adrenaline

Hormone secreted by the medulla of the adrenal glands. Adrenaline is synthesized from a closely related substance, noradrenaline, and the two hormones are released into the bloodstream in situations of fear or stress.

Adrenaline's action on the liver raises blood-sugar levels by stimulating glucose production and its action on adipose tissue raises blood fatty-acid levels; it also increases the heart rate, increases blood flow to muscles, reduces blood flow to the skin with the production of sweat, widens the smaller breathing tubes (bronchioles) in the lungs, and dilates the pupils of the eyes.

Adrenaline and noradrenaline can be synthesized in the laboratory; adrenaline is used to treat the narrowed bronchioles found in asthma, and noradrenaline (by virtue of its narrowing effect on the small arteries of the intestines and skin) to raise blood pressure when this is dangerously low. Adrenaline and noradrenaline have largely been replaced in therapy by synthetic drugs having fewer side effects on the patient, but this does not lessen the great importance of the two substances in understanding many basic mechanisms of the body.