OCR GCSE Biology Practice Exam 2026 – Comprehensive All-in-One Guide to Exam Success

The release of adrenaline in response to stress is triggered by nervous impulses from the brain to the adrenal glands. When an individual experiences stress, the hypothalamus in the brain acts as a control center that initiates a response. It sends signals through the autonomic nervous system, specifically activating the sympathetic branch, which then communicates with the adrenal glands.

The adrenal glands, located on top of the kidneys, respond to these nervous signals by secreting adrenaline (also known as epinephrine) into the bloodstream. This hormone is crucial for the "fight or flight" response, preparing the body to either confront or flee from a perceived threat. It increases heart rate, enhances blood flow to muscles, and boosts energy availability, among other physiological changes.

The other options do not correctly explain the mechanism behind adrenaline release in stress responses. Hormonal signals from the pancreas pertain to blood sugar regulation rather than stress response. Direct stimulation from the heart is not related to the initial trigger for adrenaline release; instead, adrenaline acts on the heart once released. Changes in blood glucose levels are a consequence of adrenaline's effects rather than a triggering factor for its release.