The volume inside the mouth cavity decreases and so the pressure inside the cavity rises, forcing water back over the gills
The operculum opens and water flows Out
12. A countercurrent system: Bony fish have gills that use a counter
(a) current principle.
(b) In the lamellae, the blood in the capillaries flows in the opposite direction to the water flowing over the surface.
(c) A diffusion gradient is maintained between the blood and the water right across the gill plate:
:Blood with a relatively high oxygen concentration meets water with a lot oxygen in it
:Hence, there is still a diffusion gradient for oxygen into the blood
: Blood with a relatively low oxygen concentration meets water which has had most of its oxygen removed.
(d) The countercurrent system allows the gills of a bony fish to achieve an 80% extraction of oxygen from water. That's three times the rate of extraction of human lungs from the air.
(e) This helps to overcome the problem of low oxygen content in water. mphibians Most amphibians such as frogs can live in water