1. Exchange and Transport
13.3 Gas exchange in fish

2. Learning outcomes Students should be able to understand the following:
The structure of fish gills and how water is passed along them
The difference between parallel flow and countercurrent flow
How countercurrent flow increases the rate of gas exchange
Candidates should be able to:
Describe gas exchange across the gills of a fish (gill lamellae and filaments including the countercurrent principle)

3. Water vs Air Or why fish and mammals are designed differently! Air
Oxygen Content
21% 1%
Stability
stable
varies
Diffusion rate
fast
slower
Density
Less dense
More dense

5. The circulation of blood in fish
Simple closed-circle circulatory system.
The heart only has two chambers.
This is because the fish heart only pumps blood in one direction.
The blood enters the heart through a vein and exits through an artery on its way to the gills.
In the gills, the blood picks up oxygen from the surrounding water and leaves the gills in arteries, which go to the body.
The oxygen is used in the body and goes back to the heart.

6. Gills and gas exchange Gills are the gas exchange surfaces in fish
Composed of gill filaments stacked like pages in a book
Gill lamellae project at right angles from the filaments and serve to increase the surface area of the gills for gas exchange
Gill lamellae are just a few cells thick and contain blood capillaries
How does this help facilitate gas exchange?
Why will a fish suffocate if left out of water?
Unit 2 Chap 13 Flash Files\fish gill lamellae.swf

7. How do fish get enough oxygen?
Large surface area of gill filaments and lamellae
Counter-current flow
In the fish gill, low-oxygen blood enters the capillaries, encountering water at the end of its travel through the gills, which is thus relatively low in oxygen.
As blood travels in the direction opposite to the water, it encounters "fresher" water with ever-higher oxygen concentrations. Thus, along the capillary, a steep diffusion gradient favors transfer of oxygen into the blood.

8. Fish gill anatomy
In fast moving fish, the surface area of the gills may be ten times that of the actual animal.

9. Water flow over fish gills
Unit 2 Chap 13 Flash Files\fish gill filaments.swf
Oxy blood flow

10. What’s the difference?

11. Gas exchange across gill lamellae

12. Why do fish gills need to be so efficient?
Oxygen not very soluble in water
Surface water = 5 ppm cf 210,000 ppm in air
Water contains about one-thirtieth as much oxygen per volume as the atmosphere above it.
Water more dense than air – more difficult to move over exchange surface than air
Fish countercurrent system ensures 80% absorption of oxygen from water
Our lungs can extract only 25% from air
If we were somehow able to "breath" water, we would need to take about 450 "breaths" per minute just to get enough oxygen into our lungs!

13. Ventilation
Gill efficiency is further increased by ventilation, the increase in flow of the respiratory medium over the respiratory surface.
Fish ventilate by swimming and by opening and closing the flaps that cover the gills, the opercula. This draws fresh water into their mouths to pass over their gills and out their gill slits.

14. Fish and ventilation
  --Ram Ventilation: Swim through the water and open your mouth (such as a shark would). Very simple, but the fish must swim continuously in order to breathe, not so simple.
     --Normal Ventilation: Occurs by the fish taking in water through the mouth. The mouth closes, forcing the water back over the gill filaments and out through the gill slits.

15. Countercurrent exchange principle
In countercurrent flow blood is always coming into contact with water that has a higher dissolved oxygen concentration
The diffusion gradient for oxygen is therefore maintained along the entire length of the gill structure
This facilitates maximun possible gas exchange across gill lamellae
Unit 2 Chap 13 Flash Files\counter-current flow.swf
In parallel flow the concentration gradient will level out when blood and water are both 50% saturated with oxygen
Diffusion therefore stops when the blood is only 50% saturated with oxygen
In the countercurrent system blood will continue absorbing oxygen from water as the concentration gradient does not level out
Unit 2 Chap 13 Flash Files\parallel flow.swf

16. By having the blood flow in the opposite direction, the gradient is always such that the water has more available oxygen than the blood, and oxygen diffusion continues to the place after the blood has acquired more than 50% of the water's oxygen content.

17. 1 (a) (i) many filaments/lamellae;
so large surface area; large numbers of capillaries;
maintains a diffusion gradient/removes oxygen;
thin epithelium/lamellae wall; short diffusion pathway; 2 max
To gain both marks your explanation must relate directly to the feature you have described. You will not gain full marks for describing two features without an explanation.
Simply stating ‘a good blood supply’ will not gain a mark!
(ii) maintains diffusion/concentration gradient/equilibrium not reached;
diffusion occurs across whole length of lamellae/gill; 2
This requires understanding of the countercurrent system.
(b) less energy needed/continuous flow of water or O2; 1
(c) (i) larger surface area for gaseous exchange; 1
References to ‘more space’ for gaseous exchange is not sufficient for a mark.
(ii) lowers water potential of tissues;
liquid moves in by osmosis; 2
(iii) diffusion pathway is too long/related to surface area to volume ratio;
to supply enough oxygen; 2

18. Parallel Hot Cold Start Temp (0C) 60 End Temp (0C) 24 22
Parallel flow
Counter- current flow
Parallel
Hot
Cold
Start Temp (0C) 60
End Temp (0C) 24 22
Countercurrrent
Hot
Cold
Start Temp (0C) 60
End Temp (0C) 40 23

19. Summary
List three similarities and three differences between the methods used by mammals and fish to obtain oxygen.
Differences:
Mammals obtain oxygen from air, fish from water
Mammals have alveoli, fish have gill filaments
Ventilation is tidal in mammals, unidirectional in fish
Similarities:
Both have a large gas exchange surface
Both have a good blood supply to the exchange surface
Both ventilate the exchange surface

20. Summary of gas exchange in fish by countercurrent flow
AQA AS Biology textbook pg 181
Answer summary questions 1 to 4
Unit 2 Chap 13 Flash Files\gas exchange in fish summary.swf
Build a model to explain the principle of gas exchange in fish gills by countercurrent flow

21. 1 (a) (i) many filaments/lamellae;
so large surface area; large numbers of capillaries;
maintains a diffusion gradient/removes oxygen;
thin epithelium/lamellae wall; short diffusion pathway; 2 max
To gain both marks your explanation must relate directly to the feature you have described. You will not gain full marks for describing two features without an explanation.
Simply stating ‘a good blood supply’ will not gain a mark!
(ii) maintains diffusion/concentration gradient/equilibrium not reached;
diffusion occurs across whole length of lamellae/gill; 2
This requires understanding of the countercurrent system.
(b) less energy needed/continuous flow of water or O2; 1
(c) (i) larger surface area for gaseous exchange; 1
References to ‘more space’ for gaseous exchange is not sufficient for a mark.
(ii) lowers water potential of tissues;
liquid moves in by osmosis; 2
(iii) diffusion pathway is too long/related to surface area to volume ratio;
to supply enough oxygen; 2

22. 2 a
82% 1 b
Concentration gradient is maintained over whole lamella
Diffusion gradient is maintained/oxygen is taken up over whole lamella
Equilibrium is never reached c
Ticks in first and third rows of column 1 and middle row of column 2 gains 2 marks
1 mark lost for each omitted or wrongly placed tick

23. Complete the exam style questions on gas exchange in fish
(a) countercurrent mechanism; helps maintain diffusion gradient; 2
 (b) mackerel has shortest total distance; for diffusion; (activity requires) oxygen for respiration / ATP production; [5]
 Qu 1
(a)(i) Arrow from vein towards artery, across lamella. 1
(ii) Water with high oxygen conc. meets blood with low oxygen conc; (may be derived from correct diagram) Difference in concentration maintained (across lamella); Diffusion gradient maintained. 2
(b)(i) 79 (Allow 78-80) – gains 2 marks. Length of cycle calculated from graph ( s) - gains 1mark Correct method (60 divided by cycle time), but cycle wrong time - gains 1 mark 2
(ii) Floor of mouth cavity lowered (increasing volume) [6]

24. Using worksheet – compare human and fish ventilation
How are they similar?
How are they different?
Why is ventilation essential for efficient gas exchange?

25. Learning outcomes Students should be able to understand the following:
The structure of fish gills and how water is passed along them
The difference between parallel flow and countercurrent flow
How countercurrent flow increases the rate of gas exchange
Candidates should be able to:
Describe gas exchange across the gills of a fish (gill lamellae and filaments including the countercurrent principle)