The hypothalamus is the part of the brain in charge of the stress response.
When a stress response is triggered, it sends signals sympathetic branch of ANS.
the sympathetic ANS is connected to the internal organs and when it is activated it affects the digestive system and created the fight or flight response.
it is also connected to the adrenal medulla
the adrenal medulla releases adrenaline and adrenaline will stimulate the sympathetic ANS even more