[David L.]

'fight or flight' refers to the two possibilities open to a person when confronted with life-threatening danger - he can either stand and fight off this danger to his person, or run like crazy - flee from the scene, that is, 'take flight'.
Adrenalin is released, which triggers release of Glycogen from the liver from the immediate supply of energy for this exertion; and the adrenalin also causes the heart to beat faster and breathing rate to increase. The faster heart beat means that blood is circulated more quickly around the body, (and so, to the muscles for all this fighting or fleeing), and it is the blood which carries the glycogen and oxygen to the muscles.

The problem is, we don't come across many lions roaming around the supermarket aisles as we do our food collecting there - many of the old dangers are gone. But panic attacks have zoomed up, and it is this triggering of the fight-or flight response that causes panic attacks. The important point about this is, it is not the objective nature of the danger, but that the person perceives some situation as threatening. Hence, this could be some really important social situation, where they don't want to make a fool of themselves in front of their boss and colleagues by being gauche. So they feel anxious, feel their heart beating faster, hand shaking, and then are even more anxious in case these signs of anxiety are obvious to others. One cannot seem cool and sophisticated if one is shaking like a leaf. So they become anxious about how anxious they are getting, and will it get out of control, so in their mind, the danger increases; and the body responds with more adrenalin, and hence, more (quite normal) bodily symptoms of the anxiety reaction, and it snowballs into a panic attack.