When you hold your breath, which of the following gas changes in blood would first lead to the urge to breathe?

Oddly, your breathing reflex comes from increasing acidity in your blood caused by increasing levels of carbon dioxide. This is why sticking your head in a nitrous oxide, or helium, balloon is not a good idea. You're not getting any oxygen, but you don't feel like you're suffocating, because you don't get a carbon dioxide buildup. When you breathe in, you breathe in oxygen which is immediately converted into Carbon Dioxide. The longer you hold your breath, the urge to breathe in more oxygen is evident.