Without any conscious effort, your acute stress response prepares you to flee or fight dangerous situations (Rupia et al., 2019). During such reactions, the body releases certain hormones, like adrenalin and cortisol. As a result, the heart rate hastens, blood flow to major muscle groups shunts, while digestion stops. These actions give the body a burst of energy and strength to fight or fly from threatening circumstances.
Rupia, E. J., Binning, S. A., Roche, D. G., & Lu, W. (2016). Fight‐flight or freeze‐hide? Personality and metabolic phenotype mediate physiological defense responses in flatfish. Journal of Animal Ecology, 85(4), 927-937.