Homeostasis is dynamic stability
The internal environment is never perfectly motionless. Body temperature, blood glucose, water potential and carbon dioxide concentration fluctuate continuously. Homeostasis keeps those variables within limits compatible with normal cell function rather than freezing them at a single number.
A set point is the reference value used by a control system, while the normal range is the small interval in which the variable may move without causing serious disruption. Negative feedback is the main mechanism that reduces departures from that range.
The four jobs in every control loop
First identify the regulated variable. Then locate the deviation, the receptor that detects it, the coordinating centre that interprets information, and the effector that changes the variable. Many difficult questions simply exchange the jobs of these components.
A receptor does not necessarily correct the condition, and an effector does not decide what the target should be. The receptor supplies information; the control centre compares and coordinates; the effector performs the response.
A variable leaves its normal range.
A receptor senses the change.
A control centre selects a response.
An effector reduces the deviation.
Thermoregulation: one variable, opposite responses
When body temperature rises, thermoreceptors in the skin and hypothalamus detect the change. The hypothalamus coordinates vasodilation of skin arterioles and increased sweating. More warm blood approaches the surface and evaporation removes heat, so temperature falls toward its normal range.
When temperature falls, the coordinated response reverses: vasoconstriction reduces heat loss, skeletal muscles may shiver to release heat through respiration, and metabolic heat production can increase. The regulated variable is the same, but the effectors are chosen according to the direction of deviation.
| Condition | Main effectors | Net effect |
|---|---|---|
| Temperature too high | Sweat glands; skin arterioles dilate | Heat loss increases |
| Temperature too low | Skeletal muscles; skin arterioles constrict | Heat production rises and loss falls |
Blood glucose: hormones as coordinating signals
After a carbohydrate-rich meal, rising blood glucose stimulates pancreatic beta cells to release insulin. Insulin increases glucose uptake in responsive tissues and promotes glycogen formation, reducing blood glucose. When blood glucose falls, alpha cells release glucagon, promoting glycogen breakdown and glucose release by the liver.
Insulin and glucagon are often called antagonistic because their overall effects oppose each other. Do not reduce the system to 'insulin down, glucagon up' without identifying the condition that triggers each hormone and the tissues that produce the correction.
How examiners hide the concept
A question may describe the response without naming negative feedback. Look for a variable moving away from a reference and a response that reduces that movement. Another common distractor is a response that continues to amplify the original change; that is positive feedback, as in oxytocin-driven uterine contractions.
Finish by checking the final arrow. If the response weakens the stimulus that caused it, the loop is negative. If it strengthens that stimulus and drives the process toward a defined endpoint, the loop is positive.
The ideas to carry forward
- Homeostasis maintains variables within limits, not at perfect constancy.
- Receptors detect, control centres coordinate and effectors correct.
- The response must oppose the original deviation.
- Always identify the regulated variable before naming the components.
Answer first. Then reveal the marking logic.
01Why does sweating decrease once body temperature returns toward normal?2 marks · show the biological link
Answer: The temperature deviation becomes smaller, so thermoreceptor input and the coordinating signal to sweat glands are reduced.
02What separates positive feedback from negative feedback?2 marks · show the biological link
Answer: Negative feedback reduces the initiating change; positive feedback amplifies it until a particular endpoint is reached.
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