10.1 Principles of Homeostasis

A free GCSE Biology lesson for AQA (8461) from OBA School: why the internal environment is kept stable, control systems and negative feedback. 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 is important for cells and enzymes
• Name conditions controlled in the human body
• Describe the parts of an automatic control system: receptors, coordination centres and effectors
• Explain negative feedback using an example

Key words

• Homeostasis: The regulation of the internal conditions of a cell or organism to keep them at optimum levels in response to internal and external changes.
• Internal environment: The conditions inside the body, such as the blood and tissue fluid around the cells.
• Receptor: A cell (or group of cells) that detects a stimulus.
• Coordination centre: An area that receives and processes information from receptors, e.g. brain, spinal cord, pancreas.
• Effector: A muscle or gland that brings about a response.
• Negative feedback: A change away from normal triggers a response that brings the level back to normal.

Why homeostasis matters

Your cells work best in steady conditions. Enzymes, for example, have an optimum temperature and pH: too hot and they denature. Water levels must be right or cells gain or lose too much water by osmosis. Cells also need a steady supply of glucose for respiration.

Conditions outside the body change all the time (a cold day, a run, a big meal), but homeostasis keeps the inside steady. It maintains optimal conditions for enzyme action and all cell functions.

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

Automatic control systems

Homeostasis uses automatic control systems. They can involve nervous responses (fast, using electrical impulses) or chemical responses (slower, using hormones). All control systems have three parts:

Worked example: naming the parts

When blood glucose rises after a meal, cells in the pancreas detect the rise. The pancreas releases the hormone insulin, and the liver takes up glucose.
Stimulus: rise in blood glucose. Receptor and coordination centre: pancreas. Effector: liver.

Answer: The pancreas acts as both receptor and coordination centre

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

Negative feedback

Most homeostasis works by negative feedback. 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 the set point but stays close to it.

Worked example: reading the graph

The highest reading on the graph is about 37.2 °C and the lowest about 36.8 °C. What is the range?
37.2 − 36.8 = 0.4 °C. A small range shows the control system is working.

Answer: About 0.4 °C

Common mistakes

• Saying homeostasis keeps conditions “constant”. They vary slightly around a set point.
• Naming the brain as an effector. The brain is a coordination centre; effectors are muscles or glands.
• Mixing up stimulus and receptor. The stimulus is the change; the receptor detects it.
• Describing negative feedback as “making things worse”. It reverses the change.

Revision questions

Try these, then check your answers below each one.

Question 1. What is homeostasis? (Choose from: Responding to a stimulus with a reflex, The production of hormones by glands, Keeping the external environment constant, Regulating internal conditions to keep them at optimum levels.)

Answer: Regulating internal conditions to keep them at optimum levels

Working: Homeostasis regulates internal conditions, in response to internal and external changes, to keep them at optimum levels.

Question 2. Which of these is not controlled by homeostasis in humans? (Choose from: Water levels, Eye colour, Body temperature, Blood glucose concentration.)

Answer: Eye colour

Working: Temperature, blood glucose and water levels are all controlled. Eye colour is an inherited characteristic.

Question 3. Effectors are either glands or ____.

Answer: muscles

Working: Effectors are muscles (which contract) or glands (which secrete chemicals).

Related lessons

• Previous lesson: 9.4 Metabolism and the Liver
• Next lesson: 10.2 The Structure and Function of the Human Nervous System
• In this chapter: 10.3 Reflex Actions
• In this chapter: 10.4 The Brain
• In this chapter: 10.5 The Eye

Continue in the full course

This lesson is section 10.1 (AGCBH1001) of our free GCSE Biology 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 Biology course.