Final answer:
The fight-or-flight response is triggered by the adrenal glands releasing adrenaline, leading to an increase in heart rate and heart contractility.
Step-by-step explanation:
The fight-or-flight response is due to a discharge of the adrenal glands which leads to an increase in heart rate and contractility of the heart.
When faced with a perceived threat, the sympathetic division of the autonomic nervous system is activated, releasing hormones like epinephrine (adrenaline) and norepinephrine (noradrenaline) into the bloodstream.
This causes a variety of physiological effects, including an increased heart rate, dilation of the pupils, activation of sweat glands, and an increase in energy levels through higher blood glucose levels, all intended to prepare the body for immediate physical action.