• There is very little dissolved oxygen in the water.
• Air contains 30 times more.
• Oxygen concentration in water decreases further with increasing temperature.

• 4 gill arches either side of head.
• Separate buccal and opercular cavities.
• Each arch has 2 rows of filaments in a V shape.
• Each filament is flattened with an upper and lower row of lamellae (gill plates).
• Lamellae are covered in a thin layer of epithelial cells .
• Lamellae are in close contact with those of neighbouring filaments.
• Excellent blood supply.
• The tips of the filaments overlap.
• Whole thing forms a sieve like structure.
• This provides resistance to flow of water.
• This increases time for gaseous exchange.
• Gill slits are the spaces between gill arches.
• Approx. 0.05mm wide.
• Operculum is the bony flap that covers gills in the opercular cavity.
• This protects gills and moves to aid ventilation

• Blood in lamellae flows in opposite direction to water.
• Blood always meets water with a relatively higher concentration of dissolved oxygen.
• This maintains the concentration gradient between the blood and water for the whole length of the lamellae.
• 80% of oxygen in water can be extracted.

• Some fish achieve this by swimming forwards with mouth open.
• Slow swimmers or fish in still water use pumping system to move water.
• Coordinated activity of both chambers ensures continuous flow of water.

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