I realized I wrote that poorly. Let me clarify... Your body still needs oxygen, in fact it uses more. But your blood pressure goes up, muscles get more oxygenated blood, really a host of things happen to make your body way more efficient at transporting oxygen to the parts that are going to "protect you". That's why you'll see somebody who is typically a sloth bust into an Olympic run when a gun goes off near him or a loud noise scares him good. All that adrenaline maximized oxygen flow and glucose processing and turned that Twinkie eater into a track star for a few moments.