IB Biology SLTopic 3 — Coordinating Body SystemsPaper 1 & 2Core idea~13 min read
Control Mechanisms
Right now your heart rate, your breathing and the muscles in your gut are all being adjusted, second by second, and you have not thought about any of it. Three different processes — but underneath they all run on the same simple loop. Learn the loop once and this page becomes three examples of one idea.
📘 What you need to know
The endocrine system is controlled mainly by the hypothalamus and the pituitary gland.
The anterior pituitary makes and releases its own hormones; the posterior pituitary stores and releases hormones made by the hypothalamus, such as ADH and oxytocin.
Negative feedback means the response reverses the original change, bringing the body back towards normal.
Heart rate is controlled by the cardioregulatory centre in the medulla, which has an acceleratory centre and an inhibitory centre, both connected to the SAN.
Chemoreceptors and baroreceptors in the aorta and carotid arteries detect the internal changes.
Ventilation is controlled by respiratory centres in the medulla, which respond to a fall in blood pH caused by carbon dioxide.
Peristalsis is controlled unconsciously by the enteric nervous system using two motor neurones: one excitatory, one inhibitory.
The two organs in charge
The hypothalamus monitors the blood as it flows through the brain. Depending on what it finds, it either releases hormones itself or tells the pituitary gland just below it to release hormones. That makes it the link between sensing and hormonal action.
Hypothalamus function
What it does
Regulating body temperature
Monitors blood temperature and starts a response if it is too high or too low
Osmoregulation
Monitors the water balance of the blood; releases ADH if the blood is too concentrated, which increases water absorption in the kidneys
Regulating digestion
Controls hormones affecting appetite and the secretion of digestive enzymes
Controlling the endocrine system
Makes the pituitary release hormones controlling metabolism, growth, puberty, sleep and mood
The pituitary itself comes in two halves, and the difference is examinable:
Anterior pituitary — produces and releases its own hormones.
Posterior pituitary — stores and releases hormones that were made in the hypothalamus, such as ADH and oxytocin.
A useful memory hook: the posterior pituitary is a warehouse, not a factory. Nothing is made there. If a question asks where ADH is produced, the answer is the hypothalamus, even though it is released from the pituitary.
The loop behind everything: negative feedback
Every control mechanism on this page has the same four parts, in the same order.
The dashed purple arrow is the part that makes it “negative”: the response undoes the change that started it.
Watch the word “negative”. It does not mean harmful. It means the response works in the opposite direction to the change. Blood too acidic, so breathe faster and remove acid. Body too hot, so lose heat.
Feedback control of heart rate
Your heart rate has to change during exercise, but nobody consciously decides that. The medulla, at the base of the brain near the top of the spinal cord, does it for you. It contains the cardioregulatory centre, which is made of two parts:
The acceleratory centre, which speeds the heart up.
The inhibitory centre, which slows it down.
Both are connected by nerves to the sinoatrial node (SAN). These nerves are not the ones you use for conscious activities — they belong to the autonomic nervous system, which is self-controlling.
What is detected, and where
Exercise changes conditions inside the body, and those changes are the stimulus:
The concentration of carbon dioxide in the blood increases.
Blood pressure falls at first, because the arterioles in the muscles dilate.
These are picked up by chemoreceptors (which detect blood pH, oxygen and carbon dioxide) and baroreceptors (which detect blood pressure). Both sit in the aorta, close to the heart, and in the carotid arteries, which supply the head with oxygenated blood.
The receptors do not switch on and off — they change the frequency of the impulses they send. Higher frequency activates the acceleratory centre; lower frequency activates the inhibitory centre.
Revise this as two mirrored columns. Almost every heart-rate question is asking you to pick the correct column and read it out in order.
Once the heart rate has risen, more carbon dioxide is carried away and blood pressure recovers — so the stimulus disappears and the inhibitory centre brings the rate back down. That return to the resting rate is the negative feedback loop closing.
Feedback control of ventilation rate
Breathing is controlled by respiratory centres, also in the medulla. At rest they send action potentials to the diaphragm and intercostal muscles at a slow, steady pace.
During exercise, respiration in the muscles produces much more carbon dioxide, which diffuses from the tissues into the blood. It is carried in three ways:
How carbon dioxide travels
Roughly how much
As hydrogen carbonate ions in the plasma
About 85 %
Bound to haemoglobin as carbaminohaemoglobin
About 10 %
Dissolved directly in the plasma
About 5 %
The important one is the first. Inside red blood cells, carbon dioxide combines with water to form carbonic acid, in a reaction catalysed by the enzyme carbonic anhydrase:
Step 1 — inside the red blood cell
CO2 + H2O ⇌ H2CO3
Carbonic acid then dissociates readily:
Step 2 — and the hydrogen ions are the signal
H2CO3 ⇌ HCO3− + H+
Without carbonic anhydrase this reaction is very slow, which is why carbonic acid forms far more slowly in plasma than inside red blood cells.
🧩 Why exercise makes you breathe harder
Respiring muscles produce more carbon dioxide, which enters the blood.
Inside red blood cells it forms carbonic acid, which dissociates and releases hydrogen ions.
More hydrogen ions means a lower blood pH.
Chemoreceptors in the medulla detect the fall in pH.
The respiratory centres send action potentials to the diaphragm and intercostal muscles at a higher rate, so ventilation rate and depth increase.
More carbon dioxide is breathed out, pH returns to normal, and the respiratory centres go back to their resting rate.
Notice what is actually being detected here. It is not carbon dioxide itself and it is definitely not a lack of oxygen — it is the drop in pH caused by hydrogen ions. Getting that right lifts an answer from three marks to four.
Control of peristalsis
Peristalsis is a wave of muscle contraction passing along the walls of the oesophagus or small intestine, pushing the bolus of food along the alimentary canal.
It is controlled unconsciously by part of the autonomic nervous system called the enteric nervous system (ENS) — a web of sensory, relay and motor neurones embedded in the wall of the gut itself. The muscles involved are smooth muscle (not striated), arranged in circular and longitudinal layers.
Think of squeezing toothpaste with the cap still on. Peristalsis only works because the tube ahead of the bolus is opening at the same time as the tube behind it is closing.
🧩 The peristaltic reflex
The bolus stretches the gut wall as the canal becomes distended.
Stretch receptors (sensory neurones of the ENS) detect this.
An action potential passes to relay neurones, which synapse with two different motor neurones.
One motor neurone releases an excitatory neurotransmitter behind the bolus: longitudinal muscle contracts to shorten that section and force the food forwards, and circular muscle contracts to narrow the lumen so food cannot move backwards.
The second motor neurone releases an inhibitory neurotransmitter ahead of the bolus, so that smooth muscle relaxes and the lumen opens.
The bolus is pushed into the open section, which stretches the wall further along — and the whole thing repeats as a wave.
Worked examples
WORKED EXAMPLE
During exercise the heart rate increases. Identify the receptor, the coordinator and the effector in this response. [3]
ReceptorChemoreceptors and baroreceptors in the aorta and carotid arteries.CoordinatorThe acceleratory centre of the cardioregulatory centre, in the medulla.EffectorThe sinoatrial node (SAN) of the heart.Receptors in the vessels, coordinator in the medulla, effector in the heartthe SAN, not “the heart” — be as precise as the question allows
WORKED EXAMPLE
Explain how an increase in respiration in muscle cells leads to an increase in ventilation rate. [4]
1. The changeMore carbon dioxide is produced and diffuses into the blood.2. The chemistryIn red blood cells it forms carbonic acid, which dissociates into hydrogen carbonate ions and hydrogen ions, lowering blood pH.3. DetectionChemoreceptors in the medulla detect the fall in pH.4. ResponseRespiratory centres send action potentials at a higher rate to the diaphragm and intercostal muscles, increasing the rate and depth of breathing.4 marks: CO2 up → pH down → detected in medulla → muscles stimulatedadd “this is negative feedback” if the question asks what type of control it is
WORKED EXAMPLE
Explain why two different motor neurones are needed for peristalsis. [3]
Neurone 1An excitatory neurotransmitter makes smooth muscle behind the bolus contract, shortening the section and narrowing the lumen so food is pushed forwards and cannot move back.Neurone 2An inhibitory neurotransmitter makes smooth muscle ahead of the bolus relax, opening the lumen.Why both are neededContracting alone would squeeze a closed tube; the food can only move if the section in front opens at the same time.One contracts behind, one relaxes ahead — and they must happen togetherthe third mark is for saying why one neurone would not be enough
💡 Exam tip
Build every control answer from the same four words: stimulus, receptor, coordinator, effector. Then add the response.
Name the exact structure. “The medulla” is good; “the acceleratory centre in the medulla” is better and often the mark.
Learn the two neurotransmitters as a pair: noradrenaline speeds up, acetylcholine slows down.
For ventilation, say pH falls rather than “carbon dioxide is detected”. That is the detail examiners are looking for.
If a question asks what kind of control something is, the answer on this page is nearly always negative feedback — and say why: the response reverses the change.
Do not forget the second half of peristalsis. Relaxation ahead of the bolus is worth as much as contraction behind it.
⚠ Common mix-up
Saying oxygen levels control breathing rate. In this course the trigger is the fall in blood pH caused by carbon dioxide.
Mixing up the medulla oblongata with the adrenal medulla. Brainstem versus adrenal gland — two very different places.
Swapping the two neurotransmitters at the SAN. Sympathetic and noradrenaline speed it up; parasympathetic and acetylcholine slow it down.
Saying the pituitary makes ADH. The hypothalamus makes it; the posterior pituitary stores and releases it.
Thinking negative feedback means something is going wrong. It is the normal, healthy way the body holds itself steady.
Calling gut muscle “striated”. Peristalsis uses smooth muscle.
Forgetting the receptors are in blood vessels. The aorta and carotid arteries — not in the heart itself.
Up next: Defence Against Disease — what happens when something gets past the body’s outer barriers, and how the immune system is coordinated to deal with it.
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