The Endocrine System and Fight or Flight

The endocrine system is a network of glands that secrete chemical messengers called hormones directly into the bloodstream. From there, hormones travel to target organs and cells, where they bind and alter the cell's activity, ultimately affecting behaviour.

A gland is an organ that produces and releases hormones. A hormone is the chemical messenger it releases. Because hormones move through the blood rather than along a fixed wire, they reach many parts of the body at once.

It helps to compare the endocrine system with the nervous system, since AQA often sets the two side by side:

Feature	Nervous system	Endocrine system
Signal type	Electrical impulses along neurons	Chemical hormones in the bloodstream
Speed of action	Very fast (milliseconds)	Slower (seconds to minutes)
Spread of effect	Targeted to specific cells	Widespread across the body
Duration	Short-lived	Longer-lasting

The endocrine system is slower to act than the nervous system, but its effects are more widespread and longer-lasting. This trade-off is a common exam comparison.

The two systems are not separate. They work together, and the pituitary gland links them by responding to signals from the brain.

You do not need every gland in the body, but you should recognise the major ones and, above all, the pituitary and adrenal glands, which drive the fight or flight response.

Gland	Key hormone(s)	Main effect
Pituitary	Controls release from other glands	The "master gland" — directs the endocrine system
Adrenal medulla	Adrenaline, noradrenaline	Triggers fight or flight bodily changes
Adrenal cortex	Cortisol	Regulates the longer-term stress response, metabolism
Thyroid	Thyroxine	Regulates metabolism
Ovaries	Oestrogen	Female reproductive development
Testes	Testosterone	Male reproductive development
Pancreas	Insulin	Regulates blood glucose

The pituitary gland is often called the master gland because it controls the release of hormones from many other glands. It does not act alone: it is directed by the hypothalamus, a region of the brain that links the nervous and endocrine systems.

The adrenal glands sit on top of the kidneys and have two parts. The adrenal medulla (the inner part) secretes adrenaline and noradrenaline. The adrenal cortex (the outer part) secretes cortisol.

Focus your revision on the pituitary (master gland, directed by the hypothalamus) and the adrenal medulla (source of adrenaline). These two are central to the fight or flight response that follows.

When an acute stressor is perceived — a sudden threat such as a car swerving towards you — the body launches a rapid, coordinated reaction called the fight or flight response. It prepares you either to confront the danger or to escape it.

The route this response takes is the sympathomedullary pathway (often abbreviated to the SAM pathway, for sympathetic–adrenal–medullary). It runs in a fixed order:

1. The hypothalamus identifies the threat.
2. It activates the sympathetic branch of the autonomic nervous system.
3. This stimulates the adrenal medulla to release adrenaline (and noradrenaline) into the bloodstream.
4. Adrenaline triggers physiological changes across the body that prepare it to fight or flee.
5. Once the threat passes, the parasympathetic branch returns the body to its resting state.

The key sequence to memorise is: stressor → hypothalamus → sympathetic branch → adrenal medulla → adrenaline → bodily changes. Adrenaline is released by the adrenal medulla, not the pituitary.

Adrenaline is the key hormone of the fight or flight response. Released from the adrenal medulla into the bloodstream, it acts almost instantly to produce the physical changes that ready the body for sudden physical effort.

Each change has a survival purpose:

Bodily change	Why it helps you fight or flee
Increased heart rate	Pumps oxygenated blood to muscles faster
Increased breathing rate	Takes in more oxygen for respiration
Dilated pupils	Lets in more light to improve vision of the threat
Release of stored glucose	Supplies fuel for a rapid burst of energy
Blood redirected to the muscles	Prioritises the muscles needed for action
Inhibited digestion	Diverts energy away from non-urgent processes
Sweating	Cools the body during exertion

Because adrenaline travels in the blood, these effects appear together within seconds. This is why the response feels so physical: a pounding heart, quick shallow breaths and a dry mouth as digestion shuts down.

When the danger has passed, the parasympathetic branch takes over and reverses these changes — slowing the heart, restarting digestion and returning the body to normal. This is often called the rest and digest response.

Fight or flight is well supported, but AQA rewards a critical view. Weigh up its strengths and limitations.

Adaptive and evolutionary value. Fight or flight is widely seen as an adaptive survival mechanism. Ancestors who could respond to danger with a fast burst of energy were more likely to survive and reproduce, so the response is thought to have been naturally selected. This gives the model strong evolutionary plausibility.

A possible beta bias — the response may be androcentric. Much of the early research was conducted on males, and the fight or flight model was then generalised to everyone. Taylor et al. (2000) argued this ignores a distinct female response. They proposed a tend and befriend pattern, in which women under stress protect their offspring (tend) and form alliances with other women (befriend), possibly linked to the hormone oxytocin. If correct, the traditional model does not fully describe how women respond.

Fight or flight is not the only reaction — the freeze response. Gray (1988) argued that the first response to a threat is often not to fight or flee but to freeze: a state of hypervigilance in which the animal stops, stays alert and assesses the danger before acting. This suggests the model is incomplete because it leaves out an initial, and common, reaction to threat.

Negative consequences in modern life. The response evolved to handle acute, physical threats that were quickly resolved. Modern stressors — deadlines, money, relationships — are often chronic, repeatedly activating the same system. Prolonged activation is linked to health problems such as raised blood pressure and cardiovascular disease, so a once-adaptive response can become harmful.

A strong evaluation does not just list these points. Explain why each one strengthens or weakens the model, then reach a supported conclusion.

1. Confusing the endocrine system with the nervous system

The endocrine system uses hormones carried in the blood, so it is slower but widespread and long-lasting. The nervous system uses electrical impulses along neurons, so it is fast, targeted and short-lived. Mixing up which system is fast and chemical loses easy comparison marks.

2. Saying adrenaline is released by the pituitary

Adrenaline comes from the adrenal medulla, not the pituitary. The pituitary is the master gland that controls other glands, but it is not the source of adrenaline in fight or flight.

3. Forgetting the parasympathetic return to rest

The response does not stop at the release of adrenaline. Once the threat passes, the parasympathetic branch returns the body to its resting state. Leaving this out gives an incomplete answer to a "describe the fight or flight response" question.

4. Treating fight or flight as universal

Fight or flight has been challenged. Taylor et al. (2000) proposed a tend and befriend response in women, and Gray (1988) argued the first reaction to threat is often to freeze. Presenting the model as the single, universal response to all stressors misses the evaluation the exam wants.

5. Describing the SAM pathway in the wrong order

The order is fixed: stressor → hypothalamus → sympathetic branch → adrenal medulla → adrenaline → bodily changes. Jumping straight from the hypothalamus to adrenaline, or omitting the sympathetic branch, breaks the causal chain examiners look for.