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Before an exam your heart thumps, your mouth goes dry and your stomach seems to stop working. Your NCERT book has a name for the hormones behind this: "emergency hormones or hormones of Fight or Flight". The person who did most to work out why the body reacts this way was the American physiologist Walter Cannon. This post covers what he did, how his experiments worked, and how to use his idea in a biology answer.
Who was Walter Cannon?
The National Academy of Sciences lists Cannon as born on 19 October 1871 and died on 1 October 1945, elected a member in 1914. He worked at Harvard; the title page of his 1915 book names him George Higginson Professor of Physiology at Harvard University. A 2024 article in the Journal of Medical Biography describes his department at Harvard Medical School as home to scientists from around the world.
His first big research used the new X-rays. In 1898 he published The movements of the stomach studied by means of the Röntgen rays in the first volume of the American Journal of Physiology. Watching digestion on a screen led him somewhere unexpected.
How did digestion lead him to fear?
In the preface to Bodily Changes in Pain, Hunger, Fear and Rage (1915), Cannon explains that his interest in pain and strong emotion grew out of his studies of the gut. While watching animals digest food under X-rays he had seen that emotional states disturb digestion. His first chapter covers emotions that favour and emotions that stop the flow of digestive juices and the movements of the stomach.
So he asked a bigger question. Strong emotion already sends nerve signals along the sympathetic system: hair stands up, pupils widen. Do the adrenal glands join in?
What was the cat-and-dog experiment?
Cannon took up the question with Daniel de la Paz in 1910 and published Emotional stimulation of adrenal secretion in 1911. In his book he describes the method. They used "the natural enmity between two laboratory animals, the dog and the cat". A cat was placed in a comfortable holder near a barking dog. They took blood from near the adrenal veins before and after the fright.
To detect adrenaline (then called adrenin) they needed a sensitive test. Many substances make smooth muscle contract, but few make it relax, and adrenaline is one of them. So they used a strip of intestinal muscle as the detector: if the blood relaxed the strip, adrenaline was present. Blood taken after fright showed the effect; when the adrenal glands had been removed, it did not.
What did he conclude?
Across the book's chapters, Cannon and his co-workers found that pain and strong emotion:
raise blood sugar, enough that sugar appeared in the urine of frightened cats;
help tired muscle contract again, through adrenaline and higher blood pressure;
make blood clot faster, an effect that vanished when the adrenal glands were absent;
deepen and quicken breathing.
He then asked what these changes are for. He took up William McDougall's idea linking fear to an instinct for flight and anger to an instinct for fighting, and argued that every change prepares muscles for intense effort or limits blood loss from injury. He summed them up as adaptations to "the necessities of fighting or flight". His 1914 paper, The emergency function of the adrenal medulla in pain and the major emotions, is still taught in physiology courses as a classic, as Gwirtz (2008) describes.
How does this connect to homeostasis?
Cannon later turned to how the body keeps itself steady. In 1929 he published Organization for physiological homeostasis, and in 1932 a book for general readers, The Wisdom of the Body. Modell and colleagues (2015) credit him with introducing the word "homeostasis" and making Claude Bernard's abstract idea concrete. Our post on Claude Bernard and the internal environment tells the earlier half of that story.
The two ideas fit together. An emergency response pushes the body away from its resting state for a short time; homeostasis brings it back.
How do you use this in a NEET or CBSE answer?
The NCERT Class 11 Biology chapter on chemical coordination says the adrenal medulla secretes adrenaline and noradrenaline, called catecholamines, rapidly in response to stress and emergencies. It lists their effects: alertness, pupil dilation, raised hair, sweating, faster and stronger heartbeat, faster breathing and breakdown of glycogen, which raises blood glucose.
A strong answer links each effect to its purpose, as Cannon did:
More glucose in blood: fuel for working muscle.
Faster, stronger heartbeat: more blood, and oxygen, to muscle.
Faster breathing: more oxygen in, more carbon dioxide out.
Pupil dilation and alertness: better detection of the threat.
Common mistakes: writing "adrenal cortex" instead of "adrenal medulla", and mixing up the sympathetic system (which drives these changes) with the parasympathetic. If that pair keeps tangling, the Fermi Concept Library covers every concept in biology, so you can review the autonomic system before revisiting this chapter. Our post on chemical coordination and hormones covers the rest of the chapter.
Pre-exam nerves are this same system at work. They are normal, but if stress stays heavy for weeks, read our guide on exam stress and when to get help.
What can a student take from Cannon's way of working?
Follow the side observation. He set out to study digestion and noticed that emotion interfered with it.
Ask what a response is for. Listing effects is description; explaining their purpose is understanding.
Build the right instrument. A relaxing strip of intestine turned a vague question into a measurable one.
Next step: write the four adrenaline effects above from memory, each with its purpose in one line.
