The picture below, from the fantastic website Psychology Tools, shows what happens in our body when our brains perceive a possible threat:
Just like our ancestors used to do when living in caves tens of thousands of years ago, whenever we think there is a threat coming our way (e.g. "there comes a sabre-toothed tiger!"), we prepare to fight it or quickly run away from it. Our bodies, being the well-oiled machines that they are, immediately go into survival mode and get ready to deal with the threat by activating an internal "alarm system". In simple terms, this system is called the fight or flight response.
Once our brain identifies a possible danger, stress hormones such as cortisol and adrenaline are released by the adrenal glands. I response, muscles tense up, particularly the larger ones, to prepare for a possible battle or long run. The heart starts beating faster, pumping more blood around the body, which elevates its temperature. The rise in temperature triggers sweating to cool the body down. As blood vessels in the skin contract to force blood towards those all-important major muscle groups, areas such as palms and feet become both cold and sweaty.
As breathing becomes faster and shallower to take in more oxygen, we may feel a bit dizzy or lightheaded if the excess oxygen is not being used right away. Thoughts also start racing to keep up with the changes in the environment. As digestive and elimination systems are not vital, they receive less blood and we might feel nausea, butterflies in the stomach, along with urges to use the bathroom.
All of these reactions get us ready to deal with that tiger. And in those situations, they are indeed indispensable, life saving. Indeed, if we go back many millennia, we are all descendants of the prehistoric men and women with the best "alarm systems - those that did not get eaten by the tigers!
However, nowadays, there aren't many saber-toothed tigers walking around. The perceived threats come from our financial troubles, fear of rejection, loneliness, arguments with a spouse, concerns about a job, memories of a traumatic event, self-doubt, regrets, and many, many other ideas that we ourselves label as dangerous. In those instances, the fight 0r flight response is unnecessary. Worse, it can interfere with just being able to live a fulfilling life.
If that is happening to you, talk to a therapist. You can learn to fine tune your fight or flight response so that it works for you, not against you!