Eustress (or good stress) and bad stress (acute or chronic) cause the exact same reaction in the human body. Even during voluntary “stressful” activities such as sport or exercise or when we receive unexpected good news, the brain stem, the oldest and more primordial part of the human brain, immediately mobilizes the body’s resources. The brain stem does not know, and one might say does not care, what triggers the sudden demand for additional physical activity. All the brain stem knows, prior to any higher brain intervention such as a decision to be afraid of something, or a decision to exercise, is that more blood is needed immediately to fulfill physical demands that may already be occurring (in the case of exercise or a real and impending threat) or that may be presumed to occur (in the case of perceived danger in a situation).
When the motor areas of the brain and the limbic system become activated by a positive but sudden event, most of the reticular activating system of the brain stem is also mobilized. This activation includes greatly increased stimulation of the vasoconstrictor and cardioacceleratory areas of the vasomotor center of the brain stem. Thus, the increase in arterial pressure permits to keep pace with an expected increase in muscle activity. A similar rise in pressure occurs during negatively stressful situations. The need to prepare to meet the danger posed by the stressor mobilizes the reticular activating system and the vasomotor center of the brain stem.
During dangerous situations (real or perceived), arterial pressure rises to as high as twice its normal value within a few seconds. This dramatic increase can immediately supply blood to any or all muscles of the body needed to respond. This translates into an enormously increased ability to fight against or to flee from the cause of danger. It is indeed a significant survival factor that no conscious decision is needed when this split-second mobilization is required.