What hormone increases heart rate and energy levels during stress response?

The hormone that increases heart rate and energy levels during a stress response is adrenaline, also known as epinephrine. This hormone is released by the adrenal glands in response to stress, activating the body's fight-or-flight response. When released into the bloodstream, adrenaline causes a series of physiological changes: it increases heart rate, enhances the force of heart contractions, dilates air passages to improve oxygen intake, and mobilizes energy stores by facilitating the release of glucose into the bloodstream.

These effects prepare the body to react quickly in stressful situations, providing the energy and physical readiness necessary for immediate action. While cortisol also plays a role in the stress response by regulating various bodily functions, including increasing blood sugar and suppressing non-essential functions, it does not directly increase heart rate to the same extent as adrenaline does. Insulin is involved in glucose metabolism and lowering blood sugar, and serotonin is primarily associated with mood regulation rather than direct physiological responses to stress. Thus, adrenaline is the key hormone responsible for the acute increases in heart rate and energy levels during stress.