Ventilation mechanism

• Countercurrent exchange mechanism ventilation is in one direction only - not tidal
• Fish use muscles in floor of the mouth to pump water over the gills
• Use muscles on the floor of mouth to pump water over the gills
• Use valves on mouth and opercula to ensure unidirectional flow
• Water moves into mouth cavity when mouth is open and operculum is closed - water moves out over hills when mouth closed and operculum open
• Pressure in bucks cavity (mouth) is greater than in the opercular cavity so water flows from mouth to gills To operculum