A sudden fright sets your heart pounding within moments. That is adrenaline, a hormone made in your adrenal glands.
You have two adrenal glands, one on top of each kidney. When you are frightened, excited or under stress, their inner part releases adrenaline into the blood. Its names say where it comes from: adrenaline from the Latin ad renes, at the kidneys, and its other name, epinephrine, from the Greek for on the kidney.
Fight or flight
In 1915 the American physiologist Walter Cannon described adrenaline as the hormone that gets the body ready for fight or flight. Different cells carry different kinds of adrenaline receptor, so the one hormone does something different in each part of the body.
| Part of the body | What happens |
|---|---|
| heart | beats faster and harder |
| airways | widen, so more air flows in and out |
| liver | releases glucose into the blood, fuel for muscles |
| blood vessels | widen in muscles and narrow in skin and gut |
| pupils | widen |
One hand, made from dopamine
Your adrenal glands build adrenaline from tyrosine, along the path that makes dopamine, with two more steps. An enzyme adds an -OH group to the chain carbon next to the ring, which makes noradrenaline. Another enzyme adds a methyl group, -CH₃, to the nitrogen, and that makes adrenaline, C₉H₁₃NO₃.
The new -OH turns that chain carbon into a stereocentre: it now carries -OH, -H, the ring and the rest of the chain, four different groups. The enzyme is a handed protein, and it adds the -OH on one side only, so your body makes just one hand, (R)-adrenaline.
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In 1933 the chemists Leslie Easson and Edgar Stedman explained why the (R) hand works far better than its mirror image. The receptor has three spots that match three of the groups on the stereocentre: the ring, the -OH and the chain that ends in the amine. Like the medicine in a three-spot pocket, (R)-adrenaline can rest all three groups on their spots at once. (S)-adrenaline can match only two.