IB Biology HLCoordinating Body SystemsPaper 1 & 2~12 min read
Control Mechanisms
Sprint up a flight of stairs and your heart rate roughly doubles within seconds, then slides back down over the next few minutes without you doing anything about it. Nobody decided that. A few grams of tissue at the base of your brain is reading your blood and adjusting the settings, continuously, for your whole life.
📚 What you need to know
The endocrine system is controlled mainly by the hypothalamus and the pituitary gland.
The anterior pituitary produces and releases hormones; the posterior pituitary stores and releases hormones made by the hypothalamus, such as ADH and oxytocin.
Heart rate is controlled by the cardioregulatory centre in the medulla, which has an acceleratory and an inhibitory centre, both connected to the SAN.
These run through the autonomic nervous system: sympathetic neurones and noradrenaline speed the heart, parasympathetic neurones and acetylcholine slow it.
Chemoreceptors and baroreceptors in the aorta and carotid arteries detect the internal stimuli.
Ventilation is controlled by respiratory centres in the medulla, responding to hydrogen ions from dissolved carbon dioxide. This is negative feedback.
Peristalsis is controlled unconsciously by the enteric nervous system, using two motor neurones with opposite effects.
Control of the endocrine system
A hormone is a chemical messenger produced by an endocrine gland and carried by the blood. Hormones transmit information from one part of the organism to another and bring about a change, altering the activity of one or more specific target organs. They control functions that do not need instant responses.
The hypothalamus
The hypothalamus monitors the blood as it flows through the brain, and responds either by releasing hormones itself or by stimulating the neighbouring pituitary gland to release them. Its jobs include:
Regulating body temperature — it monitors blood temperature and starts a homeostatic response if it drifts too high or too low.
Osmoregulation — cells in the hypothalamus monitor the water balance of the blood and release ADH if the blood becomes too concentrated. ADH increases water absorption in the kidneys.
Regulating digestive activity — it controls the hormones governing appetite and the secretion of digestive enzymes.
Controlling endocrine function — it makes the pituitary release hormones affecting metabolism, growth and development, puberty, sexual function, sleep and mood.
The pituitary gland
The pituitary sits below the hypothalamus. Some of its hormones act directly on body processes; others stimulate the release of further hormones from other endocrine glands. It has two sections:
Anterior pituitary — produces and releases hormones.
Posterior pituitary — stores and releases hormones that were produced by the hypothalamus, such as ADH and oxytocin.
Anterior makes, posterior stores. That single distinction is worth a mark on its own, and it is the part students most often blur. The posterior pituitary is a release point, not a factory.
Feedback control of heart rate
Heart rate needs to change — during exercise, for example — but you never think about it. The brain is involved, yet no thinking is required: this is unconscious control.
The region responsible is the cardioregulatory centre in the medulla, at the base of the brain near the top of the spinal cord. It has two distinct parts:
The acceleratory centre, which causes the heart to speed up.
The inhibitory centre, which causes the heart to slow down.
Both are connected to the sinoatrial node (SAN) by nerves. These nerves are not the ones controlling conscious activity — they make up the autonomic nervous system, which is self-controlling.
🧠
Two S words, two P words
Sympathetic — Speeds it up, using noradrenaline. Parasympathetic — Puts the brakes on, using acetylcholine. Match the first letters and you will never get the pairing the wrong way round in an exam.
Speeding up and slowing down
Exercise changes conditions inside the body, and those changes are the internal stimuli:
Carbon dioxide concentration in the blood increases.
There is an initial fall in blood pressure, caused by dilation of the muscle arterioles.
These are detected by receptors in the aorta (close to the heart) and in the carotid arteries (which supply the head with oxygenated blood):
Chemoreceptors detect changes in blood pH, oxygen and carbon dioxide.
Baroreceptors monitor changes in blood pressure. They sit on the arch of the aorta and on the enlargement of the carotid artery called the sinus.
These receptors send nerve impulses to the acceleratory and inhibitory centres, which act as coordinators. The frequency of those impulses does the talking: higher frequency activates the acceleratory centre and speeds the heart up, lower frequency activates the inhibitory centre and slows it down.
Stage
Speeding up
Slowing down
Stimulus
Increased CO2, decreased blood pressure
Decreased CO2, increased blood pressure
Receptors
Increased stimulation of chemoreceptors and baroreceptors
Decreased stimulation of chemoreceptors and baroreceptors
Coordinator
Acceleratory centre in the medulla, via sympathetic neurones
Inhibitory centre in the medulla, via parasympathetic neurones
Neurotransmitter
Noradrenaline at the synapse with the SAN
Acetylcholine at the synapse with the SAN
Effector and response
SAN fires more frequently, so heart rate and stroke volume increase
SAN fires less frequently, so heart rate returns towards resting
Feedback control of ventilation rate
Ventilation is controlled by respiratory centres in the medulla. At rest, action potentials produced at random travel to the diaphragm and intercostal muscles to cause contraction, at a stable, slow pace.
During exercise more carbon dioxide is produced by respiration, and it diffuses from the tissues into the blood. It is carried in three ways:
About 85% as hydrogencarbonate ions in the blood plasma.
About 5% dissolved directly in the plasma.
About 10% bound to haemoglobin as carbaminohaemoglobin.
Why carbon dioxide changes pH
Carbon dioxide diffuses into the cytoplasm of red blood cells, where it combines with water to form carbonic acid:
Carbonic acid formation
CO2 + H2O ⇌ H2CO3
Red blood cells contain the enzyme carbonic anhydrase, which catalyses this reaction. Without it the reaction is very slow, which is why carbonic acid forms much more slowly in plasma than inside red blood cells. The carbonic acid then dissociates:
Dissociation of carbonic acid
H2CO3 ⇌ HCO3− + H+
Those hydrogen ions lower the pH of the blood, and that is what the chemoreceptors in the medulla actually detect. Action potentials are then sent at a higher rate to the diaphragm and intercostal muscles, increasing the ventilation rate and the volume of air moved.
As ventilation increases, carbon dioxide is removed, pH returns to normal, and the respiratory centres stop sending the extra action potentials. Ventilation returns to resting rates. That is negative feedback.
The receptors are not detecting carbon dioxide. They are detecting the hydrogen ions that carbon dioxide produces once it dissolves. Saying "chemoreceptors detect a fall in pH" is the precise answer, and it explains why the enzyme carbonic anhydrase matters.
Peristalsis is a series of muscle contractions in the walls of the oesophagus or small intestine that pass like a wave along the alimentary canal, forcing the bolus of food along.
It is controlled unconsciously by the autonomic nervous system — specifically the enteric nervous system (ENS), a web of sensory, relay and motor neurones embedded in the tissues of the alimentary canal. The mechanism is called the peristaltic reflex, and the muscles involved are circular and longitudinal smooth muscle (not striated).
The sequence runs like this. The bolus stretches the canal, and stretch receptors — sensory neurones of the ENS — detect the distension. An action potential passes to relay neurones, which synapse with two different motor neurones that do opposite things:
One releases an excitatory neurotransmitter, which makes smooth muscle behind the bolus contract. Longitudinal muscles contract to shorten that section, forcing food forwards, and circular muscles contract to narrow the lumen, stopping food moving backwards towards the mouth.
The other releases an inhibitory neurotransmitter, which makes smooth muscle ahead of the bolus relax and open the lumen, so there is somewhere for the food to go.
Worked examples
WE 1
Calculate a rate of change from a graph
Using the heart rate graph above, the rate rises from 70 bpm at 2 minutes to 138 bpm at 6 minutes. Calculate the mean rate of increase, and name the pathway responsible. (3 marks)
Step 1: find the change
138 − 70 = 68 bpmStep 2: divide by the time taken
68 ÷ 4 = 17 bpm per minuteStep 3: name the pathway
Chemoreceptors and baroreceptors send higher frequency impulses to the acceleratory centre in the medulla, which sends impulses along sympathetic neurones to the SAN, where noradrenaline increases the frequency of excitation.
17 bpm per minute, via the acceleratory centre and sympathetic neuronesgive the unit as a rate, per minute, not just "bpm". Rate questions want a quantity divided by a time
WE 2
Calculate minute ventilation and explain the control
At rest a student breathes 12 times per minute with a tidal volume of 0.5 dm3. During exercise this becomes 30 breaths per minute with a tidal volume of 2.4 dm3. Calculate both minute ventilations and explain how the increase is brought about. (4 marks)
Step 1: at rest
12 × 0.5 = 6 dm3 per minuteStep 2: during exercise
30 × 2.4 = 72 dm3 per minute, which is 12 times greater
Step 3: the stimulus
More respiration produces more carbon dioxide, which dissolves in the blood to form carbonic acid. This dissociates and releases hydrogen ions, lowering blood pH.
Step 4: the responseChemoreceptors in the medulla detect the fall in pH, and the respiratory centres send action potentials at a higher rate to the diaphragm and intercostal muscles.
6 and 72 dm3 per minute; a fall in pH drives the increaseboth numbers must carry the unit per minute. And do not say the receptors detect carbon dioxide directly — they detect the hydrogen ions
WE 3
Explain why two motor neurones are needed
Explain why the peristaltic reflex requires two motor neurones releasing different neurotransmitters. (3 marks)
Step 1: what has to happen behind the bolus
An excitatory neurotransmitter makes circular and longitudinal muscle behind the bolus contract, narrowing and shortening that section so the food is pushed forwards and cannot move back.
Step 2: what has to happen in front of it
An inhibitory neurotransmitter makes smooth muscle ahead of the bolus relax, opening the lumen so there is space for the bolus to move into.
Step 3: why one neurone cannot do both
The two regions need opposite responses at the same moment. A single neurotransmitter would produce the same effect everywhere, so the wave could not move directionally.
Contraction behind and relaxation ahead must happen simultaneouslythe word "directional" is worth including. Peristalsis is not just squeezing — it is squeezing in an order
💡 Exam tips
Use the standard chain in feedback answers: stimulus, receptor, coordinator, effector, response. Label each one.
Name the medulla as the coordinator for both heart rate and ventilation. It does both jobs.
For pH questions, always route through hydrogen ions, not carbon dioxide directly.
Learn the three carbon dioxide transport percentages — 85, 5 and 10 — as a set.
Say negative feedback explicitly when the response reverses the original change.
⚠ Common mistakes
Saying the brain consciously controls heart rate. The brain is involved, but no thinking happens — it is autonomic.
Swapping the neurotransmitters. Sympathetic uses noradrenaline; parasympathetic uses acetylcholine.
Writing that the posterior pituitary makes ADH. The hypothalamus makes it; the posterior pituitary stores and releases it.
Saying chemoreceptors detect carbon dioxide in the medulla. They detect the fall in pH caused by hydrogen ions.
Forgetting carbonic anhydrase. Without the enzyme the reaction is far too slow to be useful.
Describing peristalsis as gravity. You can swallow upside down; the muscle does the work.
Calling smooth muscle striated. The muscle of the alimentary canal is not striated.
Up next: Observing Tropic Responses — the skills page. How to measure a plant bending towards light, what separates a qualitative observation from a quantitative one, and why precision and accuracy are not the same thing.
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