1. Review – factors needed for efficient gas exchange
Large surface area:volume
Moist surface to dissolve gases
Thin membranes for a short diffusion path
Concentration gradient is maintained

2. Gas Exchange in Fish
Fish have internal gills over which water continually flows.
The water has oxygen dissolved in it.
Bony fish have a ventilation mechanism.
Cartilaginous fish need to continuously move, or sit in flowing water.

3. Fish Anatomy
Mouth (Buccal Cavity)
Operculum
Buccal Floor

4. …lie the gills – the fish’s equivalent of our lungs
Behind the Operculum…
…lie the gills – the fish’s equivalent of our lungs

5. Gill Structure
Gill rakers
Gill arches/bars
Gill filaments
Lamellae

6. Structure of Gills
The gills are made of numerous thin filaments supported by a bony arch.

8. Adaptations of the gills for efficient gas exchange – which features?
Large surface area : volume
allows more diffusion of gases
Permeable membranes
allows gases to diffuse through tissues
Thin (flattened cells)
short diffusion distance
Good vascular (blood) supply
maintains concentration gradients

10. Each filament is made of thin delicate plates (lamellae) containing many capillaries, so they look dark red.

11. Ventilation
The fish opens its mouth and lowers its buccal floor, increasing the volume in the mouth and so decreasing the pressure
Water rushes in
The fish closes its mouth and raises its buccal floor
This decreases the volume, and so increases the pressure in the mouth
Water is forced over the gills and out of the operculum
Gas exchange happens in the gills

12. Gas Exchange O2 into capillaries CO2 into water
Capillaries and lamellae have thin walls. O2 and CO2 diffuse through easily.
Gases move by diffusion.
O2 into capillaries
CO2 into water

13. How does it work?
In a concurrent (parallel) flow system, blood and water flow in the same direction.
O2 concentration will reach an equilibrium.

14. CONCURRENT FLOW

15. Counter-Current Exchange
The blood flow in the capillaries is opposite to the flow of water over the gills.
This means that the blood in the capillary is always meeting new water with a full load of oxygen.
Counter current exchange of water flow and bloodstream maintains concentration gradients for diffusion.

16. COUNTER-CURRENT FLOW

18. Why don’t fish drown? Oxygen is 21% of air; < 1% of water
More water can flow over (external) gills than in and out of (internal) lungs.
Countercurrent flow in gills extracts more of the oxygen from the water in a low O2 environment.
Fish demand for O2 supply and removal of CO2 is lower than larger animals

19. The gill gas exchange system limits fish to a watery environment.
The watery environment does not require the gas exchange surface to be internal.

20. How is the structure of the gills related to its function?
Think:
Specialised exchange surface
SA/V ratio
Gas Exchange
Blood flow

21. Bring a fish head to the lesson on Wednesday.
Homework
Bring a fish head to the lesson on Wednesday.