OCR GCSE Biology A: Homeostasis, temperature control and blood glucose regulation (J247)

This study guide is aligned to the OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), for first assessment in 2018. It covers statements B3.3a to B3.3e of module B3.3, Maintaining internal environments. Statement B3.3d is in bold in the specification, so it is tested only in the Higher Tier papers. The other four statements apply to both tiers. Osmotic balance, the kidney and ADH (B3.3f onwards) are taught elsewhere.

Use this guide first, then the revision notes for fast recall and the practice questions to test yourself. Course pages: OCR GCSE Biology hub and printable checklist. To find weak spots, try the free 10-minute diagnostics. The hormone ideas build on the endocrine system page and the nervous system page.

What this unit covers

Statement	What you must be able to do	Tier
B3.3a	Explain why a constant internal environment matters, in response to internal and external change: reactions must run at appropriate rates.	Both
B3.3b	Describe the skin’s role in body temperature control: detecting external temperature, sweating, shivering, and blood flow changes (vasoconstriction and vasodilation).	Both
B3.3c	Explain how insulin controls blood sugar levels.	Both
B3.3d	Explain how glucagon interacts with insulin to control blood sugar levels.	Higher Tier only
B3.3e	Compare type 1 and type 2 diabetes and explain how they can be treated.	Both

The mathematical outcome is to extract and interpret data from graphs, charts and tables.

B3.3a: Why the internal environment must stay constant

Homeostasis means keeping conditions inside the body steady, even when conditions outside, or inside, change. Outside, it can be hot or cold. Inside, a meal raises blood sugar and exercise releases heat. Without control, cells would leave the conditions they need.

Metabolic reactions are controlled by enzymes, and enzymes only work well in a narrow set of conditions.

• Temperature: at low temperature, particles have less kinetic energy, so there are fewer successful collisions between enzyme and substrate and reactions run too slowly. If the temperature rises too far, the enzyme’s shape changes (it denatures), the active site no longer fits the substrate, and the reaction rate falls sharply. Core body temperature is kept close to 37 °C.
• Blood glucose: cells use glucose in respiration to release energy. If it falls too low, cells (brain cells especially) cannot respire fast enough. If it stays too high for long periods, it damages the body, as in diabetes.

Extreme cold slows reactions throughout the body (hypothermia). Extreme heat risks enzymes denaturing. Staying near the optimum keeps reactions at appropriate rates.

Most homeostasis uses negative feedback: a receptor detects a change, a control centre processes the information, and an effector makes a response that reverses the change.

Worked example 1: a rise in core temperature

A runner’s core temperature is measured while exercising in a warm room.

Time (min)	0	10	20	30
Core temperature (°C)	37.0	37.6	38.1	38.5

Calculate the rate of temperature rise over the 30 minutes and explain why the rise matters.

1. Rise = 38.5 − 37.0 = 1.5 °C.
2. Rate = 1.5 ÷ 30 = 0.05 °C per minute, or 3.0 °C per hour.
3. Muscle respiration releases heat faster than the body loses it. If the rise continued, enzymes would eventually denature and reactions would slow. Sweating and extra skin blood flow (below) reverse the rise.

The rise slows (0.6, 0.5, then 0.4 °C per 10 minutes), which fits negative feedback starting to work.

B3.3b: The skin and body temperature control

Temperature receptors in the skin detect external temperature, and the brain monitors the temperature of the blood. Impulses pass along sensory neurones to the brain, which sends impulses to effectors: sweat glands, muscles and the blood vessels of the skin.

Too hot

• Sweating: sweat glands release sweat onto the skin. As the water evaporates, it transfers energy from the skin to the surroundings, which cools you.
• Vasodilation: the small arteries (arterioles) leading to capillaries near the skin surface widen. More blood flows near the surface, so more heat is transferred from the blood to the surroundings.

Too cold

• Shivering: skeletal muscles contract and relax rapidly. This needs respiration, which releases heat, and that warms the blood.
• Vasoconstriction: the arterioles supplying the surface capillaries narrow, so blood is kept away from the skin and the body loses less heat.
• Less sweating: sweat production falls, so there is less evaporative cooling.

Blood vessels do not “move” towards or away from the skin. The diameter of the arterioles changes, which changes how much blood flows through the surface capillaries.

Worked example 2: skin blood flow and air temperature

A volunteer rests in rooms at different air temperatures. Blood flow through the skin of the forearm is measured in arbitrary units. Core temperature stays at 37.0 °C throughout.

Air temperature (°C)	10	20	30	40
Skin blood flow (arbitrary units)	15	40	110	240

(a) Calculate the percentage rise in forearm skin blood flow between 20 °C and 40 °C. (b) Explain the trend.

(a) Increase = 240 − 40 = 200. Percentage increase = 200 ÷ 40 × 100 = 500%. Divide by the starting value.

(b) Skin blood flow increases as air temperature rises. Vasodilation widens the arterioles to the skin, which sends extra blood close to the surface and transfers more heat to the surroundings. Core temperature stays constant, so the response keeps the internal environment steady. At 10 °C, vasoconstriction keeps flow low and reduces heat loss.

In very humid air sweat evaporates slowly, so cooling is less effective.

B3.3c: How insulin controls blood sugar

Blood glucose is kept in a narrow range, roughly 4 to 6 mmol/dm³ in a healthy person (in questions, use the values given). After a meal, carbohydrate is digested to glucose, which is absorbed into the blood, so the concentration rises.

1. The rise is detected in the pancreas, which is an endocrine gland.
2. The pancreas secretes the hormone insulin into the blood.
3. Insulin travels in the blood to target cells, mainly in the liver and muscles. Their receptors bind it.
4. The cells take up glucose from the blood. In the liver (and muscles), glucose is converted to the storage carbohydrate glycogen.
5. Blood glucose concentration falls back towards normal. Insulin secretion falls as the level drops, which is negative feedback.

Worked example 3: glucose and insulin after a meal

A healthy person eats a meal at time 0. Blood glucose and insulin are measured.

Time (min)	0	30	60	90	120	150	180
Glucose (mmol/dm³)	5.0	7.4	6.6	5.6	5.1	4.9	4.9
Insulin (arbitrary units)	10	45	60	40	20	12	10

(a) Calculate the percentage rise in glucose at its peak. (b) Calculate the mean rate of fall from 30 to 120 minutes. (c) Explain why insulin peaks after glucose.

(a) Rise = 7.4 − 5.0 = 2.4. Percentage = 2.4 ÷ 5.0 × 100 = 48%.

(b) Fall = 7.4 − 5.1 = 2.3 mmol/dm³ over 90 minutes. Rate = 2.3 ÷ 90 = 0.0256 mmol/dm³ per minute, or 1.5 mmol/dm³ per hour (2.3 ÷ 1.5 = 1.53).

(c) Glucose peaks at 30 minutes and insulin at 60 minutes. The pancreas responds to high glucose by secreting insulin, which takes time to be released and carried in the blood. Glucose falls as insulin acts on liver and muscle cells, and insulin returns to its starting value as glucose returns to about 5 mmol/dm³.

B3.3d: How glucagon interacts with insulin (Higher Tier only)

Blood glucose can also fall too low, for example after fasting or long exercise. A second pancreatic hormone, glucagon, deals with this. Insulin and glucagon have opposite effects, and together they keep glucose near the normal level.

• Blood glucose too high: pancreas releases insulin, cells take up glucose, liver converts glucose to glycogen, glucose falls.
• Blood glucose too low: pancreas releases glucagon, which acts on the liver. The liver converts stored glycogen to glucose and releases it into the blood, so glucose rises.

Each response reverses the change that triggered it. Because both are negative feedback, glucose rises and falls around a set point rather than staying exactly fixed.

Worked example 4: an overnight fast

Blood glucose falls from 5.0 to 4.4 mmol/dm³ in 8 hours without food.

1. Fall = 0.6, which is 0.6 ÷ 5.0 × 100 = 12%. Mean rate = 0.6 ÷ 8 = 0.075 mmol/dm³ per hour.
2. The fall is small because the falling level stimulates glucagon secretion. Glucagon makes the liver convert glycogen to glucose and release it, which slows the fall.

B3.3e: Type 1 and type 2 diabetes

In diabetes, blood glucose cannot be controlled properly and can rise high after meals and stay high.

	Type 1	Type 2
Problem	The pancreas does not produce enough insulin (usually because the immune system has destroyed the insulin-producing cells)	Body cells no longer respond properly to insulin (insulin resistance)
Usual onset	Often in childhood or early adult life	More often later in life
Linked to	Not caused by diet	Obesity, inactivity, poor diet, age, family history
Main treatment	Insulin injections (or a pump), with blood glucose monitoring	Healthy diet, regular exercise, weight loss; medicines if needed

Treating type 1

Diet alone cannot control type 1, because there is too little insulin. People inject insulin, usually around meals, and monitor blood glucose, planning diet and activity to match. Insulin is a protein, so it is injected (or given by pump); a tablet would be digested in the gut.

Treating type 2

Type 2 is often treated first by lifestyle change: a diet lower in refined sugars and fat, regular exercise and weight loss, which improve the response of cells to insulin. If glucose is still too high, medicines can be added (for example metformin), and some people eventually need insulin. Test strips such as Clinistix detect glucose in urine, which can indicate diabetes because glucose appears in urine when blood levels are very high.

Worked example 5: glucose tolerance test

Three people drink a glucose solution after fasting. Their blood glucose (mmol/dm³) is measured.

Time (h)	0	1	2	3
P	4.8	7.6	5.2	4.7
Q	9.5	16.0	17.5	16.8
R	7.4	13.2	14.8	12.0

Identify the healthy person and compare Q and R.

• P: peaks at 1 hour (rise of 2.8, or 58%) and is back to 4.7 by 3 hours, so P regulates glucose normally.
• Q: starts high (9.5), peaks at 17.5 (a rise of 8.0) and is still 16.8 at 3 hours. Almost nothing is removed, as expected if little insulin is made, so Q fits type 1.
• R: starts at 7.4, peaks at 14.8 and has fallen only to 12.0 by 3 hours. The level falls slowly, as if cells respond poorly to insulin, so R fits type 2.

Worked example 6: risk of type 2 diabetes

A survey of 3500 adults gives this table.

Group	Adults	With type 2 diabetes
BMI under 25	1200	18
BMI 25 to 29.9	1500	66
BMI 30 or over	800	88

Percentages: 18 ÷ 1200 = 1.5%; 66 ÷ 1500 = 4.4%; 88 ÷ 800 = 11%. The higher the BMI group, the higher the percentage with type 2 diabetes: a positive correlation. The 11% group is about 7.3 times the 1.5% group. A correlation does not prove that weight causes the disease, but it is consistent with obesity being a risk factor.

Common errors

• Saying insulin “breaks down” glucose. It makes cells take up glucose and the liver store it as glycogen.
• Describing type 2 as “no insulin”. Cells do not respond properly.
• Dividing by the final value, not the starting value, in percentage change.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247), specification Version 4.0 (August 2026), for first assessment in 2018, published by OCR. Module B3.3, statements B3.3a to B3.3e.