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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

Learning objectives

D3.3.1Homeostasis definition• Homeostasis maintains a stable internal environment within narrow limits• Variables include body temperature, blood pH, glucose, gases, ions, and osmotic concentrationD3.3.2Negative feedback loops• Negative feedback detects deviation from a set point and reverses it• Receptors, coordinators, effectors, and feedback loops restore normal conditionsD3.3.3Blood glucose regulation• Beta cells secrete insulin when blood glucose rises• Alpha cells secrete glucagon when blood glucose falls, affecting liver and muscle storesD3.3.4Diabetes• 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 factorsD3.3.5Thermoregulation• Thermoregulation uses negative feedback to maintain core temperature near 37 °C• Thermoreceptors signal the hypothalamus, which coordinates skin, muscles, liver, and hormonesD3.3.6Thermoregulation mechanisms• Cooling uses vasodilation, sweating, and hairs lying flat• Warming uses vasoconstriction, reduced sweating, shivering, metabolic heat, and brown fat

Homeostasis Stabilizes the Fluid Environment Around Cells

Homeostasis maintains selected variables of the internal environment within narrow limits despite internal and external change. The internal environment is blood plasma and tissue fluid—not the world outside the body.

Regulated variable Why large deviation matters
core temperature enzyme and membrane processes change rate
blood pH protein shape and reaction rates change
blood glucose cells lose a reliable respiratory substrate; osmotic effects rise when excessive
O₂ and CO₂ respiration and acid–base balance are disrupted
ions and osmotic concentration water movement, membrane potentials and cell volume change

Stable does not mean perfectly constant. A healthy value fluctuates within an acceptable range, and different variables can have different set ranges.

A Control Loop Links Deviation to a Corrective Response

change in a regulated variable → receptor detects the value → coordinator compares it with the set range → signal reaches an effector → effector changes the variable

Component Question it answers
stimulus / deviation what moved away from the range?
receptor what measured the variable?
coordinator where was information integrated?
effector what tissue carried out the response?
response how did the variable change?

Signals may travel as nerve impulses, hormones, or both. A valid explanation names the regulated variable and the direction of correction, not only the organs involved.

Negative Feedback Opposes Both Upward and Downward Deviations

Deviation Corrective response When correction succeeds
value above the range response lowers the value response weakens or switches off
value below the range response raises the value response weakens or switches off

The feedback is negative because the response reduces the original deviation. The output feeds back to decrease the stimulus that activated the loop.

Positive feedback amplifies a change and is useful for endpoint processes such as childbirth. By itself it does not restore a homeostatic set range.

Blood Glucose Must Supply Cells Without Creating Osmotic Stress

State Immediate problem Serious consequence if prolonged
hypoglycaemia too little glucose available, especially to the brain fainting, convulsions or coma
hyperglycaemia plasma water potential falls and water is drawn from tissues dehydration and circulatory damage

Blood glucose rises after carbohydrate absorption. It falls during fasting and prolonged activity as cells remove glucose for respiration.

Glycogen stores buffer these changes. Liver glycogen can support blood glucose; muscle glycogen is primarily a local fuel reserve for muscle activity.

When Glucose Rises, Beta Cells Use Insulin to Promote Removal and Storage

  1. A rise in blood glucose is detected directly by pancreatic beta cells in the islets of Langerhans.
  2. Beta cells secrete insulin into the blood.
  3. Insulin increases glucose uptake by target cells and promotes respiration.
  4. Liver and skeletal muscle convert glucose to glycogen; other excess can be converted to fat.
  5. As blood glucose falls toward the range, insulin secretion decreases.

Insulin is a hormone signal. It does not itself convert glucose into glycogen; it changes target-cell activity so enzymes carry out the conversions.

Two feedback pathways show pancreatic beta cells releasing insulin after blood glucose rises and alpha cells releasing glucagon after blood glucose falls.

When Glucose Falls, Alpha Cells Use Glucagon to Mobilize Liver Stores

  1. A fall in blood glucose is detected directly by pancreatic alpha cells.
  2. Alpha cells secrete glucagon into the blood.
  3. In the liver, glucagon promotes glycogen breakdown and glucose production from non-carbohydrate substrates.
  4. Glucose is released to the blood.
  5. As blood glucose rises toward the range, glucagon secretion decreases.
Feature Insulin branch Glucagon branch
source beta cells alpha cells
trigger high blood glucose low blood glucose
main effect uptake and storage liver glucose release
net result lowers blood glucose raises blood glucose

Glucagon is a hormone; glycogen is a glucose-storage polymer. Glucagon activates cellular pathways—it is not the enzyme that digests glycogen.

Type 1 and Type 2 Diabetes Disrupt Different Links in Glucose Control

Feature Type 1 diabetes Type 2 diabetes
primary failure autoimmune destruction of beta cells reduced target-cell response to insulin; beta-cell function may later decline
insulin signal little or none produced present but less effective
common management principle replace insulin and monitor glucose improve response and glucose balance through individualized diet, activity and medication
association genetic susceptibility and environmental triggers age, family history, inactivity and excess body fat are risk factors

A risk factor changes probability; it is not a single sufficient cause. Type 2 diabetes should not be reduced to “eating sugar,” and type 1 is not caused by lifestyle.

Both can produce persistent hyperglycaemia, glucose in urine and long-term damage to blood vessels, kidneys, retina and nerves if control remains poor.

A Glucose Tolerance Curve Tests How Quickly the Control System Recovers

  1. Measure fasting blood glucose.
  2. Give a standard oral glucose dose.
  3. Measure blood glucose at defined times after ingestion.
  4. Compare peak height and recovery toward the fasting range.
Curve feature Effective regulation Impaired regulation
starting value within expected fasting range may already be elevated
post-dose peak rises then turns downward often higher or prolonged
later value approaches the starting range remains elevated longer

The curve shows glucose handling under a standardized challenge. Diagnosis uses clinical thresholds and context; one graph alone does not identify the exact cellular cause.

The Hypothalamus Coordinates Opposing Temperature Responses

skin thermoreceptors monitor environmental influence + hypothalamic thermoreceptors monitor blood temperature → hypothalamus integrates the signals → skin, skeletal muscle, liver and endocrine pathways alter heat loss or production

Control feature Core temperature rises Core temperature falls
corrective direction increase heat loss and reduce heat gain reduce heat loss and increase heat production
shut-off condition response fades as temperature returns toward the range response fades as temperature returns toward the range

Human core temperature is regulated near 37 °C but is not fixed at exactly 37.0 °C; normal values vary with time and physiological state.

Skin and brain thermoreceptors signal the hypothalamus, which activates cooling above the normal range and warming below it.

A Hot Body Increases Heat Transfer and Evaporative Cooling

Effector response How it cools
skin arterioles dilate more warm blood reaches surface capillaries, increasing heat transfer to surroundings
sweat secretion increases evaporation transfers latent heat from the skin
hair erector muscles relax hairs lie flat, reducing the insulating air layer
behaviour changes shade, reduced activity or exposed skin decreases heat gain and increases loss

Sweating cools only when sweat evaporates. Sweat that drips away without evaporating removes far less heat.

A hot-skin cross-section shows active sweat glands, evaporation, dilated skin blood vessels and relaxed hair muscles.

A Cold Body Conserves Heat and Raises Metabolic Heat Production

Effector response How it warms or conserves heat
skin arterioles constrict less warm blood reaches surface capillaries, reducing heat loss
sweating decreases less evaporative heat is lost
skeletal muscles shiver rapid contractions increase respiration and heat production
liver metabolic activity rises greater respiration releases more heat
brown adipose tissue uncouples respiration fuel energy is released as heat rather than captured mainly in ATP

Brown fat is especially important in newborns, whose large surface-area-to-volume ratio causes rapid heat loss.

Raised hairs have little insulating effect in humans because human body hair is sparse; the mechanism is more effective in furred mammals.

Homeostasis Uses the Same Logic with Different Sensors and Effectors

regulated variable leaves its acceptable range → receptor detects the direction of deviation → coordinator selects an opposing response → effectors alter transfer, storage or metabolism → deviation shrinks → corrective signal declines

System Sensor / coordinator High-side response Low-side response
blood glucose pancreatic islet cells beta cells, insulin, uptake and storage alpha cells, glucagon, liver glucose release
core temperature thermoreceptors and hypothalamus vasodilation and sweating vasoconstriction, shivering and metabolic heat

For any homeostatic explanation, name the variable, direction of deviation, detector, signal, effector action and why that action reverses the deviation.

Homeostasis definition

6 marks

Explain how the pH of blood is kept constant during exercise.

Negative feedback loops

4 marks

Discuss the use of positive and negative feedback to control levels of variables.

Blood glucose regulation

8 marks

Explain the control of blood glucose concentrations in humans.

Diabetes exam focus

5 marks

Outline type II diabetes.

Thermoregulation exam focus

8 marks

Explain the control of body temperature in humans.

Thermoregulation mechanisms

1 mark

Outline one change that happens in the human body in response to a rise in body temperature above 36.4∘C36.4^{\circ} \mathrm{C}.