When the brain senses a threat, it switches on the sympathetic system and the adrenal glands within seconds. The body is pushed to fight or escape, at the cost of digestion and rest.
Fast response
The hypothalamus drives the sympathetic nerves and the adrenal medulla, which releases epinephrine (adrenaline) and norepinephrine into the blood. The effects appear within seconds.
Body changes
Heart rate, contractility and blood pressure rise, the airways open and the pupils dilate. Blood shifts from the skin and gut to the muscles, the liver releases glucose and sweating increases.
Slow response
Over minutes to hours the hypothalamus and pituitary drive the adrenal cortex to release cortisol. Cortisol raises blood sugar and helps maintain blood pressure during prolonged stress.
When stress continues
Long-lasting stress keeps cortisol high, which can raise blood pressure and blood sugar, weaken immunity and disturb sleep.
At the bedside
Pale, cool, sweaty skin with a fast pulse is the sympathetic response trying to protect blood pressure in early shock. Pain and anxiety also raise heart rate and blood pressure, so treat them before judging the vital signs.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).