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.
