A panic attack occurs when the body's fight or flight response is triggered. The fight or flight response is the body's primitive, inborn, automatic response that prepares the body to either fight or flee from an attack.
In our early days as cave men and women, this fight or flight response would kick in when we'd see, for example, a saber toothed tiger crouching nearby and need to escape quickly. We didn't understand then the hows and whys, we just knew that when we became afraid we were able to run more quickly to (hopefully) escape the threat. Our very lives depended on this response.
Nowadays we don't have many saber toothed tigers to worry about, but we do have modern day stressors such as a fight with our spouse, trouble at work, sick children, traffic, money problems, etc. Unfortunately, our body cannot differentiate between a true threat to our physical survival (the saber tooth tiger) or a stressor such as an argument with a spouse, so it reacts the same way, by activating the fight or flight response.
Once activated, our fight or flight response causes a surge of chemicals (adrenaline, cortisol, etc.) to be released into our bloodstream. These chemicals cause changes in our body to prepare us to either fight or to flee. Our heart rate increases to circulate blood more quickly to vital organs, respiration increases to provide increased oxygen to the rapidly circulating blood, the muscles tense in the arms and legs in order to move quickly and precisely. Our blood sugar levels increase, our eyes dilate, we begin to perspire, and our mouths become dry. Blood is diverted from such places as the stomach (where it may have been helping to digest food) and begins to pool in other areas, such as the head.
These physiological changes make us stronger and faster and oftentimes we can do things we otherwise would not have been able to do. (Ever heard the story of a mother who was able to lift a car off her child by herself?) If we have to run or fight our body uses up these chemicals in the activity. Problem is, if the threat we are fighting is rush hour traffic, there is no way for us to "burn off" these chemicals, and we experience a variety of troubling symptoms, such as a racing heart or shortness of breath.
When the symptoms begin we may become even more afraid, not knowing or understanding what is happening to us and fearing that we are having a heart attack or going insane. This fear causes the body to release even more chemicals, compounding the situation and making the symptoms last much longer than they need to.