Reader Noah wrote in with a question—"Why does the human body need adrenaline?"
Adrenaline, or epinephrine, is a hormone and neurotransmitter produced by the adrenal glands that participates in the sympathetic nervous system's "fight-or-flight response" (or "acute stress response") to situations that are high-stress, dangerous and/or physically exhilarating. This release is an evolutionary adaptation that allows us to better cope with these situations.
Upon its release into the body, adrenaline binds to a variety of adrenergic receptors and causes several metabolic changes, like inhibition of insulin secretion and promotion of glucagon secretion by the pancreas, stimulation of glycogenolysis in the liver and muscles and stimulation of glycolysis in muscles. These changes and others together lead to increased blood glucose and fatty acids in the body and more energy production within the body's cells. The release of adrenaline further results in increased heart rate, contracted blood vessels and dilated air passages.
Ultimately, these changes allow more blood to get to the muscles and get more oxygen to the lungs quickly—so your physical performance improves and you're primed to either flee from danger or defend yourself.