill arches – support the gills and associated blood vessels
Gill filaments – increase the surface area of the gills
Gill lamellae - at right angles to gill filaments further increase the surface area - site of gas exchange

Question 3

Why do fish need a specialised gas exchange surface?

Answer 3

• They have a small surface area to volume ratio for gas exchange
• They also have an impermeable membrane so gases can’t diffuse through their skin
• Fish are very active
• Conc. of gases in the water will be lower than in the air

Question 4

Describe the process of ventilation in fish to maintain a continuous unidirectional flow?

Answer 4

• The fish opens its mouth, lowering the floor of (buccal) cavity. This enables water to flow in. - The fish closes its mouth, causing the (buccal) cavity floor to raise, decreasing the volume and increasing the water pressure.
• The water is forced over the gill filaments by the pressure gradient between the mouth cavity and opercular cavity.
• The operculum acts as a valve and pump and lets water out and pumps it in.

Question 5

Describe counter-current flow in fish

Answer 5

• 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
• Therefore diffusion of oxygen into the blood can occur across the whole length of the gill
• This ensures maximum possible gas exchange is achieved

Question 6

How does parallel flow work in fish?

Answer 6

(Blood and water flow in the same direction), the diffusion gradient would only be maintained across part of the length of the gill lamellae and only 50% of the oxygen available would be absorbed by the blood (80% with counter current flow)