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.