Detail the pathway of oxygenated water in the ventilation of a fish. Name and briefly describe the mechanism by which gaseous exchange takes place.

Oxygen rich water enters the mouthdue to buccal expansion > water is accelerated across the gill filamentsdue to simultaneous contraction of the buccal cavity and expansion of the opercula > gaseous exchange takes place via a countercurrent system.The countercurrent system is the name given to the mechanism used for gaseous exchange in afish, whereby the direction of travel for oxygenated water and blood is opposite. This means that high oxygenatedwater untakes gaseous exchange with already oxygenated blood, while water witha lower oxygen content exchanges will less well oxygenated blood. This networkmaintains an active concentration gradient for a longer period of time.