1. Module 2

1.7. Page 5

Lesson 1

Module 2—The Endocrine System

 

Fluctuation Around the Set Point

 

This photograph shows three young children, a girl and two boys, playing on a seesaw. It is very difficult to balance a seesaw. Usually one person is up and the other is down around the balance point.

© GOH SIOK HIAN/shutterstock

 

Do you recall playing on the seesaw at the playground? Do you remember how hard it was for you and your friend at the other end to balance the seesaw evenly? Usually, one of you would be above and the other would be below the balance point.

 

This graph shows how corrective actions allow internal conditions to fluctuate around the set value. In negative feedback, when there is too much or too little of something, the body is adjusted in the opposite way so that homeostasis can be achieved.
Like the seesaw, negative feedback loops in living organisms usually are not successful in keeping a particular characteristic of a system constant. There is usually some fluctuation around the balance point or 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 is, the greater the fluctuation will be. Fluctuation around the set point is shown in the illustration to the right.

 

This photograph shows a young woman, who is wrapped in a yellow blanket, checking her temperature with a digital thermometer.

© Tomasz Trojanowski/shutterstock

In humans, the sensors and 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. The overall response is co-ordinated by some kind of control centre or boss. In the endocrine system, the control centre is the hypothalamus-pituitary complex.

 

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. At the Diploma-Exam level, your response must be complete and well-expressed using Biology 30 vocabulary. Check your answers and store your work in your course folder.

 

SC 3. Define homeostasis and explain how the endocrine system helps to maintain homeostasis in the body.

 

Use the following information describing how the temperature in your house is maintained to answer question SC 4.

 

  • This photograph shows an old-fashioned round thermostat control. The regulation of temperature in your house is similar to negative feedback systems in your body.

    © Lori Sparkia/shutterstock

    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.

 

SC 4. 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 question SC 5.

 

  • The set point for human body temperature is 37°C.

  • Thermoreceptors in the skin sense temperature.

  • The brain receives information.

  • Effectors for regulating body temperature include the following:
    • sweat glands that, when activated, produce sweat for cooling the body
    • blood vessels in the skin that constrict to direct blood away from the skin, thus warming the body, or dilate to direct blood to the skin, thus cooling the body
    • shivering, a rapid contraction of muscles that warms the body
    • behaviour such as putting on a hoodie to warm the body or putting on a t-shirt and shorts to cool the body

 

SC 5. Draw a positive feedback loop that results in a fever.

 

Check your work.
Self-Check Answers

 

SC 3. 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.

SC 4. A negative feedback loop for a mechanical system, such as your heating system, might look like the following.

The feedback loop illustrates how dropping room temperature causes the thermostat to turn on the furnace. Rising room temperatures would then result in the thermostat turning off the furnace.

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

SC 5. A positive feedback loop for body temperature resulting in a fever might look like the following.


This positive feedback loop indicates how illness can result in escalated body temperature. The brain reduces sweating and constricts blood vessels so that heat is conserved. Furthermore, shivering with the fever is increased so that heat is generated in the muscle contractions. An increased body temperature, or fever, is maintained.