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4.9.1Additional lessons

Homeostasis: Maintaining the Internal Environment — IB Biology HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 79 of 101

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Part 1 of 4What Homeostasis Is

4.9.1 Homeostasis: Maintaining the Internal Environment
Definition
  • Homeostasis is the maintenance of the internal environment of the body within narrow limits, even when the external conditions change.
Variables controlled
  • Body temperature, blood pH, blood glucose concentration, carbon dioxide level and the water content of the blood are all kept in a narrow range.
Why it matters
  • Enzymes and cell processes work best in a limited range of conditions, so large changes would slow reactions or damage cells.
Tissue fluid
  • Cells are bathed in tissue fluid, so keeping the blood stable keeps the surroundings of every cell stable.
VariableApproximate normal value
Core body temperatureAbout 37 °C
Blood pHAbout 7.35 to 7.45
Blood glucoseClose to 5 mmol dm⁻³ in a fasting adult

Part 2 of 4The Negative Feedback Loop

Each loop has the same components and the same logic.

1
A receptor detects a change in the variable, for example thermoreceptors detecting a fall in temperature.
2
The receptor sends a signal, along a nerve or by a hormone, to a control centre that holds the set point, such as the hypothalamus or pancreas.
3
The control centre compares the value with the set point and sends instructions to an effector.
4
The effector, a muscle or a gland, produces a response that opposes the original change and brings the variable back toward the set point.
5
As the variable returns to normal, the stimulus falls and the response is switched off, so the system does not overshoot.

Part 3 of 4Control Systems and Their Limits

Control Systems and Their Limits
Two control systems
  • The nervous system gives fast, short-lived responses through nerve impulses,
    • while the endocrine system gives slower, longer-lasting responses by hormones.
Fluctuation
  • Variables never stay exactly at the set point;
    • they oscillate slightly above and below it as the feedback corrects each change.
Positive feedback
  • Here the response amplifies the original change, as in childbirth, so it is not used for homeostatic control.
Failure
  • If feedback fails, as in diabetes, the variable can drift out of range and cause illness.

Part 4 of 4Exam Checklist & Tips

4.9.1 – Key Points for the Exam

  • Homeostasis The maintenance of a stable internal environment within tolerable limits.
  • Set point The normal value around which a variable is regulated.
  • Negative feedback A response that reverses a change and returns the variable to the set point.
  • Effector A muscle or gland that carries out the response.
  • Receptor A cell or structure that detects a stimulus.

Common Mistakes

  • Describing negative feedback as 'keeping a variable constant';
    • it keeps it within narrow limits that fluctuate around a set point.
  • Saying the effector detects the change; the receptor detects and the effector responds.
  • Confusing negative feedback with a negative or harmful effect.
  • Naming only the nervous system, when many homeostatic controls use hormones.

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