Depending on what stress one feels (eustress or distress), the body undergoes a stress response (general adaptation syndrome or GAS) that is controlled by the hypothalamus leading to a fight-or-flight response, a slower resistance reaction, and lastly, exhaustion.
As a result of nerve impulses travelling to the sympathetic division of the ANS from the hypothalamus, the fight-or-flight response aims to bring glucose and oxygen to organs most active during this process, which are the brain, the skeletal muscles, and the heart. These three work together to be able to move quickly from the negative stressor. This is why we are more alert, are able to move faster, and our heart beat seems faster during this instance.
Resistance reaction takes place after, initiated by the hormones CRH, GHRH, and TRH. CRH stimulates the adrenal glands to increase cortisol, reducing inflammation, producing ATP or repairing damaged cells. GHRH secretes the growth hormone, which stimulates the breakdown of glycogen in the liver. TRH then triggers secretion of thyroid hormones, leading to the increased use of glucose for ATP production. During this time, you will feel your heart still pounding even if the stressor is out of sight.
Last would be exhaustion, which the body enters since the resources have been used up during the resistance stage. Pathological changes may or may not occur since resistance reactions will still be there even after the stressor has been removed.
Reference:
Tortora, G.J., Derrickson, B. (2014). Principles of Anatomy and Physiology (14th ed.). John Wiley & Sons, Inc.