Health Science I Practice Test 2026: Anatomy, Physiology, and Medical Conditions

During emergencies the body activates the fight-or-flight system, and adrenaline (epinephrine) is the key hormone that makes the heart race. Adrenaline is released from the adrenal medulla when the sympathetic nerves are stimulated, and it acts on beta-1 receptors in the heart. This receptor activation raises the intracellular messenger cAMP, which starts a cascade that makes pacemaker cells in the SA node fire more quickly and enhances conduction through the AV node. The net effect is a faster heart rate (positive chronotropic effect) and stronger contraction, boosting cardiac output to deliver oxygen and energy to muscles.

Other hormones listed have different roles and don’t produce this rapid increase in heart rate during an emergency. Insulin mainly controls glucose uptake and storage; thyroxine increases metabolic rate more gradually, which can slowly raise heart rate over time; cortisol supports the stress response but its effects on heart rate are not as immediate or direct as adrenaline’s.