A frog’s respiratory system is much different than humans. First, the lungs in a frog have thin walls and are shaped like balloons. When frogs swim, they fill their lungs with air, which helps them stay buoyant while swimming. Frogs also do not have diaphragms compared to humans. They only breathe through their nose. They use muscles in their throat sacs to help move the air in, and then is pushed to the lungs. During this time, their mouth is closed. Body contractions allow the frogs to release carbon dioxide. When us humans breathe in, the diaphragm lets the lungs stretch out so air can move in. The skin of a frog plays a vital role for its breathing. The skin of a frog has a thin membrane, which allows the oxygen and carbon dioxide to pass through. This process is called cutaneous gas exchange. For this process to work, the skin has to be moist, so this only works when they are in aquatic environments. This is a major difference to humans because they do not use their skin to help them breathe. For a frog to make sound, they croak. This croak sound is used by males to: attract females, or to repel other males. This sound happens when a frog forces air from its lungs through the larynx, causing the vocal cords to vibrate and create a sound. The vocal sacs are below the mouth. When they create the sound, the mouth and nostrils are closed so that the muscles in the body and throat cause the throat to puff up and create the loud sound.
http://www.learner.org/jnorth/search/FrogNotes1.html
http://www.wildsingapore.per.sg/discovery/articles/FrogField_Call.htm
http://animals.pawnation.com/differences-respiratory-systems-frogs-humans-2684.html