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Module 2 The Endocrine System

Lesson 1.2.1

1.2.1 page 5

Fluctuation Around the Set Point

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Do you recall playing on the seesaw on your school playground? Remember how hard it was for you and your friend to balance the seesaw evenly? Usually it was a case of you being up and your friend being down around that balance point, or vice versa. Like the seesaw, negative feedback loops in living organisms do not usually succeed in keeping a particular feature constant. There is usually some fluctuation about the set point. This happens because it takes time for information to be passed from the sensor to the effector, and for the actions of the effector to have their effect. The longer this time delay, the greater the fluctuation will be.  Fluctuation around the set point is shown in the illustration to the right.

 

 

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In humans the sensors/receptors are specialized cells. Some of these cells are in the brain, but cells in other organs, such as the pancreas, also act as detectors for particular substances. Many different organs act as effectors. For example, the skin is an effector in temperature regulation, while the kidneys are effectors in the regulation of water content. Information passes from sensors to effectors either along nerves, as in the fight or flight regulation of epinephrine from the adrenal medulla, or via hormones in the blood, as in regulation of water content for example. The overall response is coordinated by some kind of control centre or boss. In the endocrine system the control centre is the hypothalamus/pituitary complex.

homeostasis: the maintenance of a constant internal environment, or one within narrow limits, despite possible fluctuations in the external environment

 

negative feedback: a mechanism where deviation from the optimum state causes a return to the optimum state; acts to eliminate any deviation from optimal conditions and leads to stability

 

positive feedback: a mechanism where deviation from the optimum state causes further deviation from the optimum state; usually leads to instability and is tolerated by the body only for a short time

set point (set value): ideal or optimum conditions

 

 

Try This

TR 3.

To ensure that you understand how negative feedback loops contribute to homeostasis and how positive feedback affects this balance, answer the following questions. Where appropriate, remember to phrase your answers in complete sentences. Check and then file your work in your course folder.

  1. Define homeostasis and explain how the endocrine system helps to maintain homeostasis in the body.


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Use the following information describing how the temperature in your house is maintained to answer the next question.

  • the thermostat is set at 20°C

  • room temperature cools below the set value of 20°C

  • the thermostat detects the lowered temperature

  • an electronic signal is sent to the furnace

  • the furnace is turned on and the temperature in the room increases

  1. Draw and describe a negative feedback mechanism for a thermostat connected to the furnace in your house. How is this feedback mechanism similar to how some hormones are regulated in the body?

Use the following information to answer the next question.

  • set point for human body temperature is 37°C
  • thermoreceptors in skin sense temperature
  • brain receives information
  • effectors for regulating body temperature include:
    • sweat glands which when activated produce sweat for cooling the body
    • skin blood vessels which constrict to direct blood away from the skin, thus warming the body or they dilate to direct blood to the skin, thus cooling the body
    • shivering—a rapid contraction of muscles—warming the body,
    • behaviour such as putting on a hoodie to warm the body or putting on tank tops and shorts to cool the body
  1. Draw a positive feedback loop that results in a fever.
Check your work.
Self-Check Answers

TR 3.

  1. Homeostasis is the maintenance of relative constancy of the internal environment around an optimal value called the set point. The endocrine system is self-regulating and helps to regulate other body systems through negative feedback mechanisms.
  2. A negative feedback loop for a mechanical system such as your heating system might look like the following:



    This mechanism of negative feedback is similar to how hormones regulate the internal environment.

  3. A positive feedback loop for body temperature resulting in a fever might look like the following: