An adrenaline rush is a sudden increase in the secretion of adrenaline from the adrenal glands

" The internal boxer must learn to fight without letting his adrenal glands fire. This begins by learning to spar with diaphramic continuous breathing and relaxed muscles. Then the internal fighter must learn to relax his emotions while sparring. Finally, he must learn to feel his adrenal and be especially careful not to tense anything near them while sparring. "

Adrenaline, or epinephrine, is a stress hormone secreted from the adrenal glands on the kidneys. It plays a major role in preparing the body for a fight-or-flight reaction in threatening environments.

An adrenaline rush is a sudden increase in the secretion of adrenaline from the adrenal glands. This happens when the brain communicates to the glands that there will be a need for a fight-or-flight response.

The cause of an adrenaline rush need not be an actual physical threat but can also be an imagined threat, strenuous exercise, heart failure, chronic stress, anxiety or a disorder of the brain or adrenal glands.

When you perceive something as threatening or exciting, the hypothalamus in the brain signals to the adrenal glands that it's time to produce adrenaline and other stress hormones. The adrenal glands produce adrenaline by transforming the amino acid tyrosine into dopamine. 

Oxygenation of dopamine yields noradrenaline, which is then converted into adrenaline. Adrenaline binds to receptors on the heart, arteries, pancreas, liver, muscles and fatty tissue. 

By binding to receptors on the heart and arteries, adrenaline increases heart rate and respiration, and by binding to receptors on the pancreas, liver, muscles and fatty tissue, it inhibits the production of insulin and stimulates the synthesis of sugar and fat, which the body can use as a fuel in fight-or-flight situations.

https://www.livestrong.com/article/203790-what-happens-during-an-adrenaline-rush/