The hormone responsible for increasing heart rate during stress is Adrenaline, also known as epinephrine. It is released from the adrenal medulla in response to stress and activates the sympathetic nervous system. This leads to increased cardiac output by stimulating β1-adrenergic receptors on the heart, causing a rise in heart rate (positive chronotropic effect) and force of contraction (positive inotropic effect). This rapid response prepares the body for "fight or flight" by enhancing blood flow to vital organs and muscles. Other hormones like cortisol and thyroxine have indirect or slower effects on heart rate, while insulin primarily regulates glucose metabolism.

Reference: Guyton and Hall, Textbook of Medical Physiology, 14th Edition.