Lesson 1 — The Endocrine System


Self-Check

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, phrase your answers in complete sentences.


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


  2. Use the following information describing how the temperature in your house is maintained to answer this 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.


    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?


  3. Use the following information to answer this question.

    • Set point for human body temperature is 37°C.

    • Thermoreceptors in skin sense temperature.

    • Brain receives information.

    • Effectors for regulating body temperature include the following:

      • Sweat glands, when activated, produce sweat for cooling the body.

      • Skin blood vessels 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 is a rapid contraction of muscles, warming the body.

      • Behaviours regulate body temperatures, such as putting on a hoodie warms the body or putting on a tank top and shorts might cool the body.


    Draw a positive feedback loop that results in a fever.

Self-Check Answers

  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 home's heating system might look like the following: 


     

    This mechanism of negative feedback is similar to that involving hormones regulating the internal environment. 

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


Fluctuation around the Set Point

Do you recall playing on the seesaw in your school playground? Remember how difficult 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 over that balance point, or vice versa. Similar to the seesaw, negative feedback loops in living organisms usually do not succeed to keep a particular feature constant.

Usually, some fluctuation occurs about the set point. This happens because time is needed 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. Fluctuation around the set point is shown in the illustration to the right.


 
© Getty Images

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 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 by way of hormones in the blood, as in regulation of water content. The response is coordinated by some kind of control centre or boss. In the endocrine system, the control centre is the hypothalamus/pituitary complex.


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