Homeostasis and the nervous system
Biology · GCSE · The School
Keeping the inside steady
HOMEOSTASIS is keeping internal conditions stable while the outside changes: body temperature near 37°C, blood glucose within a narrow band, water and ion balance. Every one of these works the same way — a receptor detects the change, a coordination centre processes it, and an effector acts to reverse it. That loop is NEGATIVE FEEDBACK.
The reflex arc
Touch something hot and your hand moves before you have decided to move it. Stimulus → receptor → sensory neurone → relay neurone in the spinal cord → motor neurone → effector muscle. The signal never waits for the brain, which is exactly the point: the pathway is short because the delay of conscious thought would be the difference between a scare and a burn.
Blood glucose
After a meal, glucose rises. The pancreas releases INSULIN, which makes the liver and muscles take glucose out of the blood and store it as glycogen. When glucose falls too low, the pancreas releases GLUCAGON, which converts glycogen back. In Type 1 diabetes the pancreas cannot make insulin, so it is injected; in Type 2 the body stops responding to it properly.
After a meal, blood glucose rises. The pancreas releases INSULIN; the liver and muscles take glucose in and store it as glycogen; the level falls back to normal. Hours later it drops too low. The pancreas releases GLUCAGON; the liver converts glycogen back to glucose; the level rises to normal. Two opposite corrections, each triggered by the deviation itself — that is negative feedback, and it is why the range is narrow.
Nervous and hormonal, compared properly
Both are control systems and questions frequently ask you to compare them. NERVOUS: electrical impulses along neurones, very fast, very short-lived, and targeted at a precise place. HORMONAL: chemicals in the blood, slower to act, longer lasting, and reaching everywhere the blood goes even though only cells with the right receptor respond. That last point explains a lot: it is why adrenaline affects heart, lungs and liver at once, and why a nerve impulse moves one muscle. A comparison answer should name speed, duration and specificity, not just say one is faster.
Negative feedback in words that transfer
Every negative feedback answer has the same four stages, and learning them as a frame means you can answer about temperature, glucose or water with the same structure. A receptor DETECTS the change from the set point; a coordination centre PROCESSES it; an effector — a muscle or gland — ACTS; and the action moves the level back towards the set point, which the receptor then detects. Name the actual receptor, centre and effector for the example in front of you. The word 'negative' means the response opposes the change, not that anything is harmful, and that is worth a mark on its own.