Homeostasis as the regulation of internal conditions, the conditions controlled (blood glucose, temperature and water levels), and the general structure of a control system: receptors, coordination centres and effectors with negative feedback.

What this dot point is asking

AQA wants you to define homeostasis, state the internal conditions it controls, and describe the general structure of an automatic control system: receptors, coordination centres and effectors, working together by negative feedback.

What is homeostasis?

The internal conditions controlled in the human body include:

• Blood glucose concentration, so that cells always have fuel for respiration.
• Body temperature, so that enzymes work at their optimum and do not denature.
• Water levels (and the related balance of dissolved ions), so that cells do not gain or lose too much water by osmosis.

These need to be steady because enzymes have an optimum temperature and pH, and even small changes can slow reactions down or, in the case of temperature above about 40 degrees Celsius, denature enzymes so they stop working. Water and ion levels matter because cells gain or lose water by osmosis: if the blood became too concentrated, cells would lose water and shrink; if too dilute, they would swell and could burst. Blood glucose must stay steady because every cell needs a constant supply for respiration, while too high a level damages blood vessels.

Homeostasis can be controlled by the nervous system, the hormonal system, or both. The nervous system gives fast, short-term control (for example shivering when cold), while the hormonal system gives slower, longer-term control (for example insulin and glucagon controlling blood glucose). Whichever system is used, the same three-part structure of receptor, coordination centre and effector applies, which is why AQA treats this as a single general principle that you then apply to specific examples such as temperature, glucose and water control.

The structure of a control system

All automatic control systems, whether they use nerves or hormones, share three parts that work in sequence:

These systems work by negative feedback: when a level rises too high or falls too low, the system acts to bring it back towards normal, and then switches off once normal is restored. This is why your body does not overshoot, for example you stop sweating once you have cooled down.

Try this

Q1. Define homeostasis. [2 marks]

• Cue. The regulation of internal conditions to maintain a stable optimum for cell function.

Q2. Name the three parts of a control system. [3 marks]

• Cue. Receptors, coordination centres and effectors.