Homeostasis: All control systems include:

The idea

This statement introduces the shared architecture of every automatic control system, whether nervous or chemical. Each system needs cells that detect a change, called receptors; a coordination centre that receives and processes the information, such as the brain, spinal cord or pancreas; and effectors that carry out the response, namely muscles or glands. Negative feedback then reverses the original change, returning the internal environment towards its set point. For example, a rise in blood glucose is detected by pancreatic cells, which release insulin; liver and muscle cells take up glucose, and the level falls. If the level drops too far, glucagon reverses the process. When describing any control system, identify each component and explain how the response counteracts the stimulus, rather than simply listing organs.

What earns the marks

• Receptors detect a stimulus, which is a change in the internal or external environment.
• A coordination centre, such as the brain, spinal cord or pancreas, receives and processes information from receptors.
• Effectors are muscles or glands that produce the response, either by contracting or by secreting a substance.
• Negative feedback reverses the initial change, helping to restore conditions towards a set point.
• The pathway can be summarised as stimulus → receptor → coordination centre → effector → response.
• Different coordination centres suit different systems: the brain for temperature and the pancreas for blood glucose.

Common mistakes and how to put them right

• Confusing receptors with effectors.

  Put it right by stating receptors detect change while effectors bring about the response.

• Omitting the coordination centre when describing a pathway.

  Put it right by naming the brain, spinal cord or pancreas as the processing link.

• Describing positive feedback as the usual mechanism.

  Put it right by explaining that negative feedback counteracts the change and restores the set point.

Exam tips

• Learn the generic sequence and apply it to each named example rather than memorising separate lists.
• State the role of each component in one clause, for example 'the pancreas detects a rise in blood glucose'.
• Use the phrase 'negative feedback' and explain what is reversed, not just that it happens.