D3.3 Homeostasis
Homeostasis maintains internal conditions through feedback control of blood pH, glucose, temperature, kidney filtration, osmoregulation and blood flow in human physiology.
- Syllabus
- First assessment 2025
- Topic
- D3.3
- Level
- SL
Homeostasis Keeps the Internal Environment within Limits
Homeostasis maintains variables in an organism's internal environment within preset narrow limits despite external fluctuations.
| Human homeostatic variable | Why regulation matters |
|---|---|
| Body temperature | Keeps enzyme and membrane processes in a functional range |
| Blood pH | Preserves protein shape and reaction conditions |
| Blood glucose concentration | Maintains a usable respiratory substrate supply |
| Blood osmotic concentration | Limits harmful water movement into or out of cells |
Stable tissue fluid lets cells function predictably even when temperature, food intake or water availability outside the body changes.
After a meal raises blood glucose, hormonal regulation brings the concentration back toward its preset range.
Homeostasis is dynamic: values fluctuate around a set point or within limits rather than remaining perfectly constant.
Homeostasis definition
Assessment in practice
1–6 marksHow it is assessed
This objective is assessed through structured response, commonly using Explain / Identify / Outline.
Command terms
Explain / Identify / Outline
What earns marks
Build the answer around this relationship: Homeostasis keeps internal variables within narrow limits.
Representative question
Question 1
[Maximum number: 6]
Explain how the pH of blood is kept constant during exercise.
a. pH of blood is regulated to stay within a narrow range/7.35 to 7.45
b. increase in CO2 produced during aerobic respiration «during exercise»
c. CO2 reacts with water to form carbonic acid
d. chemoreceptors detect drop in blood pH «when CO2 concentration rises» OR «increase in» CO2 lowers blood pH
e. carbonic acid dissociates to form hydrogen carbonate ions and hydrogen ions
f. hydrogencarbonate is alkaline/increases pH / neutralizes H+ions
g. hydrogen ions bind to plasma proteins/hemoglobin
h. stimulation of breathing centre/medulla oblongata
OR ventilation rate increased
i. faster diffusion/removal of CO2 «in alveoli/lungs»
Marking guidance:
Allow formula OWTTE
6 max
Negative Regulation Reverses a Deviation
Negative regulation reduces the original change so a regulated variable returns toward its normal range.
The response opposes the disturbance: a rise triggers actions that lower it, and a fall triggers actions that raise it. This stabilizes rather than amplifies the system.
Ask whether the response moves the variable in the opposite direction to the initial deviation.; identify the signal, controller and effector
If body temperature rises, sweating and vasodilation increase heat loss, reducing the rise. This gives a concrete prediction from the stated condition.
Negative means opposing the deviation, not harmful or always below the set point. Interpret the result within the stated biological model and limits.
Negative feedback loops
Assessment in practice
4 marksHow it is assessed
This objective is assessed through essay response, commonly using Discuss.
Command terms
Discuss
What earns marks
Build the answer around this relationship: Negative feedback opposes the original change.
Representative question
Question 1
[Maximum number: 4]
Discuss the use of positive and negative feedback to control levels of variables.
Negative feedback max 3
a. negative feedback used in homeostasis
OR
negative feedback to maintain equilibrium;
b. negative feedback returns level of variable to the set point/norm;
c. negative feedback used to keep level of variable constant/within (narrow) limits;
d. negative feedback causes increase following a decrease / decrease following an increase;
Positive feedback
e. positive feedback amplifies changes;
f. positive feedback disrupts equilibrium;
g. positive feedback is generally for a short time;
Use of an example to illustrate a marking point may be given credit.
d. Allow a labelled graph.
4 max
Insulin and Glucagon Regulate Blood Glucose
Pancreatic endocrine cells detect blood glucose: beta cells release insulin when it rises, while alpha cells release glucagon when it falls.
| Change | Hormone carried in blood | Main target effects | Result |
|---|---|---|---|
| Glucose above set point | Insulin | Increased glucose uptake by target cells; glycogen synthesis in liver and muscle | Blood glucose falls |
| Glucose below set point | Glucagon | Liver glycogen breakdown and glucose release | Blood glucose rises |
The two opposing hormone responses form negative-feedback loops that reduce the original deviation.
After a carbohydrate-rich meal, rising glucose stimulates beta cells; insulin promotes uptake and storage until secretion falls as the set point is approached.
Glucagon acts mainly on the liver to raise circulating glucose; muscle glycogen is primarily a local fuel store and is not released as blood glucose in response to glucagon.
Blood glucose regulation
Assessment in practice
1–4 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Explain / Describe.
Command terms
Identify / Explain / Describe / Outline / State / Discuss
What earns marks
Build the answer around this relationship: Beta cells secrete insulin when blood glucose is high.
Representative question
Question 1
[Maximum number: 8]
Explain the control of blood glucose concentrations in humans.
pancreatic cells monitor the blood glucose concentrations;
alpha and beta cells are in the islets of Langerhans;
negative feedback mechanisms;
send hormones (through bloodstream) to target organs;
if too high, β cells (in pancreas) produce insulin;
insulin stimulates liver/muscle cells to take up glucose;
glucose is converted into glycogen (stimulated by insulin);
lowering blood glucose level;
other cells are stimulated to absorb glucose and use it in cell respiration;
if glucose levels too low, α cells (in pancreas) produce glucagon;
glucagon stimulates liver/muscle cells to break down glycogen;
and release glucose into the blood;
raising the blood glucose level;
Type 1 and Type 2 Diabetes Disrupt Different Parts of Control
Diabetes mellitus causes persistent difficulty controlling blood glucose, but type 1 and type 2 begin with different physiological failures.
| Feature | Type 1 | Type 2 |
|---|---|---|
| Main physiological change | Autoimmune destruction of pancreatic beta cells causes little or no insulin secretion | Target cells respond poorly to insulin; beta-cell function may later decline |
| Important risk pattern | Autoimmune susceptibility; not prevented by lifestyle | Risk rises with genetic susceptibility, excess body fat and low physical activity |
| Management | Insulin replacement, glucose monitoring and coordinated diet/exercise | Activity, diet and healthy body mass can reduce risk and aid control; medication and sometimes insulin may be required |
With too little effective insulin signalling, uptake and storage do not adequately reduce blood glucose after a meal, so hyperglycaemia persists.
Lifestyle is a risk modifier for type 2, not a moral diagnosis or the sole cause. A single high reading does not distinguish the two types.
Diabetes exam focus
Assessment in practice
1–2 marksHow it is assessed
This objective is assessed through structured response, commonly using Describe / Identify / Explain.
Command terms
Describe / Identify / Explain / State / Discuss / Analyse / Outline
What earns marks
Build the answer around this relationship: Type I diabetes involves insufficient insulin production.
Representative question
Question 1
[Maximum number: 5]
Outline type II diabetes.
excess glucose in blood / hyperglycemia;
symptoms are excessive thirst / frequent urination / dehydration / fatigue;
Marking guidance:
Reject weight loss as a symptom of Type II diabetes. unresponsive to insulin / insulin resistance / not enough insulin produced;
Reject no insulin.
linked to/risk factor is obesity/diets high in sugar/fat;
late onset / onset is usually adulthood/after childhood;
insulin not required (usually) / insulin ineffective as a treatment;
treated with low sugar diets/low GI/glycemic index foods;
[5 max]
Reject low carbohydrate diet.
Do not award a mark simply for stating that glucose is present in the urine as this is in the question.
Thermoregulation Is a Negative-Feedback Control System
Human thermoregulation detects deviation in core temperature and coordinates effectors that reverse the change.
| Control component | Role |
|---|---|
| Peripheral thermoreceptors | Detect temperature changes, especially at the skin |
| Hypothalamus | Integrates peripheral and central temperature information |
| Pituitary/thyroid pathway | Alters thyroxin signalling and therefore metabolic heat production |
| Skeletal muscle | Shivering raises respiration and heat production |
| Brown adipose tissue | Uncoupled respiration releases energy as heat |
A fall in temperature is detected, the hypothalamus coordinates reduced heat loss and increased muscle/adipose heat production, and the response decreases as core temperature recovers.
The regulated variable is core temperature; skin temperature can change more rapidly and acts partly as an early environmental signal.
Thermoregulation exam focus
Assessment in practice
1–8 marksHow it is assessed
This objective is assessed through structured response, commonly using Describe / Explain / Identify.
Command terms
Describe / Explain / Identify / Outline
What earns marks
Build the answer around this relationship: The hypothalamus coordinates body temperature control.
Representative question
Question 1
[Maximum number: 8]
Explain the control of body temperature in humans.
normal body core temperature constant/ 36.5 to 37.5∘C;
(accept single values within this range);
regulated by negative feedback/homeostatic mechanisms;
hypothalamus is the centre of thermoregulation;
hypothalamus sends impulses to the body to increase/decrease temperatures;
release of sweat (by sweat glands in the skin) if skin temperature rises;
evaporation of water cools the body;
(concept of evaporation must be mentioned)
heat is transferred by blood;
transfer of heat from body core in blood to surface;
if temperature rises, increased flow of blood/heat to the skin/vasodilation of skin blood vessels/arterioles;
(do not accept veins, arteries or capillaries)
if temperature drops, decreased flow of blood/heat to the skin/vasoconstriction of skin blood vessels/arterioles;
(eg reducing activity (to lower body temperature) / reducing exposed surfaces (to reduce heat loss)
Human Effectors Alter Heat Loss and Production
Human thermoregulation combines physiological and behavioural responses; each effector changes heat transfer or metabolic heat production.
| When hot | Effect | When cold | Effect |
|---|---|---|---|
| Skin vasodilation | More warm blood near the surface increases heat loss | Skin vasoconstriction | Less warm blood near the surface reduces heat loss |
| Sweating | Evaporation removes latent heat | Shivering | Rapid muscle contraction increases respiration and heat production |
| Hairs lie flatter | Reduces the trapped insulating air layer | Hair erection | Traps more air, though the effect is small in humans |
| Behaviour seeks shade/cooling | Reduces heat gain or raises loss | Brown-fat uncoupled respiration/warmer behaviour | Produces or conserves heat |
Sweating is most effective when sweat evaporates; high humidity reduces evaporation and therefore reduces cooling.
Vasodilation transfers internal heat toward skin but does not itself remove heat from the body; the environment must accept that heat.
Thermoregulation mechanisms
Assessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Outline.
Command terms
Identify / Outline
What earns marks
Build the answer around this relationship: Evaporation of sweat removes heat from the body.
Representative question
Question 1
[Maximum number: 1]
Outline one change that happens in the human body in response to a rise in body temperature above 36.4∘C.
a. sweating/sweat secretion;
b. vasodilation / skin arterioles widen / increased blood flow to skin;
c. shunt vessels in the skin close;
b. Do not accept' blood
vessels widen' unless the answer indicates that more blood will flow to the skin.
1 max
Retrieve the Core Homeostasis Route
Core D3.3 is secure when every example becomes a feedback route: identify the variable, detect deviation, coordinate a response, activate effectors, and reverse the change. Glucose and temperature are the key worked examples.
- stable internal environment within narrow limits
- detects deviation from set point and reverses it
- insulin lowers high glucose; glucagon raises low glucose
- hypothalamus coordinates cooling or warming responses
Core Homeostasis
Core homeostasis answers should use a control-loop structure, not a list of responses. The response starts with the variable and set point, then explains how the body detects deviation and activates the response that reverses it. Apply that loop to glucose, diabetes, or temperature.
- Define homeostasis as maintaining stable internal conditions within narrow limits.
- Use negative feedback language: receptor, coordinator, effector, set point, and reverse the deviation.
- Apply the loop to insulin/glucagon, diabetes types, or hot/cold thermoregulation responses.
Objective notes
6 learning objectivesD3.3.1Homeostasis definition• Homeostasis maintains a stable internal environment within narrow limits• Variables include body temperature, blood pH, glucose, gases, ions, and osmotic concentration4% of analysed papers 5 papers · 5 questionsViewD3.3.2Negative feedback loops• Negative feedback detects deviation from a set point and reverses it• Receptors, coordinators, effectors, and feedback loops restore normal conditions1% of analysed papers 1 paper · 1 questionViewD3.3.3Blood glucose regulation• Beta cells secrete insulin when blood glucose rises• Alpha cells secrete glucagon when blood glucose falls, affecting liver and muscle stores14% of analysed papers 20 papers · 26 questionsViewD3.3.4Diabetes• Type 1 diabetes results from autoimmune destruction of pancreatic beta cells• Type 2 diabetes involves insulin-receptor/response failure and is linked to lifestyle risk factors11% of analysed papers 15 papers · 24 questionsViewD3.3.5Thermoregulation• Thermoregulation uses negative feedback to maintain core temperature near 37 °C• Thermoreceptors signal the hypothalamus, which coordinates skin, muscles, liver, and hormones3% of analysed papers 4 papers · 4 questionsViewD3.3.6Thermoregulation mechanisms• Cooling uses vasodilation, sweating, and hairs lying flat• Warming uses vasoconstriction, reduced sweating, shivering, metabolic heat, and brown fat3% of analysed papers 4 papers · 4 questionsView