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D3.3.1—Homeostasis definition

Homeostasis maintains internal conditions within narrow limits, including blood pH, water balance, blood glucose and body temperature in human physiology.

Syllabus
First assessment 2025
Objective
D3.3.1
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2018
Most common paperPaper3
Typical marks1–6

Common command terms

  • Explain
  • Identify
  • Outline

Recent exam appearances

November 2018Paper3 ["HL"] · TZ021[ 6 ]D3.3.1—Homeostasis definition
May 2017Paper3 ["HL"] · TZ219(iv)[ 1 ]D3.3.1—Homeostasis definition
Practice this objective

Coverage 2017–2018 · Updated 16 Jul 2026

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 marks
How 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.

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.

Concept essentials

  • Homeostasis keeps internal variables within narrow limits.
  • Negative feedback restores variables toward a set point.
  • Blood pH, water balance, glucose and temperature are common homeostatic variables.