How It Works: Adrenaline and the Fight-or-Flight Response

How It Works: Adrenaline and the Fight-or-Flight Response

When the brain perceives a threat or stressful situation, it sends a signal via the nervous system to the adrenal glands (located above the kidneys). The adrenal glands respond almost instantly by secreting adrenaline directly into the bloodstream.

Adrenaline prepares the body for rapid physical action by targeting multiple organs simultaneously. The heart rate increases to deliver more oxygenated blood to muscles. Breathing rate increases to take in more oxygen. Blood is diverted away from the digestive system towards skeletal muscles. The liver breaks down glycogen to release glucose into the blood, providing more fuel for respiration in muscles. Pupils dilate to improve vision. This coordinated response happens within seconds — unlike hormonal responses that can take minutes or hours.

Thyroxine operates on a much longer timescale. It is released by the thyroid gland in the neck and regulates the body's overall metabolic rate — the speed at which chemical reactions occur in cells. Thyroxine levels are controlled by a negative feedback loop: low thyroxine levels stimulate the pituitary gland to release TSH (thyroid-stimulating hormone), which prompts the thyroid to produce more thyroxine. When thyroxine levels rise sufficiently, TSH secretion is suppressed, reducing further thyroxine production.