interesting. IIUC, this should be easy (but perhaps dangerous) to test by inhaling a couple of times from a helium balloon, and exhaling to the atmosphere. you won't get oxygen in, but you will get CO_2 out. breathing helium in would presumably enable the lungs to get CO_2 out of the blood stream, lowering the urge to breathe even as no oxygen takes its place in hemoglobin. but how does that work, chemically? would the CO_2 really move from blood to lung "air" without being replaced by O_2? this probably wouldn't be testable otherwise, but what would hemoglobin bind to? curious...