A good interactive explanation of breathing and gas exchange in humans with diagrams to label, animations to watch and questions to answer.
Although this is of the human respiratory system there is a good diagram that gives the functions of the various parts as you move your mouse over it. Also an animation of gas exchange and a quiz to test your understanding of it.
- http://en.wikipedia.org/wiki/Lung Wikipedia
Wikipedia on the lungs. Lots of good information on the human respiratory system with all sorts of links if you are interested.