COUNTERCURRENT EXCHANGE
The efficiency of fish gills stems from a simple adaptation known as countercurrent exchange: The blood in the capillaries flows in the opposite direction from the water in the adjacent channels. Dissolved gases diffuse faster between fluids with a large difference in gas concentration (a high concentration gradient) than between fluids with only a small difference.

In the fish gill, low-oxygen blood enters the capillaries, encountering water at the end of its travel through the gills, which is thus relatively low in oxygen. As blood travels in the direction opposite to the water, it encounters "fresher" water with ever-higher oxygen concentrations. Thus, along the capillary, a steep diffusion gradient favors transfer of oxygen into the blood.

Gill efficiency is further increased by ventilation, the increase in flow of the respiratory medium over the respiratory surface. Fish ventilate by swimming and by opening and closing the flaps that cover the gills, the opercula. This draws fresh water into their mouths to pass over their gills and out their gill slits.