Key Idea: No organ works alone. Topic 3.6 is the story of how the body's systems are coordinated so the whole organism stays alive and stable. Two control systems run the show: the fast, electrical nervous system and the slower, chemical endocrine system (hormones in the blood). Together they keep the internal environment steady — a job done again and again by one idea: negative feedback, where the response always opposes the change. You will see that same loop control the heartbeat, blood glucose and breathing rate; you will follow food through the gut, watch the liver balance blood nutrients, and link diet to health. This topic is a regular on Paper 1 (data/identify items — match a hormone to its gland, read an ECG or a CO₂-vs-ventilation graph) and a favourite on Paper 2 / Paper 3 (explain the conduction of the heartbeat, outline absorption, or explain how a high-fat diet causes heart disease).
🔗 Integrating the nervous & endocrine systems
The body has two signalling systems. The nervous system sends fast electrical impulses along neurons; the endocrine system sends slower chemical signals — hormones — through the blood. They are linked by the brain: the hypothalamus signals the pituitary gland, which controls other endocrine glands. So the nervous system can drive the endocrine system — and the signal from the CNS to an endocrine gland is carried by neurons (nerves).
| Feature | Nervous system | Endocrine system |
|---|---|---|
| Signal | electrical impulse along neurons | chemical hormone in the blood |
| Speed | very fast (a fraction of a second) | slower (seconds to minutes) |
| Reach | precise — one muscle or gland | widespread — any cell with the receptor |
| Duration | short-lived | longer-lasting |
Nervous = Now (fast, electrical, brief). Endocrine = Enduring (slower, hormonal, long-lasting). And negative feedback always does the opposite of the change — that is the whole point of the word negative.
❤️ Controlling the heartbeat
The heart is myogenic — the beat starts inside the heart muscle itself, not from the brain. A patch of special muscle, the sinoatrial (SA) node in the right-atrium wall, is the natural pacemaker that fires every beat. The impulse spreads across the atria (they contract), reaches the atrioventricular (AV) node (which delays it so the atria empty first), then sweeps through the ventricles (they contract). Intercalated discs between cardiac-muscle cells let the impulse pass quickly from cell to cell.
The heart's four chambers. The SA node — the heart's natural pacemaker — sits in the wall of the RIGHT ATRIUM; each beat begins there and the wave of contraction spreads across the atria first, then down to the ventricles.
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| Step | Structure | What happens |
|---|---|---|
| 1 | SA node (right-atrium wall) | fires the impulse on its own (myogenic) |
| 2 | Both atria | contract, pushing blood into the ventricles |
| 3 | AV node (atria-ventricle border) | delays the impulse so the atria finish emptying |
| 4 | Both ventricles | contract together, pushing blood into the arteries |
Key Idea: The SA node sets a resting rhythm, but the medulla sends nerves to speed it up or slow it down, and the hormone adrenaline always speeds it up. If the SA node fails, an artificial pacemaker is implanted to send regular impulses and keep a normal rhythm.
SA node = Starts the beat (Above, in the atria). AV node = passes the signal After, from Atria to Ventricles (with a delay). And adrenaline always speeds the heart up — it never slows it down.
🍽️ The digestive system
Food is too large to enter cells, so the gut breaks it down and absorbs the small products. It travels one way — mouth → stomach → small intestine → large intestine — pushed by peristalsis (waves of involuntary smooth muscle controlled by the autonomic / enteric nervous system). The stomach secretes hydrochloric acid (low pH) to kill bacteria and let pepsin digest protein. The exocrine pancreas adds amylase, protease and lipase to finish digestion in the small intestine, which absorbs the products through villi. The large intestine reabsorbs water.
| Region | Main job | Key feature |
|---|---|---|
| Stomach | begins protein digestion | low pH (acid + pepsin); kills bacteria |
| Small intestine | finishes digestion and absorbs | villi: large surface area, thin wall, rich blood supply |
| Large intestine | reabsorbs water | no enzyme digestion; forms faeces |
| Food molecule | Enzyme | Absorbable products |
|---|---|---|
| Starch | amylase | glucose |
| Protein | protease (pepsin, then pancreatic) | amino acids |
| Triglyceride (fat) | lipase | fatty acids and glycerol |
Peristalsis = involuntary muscle waves. Stomach = acid + pepsin (protein). Pancreas + small intestine = the rest of the enzymes, then absorption through the villi.
🫀 The liver & regulating blood nutrients
All blood leaving the gut passes through the liver first, making it the body's chemical hub. Its cells, hepatocytes, keep blood nutrients steady. For glucose the liver is the effector in a negative-feedback loop: when glucose is high (after a meal) insulin makes it store glucose as glycogen; when glucose is low (between meals) glucagon makes it break glycogen back into glucose. The liver also removes excess cholesterol into bile, detoxifies alcohol, and breaks down old red blood cells (making the pigment bilirubin).
Negative feedback is the engine of this whole topic: a change moves a level away from its set point, receptors detect it, a control centre signals effectors, and the response OPPOSES the change — returning the level to normal. Glucose, heart rate, CO₂ and breathing are all held steady this way.
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| Blood glucose | Hormone | Liver response | Result |
|---|---|---|---|
| Too high (after a meal) | insulin | store glucose as glycogen | glucose falls to normal |
| Too low (between meals) | glucagon | break glycogen into glucose | glucose rises to normal |
Insulin → IN (glucose goes into store as glycogen, lowering blood glucose). Glucagon → Glucose comes back (glycogen broken down, raising blood glucose). If the liver cannot clear bilirubin, it builds up → yellow skin and eyes (jaundice).
🫁 Controlling ventilation rate
Breathing rate is controlled by blood CO₂, not oxygen. A rise in CO₂ (which lowers blood pH) is detected by chemoreceptors in the medulla, aorta and carotid arteries. The medulla then raises the ventilation rate and depth by signalling the diaphragm and intercostal muscles. Faster breathing exhales more CO₂, so CO₂ (and pH) return to normal — another negative-feedback loop that works in both directions.
Ventilation = breathing in and out. Raising the ventilation RATE means more, deeper breaths per minute, so more air — and more CO₂ — is moved out of the lungs each minute.
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| Stage | What happens | Structure |
|---|---|---|
| Change | blood CO₂ rises; pH falls | respiring cells / exercise |
| Detect | the rise is sensed | chemoreceptors (medulla, aorta, carotid) |
| Control | more impulses to the breathing muscles | the medulla |
| Respond | ventilation rate & depth increase | diaphragm + intercostal muscles |
| Result | more CO₂ exhaled → CO₂ back to normal | the lungs |
More CO₂ in → breathe more out. Rising CO₂ makes you breathe harder; harder breathing blows the CO₂ away; once CO₂ is back to normal, breathing settles again. The body watches CO₂, not oxygen.
🥗 Nutrition, diet & health
An essential nutrient is one the body cannot make, so it must come from the diet (e.g. vitamin C, vitamin D, certain amino acids). A balanced diet supplies all nutrient groups in the right amounts. Malnutrition is any unbalanced diet. Too little of a nutrient causes deficiency diseases (lack of vitamin C → weak collagen → scurvy; lack of vitamin D → poor calcium absorption → soft bones / rickets). Too much energy causes obesity, hypertension and coronary heart disease (CHD).
| Step in the chain | What happens |
|---|---|
| 1. Diet high in saturated fat | raises LDL cholesterol in the blood |
| 2. Cholesterol deposited in artery walls | fatty plaques build up — atherosclerosis |
| 3. Coronary arteries narrow | less blood and oxygen reach the heart muscle |
| 4. Artery becomes blocked | part of the heart muscle dies — a heart attack (CHD) |
Essential = the body cannot make it. Malnutrition means too little OR too much — not just starvation. The high-fat → CHD chain is a row of dominoes: fat → cholesterol → plaque → narrowing → less oxygen → heart attack.
✍️ Worked examples
IB-style question — compare the two control systems
Compare the way the nervous system and the endocrine system transmit signals, including the type of signal, its speed and how long its effect lasts. [3]
Model answer:
The nervous system uses fast electrical impulses along neurons, whereas the endocrine system uses slower chemical hormones carried in the blood.
Nervous responses are fast and short-lived, while endocrine (hormonal) responses are slower but longer-lasting.
Nervous signals reach a precise target, whereas hormones reach widespread target cells anywhere in the body. (Mark each contrasted pair: signal type, speed, duration — up to 3.)
Nervous = fast electrical impulses (neurons), short-lived and precise; endocrine = slower chemical hormones (in the blood), longer-lasting and widespread.
IB-style question — how the SA node coordinates the beat
Explain how an electrical signal from the sinoatrial node spreads through the heart to coordinate a single heartbeat. [4]
How to score all four marks:
The SA node (in the right-atrium wall) fires an electrical impulse on its own — the heart is myogenic.
The impulse spreads across both atria, so they contract and push blood into the ventricles.
The signal reaches the AV node, which delays it so the atria can finish emptying first.
The impulse then travels through the ventricle walls, so both ventricles contract together and push blood into the arteries. (Award 1 mark per ordered step, up to 4.)
SA node fires → atria contract → AV node delays the impulse → ventricles contract a moment later, giving a coordinated beat.
IB-style question — adaptations for absorption
Explain how the structure of the small intestine is adapted to absorb the products of digestion. [3]
How to score all three marks:
The lining is folded into many villi (and microvilli), which greatly increase the surface area for absorption.
Each villus has a thin, one-cell-thick wall, giving a short diffusion distance into the blood.
A dense network of blood capillaries carries absorbed glucose and amino acids away, keeping a steep concentration gradient. (Mark 1 per structure linked to its function, up to 3.)
Villi give a large surface area; the thin wall gives a short diffusion distance; and a rich blood supply maintains a steep concentration gradient — together these maximise absorption.
IB-style question — how hepatocytes regulate blood glucose
Outline how hepatocytes help to regulate the level of glucose in the blood. [2]
How to score both marks:
When blood glucose is high (after a meal), hepatocytes take up glucose and store it as glycogen (triggered by insulin), so glucose falls back to normal.
When blood glucose is low (between meals), hepatocytes break glycogen back into glucose and release it (triggered by glucagon), so glucose rises back to normal. (Mark 1 per direction of the loop.)
High glucose → store as glycogen (insulin); low glucose → break glycogen into glucose (glucagon) — keeping the level steady by negative feedback.
IB-style question — explain a CO₂-versus-ventilation graph
A graph shows that as the carbon dioxide concentration in the air a person breathes increases, their ventilation rate also increases. Explain this result. [3]
How to score all three marks:
State the trend. As CO₂ concentration increases, the ventilation rate increases — a positive correlation.
Detection. Breathing in more CO₂ raises blood CO₂ (and lowers blood pH); this is detected by chemoreceptors in the medulla, aorta and carotid arteries.
Response. The medulla raises the ventilation rate so the extra CO₂ is exhaled, returning blood CO₂ to normal. (Mark 1: trend. Mark 2: chemoreceptors detect CO₂. Mark 3: medulla raises ventilation to remove CO₂.)
Higher CO₂ is detected by chemoreceptors; the medulla raises the breathing rate; the extra breathing exhales CO₂ to bring it back to normal — so ventilation rises with CO₂ (negative feedback).
IB-style question — a high-fat diet and heart disease
Explain how a diet high in saturated fat can lead to coronary heart disease. [5]
How to score all five marks:
A diet high in saturated fat raises the level of LDL cholesterol in the blood.
Cholesterol is deposited in the artery walls, forming fatty plaques — this is atherosclerosis.
The plaques make the coronary arteries narrower, so less blood can flow through them.
With reduced blood flow, the heart muscle receives less oxygen and cannot respire enough (chest pain).
If a coronary artery becomes fully blocked, part of the heart muscle dies — a heart attack (CHD). (Award 1 mark per linked step in the chain, up to 5.)
Saturated fat raises LDL cholesterol → deposited in artery walls (atherosclerosis) → coronary arteries narrow → less oxygen reaches the heart muscle → a blockage kills heart muscle, causing CHD.
✅ Quick self-check
Tap each card to check yourself across all six micros.
How do the nervous and endocrine systems differ, and how are they linked? Nervous = fast electrical impulses along neurons, short-lived and precise. Endocrine = slower chemical hormones in the blood, longer-lasting and widespread. They are linked by the hypothalamus, which signals the pituitary gland — so the nervous system can drive the endocrine system.
How is a single heartbeat coordinated? The heart is myogenic: the SA node (right-atrium wall) fires each beat → the atria contract → the AV node delays the impulse so the atria empty first → the ventricles contract. Intercalated discs pass the impulse cell-to-cell; the medulla and adrenaline change the rate.
How is the small intestine adapted for absorption, and how is peristalsis controlled? Villi give a large surface area, a thin wall gives a short diffusion distance, and a rich blood supply keeps a steep concentration gradient. Peristalsis is waves of involuntary smooth muscle controlled by the autonomic / enteric nervous system.
How does the liver keep blood glucose steady? When glucose is high, insulin makes hepatocytes store it as glycogen (glucose falls); when glucose is low, glucagon makes them break glycogen back into glucose (glucose rises). The response always opposes the change — negative feedback.
What controls the ventilation rate? Blood CO₂ (not oxygen). A rise in CO₂ (which lowers pH) is detected by chemoreceptors in the medulla, aorta and carotid arteries; the medulla raises the ventilation rate so more CO₂ is exhaled, returning CO₂ to normal — negative feedback.
What is an essential nutrient, and how does a high-fat diet harm the heart? An essential nutrient is one the body cannot make, so it must come from the diet (e.g. vitamin C, vitamin D). A high-fat diet raises cholesterol → fatty plaques narrow the coronary arteries (atherosclerosis) → less oxygen reaches the heart muscle → coronary heart disease.
The shared logic across every micro in this topic: whichever way an internal level drifts, the response pushes it back toward normal — glucose, heart rate and breathing are all controlled by this same loop.
🔒 Interactive diagram
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Exam Tips
- Nervous = fast/electrical/short-lived/precise; endocrine = slow/chemical/long-lasting/widespread. For a 'compare' mark, give both systems in each point.
- Signals from the CNS to an endocrine gland are carried by neurons (nerves), via the hypothalamus and pituitary.
- For 'explain how the SA node coordinates the beat [4]', give the SEQUENCE in order: SA node fires → atria contract → AV node delays → ventricles contract.
- Adrenaline ALWAYS raises heart rate — it never slows it down. The structural feature that aids conduction is intercalated discs (gap junctions).
- Peristalsis = involuntary smooth muscle controlled by the autonomic / enteric nervous system — not conscious, not skeletal.
- Two reasons for a low stomach pH: it kills bacteria AND gives pepsin its optimum pH (give both).
- For absorption adaptations link each structure to its job: villi → surface area, thin wall → short diffusion distance, capillaries → steep gradient.
- To outline liver glucose control, give BOTH directions: store as glycogen (high) AND break glycogen down (low). Excess cholesterol leaves in bile/faeces, not urine.
- The body controls breathing by sensing CO₂, not oxygen; rising CO₂ lowers blood pH. Name chemoreceptors → medulla → diaphragm/intercostal muscles in order.
- For a CO₂-vs-ventilation graph (or a diet-vs-CHD graph), state the TREND first, then give the mechanism — describing the trend alone earns no explanation marks.
- 'Essential nutrient' = one the body cannot make. 'Malnutrition' covers BOTH deficiency AND obesity — not just starvation.
- The high-fat → CHD chain scores on the logical links (fat → cholesterol → atherosclerosis → narrowing → less oxygen → heart attack), not on naming the disease.
Visual recap — the SA node fires in the right-atrium wall, the AV node relays the signal at the atria-to-ventricle border, and the impulse then sweeps through both ventricle walls so they contract together.
🔒 Interactive diagram
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When ventilation rate rises, each extra exhalation carries CO₂ out of the body — this is how faster breathing lowers blood CO₂ back toward normal.
🔒 Interactive diagram
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Key Idea: The body acts as one integrated whole, run by a fast nervous system and a slower endocrine system that are linked by the hypothalamus → pituitary — and tuned everywhere by negative feedback, where the response opposes the change. The heart is myogenic: the SA node fires, the atria contract, the AV node delays, then the ventricles contract — with the medulla and adrenaline adjusting the rate. Food travels the gut by peristalsis; the stomach (acid + pepsin) and the exocrine pancreas (amylase, protease, lipase) digest it, and villi absorb the products. The liver (hepatocytes) balances blood nutrients — storing or releasing glucose as glycogen via insulin/glucagon, removing cholesterol in bile, and clearing bilirubin. Breathing is controlled by blood CO₂: chemoreceptors detect a rise, the medulla raises the ventilation rate, and the extra CO₂ is exhaled. Finally, diet must supply the essential nutrients the body cannot make — too little brings deficiency diseases (scurvy, rickets), too much brings obesity and coronary heart disease.