The big idea: Your blood is held very close to pH 7.4 — only tiny changes are allowed, because enzymes and cells stop working properly if the pH drifts.
The body controls blood pH mainly by controlling how much carbon dioxide (CO2) is in the blood.
CO2 is acidic in water. When it dissolves in the blood it forms an acid, which lowers the pH.
So if CO2 builds up, blood pH falls; if CO2 is removed, blood pH rises. Breathing is the tap that lets the body control CO2 — and therefore pH.
- Blood pH set point
- The normal value the body keeps blood pH close to — about pH 7.4 (slightly alkaline).
- Carbon dioxide (CO2)
- A waste gas made by cell respiration. It dissolves in blood and forms carbonic acid, which lowers blood pH.
- Carbonic acid
- The weak acid formed when CO2 dissolves in water/blood. It releases hydrogen ions (H+), which makes the blood more acidic (lowers pH).
- Chemoreceptors
- Sensory cells that detect the level of CO2 / the pH of the blood. They are found in the medulla of the brain and in the walls of the aorta and the carotid arteries.
- Ventilation rate
- How fast (and how deeply) you breathe — the rate at which air is moved in and out of the lungs.
The one chain to memorise: More respiration → more CO2 → more acid → lower pH.
Faster breathing → less CO2 → less acid → higher pH.
If you can write this chain, you can answer almost every blood-pH question. The whole topic is just the body keeping these two chains in balance around pH 7.4.
It is the CO2 that is sensed — not oxygen: A common trap: the chemoreceptors that control breathing for pH are mainly watching CO2 (and the resulting pH), NOT oxygen.
This is why holding your breath becomes uncomfortable so quickly — it is rising CO2 (falling pH), not falling oxygen, that drives the urge to breathe.
Blood pH is controlled by negative feedback — a change is detected and the body acts to reverse it, bringing pH back to the set point.
Read the loop as a chain of cause and effect. The clearest case is exercise, when respiration speeds up and CO2 floods into the blood.
What happens when blood pH FALLS (too acidic)
- Stimulus: respiration (e.g. during exercise) adds more CO2 to the blood. The CO2 forms carbonic acid, so blood pH falls below 7.4.
- Receptor: chemoreceptors (in the medulla, aorta and carotid arteries) detect the rise in CO2 / fall in pH.
- Control centre: the medulla (in the brainstem) receives this and sends more nerve impulses to the breathing muscles.
- Effector & response: the diaphragm and rib muscles work harder, so ventilation rate and depth increase — you breathe faster and deeper.
- Result: faster breathing removes more CO2 from the blood, so there is less acid and pH rises back to ~7.4.
- Switch-off: once pH is back at the set point, the chemoreceptors stop signalling and breathing returns to normal — this 'reverse-the-change' shut-off is what makes it negative feedback.
Why this is negative feedback: Negative feedback means the response opposes (reverses) the original change.
Blood pH fell → the body raised breathing → CO2 dropped → pH went back up. The correction undoes the deviation, then switches off.
Compare this with positive feedback, where the response makes the change even bigger. Blood-pH control is firmly negative feedback.
| Stage of the loop | What it is here | What happens |
|---|---|---|
| Set point | The normal blood pH | Blood is held very close to pH 7.4 |
| Stimulus | Blood pH falls (more acidic) | Respiration adds CO2 to the blood; CO2 forms an acid, so pH drops below 7.4 |
| Receptor | Chemoreceptors | Detect the rise in CO2 / fall in pH (found in the medulla of the brain and in the walls of the aorta and carotid arteries) |
| Control centre | The medulla (brainstem) | Processes the signal and sends nerve impulses to the breathing muscles |
| Effector / response | Breathing muscles | Ventilation rate and depth INCREASE — faster, deeper breathing removes more CO2 |
| Return to set point | Negative feedback | Less CO2 in the blood means less acid, so pH rises back to ~7.4 and the correction is switched off |
It works both ways: The same loop also corrects an opposite deviation.
If you over-breathe (hyperventilate), you blow off too much CO2, so blood pH rises above 7.4 (too alkaline). The chemoreceptors detect this and the medulla slows breathing down, so CO2 builds back up and pH falls to the set point.
So increasing breathing fixes acidic blood; slowing breathing fixes alkaline blood — one loop, two directions.
| If blood pH... | Likely cause | Chemoreceptor + medulla response | Effect on pH |
|---|---|---|---|
| FALLS below 7.4 (too acidic) | Too much CO2 (vigorous exercise, holding breath) | Ventilation rate and depth INCREASE | More CO2 is exhaled → pH rises back to 7.4 |
| RISES above 7.4 (too alkaline) | Too little CO2 (over-breathing / hyperventilation) | Ventilation rate and depth DECREASE | CO2 is retained → pH falls back to 7.4 |
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How this is tested: On Paper 2 the classic is a 4-mark Explain: explain the body's response when blood pH falls below the optimum. Score it as separate links in the chain — CO2 makes acid → chemoreceptors detect → medulla acts → breathing increases → CO2 removed → pH restored.
On Paper 1 a 1-mark item shows a ventilation-vs-pH graph and asks what happens to breathing when blood is below pH 7.4 — the answer is that ventilation rate increases to blow off CO2 and raise pH back up.
Naming the receptor (chemoreceptors) and the control centre (medulla) is the easy extra mark students forget.
IB-style question — explain the response to falling blood pH
After a sprint, a runner's blood pH drops below its normal value. Explain how the body responds to return blood pH to its set point. [4]
How to score all four marks
- Name the cause. Hard exercise raises respiration, adding CO2 to the blood; CO2 forms carbonic acid, which lowers blood pH below the set point (~7.4).
- Name the receptor. Chemoreceptors (in the medulla and the walls of the aorta and carotid arteries) detect the rise in CO2 / fall in pH.
- Name the control centre and effector. The medulla sends more nerve impulses to the breathing muscles, so ventilation rate and depth increase (faster, deeper breathing).
- Link to the correction. Faster breathing removes more CO2, so the blood is less acidic and pH rises back to the set point — a negative-feedback correction. (Award 1 mark per distinct point, up to 4.)
Final answer
Exercise adds CO2, which forms acid and lowers blood pH; chemoreceptors (medulla/aorta/carotid arteries) detect this; the medulla increases ventilation rate and depth; faster breathing removes CO2 so pH rises back to ~7.4 — negative feedback.
✓ Why this scores full marks: Each sentence is a separate link in the chain — cause (CO2/acid), receptor (chemoreceptors), control centre + effector (medulla → faster breathing), and the result (CO2 removed → pH restored).
A 4-mark Explain needs four distinct points, not one idea ('you breathe faster') written four ways. Naming the chemoreceptors and the medulla turns a vague answer into a full-mark one.
Exam Tips:
- Always say WHY pH falls: CO2 dissolves to form an acid. 'CO2 is high' alone often misses the pH mark.
- Name the receptor (chemoreceptors) AND the control centre (medulla) — these are the marks most students drop.
- End on the correction: faster breathing removes CO2 → pH back to ~7.4. That closing link earns the final mark and shows it is negative feedback.