Situation is appraised and perceived as a threat.
Hypothalamus is alerted. This recognises the stress is acute so it activates the Sympathomedullary (SAM) Pathway.
This activates the sympathetic branch of the Autonomic Nervous System (ANS).
This then stimulates the adrenal medulla which is part of the adrenal gland. These are located just above the kidneys.
The adrenal medulla secretes the hormones adrenalin and noradrenalin into the bloodstream.
This prepares the body for fight or flight and as such causes a number of physical changes, including increased respiration rate, increased alertness and pupil dilation.
The body cannot maintain this increased level of activity for long periods of time, as it takes too much energy. After a few minutes/when the stressor has been dealt with, the parasympathetic branch of the nervous system is activated.
This is a countershock response and serves to return the body back to its normal resting state (e.g. Reduces heart rate and breathing rate).