Fish Gills and the Counter-current Principle
How gill filaments and lamellae exchange gases, and why counter-current flow keeps a diffusion gradient along the whole lamella. Includes the 5-mark exam question, a worked model answer and the examiner trap on blood and water flowing in the same direction.
Chapters (9)
Exam question [5 marks]
Water contains much less oxygen than air. Explain how the structure of a fish gill allows efficient gas exchange.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Many filaments and lamellae give a large surface area | "lots of lamellae", "folded plates", "large surface area for diffusion" | "gills have a big surface" with no reference to filaments or lamellae |
| 2 | Lamellae are thin (with a capillary network), giving a short diffusion distance | "thin walls", "one cell thick", "short distance", "capillaries carry oxygen away" | "thin" with no consequence stated |
| 3 | Blood flows through the lamellae in the opposite direction to the water (counter-current) | "water and blood flow in opposite directions" | "flow past each other"; "flow in the same direction"; "parallel flow" |
| 4 | A concentration gradient is maintained along the whole length of the lamella | "water always has a higher oxygen concentration than the blood", "gradient all the way along" | "there is a gradient" with no reference to the whole length |
| 5 | So more oxygen diffuses into the blood (equilibrium is not reached) | "more oxygen absorbed", "blood becomes highly saturated" | "fish absorb oxygen from the air" |
Marks 1 and 2 are for the exchange surface, 3 to 5 are for counter-current flow. Mark 4 needs "whole length" or equivalent; a bare "gradient" is not enough. Mark 5 needs the consequence (more oxygen absorbed or no equilibrium). Fish take in oxygen dissolved in water, not from air. Do not credit the same statement under both marks 4 and 5. Percentage figures in answers are optional; credit them only if they show the gradient is kept.
Model answer (5/5)
Gills have many filaments, each covered in lamellae, so there is a large surface area for diffusion. The lamellae are thin and contain a network of capillaries, so there is a short diffusion distance. Blood flows through the lamellae in the opposite direction to the water, which is counter-current flow. This means a concentration gradient is maintained along the whole length of the lamella, because the water always has a higher oxygen concentration than the blood next to it. So diffusion continues all the way along, equilibrium is not reached, and more oxygen is absorbed into the blood than with parallel flow.
Examiner trap
- Saying counter-current is where water and blood flow the same way
- Not saying a gradient is maintained along the whole lamella
- Saying fish absorb oxygen 'from the air'
Key terms
- gill
- filament
- lamella
- counter-current flow
- concentration gradient
- diffusion
- oxygen saturation
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:29Water flows over the gills
A fish takes water in through its mouth, passes it over its gills, and lets it out under a flap called the operculum. Water holds far less oxygen than air does, so the fish has to be very good at taking it out. Notice that fish do not absorb oxygen from the air. They absorb oxygen dissolved in the water.
0:50Gill arch and filaments
Each gill has a stack of thin gill filaments, like the teeth of a comb. And along each filament are many tiny folds called gill lamellae. The lamellae are where gas exchange happens.
1:03The lamella up close
There are lots of filaments and lots of lamellae, so there is a very large surface area. Each lamella is very thin, so oxygen has a short diffusion distance between the water and the blood. And each one has a network of capillaries, so the blood carries the oxygen away and keeps the gradient steep.
1:23Parallel flow: why it fails
Now the key idea. Imagine the blood and the water flowed in the same direction, which is called parallel flow. Let's use example figures: percentage saturation with oxygen. At the start, the water is one hundred and the blood is nought. Oxygen diffuses in, and the blood rises as the water falls. Soon they both reach fifty.
Now there is no difference, no concentration gradient, so no more diffusion. The blood can never get above about fifty percent.
1:54Counter-current flow
Now flip the blood around, so it flows the opposite way to the water. That is counter-current flow. Water comes in at one hundred, and meets blood that is already at ninety. Further along, the water has lost some oxygen, down to seventy, but it meets blood that is lower, at sixty. At the far end, where the blood first enters, the water is at just ten, and it meets blood that has nothing, at nought.
Every step along the lamella, the water has a higher oxygen concentration than the blood next to it. So a concentration gradient is maintained along the whole length of the lamella, and diffusion carries on all the way.
2:35Comparing the two with numbers
Compare the two, using example figures, not exam values. With parallel flow, the blood leaves at about fifty. With counter-current flow, the blood leaves at around ninety. The fish absorbs far more of the oxygen in the water, which is exactly what it needs when water holds so little oxygen.
3:27Examiner trap and recap
The trap. If you say the blood and water flow in the same direction, you lose the marks. Saying they just flow past each other loses them too. And always say the gradient is kept along the whole length, not just that there is a gradient.
Recap. Many lamellae give a large surface area, thin lamellae give a short distance, and opposite flows keep the gradient all the way along.
Next, we move from water to air, and look at gas exchange in the leaves of plants.
Watch first
This lesson builds on:
- Surface Area to Volume Ratio Explained · AQA 7402 · 3.3.1
- Gas Exchange: Single Cells and Insects · AQA 7402 · 3.3.2
Keep going
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- Gas Exchange: Single Cells and Insects · AQA 7402 · 3.3.2
- Gas Exchange in Leaves: Mesophyll and Stomata · AQA 7402 · 3.3.2
- Gas Exchange vs Water Loss: Xerophytes · AQA 7402 · 3.3.2
- Human Gas Exchange System and Alveoli · AQA 7402 · 3.3.2
All 10 3.3 Exchange with the environment lessons Whole playlist on YouTube
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-054.