Well, I would say that the tank and all the moving bits and bobs associated with it (pumps, filters, and so forth) are implanted in the cyberleg. From the air tank to the lungs runs a system of hoses along the outside of the arteries that then plugs in to the lung(s) and delivers oxygen and removes carbon dioxide. The carbon dioxide could be emitted through a one-way valve built into the leg expressly for that purpose. The whole of the hose system should not be extremely invasive, could be run along the artery using current medical techniques or built along the artery through nanosurgery, and has no moving parts. That's the simplest method I could think of, without resorting to turning the air tank into an artificial lung.
Hope that Helps!