AQA GCSE Biology: Homeostasis

Homeostasis is how the body keeps its internal conditions steady, whatever is happening outside. This guide explains what homeostasis is, why it matters for enzymes and cells, the main conditions the body regulates, and the automatic control systems that do the regulating. The idea of a receptor, a coordination centre and an effector runs through the whole of this topic.

What homeostasis is

Homeostasis is the regulation of the internal conditions of a cell or organism, in response to internal and external changes, to keep the optimum conditions for function.

Both halves of "internal and external" earn their place. The weather changing is an external change; running up a flight of stairs is an internal one. Homeostasis has to answer both, and it is running constantly rather than only in emergencies.

It matters because enzymes, and cells in general, only work well within a narrow range of temperature and pH. Move away from the optimum and enzyme-controlled reactions slow down; move far enough and the enzymes denature, their active sites change shape, and the reaction stops rather than slows. Denaturing is not reversible, which is why the body defends its temperature so hard.

In humans, the main conditions kept in balance are blood glucose concentration, body temperature and water levels.

Automatic control systems

Internal conditions are kept steady by automatic control systems, which have three parts.

• Receptors (for example temperature receptors in the skin) detect a stimulus, a change in the internal or external environment.
• Coordination centres (for example the brain, spinal cord or pancreas) receive and process the information from the receptors.
• Effectors (muscles or glands) bring about a response that restores the optimum level.

Learn those three as an order rather than a list, because that is how they are examined. A question gives a situation and asks you to name the parts, and the marks follow the sequence: what detected the change, what processed it, what did something about it.

Run it through an example. You walk into a cold room: temperature receptors in the skin detect the fall, the thermoregulatory centre in the brain processes it, and the effectors, the blood vessels and the skeletal muscles, narrow and shiver to keep heat in. Every control system in this topic has that same shape underneath it.

Negative feedback

These systems may use nervous responses, chemical (hormonal) responses, or both. Either way they work by negative feedback: when a level rises too high or falls too low, the system acts to bring it back, and once it is back the corrective response stops.

That last clause is the part people leave out, and it is usually worth a mark. A system that did not switch off would overshoot: you would keep shivering until you were too hot.

It also explains the shape of the graphs this topic produces. A level under negative feedback is never a flat line. It wavers around the normal level, because a correction can only start once the change has already been detected. "Kept constant" is the wrong description. "Brought back" is the right one.

How this fits together

Homeostasis relies on two coordination systems: the fast, electrical nervous system and the slower, chemical endocrine system. The nervous system is the subject of the nervous system guide.