Final answer:
An increased heart rate due to adrenaline release from the adrenal glands in response to stress is an example of endocrine system signaling, specifically the fight-or-flight response activated by the sympathetic nervous system.
Step-by-step explanation:
An increased heart rate caused by the release of adrenaline from the adrenal glands, which are located just above the kidney, is an example of endocrine system signaling. This response, often referred to as the fight-or-flight response, involves the activation of the sympathetic nervous system. When an individual perceives a threat, the sympathetic nervous system stimulates the adrenal glands to secrete adrenaline (also known as epinephrine) and norepinephrine into the bloodstream, which in turn increase heart rate, blood pressure, and energy availability. In the heart, adrenaline binds to receptors which causes the cells to increase the rate at which they fire. This series of reactions prepares the body to either confront or retreat from the threat. These reactions are mediated by various signaling pathways, including G-protein coupled receptors and the second messenger cAMP.
In more detail, this hormonal response is initiated by neural signals sent to the adrenal gland, activating the release of epinephrine and norepinephrine. These catecholamines signal various physiological changes, like increased heart rate, glucose mobilization, and blood flow redirection to essential organs. The secretion is controlled by a neural pathway originating from the hypothalamus — specifically, the sympathetic adrenomedullary (SAM) pathway.