8.2.2 Control of Homeostatic Mechanism

Homeostasis is maintained by negative feedback loops within the organism. In contrast, positive feedback loops push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine systems in mammals.

Definition

Control of Homeostatic Mechanism: Homeostasis is maintained by negative feedback loops within the organism. In contrast, positive feedback loops push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine systems in mammals.

Main Idea

• Homeostasis is maintained by negative feedback loops.
• Positive feedback loops push the organism further out of homeostasis.
• Positive feedback may be necessary for life to occur.
• Homeostasis is controlled by the nervous and endocrine systems in mammals.
• A feedback mechanism is a physiological regulation system that works to return the body to its normal internal state.
• In a living system, the feedback mechanism takes the shape of a loop.
• The loop aids in maintaining homeostasis.

Core Concept

• Negative feedback = maintains homeostasis
• Positive feedback = pushes further out of homeostasis
• Control systems = nervous and endocrine systems
• Feedback mechanism = physiological regulation system
• Feedback loop = aids in maintaining homeostasis
• Negative feedback = change in one direction causes change in opposite direction
• Negative feedback = predominant mechanism used in homeostasis

Detailed Explanation

Control of Homeostatic Mechanism

• Homeostasis is maintained by negative feedback loops within the organism.
• In contrast, positive feedback loops push the organism further out of homeostasis, but may be necessary for life to occur.
• Homeostasis is controlled by the nervous and endocrine systems in mammals.

Feedback

• A feedback mechanism is a physiological regulation system in a living body that works to return the body to its normal internal state, or commonly known as homeostasis.
• In a living system, the feedback mechanism takes the shape of a loop, which aids in maintaining homeostasis.

Negative Feedback Mechanism

• In negative feedback loops a change in a given direction causes change in the opposite direction.
• For example, an increase in the concentration of a substance causes feedback that ultimately causes the concentration of the substance to decrease.
• Any homeostatic process that changes the direction of the stimulus is a negative feedback loop.
• It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it.
• In other words, if a level is too high, the body does something to bring it down; conversely, if a level is too low, the body does something to raise it; hence, the term: negative feedback.

Example: Maintenance of Blood Glucose Levels

• When an animal has eaten, blood glucose levels rises, which is sensed by the nervous system.
• Specialized cells in the pancreas (part of the endocrine system) sense the increase, releasing the hormone insulin.
• Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system.
• However, if an animal has not eaten and blood glucose levels decrease, this is sensed in a different group of cells in the pancreas: the hormone glucagon is released, causing glucose levels to increase.
• This is still a negative feedback loop, but not in the direction expected by the use of the term "negative."

Example: Control of Blood Calcium

• Another example of an increase as a result of a feedback loop is the control of blood calcium.
• If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys.
• The effects of PTH are to raise blood levels of calcium.
• Negative feedback loops are the predominant mechanism used in homeostasis.

Summary Table of Feedback Mechanisms

Intermediate Level

Control of Homeostatic Mechanism

Negative Feedback Mechanism

Positive Feedback Mechanism

Examples of Negative Feedback

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Homeostasis is maintained by negative feedback loops.
• 2. Positive feedback loops push further out of homeostasis.
• 3. Positive feedback may be necessary for life.
• 4. Homeostasis is controlled by nervous and endocrine systems.
• 5. Feedback mechanism is a physiological regulation system.
• 6. Feedback mechanism takes the shape of a loop.
• 7. Negative feedback reverses change.
• 8. Negative feedback maintains homeostasis.
• 9. Negative feedback is the predominant mechanism.
• 10. Positive feedback amplifies change.
• 11. Positive feedback is rare.
• 12. Blood glucose control is an example of negative feedback.
• 13. Insulin lowers blood glucose.
• 14. Glucagon raises blood glucose.
• 15. Blood calcium control is another example of negative feedback.
• 16. PTH raises blood calcium.
• 17. Negative feedback loops are predominant.
• 18. Positive feedback may be necessary for life.
• 19. Feedback loops aid in maintaining homeostasis.
• 20. Control systems are nervous and endocrine systems.

Remember

• - Negative feedback = maintains homeostasis
• - Positive feedback = pushes further out of homeostasis
• - Control systems = nervous and endocrine systems
• - Feedback mechanism = physiological regulation system
• - Feedback loop = aids in maintaining homeostasis
• - Negative feedback = change in one direction causes change in opposite direction
• - Negative feedback = predominant mechanism used in homeostasis

Common Mistakes

• 1. Confusing negative and positive feedback — Negative = reverses; positive = amplifies.
• 2. Thinking positive feedback is common — Positive feedback is rare.
• 3. Mixing up the examples — Blood glucose = negative; childbirth = positive.
• 4. Forgetting the role of insulin — Lowers blood glucose.
• 5. Forgetting the role of glucagon — Raises blood glucose.
• 6. Mixing up the control systems — Nervous and endocrine systems.
• 7. Believing negative feedback amplifies — Negative feedback reverses.
• 8. Thinking positive feedback is not important — Positive feedback may be necessary for life.
• 9. Mixing up PTH and calcium — PTH raises blood calcium.
• 10. Forgetting the role of the parathyroid gland — Releases PTH.

Quick Revision

Homeostasis is maintained by negative feedback loops within the organism. In contrast, positive feedback loops push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine systems in mammals. A feedback mechanism is a physiological regulation system in a living body that works to return the body to its normal internal state, or commonly known as homeostasis. In a living system, the feedback mechanism takes the shape of a loop, which aids in maintaining homeostasis. In negative feedback loops a change in a given direction causes change in the opposite direction. For example, an increase in the concentration of a substance causes feedback that ultimately causes the concentration of the substance to decrease. Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down; conversely, if a level is too low, the body does something to raise it; hence, the term: negative feedback. An example of negative feedback is the maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rises, which is sensed by the nervous system. Specialized cells in the pancreas (part of the endocrine system) sense the increase, releasing the hormone insulin. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system. However, if an animal has not eaten and blood glucose levels decrease, this is sensed in a different group of cells in the pancreas: the hormone glucagon is released, causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term "negative." Another example of an increase as a result of a feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys. The effects of PTH are to raise blood levels of calcium. Negative feedback loops are the predominant mechanism used in homeostasis.

60-Second Revision

• Negative feedback = maintains homeostasis
• Positive feedback = pushes further out of homeostasis
• Control systems = nervous and endocrine systems
• Feedback mechanism = physiological regulation system
• Feedback loop = aids in maintaining homeostasis
• Negative feedback = change in one direction causes change in opposite direction
• Negative feedback = predominant mechanism used in homeostasis
• Warning: Negative feedback reverses change; positive feedback amplifies change

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• 8. Thermoregulation and Homeostasis

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• 8.3 OSMOREGULATION

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