In fish, the heart only has one atrium and one ventricle. The oxygen-depleted blood that returns from the body enters the atrium, and then the ventricle, and is then pumped out to the gills where the blood is oxygenated, and then it continues through the rest of the body.
Diffusion will continue until a condition of equilibrium<span> will be reached. Ions or Molecules are equally balanced on both sides or a permeable membrane.</span>
