Green Frogs are well adapted to their environment.  Not only have frogs developed lungs for breathing, but they also have the ability to breathe through their skin.  The diffusion of oxygen through the skin as well as inhalation is an efficient way of delivering oxygen to the blood stream.  Frogs use positive pressure to take a breath and then close their nose and mouth.  They then use negative pressure to force air into the lungs.

Frogs have a closed circulatory system and a less evolved heart than most animals but are more evolutionally developed than fish.  A frogs circulatory system is composed of veins, arteries, 2 complete circuits, and a 3 chambered heart.

The order anura (which also includes Rana clamitans) has developed an extremely specific hind end which helps all members of the order jump (or hop) higher and farther away from predators.  This development also helps with swimming.

Another one of the frogs adaptations is webbed feet.  Because the "toes" of the frog are connected with a thin layer of epidermis frogs can swim faster.  Think of it like swimming yourself.  Do you swim with your fingers together or spread apart?