Whether you are sprinting in the sun or asleep in the cold, your body keeps its inside conditions almost constant — temperature, water, blood glucose. This self-regulation is homeostasis, and it works by negative feedback: when a level moves away from normal, the body triggers a change that brings it back.
Why this matters Homeostasis keeps enzymes working at their optimum and cells alive. It explains diabetes, sweating, shivering and even why plants bend towards light — all popular CAIE topics that reward clear cause-and-effect answers.
Negative feedback
- A receptor detects the change.
- A coordinator (brain or hormone system) processes it.
- An effector (muscle or gland) brings about the response.
- The correction reverses the original change — that is why it is called negative feedback.
The eye
The eye is a sense organ full of light receptors. Key parts:
| Part | Job |
|---|---|
| Cornea | Refracts (bends) light — does most of the focusing |
| Iris | Controls how much light enters by changing pupil size |
| Lens | Fine-focuses light onto the retina |
| Retina | Light receptors that send impulses to the brain |
- Pupil reflex — in bright light, circular iris muscles contract, radial muscles relax, and the pupil gets smaller to protect the retina. In dim light the opposite happens and the pupil widens.
- Accommodation — focusing on near objects: the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes fatter (more curved). For distant objects the muscles relax and the lens becomes thinner.
Blood glucose, temperature and plants
- Blood glucose — controlled by the pancreas. High glucose → insulin makes the liver store glucose as glycogen, lowering it. Low glucose → glucagon makes the liver release glucose, raising it. In Type 1 diabetes the pancreas makes too little insulin, so glucose stays dangerously high; it is treated with insulin injections.
- Temperature — controlled by the skin and brain. Too hot: sweating (evaporation cools) and vasodilation (blood vessels widen, more heat lost). Too cold: shivering (respiration releases heat) and vasoconstriction (vessels narrow, less heat lost).
- Plant tropisms — growth responses controlled by the hormone auxin. In a shoot bending to light (phototropism), auxin gathers on the shaded side, making those cells grow longer, so the shoot bends towards the light.
Worked example. “Explain how the body responds when blood glucose rises after a meal (3 marks).” (1) The pancreas detects the rise and releases insulin; (2) insulin travels in the blood to the liver (and muscles); (3) it makes them convert excess glucose into glycogen for storage, so blood glucose falls back to normal — negative feedback.
Common exam mistake Don't confuse vasodilation (blood vessels near the skin widen when hot) with vessels “moving” — they stay put, but more blood flows near the surface. And insulin lowers blood glucose; glucagon raises it — don't swap them.
Memory hook “InsulIN puts glucose IN store.” For the pupil: bright light → small pupil (like a cat's eye in sunshine). Auxin makes the shaded side grow, so the shoot leans into the light.