<P> Rather than releasing a neurotransmitter, the cells of the adrenal medulla secrete hormones . </P> <P> The adrenal medulla is the principal site of the conversion of the amino acid tyrosine into the catecholamines; epinephrine, norepinephrine, and dopamine . </P> <P> Because the ANS, specifically the sympathetic division, exerts direct control over the chromaffin cells the hormone release can occur rather quickly . In response to stressors such as exercise or imminent danger, medullary cells release the catecholamines adrenaline and noradrenaline into the blood . Adrenaline composes about 85% of the released catecholamines, and noradrenaline the other 15% . </P> <P> Notable effects of adrenaline and noradrenaline include increased heart rate and blood pressure, blood vessel constriction in the skin and gastrointestinal tract, smooth muscle (bronchiole and capillary) dilation, and increased metabolism, all of which are characteristic of the fight - or - flight response . Release of catecholamines is stimulated by nerve impulses, and receptors for catecholamines are widely distributed throughout the body . </P>

When does the body secrete epinephrine and norepinephrine