Countercurrent exchange in the fish gill to maximize countercurrent exchange in the fish gill to maximize osmosis blood pressure active transport diffusion

The answer is Diffusion

Diffusion is a physical process where molecules of a material move from an area of high concentration to an area of low concentration. The gill is where fish absorb oxygen from the surrounding water into their blood. Oxygen can only be transported into the blood at the gills if the oxygen level is higher in the water than in the blood. It means oxygen needs to flow from an area of high levels to an area of low levels.