Caffeine is a psychoactive stimulant alkaloid that can be found in many natural products such as coffee, tea, guarana, yerba mate and tea. According to the International Journal of Food Sciences and Nutrition, about 80 percent of Americans consume caffeine on a daily basis, with an average intake of about 280 mg. One of the most noticeable side effects from caffeine is an increase in heart rate.
The main reason caffeine raises the heart rate is its stimulant properties. Caffeine takes about 45 minutes to be absorbed into the bloodstream. Once there, caffeine can make its way to the heart, where it acts as a stimulant by binding, or attaching itself to, specific receptors on the heart tissues causing the heart rate to increase.
Once at the heart, caffeine mimics the actions of epinephrine and binds with receptors that are commonly used by enzymes to block the production of cAMP (cyclic adenosine monophosphate). cAMp is a substance that activates protein kinase, which produces the adenosine triphosphate (ATP) needed for muscle contractions and relaxation of the heart. The increase of ATP causes the heart to beat at a faster pace.
Caffeine also attaches itself to the P-site areas of the adenosine receptors. Adenosine is a chemical made in the brain that slows down nerve cell activity. It is mostly used by the body when it's time to sleep. Caffeine has the ability to mimic adenosine, and by doing so can attach to the adenosine receptors. This blocks adenosine from binding to the cells. Because caffeine is a stimulant, nerve cell activity is increased, instead of the decrease that would normally occur from adenosine. The pituitary gland then observes the increase in nerve cell activity and interprets it as an emergency in the body. This causes the brain to activate the body's "fight or flight" response. This response causes the pituitary gland to release hormones that activate the adrenal glands. The adrenal glands in turn release the hormone adrenaline to react to the "emergency." Adrenaline then reacts, causing the heart to beat faster in preparation for the need to run or fight during the emergency, even though no emergency exists.