Final answer:
An increased heart rate due to adrenaline is an example of the fight-or-flight response initiated by the adrenal glands in response to stress or danger.
Step-by-step explanation:
An increased heart rate caused by the release of adrenaline from the adrenal glands, which are located just above the kidneys, is an example of the body's fight-or-flight response. This response is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. When faced with such a situation, the brain processes signals starting in the amygdala and then the hypothalamus, leading to the release of hormones like adrenaline.
Adrenaline, a non-steroid catecholamine hormone, produced by the medulla of the adrenal glands, stimulates the fight-or-flight response, increasing the heart rate and blood flow, leading to a physical boost and heightened awareness. This crucial hormone also triggers various other physical effects, such as dilated pupils, sweating, and increased alertness. The body prepares itself to either confront the threat or to run away from it—hence, the term 'fight-or-flight.'