Adrenaline is another word for the hormone epinephrine, part of the brain's response to exercise. Epinephrine has an effect on your body to increase the cardiorespiratory activity that facilitates exercise. It also has an affect on your metabolism and can even improve your mood.
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Even the anticipation of exercise can spark a rush of adrenaline. Upon exercise or before, your brain sends a signal to your adrenal glands. In response, your adrenal glands excrete epinephrine into the bloodstream. The release of epinephrine is greater in an untrained person than a trained person.
Epinephrine is released as part of the sympathetic nervous system initiating the "fight or flight response.'' It increase your heart rate in order to increase the amount of blood your heart beats each minute. More blood flowing means a larger amount of oxygen available to your muscles that need it. It also works to constrict your blood vessels, increasing your blood pressure and to allow more blood to your muscles.
Regular exercise training can increase your mood. With regular exercise comes a reduction in the levels of epinephrine at rest. Less sympathetic nervous system innervation leads to a reduction in the intense feelings that come from epinephrine. In addition, depletion of epinephrine on a daily basis that can come from being over-stimulated can lead to fatigue. Less fatigue can mean a happier mood.
Your adrenaline rush from epinephrine also influences your metabolism. Epinephrine stimulates the metabolism of carbohydrate and fat stores inside your body. In preparation for the "fight or flight response" your body must have ample fuel to run on. For this reason, epinephrine increases the activity of the hormones responsible for breaking down glycogen and fat and making it available for your muscles.