When the sympathetic nervous system is activated, the brain stimulates the adrenal medulla. The cells in the medulla immediately secrete adrenaline (80%) and noradrenaline (20%). These hormones stimulate adrenergic receptors throughout the whole body. Adrenaline mobilises glucose, fatty acids and energy, and increases heart rate and force of muscle contraction.
I. Acute stress results in higher blood pressure within seconds. II. Chronic stress is mediated by noradrenaline.
Extra info: Acute stress is a response to an FFF (fight, fright, flight)-situation mediated via adrenaline and immediately results in changes in many physical processes such as increased blood pressure, dilated pupils, increased breathing frequency, increased heart rate etc. In contrast, chronic stress is mediated by the ACTH-cortisol system, usually characterized by elevated cortisol levels.