How do fish ventilate their gills?

Unlike mammals fish have water as their gas exchange medium instead of air. Fish use gills for gas exchange, gills are specialised flaps of tissue which have the following features to allow efficient gas exchange: Numerous folds which give rise to a large surface areaAn efficient circulatory system which produces a concentration gradient. Gills may be external or internal. External gills are less protected but can have a higher surface area. External gills receive oxygen passively from the surrounding water. Because internal gills are covered they need to be actively ventilated. The fish does this by opening its mouth which lowers the pressure in its buccal cavity causing water to rush in. The water flows through the pharynx and paste the gill plates where gas exchange occurs. The fish then closes its mouth which raises the pressure in the buccal cavity and forces the water through the opercular openings on each side of the fished head. Flow is unidirectional (unlike mammalian lungs)