IB Biology SLTopic 3 — Coordinating Body SystemsPaper 1 & 2Core idea~11 min read
Epinephrine & Melatonin
Two hormones, two completely different jobs. One creeps up on you every evening and puts you to sleep. The other floods you in seconds when something frightening happens. Between them they show exactly what chemical control is good at — changing the whole body at once, and keeping it changed.
📘 What you need to know
A circadian rhythm is a pattern of activity that repeats on roughly a 24-hour cycle.
Melatonin is secreted by the pineal gland in the brain. It rises in the evening in response to darkness and falls at dawn in response to light.
Light is detected by the retina, which signals to the pineal gland — so day length controls the amount of melatonin made.
Rising melatonin causes tiredness and promotes sleep; falling melatonin prepares the body to wake up.
In constant darkness the rhythm carries on, but on a cycle slightly longer than 24 hours. Light resets the clock each day.
Epinephrine (adrenaline) is secreted by the adrenal medulla when sympathetic neurones stimulate it during stress, fear or excitement.
Epinephrine prepares the body for the fight or flight response: faster heart rate, more blood to muscles, less to the gut and skin.
Blood flow to the brain stays constant whatever else is happening.
Circadian rhythms
All sorts of processes in plants and animals repeat on a daily cycle. Many animals are only active during one part of the 24 hours — nocturnal animals come out at night, and humans are diurnal, meaning we are awake in daylight.
In humans, physical activity, sleep, body temperature and hormone secretion all follow regular daily patterns. These are the circadian rhythms, and a lot of them are driven by melatonin.
Where the word comes fromcirca = about • dies = day → “about a day”
The “about” matters more than it looks. If the cycle were exactly 24 hours you would never need light to correct it. The fact that it is only about 24 hours is the reason sunlight has to reset your clock every morning.
Melatonin and the sleep-wake cycle
The pineal gland secretes melatonin into the blood. How much it makes depends on light, and the pathway is worth learning in order:
🧩 From daylight to sleepiness
The retina of the eye detects how much light there is.
Signals are sent from the retina to the pineal gland.
In darkness, the pineal gland increases melatonin secretion into the blood.
Melatonin reaches target sites all over the body — the hypothalamus, the pituitary, the kidneys, blood vessels, the gut and cells of the immune system.
Rising melatonin causes tiredness and promotes sleep.
At dawn, light causes melatonin to fall, and the body prepares to wake and stay awake through the day.
Because the amount of daylight changes through the year, the pineal gland is tracking the season as well as the time of day. That is why it is sometimes described as being both a clock and a calendar.
Read the shape, not just the peak: melatonin begins climbing as daylight fades, peaks in the middle of the night and has almost gone by mid-morning.
What else melatonin does
It contributes to the drop in core body temperature that happens at night.
Receptors in the kidney respond to melatonin, which reduces urine production overnight — handy, since it means you can sleep through.
Its target sites are spread widely: the hypothalamus and pituitary, immune cells, the gonads, the kidneys, blood vessels and the gut.
The evidence: what happens in constant darkness
People living without any natural daylight still release melatonin on a roughly 24-hour cycle — but the cycle drifts, running slightly longer than 24 hours.
That result does two jobs at once. It shows the rhythm is generated internally, not simply caused by the sun. And it shows what light is actually for: resetting the internal clock each day so it stays in step with the outside world.
Jet lag, explained. Fly across several time zones and your pineal gland is still running on the old schedule — releasing melatonin in the middle of your new afternoon. It takes a few days of morning light to drag the rhythm back into step. Same reason night-shift work is so hard on the body.
Epinephrine and the fight or flight response
Now the opposite kind of hormone. When something stressful, frightening or exciting happens, neurones of the sympathetic nervous system stimulate the adrenal medulla — the inner part of the adrenal gland, which sits on top of each kidney — to secrete epinephrine, also called adrenaline.
Epinephrine is a hormone, so it travels in the bloodstream and binds to receptors on its target organs. The familiar symptoms — pounding heart, dry mouth, sweating — are simply the effects of that hormone arriving everywhere at once.
The pale face of someone who has had a fright is the skin vessels constricting. The queasy feeling is blood being taken away from the gut. Both are the same hormone doing its job.
How the heart rate rises — two routes at once
Epinephrine binds to receptors at the sinoatrial node (SAN), increasing how often it fires. Heart rate rises, so blood reaches the muscle cells faster.
More blood means more oxygen and glucose arriving, so the rate of aerobic respiration in the muscle cells increases and more energy is released.
Epinephrine also stimulates the cardiovascular control centre in the medulla oblongata, which increases impulses along the sympathetic neurones to the heart, speeding it up further.
So the change is caused by a hormone and by nerves at the same time. The whole fight or flight response is a combination of nervous and hormonal control — a very good example of integration.
If a question asks you to explain how epinephrine gives a muscle more energy, do not stop at “heart rate increases”. Follow the chain all the way: faster heart → more oxygen and glucose delivered → more aerobic respiration → more energy released. Each link is a mark.
Worked examples
WORKED EXAMPLE
Using the melatonin graph, describe how melatonin concentration changes over 24 hours. [3]
Start with the low part and give timesMelatonin stays low, near zero, through the daylight hours from about 6am to 6pm.Describe the riseIt begins to rise in the evening as daylight decreases, and keeps rising into the night.Peak and fallIt peaks during the middle of the night at around 60 units, then falls steeply after dawn.Low in daylight, rising in the evening, peak at night, falling at dawn“describe” a graph = use numbers off the axes, not just “it goes up”
WORKED EXAMPLE
Volunteers lived in constant darkness for a month. Their melatonin rhythm continued but on a cycle slightly longer than 24 hours. What does this suggest about the control of circadian rhythms? [3]
What the continuing rhythm showsThe rhythm is generated internally, since it carried on with no light and dark signals.What the drift showsThe internal clock is not exactly 24 hours, so it slowly slips out of step with the outside world.ThereforeThe role of light is to reset the melatonin system each day and keep the rhythm matched to daylight hours.Internal clock, corrected daily by lighttwo findings, two conclusions, then join them — that is the 3rd mark
WORKED EXAMPLE
Explain why blood flow to the digestive system decreases during the fight or flight response, while blood flow to the brain does not. [3]
What happens to the gutBlood vessels supplying the digestive system constrict, so less blood flows there.Why that is usefulDigestion is not needed immediately, so blood can be diverted to the skeletal muscles that may be needed for running or fighting.Why the brain is differentThe brain needs a constant supply of oxygen and glucose to function, so its blood flow is kept the same whether the body is stressed or relaxed.Divert from what can wait; protect what cannotthe word “diverted” earns credit — it shows you know the total is shared out
💡 Exam tip
Melatonin questions almost always involve a graph. Practise describing one with numbers and times from the axes.
Get the direction right: darkness increases melatonin, light decreases it. Many students reverse this under pressure.
Name the gland and where it is: pineal gland, in the brain; adrenal medulla, in the adrenal gland on the kidney.
For epinephrine, write the chain of consequences rather than a list of symptoms. Chains score, lists do not.
Remember that “epinephrine” and “adrenaline” are the same thing — the IB uses epinephrine, so use it too.
If asked for evidence that a rhythm is internal, the constant-darkness experiment is the answer to reach for.
⚠ Common mix-up
Saying melatonin makes you sleep. It promotes sleep and causes tiredness — it does not switch you off like an anaesthetic.
Thinking the rhythm stops without light. It does not. It keeps going, just on a slightly longer cycle.
Confusing the adrenal medulla with the medulla oblongata. One is inside the adrenal gland; the other is in the brainstem. Both appear in this topic.
Writing that epinephrine “gives you energy”. It does not contain energy. It increases the rate of aerobic respiration, which releases energy from glucose.
Saying blood flow increases everywhere. It is redistributed — up to the muscles, down to the gut and skin, unchanged to the brain.
Calling epinephrine a neurotransmitter here. In this response it is released into the blood by a gland, which makes it a hormone.
Up next: Control Mechanisms — how the hypothalamus, the medulla and negative feedback keep heart rate, breathing and digestion under control without you ever noticing.
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