Homeostasis — Science Sage

What is homeostasis?

Homeostasis is the maintenance of a relatively stable internal environment (e.g. body temperature, blood glucose concentration, water content), despite changes in the external environment. Keeping conditions stable allows the body's cells and enzymes to function optimally.

Negative feedback

Homeostasis works mainly through negative feedback — a control mechanism where a change from the normal (set point) value triggers a response that reverses the change, returning the variable back towards the set point. All negative feedback loops share the same basic components:

Stimulus — a change in a condition (e.g. a rise in body temperature).
Receptor — detects the change (e.g. thermoreceptors in the skin/hypothalamus).
Control centre — processes the information and coordinates a response (e.g. the hypothalamus).
Effector — carries out the response (e.g. sweat glands, muscles).
Response — the change that corrects the original deviation, restoring the set point.

Feedback loop: Stimulus → Receptor → Control centre → Effector → Response (which removes the original stimulus).

Thermoregulation

The hypothalamus in the brain monitors and controls body temperature using structures in the skin:

Structure/response	When body is too HOT	When body is too COLD
Sweat glands	Increase sweat production — evaporation cools the skin	Sweat production decreases
Blood vessels (skin)	Vasodilation — vessels widen, more blood flow to skin, more heat lost	Vasoconstriction — vessels narrow, less blood flow to skin, less heat lost
Hair erector muscles	Relax — hairs lie flat	Contract — hairs stand up, trapping an insulating layer of air
Skeletal muscles	No shivering	Shivering — rapid muscle contraction generates heat

Blood glucose regulation

The pancreas monitors and regulates blood glucose concentration using two antagonistic hormones:

Insulin — released when blood glucose is too high; stimulates the liver and muscle cells to absorb glucose from the blood and store it as glycogen, lowering blood glucose.
Glucagon — released when blood glucose is too low; stimulates the liver to break down stored glycogen back into glucose and release it into the blood, raising blood glucose.

Diabetes mellitus occurs when the body cannot produce enough insulin (Type 1) or cells stop responding properly to insulin (Type 2), leading to chronically high blood glucose levels.

Thermoregulation Simulator

Drag the slider to raise or lower body temperature away from the 37°C set point, and see which negative feedback responses the body activates.

Body temperature vs set point

Body temperature 37.0°C

Body temperature is at the normal set point (37°C) — no correction needed.

🥵 Responses to HEAT

Sweat glands secrete more sweat (evaporative cooling)
Vasodilation — skin blood vessels widen, more heat radiates away
Hair erector muscles relax (hairs lie flat)

🥶 Responses to COLD

Shivering — muscle contractions generate heat
Vasoconstriction — skin blood vessels narrow, less heat lost
Hair erector muscles contract (hairs stand up, trap air)

Answer in your exercise book.

Question 1 · (2 marks)

Define "homeostasis".

Question 2 · (4 marks)

Name and briefly describe the FOUR components of a negative feedback loop, in the correct order.

Question 3 · (4 marks)

Describe TWO ways the body responds to being too hot, and TWO ways it responds to being too cold.

Question 4 · (3 marks)

Explain the roles of insulin and glucagon in regulating blood glucose concentration.

Question 5 · (2 marks)

Explain why vasoconstriction helps the body retain heat.

Question 6 · Reading a Blood Glucose Tolerance Test (7 marks)

Two people eat identical meals. Their blood glucose concentration is measured every hour afterwards:

Time after meal (hours)	0	1	2	3	4
Person A (mmol/L)	5.0	7.8	6.5	5.2	5.0
Person B (mmol/L)	5.5	10.5	11.8	10.9	9.8

(a) Describe the trend in Person A's blood glucose after the meal. In terms of insulin, explain why the level returns close to 5.0 mmol/L by hour 4.
(b) Identify TWO clear differences between Person B's pattern and Person A's pattern, using the actual values from the table.
(c) Suggest a physiological explanation for why Person B's blood glucose rises higher and stays elevated for longer than Person A's.
(d) By hour 4, has Person B's blood glucose returned to its starting (hour 0) value? Use this observation to explain why doctors use this kind of test (a glucose tolerance test) to help diagnose diabetes.