Structure and function of the gills of a bony fish.

gill filaments one gill arch water is sucked into mouth A Bony fish with its gill cover removed. Water flows in through the mouth, over the gills, then out through gill slits. Each gill has bony gill arches with many thin gill filaments attached. gill arch gill , filament B Two gill arches with filaments respiratory surface fold with a capillary bed inside \ oxygen-poor blood oxygenated blood from deep in bOdY back toward body C Countercurrent flow of water and blood capillary D Oxygen flow from water into a capillary. Percentages indicate the degree of oxygenation of water (blue) and blood (red). All along the capillary, oxygen flows down its concentration gradient from water into blood.