B10.1 Principles of Homeostasis

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: why internal conditions are kept stable, automatic control systems, and nervous versus hormonal responses. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Define homeostasis and explain why it matters for enzymes and cells
• Name conditions controlled in the human body
• Describe the parts of a control system: receptors, coordination centres and effectors
• Compare nervous and hormonal (chemical) responses
• Explain how control systems bring a level back to normal

Key words

• Homeostasis: The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.
• Stimulus: A change in the environment, inside or outside the body.
• Receptor: A cell or group of cells that detects a stimulus.
• Coordination centre: Receives and processes information from receptors, e.g. brain, spinal cord, pancreas.
• Effector: A muscle or gland that brings about a response.
• Hormone: A chemical released by a gland into the blood, which carries it to a target organ.

Why homeostasis matters

Cells work best in steady conditions. Enzymes have an optimum temperature and pH: too hot and they denature, too cold and reactions are too slow. Cells gain or lose too much water by osmosis if water levels are wrong, and they need a steady supply of glucose for respiration. Homeostasis maintains optimum conditions for enzyme action and all cell functions.

• Blood glucose concentration; Why it matters Steady supply for respiration; far too high or too low can cause coma
• Body temperature (about 37 °C); Why it matters Enzymes work best; too hot and they denature
• Water levels; Why it matters Stops cells swelling or shrinking by osmosis

These responses are automatic: you are not aware of them. They can raise a level that is too low or lower a level that is too high.

Automatic control systems

Control systems may use nervous responses or chemical (hormonal) responses. All of them include receptors, coordination centres and effectors.

Worked example: naming the parts

During a race a runner’s body temperature rises. Temperature receptors detect the rise and send impulses to the brain. The brain sends impulses to the sweat glands, which release more sweat.
Stimulus: rise in temperature. Receptor: temperature receptors. Coordination centre: brain. Effector: sweat glands. Response: more sweat.

Answer: Sweat glands are effectors because glands release substances

Exam tip: effectors are always muscles or glands. Muscles respond by contracting; glands respond by releasing (secreting) substances such as hormones.

Nervous or hormonal?

• Signal; Nervous response Electrical impulses along neurones; Hormonal response Hormones (chemicals) in the blood
• Speed; Nervous response Very fast; Hormonal response Slower
• How long; Nervous response Short-lived; Hormonal response Lasts longer
• Where it acts; Nervous response A precise place; Hormonal response Target organs, often more widely

Bringing levels back to normal

If a level rises above normal, the response lowers it; if it falls, the response raises it. The level swings a little either side of normal but stays close to it. (At Higher tier this is called negative feedback; you will meet it again with hormones in B11.)

Worked example: reading the graph

The lowest value is 88 and the highest is about 132 mg per 100 cm³. Calculate the percentage rise after breakfast.
(132 − 88) ÷ 88 × 100 = 50%.

Answer: 50% rise, then the level returns towards 88

Common mistakes

• Saying homeostasis keeps levels exactly constant. They vary slightly.
• Calling the brain an effector. It is a coordination centre.
• Mixing up stimulus and receptor. The stimulus is the change.
• Saying hormones travel along nerves. They travel in the blood.

Revision questions

Try these, then check your answers below each one.

Question 1. Which is the best definition of homeostasis? (Choose from: Keeping the body at exactly the same conditions all the time, The regulation of the internal conditions of a cell or organism to keep optimum conditions, in response to internal and external changes, The way the body responds to changes in the outside environment only, Producing hormones to make the body grow.)

Answer: The regulation of the internal conditions of a cell or organism to keep optimum conditions, in response to internal and external changes

Working: Homeostasis regulates internal conditions to maintain optimum conditions for enzymes and cells, in response to changes inside and outside the body.

Question 2. The diagram shows the parts of a control system. What belongs in box 3? (Choose from: Effector, Coordination centre, Receptor, Hormone.)

Answer: Coordination centre

Working: Stimulus → receptor → coordination centre → effector → response. Examples of coordination centres are the brain, spinal cord and pancreas.

Question 3. True or false? Effectors are muscles or glands.

Answer: True

Working: Muscles respond by contracting and glands respond by releasing (secreting) chemicals such as hormones.

Related lessons

• Previous lesson: B9.4 Metabolism and the Liver
• Next lesson: B10.2 The Structure and Function of the Human Nervous System
• In this chapter: B10.3 Reflex Actions
• In this chapter: B10 RP Required Practical: Reaction Time

Continue in the full course

This lesson is section B10.1 (AGCSC1001) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.