Feedback in Biological Systems
After a big meal, your blood glucose climbs. A couple of hours later, it is back to normal, and you never had to think about it. On a hot hike, you sweat without deciding to. What keeps these values from running out of control, and why does your body sometimes do the opposite and make a change grow bigger? By the end of this page, you will be able to answer both.
Negative feedback and homeostasis
Your body works best within narrow ranges: core temperature near 37 °C, blood glucose near 90 mg/dL between meals. Keeping conditions in a narrow range is called homeostasis.
Think of a home thermostat set to 68 °F. When the room gets colder, the furnace turns on. When the room warms back up, the furnace turns off. The system pushes the temperature back toward the target, so the change gets reduced.
Every feedback loop has the same parts:
| Part | Job | Body temperature | Thermostat |
|---|---|---|---|
| Sensor | Detects the change | Thermoreceptors | Temperature sensor |
| Control center | Compares to the set point | Hypothalamus | Thermostat circuit |
| Effector | Carries out the response | Sweat glands, skin blood vessels, muscles | Furnace |
When body temperature rises, the hypothalamus signals sweat glands to release sweat and skin blood vessels to widen. Both cool the body. When temperature falls, shivering and narrowed blood vessels warm it. The loop is always correcting in the opposite direction of the change.
Define negative feedback* and give one example from the human body. State in your answer whether the response reduces or increases the original change.
Define negative feedback and give one example from the human body. State in your answer whether the response reduces or increases the original change.
A home thermostat is set to 68 °F. When the room gets colder than the set point, it turns the furnace on, and when the room warms to the set point, it turns the furnace…
A home thermostat is set to 68 °F. When the room gets colder than the set point, it turns the furnace on, and when the room warms to the set point, it turns the furnace off. Match the parts of this system to the parts of human body temperature regulation (set point near 37 °C): the sensor, the control center, and the effector. Then state whether this is positive or negative feedback.
Blood glucose and hormone loops
Hormones are the signals in many feedback loops. Blood glucose is the classic case.
When glucose falls too low between meals, alpha cells release glucagon, which tells the liver to break down glycogen and release glucose. Two hormones with opposite effects give tight control in both directions.
Many hormone systems also use negative feedback by the product. Thyroid hormone (T4) inhibits the pituitary's release of TSH, and TSH stimulates the thyroid to make T4.
After a meal, a student's blood glucose rises from 90 mg/dL to 160 mg/dL. Over the next two hours it returns to about 90 mg/dL. Using the roles of insulin and the cells…
After a meal, a student's blood glucose rises from 90 mg/dL to 160 mg/dL. Over the next two hours it returns to about 90 mg/dL. Using the roles of insulin and the cells of the liver and muscle, explain how negative feedback produces this return. Then predict what would happen to blood glucose after a meal in a person whose cells no longer respond to insulin.
Thyroid hormone (T4) normally inhibits the release of TSH from the pituitary gland, and TSH stimulates the thyroid to release T4. Two patients have blood tests: |…
Thyroid hormone (T4) normally inhibits the release of TSH from the pituitary gland, and TSH stimulates the thyroid to release T4. Two patients have blood tests:
| Patient | T4 level | TSH level |
|---|---|---|
| A | Low | High |
| B | High | Low |
For patient A, suppose the thyroid gland itself is damaged. Use negative feedback to explain why TSH is high. Then suggest a possible explanation for patient B's results.
Positive feedback
Sometimes a system needs a change to grow, not shrink.
Childbirth. Stretching of the cervix triggers the release of oxytocin. Oxytocin strengthens uterine contractions, which stretch the cervix even more, which releases more oxytocin. Contractions get stronger and closer together, from about 30 minutes apart early in labor to about 3 minutes apart near delivery.
The loop ends when the baby is born and the cervix is no longer stretched. Other examples: ethylene from a ripening apple speeds up ripening of nearby fruit (so one ripe apple can hasten a whole bowl), and blood clotting, where clotting factors activate more clotting factors.
Why is this not used for everyday control? Because the change keeps growing. A positive loop needs an end event to stop it, or it would run away.
Classify each process as negative feedback or positive feedback: (a) insulin lowers blood glucose after a meal; (b) oxytocin strengthens uterine contractions during…
Classify each process as negative feedback or positive feedback:
(a) insulin lowers blood glucose after a meal;
(b) oxytocin strengthens uterine contractions during childbirth;
(c) ethylene released by a ripening apple speeds up the ripening of nearby fruit;
(d) sweating cools the body when core temperature rises.
During labor, the time between contractions drops from about 30 minutes early in labor to about 3 minutes near delivery. Stretching of the cervix triggers the release of…
During labor, the time between contractions drops from about 30 minutes early in labor to about 3 minutes near delivery. Stretching of the cervix triggers the release of oxytocin, which strengthens contractions, which stretch the cervix further. (a) Describe this loop and explain why it is positive feedback. (b) Explain why positive feedback does not maintain homeostasis. (c) State what event ends the loop.
Feedback in pathways and when loops fail
Feedback also works inside cells. In a metabolic pathway, the end product often inhibits an early enzyme. This is called feedback inhibition.
When a negative loop fails or flips. Loops can break at any part. If cells cannot respond to insulin, the effector fails and glucose stays high. A heat-stroke case is different: high body temperature damages cells, including the ones that regulate temperature. Less cooling means an even higher temperature, which causes more damage. The response now amplifies the change, so a negative loop has turned into a dangerous positive one.
In a metabolic pathway, enzyme 1 converts compound A into B, enzyme 2 converts B into C, and enzyme 3 converts C into the end product D. D binds to an allosteric site on…
In a metabolic pathway, enzyme 1 converts compound A into B, enzyme 2 converts B into C, and enzyme 3 converts C into the end product D. D binds to an allosteric site on enzyme 1 and reduces its activity. (a) Explain why this is negative feedback and what it accomplishes for the cell. (b) A mutation changes the allosteric site of enzyme 1 so D can no longer bind. Predict and justify what happens to the concentration of D over time and to the supply of compound A. (c) Sketch in words what a graph of D concentration over time would look like for the normal cell compared with the mutant.
A hiker's core body temperature and sweat rate are measured during a hike on a hot day in Arizona. | Time (min) | Core temperature (°C) | Sweat rate (L/h) |…
A hiker's core body temperature and sweat rate are measured during a hike on a hot day in Arizona.
| Time (min) | Core temperature (°C) | Sweat rate (L/h) |
|---|---|---|
| 0 | 37.0 | 0.1 |
| 10 | 37.8 | 0.5 |
| 20 | 38.2 | 0.9 |
| 30 | 37.9 | 0.9 |
| 40 | 37.2 | 0.3 |
(a) Identify the set point, the stimulus, and the effector response in this system. (b) Explain why the sweat rate lags behind the temperature change and why it falls after minute 30. (c) In heat stroke, high temperature can damage cells, which can further impair temperature regulation. Explain how this could turn a negative feedback loop into a dangerous positive feedback loop.
Back to the start: your blood glucose returns to normal because insulin triggers a negative feedback loop, and childbirth speeds up because oxytocin drives a positive one that ends with delivery.
Summary
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Homeostasis is keeping conditions within a narrow range around a set point.
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A loop has a stimulus, a sensor, a control center, and an effector.
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Negative feedback reduces the change and stabilizes the system (insulin and glucagon, body temperature, thyroid hormones, feedback inhibition of enzymes).
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Positive feedback amplifies the change and needs an end event (oxytocin in labor, ethylene in fruit ripening, blood clotting).
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To classify: does the response make the change smaller or larger?
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When a part of a loop is damaged, the system can no longer correct changes, and a negative loop can even turn into a runaway positive one.