In fact, immune function, weight, circulation, mood, and sexual function are all heavily influenced by amount of nitric oxide in your blood stream. It is not produced during mouth-breathing.
Viagra works by releasing nitric oxide into the bloodstream which opens the capillaries in the genitals and elsewhere in the body.
Nasal breathing boosts nitric oxide six-fold, which is one of the reasons we can absorb 18% more oxygen than when mouth-breathing. In many case, nasal breathing can even cure erectile dysfunction!