A duck breathes like a human in that it inhales and exhales using lungs. However, there is a big difference. When humans breathe, the air goes into their lungs, lingers there for a moment, then comes back out. With ducks and other birds, though, the flow of air is always one-way. The air is inhaled into the first lung, then into the second, and finally out again. This dramatically increases the efficiency with which oxygen gets to the duck's bloodstream, allowing it to fly at altitudes where the air is thinner.
