Alright, so the HPA axis is essentially the physiological mechanism that produces the fight-flight response.
When we perceive a threat, this stimulates the hypothalamus. The hypothalamus releases a type of neurohormone called CRH (corticotropin-releasing hormone). CRH stimulates the anterior pituitary gland to produce ACTH (adrenocorticotropic hormone) into the bloodstream.
ACTH travels through the blood and eventually stimulates the adrenal glands, which are located on top of the kidneys. The adrenal glands respond to ACTH by producing adrenaline, noradrenaline and cortisol.
Adrenaline and noradrenaline stimulate the sympathetic nervous system, and result in physiological changes such as an increase in heart and breathing rate, blood pressure, etc. Cortisol stimulates the liver to release more glucose into the bloodstream.
Because the heart is pumping faster, blood, along with the nutrients (eg. glucose) it contains, can be delivered to the muscles a lot faster. Because we've got more glucose in the bloodstream, the muscles have a greater supply with which they can make energy.
The effects that are produced as a result of the HPA axis are collectively known as the fight-flight response. Essentially, because our muscles are able to work more efficiently, we're more easily able to fight a threat or flee from it. We also see a variety of other changes such as dilation of the pupils, suppression of the digestive system, etc
Hope that helped a bit
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