How does the body keep its internal environment stable?
Explain the principle of homeostasis and how negative feedback maintains a stable internal environment
Homeostasis keeps internal conditions within a narrow range using negative feedback loops of receptor, control centre and effector that reverse changes from a set point.
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What this dot point is asking
You need to define homeostasis, describe the components of a negative feedback loop, and explain how the loop keeps a variable near its set point. This is the framework for every other Topic 3 dot point.
What homeostasis is
Homeostasis is the maintenance of a relatively constant internal environment despite changes in external conditions. Cells work best within narrow ranges of temperature, pH, water balance and solute concentration, so the body must keep these variables close to a target value called the set point.
Examples of regulated variables include body temperature, blood glucose concentration, and water and salt balance.
The negative feedback loop
Homeostasis is achieved mainly by negative feedback. The word "negative" means the response opposes (reverses) the original change.
A feedback loop has three key components:
- Receptor - detects a change (the stimulus) in the variable away from the set point.
- Control centre (often the brain or an endocrine gland) - receives information from the receptor, compares it with the set point, and coordinates a response.
- Effector - a muscle or gland that carries out the response that corrects the change.
The control centre communicates using the nervous system (fast, electrical) and the endocrine system (slower, hormonal).
How the loop maintains stability
When a variable moves above or below its set point, the loop acts to push it back:
- The variable changes (for example, temperature rises).
- Receptors detect the change.
- The control centre processes the information.
- Effectors respond to reverse the change (for example, sweating to cool down).
- As the variable returns to the set point, the stimulus reduces and the response is switched off.
This means the variable does not sit perfectly still; it fluctuates within a narrow range around the set point, constantly being nudged back.
Negative versus positive feedback
In negative feedback, the response reverses the change and restores stability. In positive feedback, the response amplifies the change and pushes the variable further from the starting point (for example, during childbirth contractions). Homeostasis relies overwhelmingly on negative feedback.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 SACE Stage 23 marksExplain the role of negative feedback in the control of human body temperature.Show worked answer →
Three steps show the loop reversing a change.
A change in body temperature away from the set point (about 37 degrees C) is detected by receptors (thermoreceptors), which signal the control centre, the hypothalamus.
The hypothalamus triggers effectors that produce a response in the opposite direction to the change (for example, sweating and vasodilation if too hot, shivering and vasoconstriction if too cold).
This response returns body temperature toward the set point. Because the response counteracts (reverses) the original change, it is negative feedback, which keeps temperature stable.
2019 SACE Stage 22 marksExplain why it is necessary for human beings to maintain a body temperature at about 37 degrees C.Show worked answer →
For 2 marks, focus on enzymes.
Metabolic reactions in the body are controlled by enzymes, which work best at their optimum temperature, around 37 degrees C.
If body temperature rises too high the enzymes denature (their active sites change shape) and stop working, and if it falls too low reactions become too slow, so maintaining about 37 degrees C keeps enzyme-controlled reactions working at an effective rate and the body functioning normally.