Fish have a two-chambered heart (one atrium and one ventricle). Blood flows once through the heart in a single circulation, and oxygenated and deoxygenated blood mix.
Humans, however, have a four-chambered heart (two atria and two ventricles), completely separating oxygenated and deoxygenated blood. This double circulation ensures efficient oxygen transport throughout the body. Hence, mixing is prevented in humans but not in fishes due to differences in heart structure.