Adrenaline (Epinephrine): Fight-or-Flight Chemistry

A sudden fright makes your heart pound, your breathing quicken, your pupils widen and your mouth go dry, all within seconds. The chemical behind this is adrenaline, known in North America as epinephrine. It’s a small molecule released by the adrenal glands, and it prepares the body to deal with danger by redirecting blood, energy and oxygen to where they’re needed most. It was also the first hormone ever isolated in pure form.

Identity

Property	Value
Names	Adrenaline (UK, international); epinephrine (US)
Formula	C₉H₁₃NO₃
Molar mass	183.20 g mol⁻¹
Class	Catecholamine (hormone and neurotransmitter)
Made in	Adrenal medulla; small amounts in nerve cells
Active form	(R)-adrenaline

Both names mean the same thing: “adrenal” is Latin and “epinephrine” is Greek for “on the kidney”, because the adrenal glands sit on top of the kidneys.

Structure

Adrenaline has three parts:

1. A benzene ring with two –OH groups on neighbouring carbons. This benzene-1,2-diol unit is called a catechol, which gives the family its name: catecholamines.
2. A short side chain of two carbons, with an –OH group on the carbon next to the ring.
3. A secondary amine at the end, carrying a methyl group (–NH–CH₃).

The carbon bearing the side-chain –OH is a chiral centre (see isotopes, ions, isomers and allotropes). The body makes only the (R) enantiomer, which is many times more active than its mirror image, because receptors are chiral and fit one form better. It’s a vivid example of why the 3D shape of a drug matters.

At body pH, the amine group is protonated (–NH₂⁺–CH₃), so adrenaline is charged and water-soluble. It can’t cross cell membranes easily, so it acts through receptors on the cell surface (see hormones as chemical messengers).

How the body makes it

Adrenaline is built from the amino acid tyrosine (or from phenylalanine, which is converted to tyrosine), in four enzyme steps:

1. Tyrosine → L-DOPA: an –OH group is added to the ring (this is the slowest, rate-limiting step).
2. L-DOPA → dopamine: the carboxyl group is removed (decarboxylation).
3. Dopamine → noradrenaline: an –OH is added to the side chain.
4. Noradrenaline → adrenaline: a methyl group is added to the nitrogen, transferred from a donor molecule called S-adenosylmethionine.

So adrenaline shares its origins with the neurotransmitters dopamine and noradrenaline, which are intermediates on the same pathway (see dopamine). The last step happens mainly in the adrenal medulla, which is why that’s where most adrenaline is made. The adrenal medulla is controlled directly by nerves, so it can release adrenaline within seconds of a threat.

Fight or flight: what adrenaline does

Adrenaline binds to adrenergic receptors, proteins on the surfaces of many cell types. There are two main families, α (alpha) and β (beta), each with subtypes. Different tissues carry different receptors, so the same molecule produces different effects in different places:

Target	Receptor	Effect
Heart	β₁	Faster, stronger heartbeat
Airways	β₂	Bronchioles widen (easier breathing)
Blood vessels in skin and gut	α₁	Narrow (redirects blood away)
Blood vessels in skeletal muscle	β₂	Widen (more blood to muscles)
Liver	β₂ (and α)	Breaks down glycogen, releasing glucose
Fat tissue	β	Breaks down fat, releasing fatty acids
Eyes	α₁	Pupils widen
Gut	β₂, α	Digestion slows

Together, these changes send more oxygen and fuel to the muscles and brain, and switch off non-urgent processes like digestion. The receptors act through G proteins and second messengers such as cyclic AMP, which amplify the signal so that tiny amounts of adrenaline have large effects.

The effects are short-lived. Adrenaline is quickly taken back into nerve endings or broken down by enzymes (such as COMT and MAO), and its half-life in blood is only a few minutes.

The first hormone to be isolated

• In the 1890s, George Oliver and Edward Schäfer in London showed that extracts of the adrenal gland sharply raised blood pressure.
• In 1901, the Japanese chemist Jōkichi Takamine, with his assistant Keizo Uenaka, isolated the pure crystalline substance and named it Adrenalin. It was the first hormone ever obtained in pure form. (John Jacob Abel had earlier prepared a less pure derivative he called epinephrine, which is where the other name comes from.)
• In 1904, Friedrich Stolz synthesised it chemically, making it one of the first hormones to be made in a laboratory.

Adrenaline as a medicine

Anaphylaxis

In a severe allergic reaction (anaphylaxis), blood vessels widen and leak, blood pressure crashes and airways swell and narrow. Adrenaline reverses these effects: it tightens blood vessels through α receptors and opens airways through β₂ receptors. It’s the most important emergency treatment, which is why people with severe allergies carry adrenaline auto-injectors (such as EpiPens), typically delivering about 0.3 mg into the thigh muscle for adults.

Cardiac arrest

Adrenaline is given during cardiopulmonary resuscitation to increase blood flow to the heart and brain.

Local anaesthetics

Small amounts of adrenaline are added to local anaesthetics (for example, in dentistry). By narrowing nearby blood vessels, it keeps the anaesthetic in place longer and reduces bleeding.

Blocking adrenaline: beta-blockers

Beta-blockers (such as propranolol and atenolol) are molecules shaped enough like adrenaline to bind β receptors, but they don’t activate them. They’re competitive antagonists (see competitive vs non-competitive inhibition for the similar logic with enzymes). By blocking adrenaline’s effects on the heart, they slow the heart rate and lower blood pressure. They’re used for high blood pressure, heart conditions and sometimes to reduce the physical symptoms of anxiety. Asthmatics must use them with care, because blocking β₂ receptors can narrow the airways.

Stability

The catechol ring is easily oxidised by air, especially in light and in alkaline solution, forming pink and then brown products (such as adrenochrome). That’s why adrenaline solutions are kept in dark, sealed containers, often with antioxidants added, and why auto-injectors should be replaced if the liquid is discoloured. It’s a practical example of oxidation and reduction.

Adrenaline and everyday stress

The fight-or-flight response evolved for short, physical emergencies, such as escaping a predator. Modern stresses, like exams, traffic jams or a difficult conversation, can trigger the same chemistry without any physical action to use up the released glucose and fatty acids. The racing heart, shaky hands and “butterflies” before a presentation are adrenaline at work. Slow breathing can help, because it activates the parasympathetic nervous system, which counteracts many of adrenaline’s effects on the heart. Longer-lasting stress involves cortisol as well, whose effects build up over hours and days rather than seconds.

Key takeaways

• Adrenaline (epinephrine) is a catecholamine: a catechol ring, a side-chain –OH and a methylated amine; only the (R) form is highly active.
• It’s made from tyrosine via L-DOPA, dopamine and noradrenaline, mainly in the adrenal medulla.
• It acts through α and β adrenergic receptors, producing the fight-or-flight response: faster heart, wider airways, more glucose and fatty acids in the blood.
• It was the first hormone isolated (Takamine, 1901).
• It treats anaphylaxis and cardiac arrest; beta-blockers block its effects. For the related neurotransmitters, see neurotransmitters.