system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why and how does a panic attack occur? Is it describable by words?
gpt
Basically it's your 'fight or flight' mode going into overdrive. The psychology is a bit more complicated, but essentially because of a mental condition such as a phobia or panic disorder, your brain decides that there is an immediate and very serious threat to you even though there isn't. Your brain then tells your body to prepare to either fight or run away. 

This involves the release of adrenaline and the stimulation of the sympathetic nervous system. Your heart rate goes way up, you start breathing much faster and you sweat. 

The fast breathing (hyperventilation) is quite important in a panic attack. Your body is preparing you for the sudden strenuous exercise involved in fighting or running away, but of course you don't actually end up doing any strenuous exercise. This means that you expel loads of carbon dioxide out through your lungs without much of it being produced from exercise. A loss of a lot of carbon dioxide changes the pH of the blood, and this can cause some of the other symptoms of a panic attack, such as lightheadedness and dizziness. You may then interpret the combination of all of the symptoms as "feeling like you're going to die".