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The Role of Adrenaline in the Fight-or-Flight Response

Edmund has spent the last ten years working in clinical research. He has written many articles on human anatomy and physiology.

Potentially life-threatening situations trigger the fight-or-flight response, which causes your body to release adrenaline.

Potentially life-threatening situations trigger the fight-or-flight response, which causes your body to release adrenaline.

Have you ever been chased or attacked by a bear? If not, think of the physical and emotional stress you get when you're late for a very important appointment and multiply that feeling by a thousand.

In the face of danger (any event capable of causing harm), the nervous system and the endocrine system crank up their gears. The heart rate increases significantly to raise the supply of energy to the muscles. The body becomes completely focused and ready for action in what experts call the fight-or-flight response. This is the reaction that gets you out of trouble as soon as possible.

How Does the Fight-or-Flight Response Work?

As soon as a threat is perceived, an electrical signal is sent from the brain down to the adrenal glands (small glands located at the upper part of the kidneys). The signal comes from a part of the brain called the hypothalamus. The adrenal glands then secrete the hormone adrenaline (also known as epinephrine) into the bloodstream. This causes a spike in the concentration of adrenaline in the blood.

The bloodstream circulates adrenaline to different parts of the body where it evokes different effects. Adrenaline causes vasodilation to increase blood supply to the muscles. This focuses the body's energy supply toward the muscles where they are needed the most during an emergency situation.

The increased blood-adrenaline level triggered by the fight-or-flight reaction and its effects on different parts of the body is also referred to as "adrenaline rush."

A thrill-seeker base jumps from a tower.

A thrill-seeker base jumps from a tower.

What Is an Adrenaline Junkie?

The term "adrenaline junkie" is reserved for individuals who enjoy dangerous activities and are addicted to the adrenaline rushes that come with such activities. For adrenaline junkies who are involved in extreme sports, "normal sports" just don’t do it anymore. Adrenaline junkies tend to push their limits with the craziest and scariest of activities.

Just in case you feel like increasing the length of your bucket list, here is a short list of dangerous things to try.

  • Hang gliding
  • Freediving
  • BASE jumping
  • Cliff jumping
  • Ice climbing
  • Freerunning
  • Bungee jumping

The Effects of Adrenaline in Different Parts of the Body

When the fight-or-flight response releases adrenaline into the bloodstream, it affects many body parts and internal systems.

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In the eye, adrenaline binds to adrenoceptors, resulting in the contraction of the radial muscle of the iris. This causes the pupils to become dilated to allow more light into the eye for brighter and sharper images. This is important because in the presence of a threat, you would want a clear view of your surroundings.


Adrenaline binds to surface receptors of the liver to trigger a pathway inside liver cells. An enzyme called glycogen phosphorylase is released in the liver cells to break down glycogen into individual glucose molecules. This process is known as glycogenolysis and leads to a rise in blood sugar levels. Glucose molecules are then transported to muscle cells to provide a boost of energy. This is important because glucose can be quickly broken down to produce adenosine triphosphate (ATP), which is an energy source for cells.


Adrenaline binds to receptors on the smooth muscle cells of the bronchioles, causing them to dilate. This relaxes the smooth muscles and allows more oxygen to diffuse into the blood. Adrenaline also causes dilation of the arterioles and speeds up the breathing rate. The purpose of increasing the rate of inspiration and expiration is to allow the body to absorb more oxygen into the bloodstream and expel more carbon dioxide.


Adrenaline stimulates the cells of the heart so that it beats faster, increasing the heart rate. During a fight-or-flight reaction, it is important for oxygen, glucose, hormones, and other chemicals to be able to circulate much faster throughout the body to the cells that need them.


Adrenaline binds to receptors on the smooth muscle cells in the skin, causing them to contract. This is why the hair on the surface of your skin rises when you are under physical or emotional stress. Adrenaline also binds to a receptor that causes the contraction of sweat glands, resulting in perspiration.

Digestive System

Adrenaline causes vasoconstriction to the blood supply of the digestive system. This shuts down supply to the digestive system to give priority to musculoskeletal system. Digesting a burger is not exactly a priority during an emergency situation.

Why Is Adrenaline Important?

Adrenaline is an efficient messenger with an important role in the fight-or-flight response. It signals different parts of the body and causes different reactions in different systems. It allows us to respond long enough to potentially get out of danger by either fighting or fleeing.

This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.


Scarlett on February 27, 2016:

Thanks greatly you really aid us

Leah Kennedy-Jangraw from Massachusetts on March 20, 2013:

Great summary of Adrenaline actions in Fight or Flight- thanks for sharing. Wonder if there is anything to adrenaline junkies having attentuated effects of Adrenaline overtime and so they seek out bigger and scarier things in order to continue to feel the same rush?

Interesting stuff-thanks for sharing!

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