An acute stress reaction is a biopsychological condition arising in response to an event which is individually regarded as emotionally stressful. The onset of a stress response is associated with specific physiological actions in the sympathetic nervous system, both directly and indirectly through the release of adrenaline and to a lesser extent noradrenaline from the medulla of the adrenal glands. These catecholamine hormones facilitate immediate physical reactions by triggering increases in heart rate and breathing, constricting blood vessels. The other major player in the acute stress response is the hypothalamic-pituitary-adrenal axis.