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
- HL
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.
Kidneys Perform Excretion and Osmoregulation
HL onlyKidneys regulate blood composition by excreting metabolic wastes and by osmoregulating water and dissolved ions.
| Process | Meaning | Kidney outcome |
|---|---|---|
| Excretion | Removal of metabolic waste and unwanted substances | Urea and other unwanted solutes leave in urine |
| Osmoregulation | Regulation of osmotic concentration | Nephrons adjust water and ion reabsorption, changing urine volume and concentration |
Osmoticconcentrationisexpressedinosmolesperlitre(osmolL−1).
When body water is scarce, increased water reabsorption produces a smaller volume of more concentrated urine while urea is still excreted.
Excretion and egestion are different: kidneys remove substances from blood, whereas egestion removes undigested material from the gut.
Kidney role
HL onlyAssessment in practice
1–7 marksHow it is assessed
This objective is assessed through structured response, commonly using Explain / State / Deduce.
Command terms
Explain / State / Deduce / Outline / Identify
What earns marks
Build the answer around this relationship: Kidneys remove wastes while conserving useful substances.
Representative question
Question 1
[Maximum number: 8]
Explain the role of the kidney in osmoregulation.
a osmoregulation is regulation of water and solute/salt balance/solute concentrations;
b nephron (is the functional unit of the kidney/osmoregulates);
c ultrafiltration in glomerulus / glomerular filtrate collected by Bowman's capsule;
d loop of Henle establishes/maintains hypertonic conditions in medulla;
e osmosis/reabsorption of water (from filtrate) in the collecting duct;
f brain/hypothalamus monitors blood solute concentration / pituitary secretes ADH;
g ADH secreted when solute concentration of blood is too high/hypertonic/when dehydrated;
h ADH increases permeability of collecting duct to water;
i ADH causes more aquaporins (in membranes of collecting duct wall cells);
j more water reabsorbed resulting in more concentrated/hypertonic urine/less volume of urine;
k less/no ADH secreted when solute concentration (of blood) is too low/hypotonic;
I less water reabsorbed resulting in dilute/hypotonic urine/large volume of urine;
Marking guidance:
Reject 'water balance' and 'water concentration' for mpa
8 max
Ultrafiltration Is Followed by Selective Reabsorption
HL onlyThe glomerulus and Bowman's capsule form filtrate by ultrafiltration; the proximal convoluted tubule (PCT) then returns useful substances to blood.
| Site | Mechanism and outcome |
|---|---|
| Glomerulus → Bowman's capsule | High hydrostatic pressure forces water and small solutes through fenestrations, basement membrane and podocyte slits; cells and most plasma proteins remain in blood |
| PCT | Membrane transport selectively reabsorbs all normal glucose and amino acids, much Na⁺ and other ions; water follows by osmosis into surrounding capillaries |
Ultrafiltration is mainly size/pressure based, so it cannot distinguish useful small solutes from wastes. Selective transport in the PCT performs that recovery.
Glucose enters Bowman's filtrate because it is small, then is normally reabsorbed in the PCT rather than excreted in urine.
Large proteins or blood cells in urine are not normal products of ultrafiltration and may indicate damage to the filtration barrier.
Glomerulus, Bowman's capsule, PCT
HL onlyAssessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify.
Command terms
Identify
What earns marks
Build the answer around this relationship: Ultrafiltration occurs from the glomerulus into Bowmans capsule.
Representative question
Question 1
[Maximum number: 8]
Explain the presence of glucose in the urine of a diabetic person and its absence in the urine of a person with type I diabetes that is being successfully treated.
glucose filtered out of blood (plasma) in glomerulus;
glucose reabsorbed from filtrate in proximal convoluted tubule;
by active transport;
reference to specific pumps for glucose / limited capacity for glucose uptake;
in diabetic patients glucose concentration in plasma is high;
not all glucose can be reabsorbed (in PCT) /capacity for reabsorption exceeded;
no glucose reabsorption after the proximal convoluted tubule;
glucose still present at end of nephron/collecting duct/in the ureter/bladder;
Marking guidance:
Do not award a mark simply for stating that glucose is present in the urine as this is in the question. type I diabetes treated with insulin;
insulin reduces the glucose concentration of blood/plasma/filtrate;
Reject regulates glucose concentration. all glucose reabsorbed from filtrate in Type I diabetics if treated;
The Loop of Henle Builds a Medullary Gradient
HL onlyThe loop of Henle uses differing permeabilities and ion transport to create a high solute concentration in the medulla, enabling later water conservation.
The descending limb loses water but has low ion permeability; the ascending limb pumps ions but is water-impermeable. Countercurrent multiplication amplifies the gradient.
For each limb ask whether water can cross and whether solute is transported before predicting fluid concentration.; identify the signal, controller and effector
As filtrate descends, water leaves into the concentrated medulla; as it ascends, ions leave while water stays, making filtrate dilute. This gives a concrete prediction from the stated condition.
The loop creates the gradient; it does not by itself determine the final urine volume. Interpret the result within the stated biological model and limits.
Loop of Henle
HL onlyAssessment in practice
1 marksHow it is assessed
This objective is assessed through multiple choice, commonly using Identify / State.
Command terms
Identify / State
What earns marks
Build the answer around this relationship: The descending limb allows water to leave the filtrate.
Representative question
Question 1
[Maximum number: 1]
What is the function of the loop of Henle?
A
To reabsorb salt
B
To maintain a hypertonic solution in the medulla
C
To transport liquid from the collecting ducts to the convoluted tubules
D
To reabsorb glucose
B
ADH Moves Aquaporins to Control Collecting-Duct Water Loss
HL onlyHypothalamic osmoreceptors detect blood osmotic concentration and adjust pituitary ADH release, changing collecting-duct water permeability.
| Blood condition | ADH and aquaporin location | Urine response |
|---|---|---|
| Too concentrated / low water | More ADH; aquaporins move from intracellular vesicles into collecting-duct cell membranes | More water follows the medullary gradient into blood; low-volume concentrated urine |
| Too dilute / excess water | Less ADH; aquaporins are removed from membranes into vesicles | Less water is reabsorbed; larger-volume dilute urine |
Changing membrane aquaporin number switches permeability rapidly without rebuilding the collecting duct, closing a negative-feedback loop.
During dehydration, increased ADH inserts more aquaporins, so water leaves the collecting duct by osmosis and blood osmotic concentration moves back toward its set point.
ADH changes permeability but does not create the medullary gradient; the loop of Henle establishes the gradient that makes water reabsorption possible.
Osmoregulation by collecting ducts
HL onlyAssessment in practice
1–5 marksHow it is assessed
This objective is assessed through structured response, commonly using Describe / Explain / Identify.
Command terms
Describe / Explain / Identify / State / Outline
What earns marks
Build the answer around this relationship: High blood solute concentration stimulates ADH release.
Representative question
Question 1
[Maximum number: 7]
Explain the hormonal control of osmoregulation in the kidney by negative feedback.
a. negative feedback loops maintain homeostasis/return variables to an original set point if levels are increased or decreased;
b. osmoreceptors in the hypothalamus detect if body/blood is dehydrated/hypertonic/solute concentration is too high;
c. ADH is secreted if body/blood is dehydrated/hypertonic;
d. (ADH is released) from the pituitary;
e. (ADH) increases/stimulates more aquaporins / aquaporins open in the collecting duct;
f. collecting duct more permeable to water/reabsorbs more water (from filtrate/urine);
g. more water is reabsorbed into the blood / blood solute concentration reduced;
h. less water lost in urine / smaller volume of (more concentrated) urine;
i. increases the concentration of water/ water potential in blood/decreases the concentration of blood/plasma;
j. negative feedback / less/no ADH secreted when blood solute concentration returns to normal;
7
Marking guidance:
max
Activity Redistributes Blood among Organs
HL onlyArteriolar vasodilation and vasoconstriction redistribute blood so organ supply matches changing metabolic activity while vital functions continue.
| State | Skeletal muscle | Gut | Brain | Kidneys |
|---|---|---|---|---|
| Sleep | Lower flow to most muscle groups than when awake | Depends on digestive activity | Total flow changes little, though regions such as hypothalamus/brainstem can rise during REM | Maintained for excretion and osmoregulation; lying down can raise renal flow |
| Vigorous exercise | Strongly increased | Reduced as blood is redirected | Kept relatively stable | Reduced during prolonged vigorous exercise, but regulation limits disruption |
| Wakeful rest | Lower than during exercise | Increased after a meal for digestion and absorption | Kept relatively stable | Fairly constant overall; posture can alter flow |
Locally active tissues dilate their arterioles, while sympathetic/epinephrine signals can constrict vessels to less immediately required organs and dilate those supplying skeletal muscle.
During vigorous exercise, skeletal-muscle flow rises while gut and renal flow fall; this supplies respiration where demand is greatest.
Redistribution is relative, not complete shut-off. Brain and kidney perfusion must remain sufficient for neural control, excretion and osmoregulation.
Blood supply changes
HL onlyAssessment in practice
1 marksHow it is assessed
This objective is assessed through multiple choice, commonly using Identify.
Command terms
Identify
What earns marks
Build the answer around this relationship: Exercise increases heat production in muscles.
Representative question
Question 1
[Maximum number: 1]
What is a reason for the changes in blood flow during exercise?
A
Increased blood flow to the kidneys removes waste products from exercise.
B
Blood flow to the brain decreases so that blood is diverted to the kidneys.
C
Increased blood flow to the skin removes heat.
D
Increased blood flow to the digestive system provides more glucose to muscles.
C
Retrieve the HL Homeostasis Route
HL onlyHL D3.3 adds kidney and circulation mechanisms. The route is still feedback logic: nephrons filter and reabsorb, the loop of Henle builds a gradient, ADH changes collecting duct permeability, and blood vessels redistribute flow according to activity.
- excretion removes urea; osmoregulation adjusts water and ions
- glomerulus filters and proximal tubule selectively reabsorbs
- medulla gradient and aquaporins conserve water
- vasodilation and vasoconstriction redirect flow by activity
HL Kidney and Blood-Flow Control
HL onlyHL homeostasis transfer is about structure-function precision. In kidney answers, say where filtration, reabsorption, salt pumping, water movement, ADH, and aquaporins happen. In blood-flow answers, say which vessels dilate or constrict and how activity changes tissue demand.
- Distinguish excretion from osmoregulation and locate filtration/reabsorption in the nephron.
- Explain the loop of Henle and ADH using permeability, salt movement, aquaporins, and concentrated urine.
- Explain blood redistribution using vasoconstriction, vasodilation, exercise, epinephrine, and tissue demand.
Objective notes
11 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 concentration2% of analysed papers 2 papers · 2 questionsViewD3.3.2Negative feedback loops• Negative feedback detects deviation from a set point and reverses it• Receptors, coordinators, effectors, and feedback loops restore normal conditions0% of analysed papers ViewD3.3.3Blood glucose regulation• Beta cells secrete insulin when blood glucose rises• Alpha cells secrete glucagon when blood glucose falls, affecting liver and muscle stores7% of analysed papers 8 papers · 10 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 factors9% of analysed papers 10 papers · 14 questionsViewD3.3.5Thermoregulation• Thermoregulation uses negative feedback to maintain core temperature near 37 °C• Thermoreceptors signal the hypothalamus, which coordinates skin, muscles, liver, and hormones4% 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 fat4% of analysed papers 5 papers · 5 questionsViewD3.3.7(HL)—Kidney role• Kidneys regulate blood composition by excretion and osmoregulation• Nephrons remove urea and adjust water and ion concentrations in urine11% of analysed papers 12 papers · 12 questionsViewD3.3.8(HL)—Glomerulus, Bowman's capsule, PCT• Glomerular blood pressure drives ultrafiltration into Bowman's capsule• The proximal convoluted tubule selectively reabsorbs glucose, amino acids, ions, and water8% of analysed papers 9 papers · 11 questionsViewD3.3.9(HL)—Loop of Henle• Ascending limb pumps Na⁺/Cl⁻ and is impermeable to water• Descending limb loses water, creating a medulla gradient for water conservation5% of analysed papers 6 papers · 6 questionsViewD3.3.10(HL)—Osmoregulation by collecting ducts• Hypothalamus osmoreceptors control posterior pituitary ADH release• ADH inserts aquaporins in collecting ducts, increasing water reabsorption and concentrated urine16% of analysed papers 18 papers · 18 questionsViewD3.3.11(HL)—Blood supply changes• Vasoconstriction and vasodilation redistribute blood according to activity• Exercise and epinephrine increase skeletal muscle flow and reduce gut or renal flow1% of analysed papers 1 paper · 1 questionView