Nitric oxide as a treatment for gas exchange always amuses me. Pretty much a poison but also a great little broncodialator. My wife was an ICU nurse so I learnt a lot of her stuff while she was studying and working there .
I would not call it a poison, our bodies produce it. It is a natural vasodilator. Our bodies synthesize nitric oxide from the amino acid L-arginine by means of the enzyme nitric oxide synthase. The main site of the molecule's synthesis is the inner layer of blood vessels, the endothelium, though the molecule is also produced by other types of cells. Sildenafil, the little blue pill, is a specific phosphodiesterase type 5 inhibitor that enhances nitric oxide-mediated vasodilation.