gas exchange in fish

• fish have a waterproof, and therefore gas-tight, outer covering
• being relatively large, fish have a small SA to volume ratio
• due to their small sa to vol ratio, their body surface is not adequate to supply and remove their respiratory gases and so have evolved a specialised internal gas exchange surface - the gills
• The gills are located within the body of the fish behind the head
• the gills are made up of gill filaments
• Bony fish have four pairs of gills, each gill supported by an arch.
• Along each arch there are multiple projections called gill filaments, with lamellae on them which participate in gas exchange.
• Blood and water flow across the lamellae in a counter current direction ensuring that a steep diffusion gradient is maintained so that the maximum amount of oxygen is diffusing into the deoxygenated blood from the water
• Ventilation begins with the fish opening its mouth followed by lowering the floor of buccal cavity. This enables water to flow in.
• After water has flowed in, fish closes its mouth, causing the buccal cavity floor to raise, thus increasing the pressure.
• The water is forced over the gill filaments by the difference in pressure between the mouth cavity and opercular cavity.
• The operculum acts as a valve and pump and lets water out and pumps it in.