You are viewing a free preview of this lesson.
Subscribe to unlock all 7 lessons in this course and every other course on LearningBro.
You have now worked through the whole of Topic B3 of OCR Gateway Combined Science A — the nervous system and reflexes, receptors and the eye, the endocrine system, the control of blood glucose, temperature and water balance, and the hormones of reproduction. This final lesson pulls it all together. The big idea that links almost every part of B3 is homeostasis and the recurring receptor → coordination centre → effector pattern of negative feedback. This lesson shows how that one pattern appears again and again, recaps the nervous vs hormonal contrast, revisits the reaction-time practical and the BMI calculation, and warns about the misconceptions that catch students out. Treat it as a revision and exam-technique session.
By the end of this lesson you should be able to apply the negative-feedback model across all of B3, compare nervous and hormonal control confidently, handle the key required practical and calculation, and avoid the most common B3 errors.
As a synthesis lesson this leans on AO2 and AO3: you apply the negative-feedback framework to unfamiliar contexts, interpret the reaction-time practical and BMI calculation, and evaluate extended answers — while still drawing on the AO1 recall built through the topic.
The single most powerful idea in B3 is that the body keeps its internal conditions stable by negative feedback, using the same three-part control system every time:
flowchart LR
A["Receptor<br/>detects a change"] --> B["Coordination centre<br/>processes the information"]
B --> C["Effector<br/>brings about a response"]
C -.->|"counteracts the change,<br/>returning to normal"| A
Whenever a condition moves away from its normal level, receptors detect the change, a coordination centre processes it, and effectors produce a response that counteracts the change and brings the level back to normal. The remarkable thing is how many different systems in B3 follow exactly this template:
| Condition controlled | Coordination centre | Effector(s) | Correcting response |
|---|---|---|---|
| Blood glucose | Pancreas | Liver / body cells | Insulin stores glucose as glycogen (lowers); glucagon releases it (raises) |
| Body temperature | Thermoregulatory centre (brain) | Skin vessels, sweat glands, muscles | Vasodilation/sweating (cool); vasoconstriction/shivering (warm) |
| Water content | Brain / pituitary | Cells that reabsorb water | ADH adjusts how much water is reabsorbed |
| Menstrual cycle | Pituitary & ovaries | Ovaries / uterus | Hormones build/maintain lining and trigger ovulation |
| Thyroxine level | Pituitary | Thyroid | Negative feedback raises/lowers thyroxine |
Seeing this shared pattern is exactly the kind of synoptic understanding that lifts an answer. If a question gives you an unfamiliar example of homeostasis, you can still answer it by applying receptor → coordination centre → effector → counteract the change → return to normal.
Exam Tip: When you meet any homeostasis question — even an unfamiliar one — reach for the same framework: name the receptor, the coordination centre, the effector and the corrective response, and state that the level returns to normal. This template earns marks across the whole topic.
B3 has two coordinating systems, and the contrast between them runs through the whole topic. This table is worth knowing cold.
| Feature | Nervous control | Hormonal control |
|---|---|---|
| Message carried by | Electrical impulses along neurones | Chemical hormones in the blood |
| Speed | Very fast | Slower |
| Duration | Short-lived | Long-lasting |
| Area affected | Precise/localised | Widespread (any target organ with receptors) |
| Example in B3 | Reflex withdrawal; pupil reflex | Blood glucose control; the menstrual cycle |
The two systems are not rivals — they work together. The fight-or-flight response is the perfect illustration: the nervous system reacts instantly, while adrenaline (hormonal) sustains the raised heart and breathing rate. Being able to say which system suits a given situation, and why (its speed and duration), is a recurring B3 skill.
Exam Tip: A common misconception is to assume every quick response is nervous and every slow one hormonal without saying why. Always justify your choice with the speed and duration the situation needs.
The named practical for this part of B3 is the ruler-drop reaction-time test.
| Practical | What you change / measure | Key technique | Top marks come from |
|---|---|---|---|
| Reaction time (ruler-drop test) | Catch a dropped ruler; measure the distance fallen | Drop without warning; convert distance to time | Keeping conditions the same; repeats and a mean; ignoring anomalies |
Exam Tip: Examiners reward identifying the independent variable (what you change, e.g. whether the person has had caffeine), the dependent variable (the distance fallen / reaction time) and the control variables (same hand, same starting position, no warning). "Drop without warning" is a frequently rewarded detail.
A student does the ruler-drop test five times and records catch distances of 15,14,22,13 and 16 cm. Identify any anomaly and calculate the mean of the remaining values.
Step 1 — spot the anomaly. The value 22 cm is much larger than the others, so it is an anomaly and should be excluded.
Step 2 — calculate the mean of the remaining four:
415+14+13+16=458=14.5 cm
Answer: the anomaly is 22 cm; the mean of the other readings is 14.5 cm. A shorter distance means a faster reaction, so this student's typical catch corresponds to a fast reaction time.
Common error: including the anomalous 22 cm in the mean, which would wrongly inflate it. Always identify and exclude anomalies first.
The main calculation in B3 is the body mass index, used when discussing the obesity link to Type 2 diabetes.
BMI=[height (m)]2body mass (kg)
Worked example: A person has a mass of 72 kg and a height of 1.6 m. Find their BMI.
Square the height first: [1.6]2=2.56 m2. Then:
BMI=2.5672=28.1 (to 3 s.f.)
Answer: a BMI of about 28, which falls in the overweight range (25.0–29.9).
Exam Tip: The classic BMI mistake is forgetting to square the height. Write the squaring as its own step, and remember the units are kg/m2.
OCR uses specific command words that tell you what kind of answer to give. Reading them correctly is worth easy marks.
| Command word | What it asks for |
|---|---|
| State / Name / Give | A short fact, no explanation (e.g. "Name the gland that releases insulin") |
| Describe | Say what happens, in order, with no need for reasons |
| Explain | Give reasons why — use "because", "so that", "this means that" |
| Compare | Give similarities and differences, ideally point-by-point |
| Calculate | Work out a number — show working and give a unit |
| Evaluate | Give points for and against and a brief conclusion |
Exam Tip: The difference between describe and explain decides many marks. "Blood glucose falls after insulin is released" describes; "...because insulin makes the liver store glucose as glycogen" explains. If the command word is explain, give the reason.
Cover the right-hand column and test yourself.
Subscribe to continue reading
Get full access to this lesson and all 7 lessons in this course.