Reading time: 4 min

If you’ve ever seen Pulp Fiction you’ll likely not forget the scene where Uma Thurman’s overdose is reversed with a shot of adrenaline to the heart; wide-eyed, she bolts upright with a gasp.

“I’m an adrenaline junkie,” people say, as they thrill-seek their way across the globe, bungee jumping, diving off cliffs, swimming with sharks.  Drawn to danger, they revel in that heart-in-your-throat, blood pumping feeling.  Not everyone shares their enthusiasm.  But what is adrenaline, exactly? And what does it do?

What is Adrenaline?

One of the most interesting things about adrenaline,” says Baltimore endocrinologist Mansur Shimali, “is that it can be both a hormone, and a neurotransmitter.

Adrenaline, also known as epinephrine, is our “fight-or-flight” hormone”, says Michael R. Rickels of University of Pennsylvania Perelman School of Medicine.  “It increases heart rate and contractility in order to get our blood pumping and oxygen to our muscles and brain under conditions of physiologic stress, and will help to maintain blood pressure during blood loss or dehydration.

Adrenaline is a survival hormone.  It helps us be fast or strong under threat.  When humans encounter danger, we have two options: fight off the predator, or run for the hills. Each of these responses requires muscle strength.  When our amygdala senses danger, it reaches out to the hypothalamus,  Adrenaline then triggers the physiological changes our body will need to attack, defend, or run away.  Even before we react, or choose a course of action, our heart is already rushing oxygenated blood to our muscles, so we’ll be ready.

On your mark, get set, Go!

In early humans, adrenaline provided a survival mechanism to fight off foes, and win competition for food, land or mates. But the adrenaline response is not limited to predator danger.

At the start of a sprint, as runners hear, “On your mark!  Get set!”, they crouch, ready to run.  Their hearts race. Their muscles twitch. Adrenaline is coursing through their blood, ensuring that when they hear the command “Go!” they are prepared to race for their lives (or at least a medal). Adrenaline powers our performance whether the external stressor is unexpected (a pouncing tiger), sought-after (hang-gliding) or self-imposed (athletic competitions or haunted houses).

Effects of Adrenaline?

The immediate impact of an adrenaline surge:

  • Eyes dilate.
  • Heart beats faster.
  • Sweat increases.
  • Bronchioles dilate (so we can get more oxygen).
  • Blood vessels dilate (enlarge) in our muscles.
  • Blood vessels constrict in our digestive tract to slow digestion.
  • Kidneys make more renin (to increase blood pressure).
  • Glucose production increases, for energy.

Each of these responses is tailored to focus the body’s resources on survival.

Where does adrenaline come from?

Adrenaline is a creation of the nervous system.

The human nervous system is divided into two sections:

  • central nervous system
  • peripheral nervous system

The peripheral nervous system has two subsystems:

  • somatic nervous system (our voluntary movements).
  • autonomic nervous system (unconscious operation of  heart, digestion and breathing).

The autonomic nervous system has three components:

  • sympathetic (stimulates ‘fight or flight’).
  • parasympathetic (“breed and feed” or ‘rest and digest’).
  • Enteric (“second brain”).

Sympathetic Nervous System (Fight or Flight)

Our sympathetic nervous system is the center of our ‘fight or flight’ response, that ancient reaction to danger which enabled our ancestors to outrun, outmaneuver and outsmart predators.

The sympathetic nervous system is always mildly activated, ready. It manages stress, and returns the body to homeostasis, through the activation (or inactivation) of the adrenal gland.

Adrenal Gland

Adrenaline is manufactured within our adrenal glands. There are two.

Each adrenal gland sits atop a kidney. (The word adrenal literally means at (ad) the kidney (renal).

The adrenal gland has two sections.

  • Adrenal cortex (outer portion)
  • Adrenal medulla (inner portion)

The adrenal cortex produces cortisol (another stress hormone), which regulates metabolism.  Aldosterone, which helps control blood pressure, is also secreted here.

The adrenal medulla.  It’s here that adrenaline (a catecholamine:) is generated to help you fight tigers, or meet that deadline. Unlike the adrenal cortex, the adrenal medulla is not essential to human life.  The adrenal medulla connects to the sympathetic nervous system via a sympathoadrenal system that regulates the stress response.

Adrenaline and Diabetes

But adrenaline is more than a spur to physical exertion.  “Adrenal hormones also regulate salt and water balance in the human body,“  says Dr. Shimali, “They do that by signaling to the kidneys.

Adrenaline also affects metabolism”, says Dr. Rickels, “releasing fatty acids from fat tissue that our muscles can burn, and releasing glucose from our liver that on which our brain depends, and so helps to maintain blood glucose levels during prolonged fasting or periods of famine.

This is especially significant for diabetics. “Adrenaline features prominently in the defense against low blood glucose (hypoglycemia)”, says Dr. Rickels, “and is thus critical for patients with insulin-dependent diabetes to help them avoid severe insulin reactions that can result in loss-of-consciousness, or seizures.”

Other health applications

But there are medical applications as well. Supplemental adrenaline is used widely, for a variety of reasons, in sometimes unexpected ways.  Did you ever sit in the dentist’s chair, lip puffy with novocaine, heart pounding as the dentist approached you with a drill?  It might not be your fear itself, making your heart pound. Dentists add epinephrine (adrenaline) to novocaine, to staunch bleeding and make the effects last longer.

Adrenaline (epinephrine) has also saved countless lives by reversing anaphylactic shock from allergic reactions. It restarts hearts after cardiac arrest.  It is used daily to ease breathing in asthmatics or children with croup.  It reverses blood pressure due to blood loss.  Spikes in adrenaline have also been linked to early morning cardiac events. Adrenaline even plays a role in our circadian rhythm  Despite this significant impact on medicine, no Nobel Prize was ever awarded in connection with adrenaline’s discovery.

It’s a hormone. It’s a neurotransmitter. It’s a life-saving intervention. It’s a blast! For something deemed ‘non-essential to human life, adrenaline certainly has a key role to play in our physical performance, stress mediation, and the quality of our life.

So when confronted with danger, whether we decide to fight or run, the physiological changes are the same. Cowards and warriors alike have adrenaline to thank for fueling choices made in an instant. And that adrenaline rush you feel? Some people even pay for that.

Did we miss anything?

If you have any questions, suggestions or topic requests, please reach out.