Why Does Epinephrine Increase Heart Rate?

Epinephrine comes to the rescue when your body needs a turbo boost. During times of alarm your body may need extra strength, speed or alertness, so your hormonal system almost instantaneously responds to these unexpected events. Your adrenal glands, located in your kidneys, play an important role in this process through the release of epinephrine. One of the easily recognized symptoms of epinephrine in your system is an increased heart rate.

Adrenal Medulla

• Located in the kidneys, your adrenal medulla glands are essentially a part of your central nervous system -- which regulates all the functions of your body, including your heart rate. Two hormones are synthesized within the adrenal glands: epinephrine and norepinephrine. For quick release, your body stores these hormones in vesicles called chromaffin granules. Central nervous system triggers such as fear, anxiety, pain, changes in environmental temperature, low blood sugar and increased energy demands signal chromaffin granules to release hormones to assist your body in meeting its increased needs.

Epinephrine Function

• Both epinephrine and norepinephrine are released at the same time in response to central nervous system triggers. Epinephrine acts immediately to increase the frequency of heartbeats, while norepinephrine causes blood vessels to constrict. The effect of increased heart rate and blood vessel constriction is an increase in the strength of your heartbeats, resulting in an increase in blood pressure and respiration, as well as more blood flow to your muscles. This is very helpful when you need quick physical responses and actions.

Cardiac Output

• In times of stress, epinephrine facilitates greater cardiac output, meaning a greater volume of blood pumped during a given time period. For most, the entire volume of blood within the body is pumped through the heart once every minute; this is the resting heart rate. However, during times of vigorous exercise, your heart pumps up to seven times more output.

Epinephrine Receptors

• As epinephrine flows through the bloodstream, it interacts with receptor cells, called alpha receptors and beta-2 receptors. All arterioles -- small branches leading off of arteries to capillaries -- contain alpha receptors. When stimulated by epinephrine, alpha receptors cause blood vessels to constrict. Located primarily in skeletal and heart muscle arterioles, beta-2 receptors cause blood vessels to dilate. Activation of beta-2 receptors effectively pulls the body's energy reserves away from functions such as digestion, and provides greater energy to muscles and the cardio-respiratory system for emergency situations. Imperative to survival in times of danger and integral in providing the body with the ability to enjoy competitive activities, epinephrine's effect to increase heart rate is essential to life.