1. Gas exchange in fish
Structure

2. Gas exchange in fish
Gas exchange is more difficult for fish than for mammals because the concentration of dissolved oxygen in water is less than 1%, compared to 20% in air.

3. Gas exchange in fish
Gas exchange is more difficult for fish than for mammals because the concentration of dissolved oxygen in water is less than 1%, compared to 20% in air.
Fish have developed specialised gas- exchange organs called gills, which are composed of thousands of filaments.

4. Gas exchange in fish
Gas exchange is more difficult for fish than for mammals because the concentration of dissolved oxygen in water is less than 1%, compared to 20% in air.
Fish have developed specialised gas- exchange organs called gills, which are composed of thousands of filaments.
Each gill is covered by a muscular flap (the operculum) on the side of a fish's head.

5. Gas exchange in fish
The filaments in turn are covered in feathery lamellae which are only a few cells thick and contain blood capillaries.

6. Gas exchange in fish
The filaments in turn are covered in feathery lamellae which are only a few cells thick and contain blood capillaries.
This structure gives a large surface area and a short distance for gas exchange.

7. How it works
Water flows over the filaments and lamellae, and oxygen can diffuse down (a concentration gradient; high to low) the short distance between water and blood

8. How it works
Water flows over the filaments and lamellae, and oxygen can diffuse down (a concentration gradient; high to low) the short distance between water and blood.
Fish ventilate their gills to maintain the gas concentration gradient.
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9. How it works
Water flows over the filaments and lamellae, and oxygen can diffuse down (a concentration gradient; high to low) the short distance between water and blood.
Fish ventilate their gills to maintain the gas concentration gradient.
An opercula valve ensures the one-way flow that the high density of water requires.

10. How Lamellae work
The gill lamellae are arranged as a series of flat plates sprouting from the gill arch.

11. How Lamellae work
The gill lamellae are arranged as a series of flat plates sprouting from the gill arch.
On their upper and lower surfaces there are many thin vertical flaps which contain blood capillaries.

12. How Lamellae work
The gill lamellae are arranged as a series of flat plates sprouting from the gill arch.
On their upper and lower surfaces there are many thin vertical flaps which contain blood capillaries.
The blood flows through these capillaries in the opposite direction to the flow of water over the gills.

13. How Lamellae work
The gill lamellae are arranged as a series of flat plates sprouting from the gill arch.
On their upper and lower surfaces there are many thin vertical flaps which contain blood capillaries.
The blood flows through these capillaries in the opposite direction to the flow of water over the gills.
This is called a counter-current flow system
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14. Counter-current flow/exchange
The efficiency of fish gills stems from a simple adaptation known as countercurrent exchange: The blood in the capillaries flows in the opposite direction from the water in the adjacent channels. 

15. Counter-current flow/exchange
The efficiency of fish gills stems from a simple adaptation known as countercurrent exchange: The blood in the capillaries flows in the opposite direction from the water in the adjacent channels. 
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 high in oxygen. 

16. Counter-current flow/exchange
The efficiency of fish gills stems from a simple adaptation known as countercurrent exchange: The blood in the capillaries flows in the opposite direction from the water in the adjacent channels. 
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.

17. Counter-current flow/exchange
This method removes almost all of the oxygen (80-90%) from the water that passes over the gills and then transfers it to the blood.

18. Counter-current flow/exchange
This method removes almost all of the oxygen (80-90%) from the water that passes over the gills and then transfers it to the blood.
The lamellae are very important in this process, as this is the only place where countercurrent occurs, they can be found protruding from the gill filaments. ec