Outside, the temperature swings and meals come and go. Inside, the tissue fluid around your cells stays almost the same. Homeostasis is the set of control systems that makes that happen, and nearly all of them run on negative feedback.
Homeostasis is the maintenance of a constant internal environment: core temperature, blood glucose concentration and blood water potential are all kept within narrow limits around a set point. This matters because enzymes work best within a narrow range of temperature and pH, and cells shrink or swell if the water potential of the tissue fluid changes. A change away from the set point is a stimulus, which may be internal (more heat from respiration during exercise) or external (a cold day). Receptors detect it; a coordination system, either the nervous system (fast, short-lived, impulses along neurones) or the endocrine system (slower, longer-lasting, hormones in the blood), sends instructions; effectors, which are muscles or glands, bring about a response. In negative feedback the response reverses the original change, so the value returns towards the set point.
Fig. 1: the components of a negative feedback loop, using a rise in core temperature as the stimulus
Section 01
A control loop with five parts.
Press Too hot (core temperature jumps to 38.5 °C) and watch the light travel round the loop from stimulus to receptor, coordinator, effector and response. Then press Too cold (35.6 °C). In both cases the graph returns to 37 °C.
Why homeostasis matters: enzymes have an optimum temperature and pH, so metabolism runs efficiently only if these are steady; a stable water potential stops cells losing or gaining too much water by osmosis; a steady glucose supply keeps respiration going, especially in brain cells.
Stimulus: any change in a factor away from its set point. Internal stimuli come from inside the body (heat from respiring muscles, glucose absorbed after a meal); external stimuli come from the surroundings (air temperature, lack of drinking water).
Receptor: a cell or group of cells that detects the stimulus, such as thermoreceptors in the skin and hypothalamus, osmoreceptors in the hypothalamus, or the α and β cells of the pancreas.
Coordination system: processes the information and sends instructions to the effectors, by nerve impulses or by hormones.
Effector: a muscle or gland that carries out the response, such as sweat glands, arteriole muscles, liver cells or the collecting duct of the kidney.
Negative feedback: the response reduces the size of the stimulus, so the factor moves back towards the set point and the corrective action then switches off. The value fluctuates a little around the set point rather than staying perfectly constant.
Examiner trap
Negative feedback does not mean that something bad happens. It means the response is in the opposite direction to the change: a rise causes responses that lower the value, a fall causes responses that raise it.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the simulator you press Too hot. Which part of the loop is played by the hypothalamus?
Quick check+10 XP
After Too hot, the graph returns from 38.5 °C towards 37 °C. Which statement explains this?
Quick check+10 XP
Which effectors does the simulator list for the Too cold response?
Quick check+10 XP
Which of these is an internal stimulus that can raise core temperature?
Section 02
Nerves or hormones: two coordination systems.
Core temperature is controlled mainly through the nervous system; blood glucose and blood water potential are controlled through hormones. Compare the two systems.
Feature
Nervous system
Endocrine system
signal
electrical impulses along neurones, chemical transmission at synapses
hormones secreted by endocrine glands into the blood
speed of response
very fast (milliseconds)
slower (seconds to hours)
duration
short-lived
often longer-lasting
target
a specific muscle or gland reached by the neurone
any cell with receptors for that hormone
example in homeostasis
hypothalamus to sweat glands and skin arterioles
insulin, glucagon, ADH
Too hot: thermoreceptors in the skin and in the hypothalamus detect the rise; the hypothalamus sends impulses so that sweat glands secrete more sweat (evaporation removes heat) and arterioles in the skin dilate (vasodilation), so more blood flows near the surface and more heat is lost.
Too cold: skeletal muscles shiver (respiration releases heat), arterioles in the skin constrict (vasoconstriction) to reduce heat loss, and hair erector muscles contract to trap a layer of air. Over longer periods, more thyroxine raises the metabolic rate.
Core temperature means the temperature of the blood and internal organs, held near 37 °C in humans; the skin may be much cooler.
Examiner trap
Arterioles do not move up and down in the skin. Vasodilation and vasoconstriction change the diameter of arterioles, which changes how much blood flows through the surface capillaries.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which statement best explains why core temperature is kept close to 37 °C?
Exam style+20 XP
Which correctly compares control by the nervous system with control by the endocrine system?
Exam style+20 XP
On a cold day, which change in the skin reduces heat loss?
Exam style+20 XP
Which is a correct pairing of a homeostatic factor with a receptor that detects changes in it?
Skill unlocked
Principles of homeostasis, mastered.
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