P laying volleyball is hard work, especially in the hot sun. swimming The players’ bodies were using their mitochondria to burn sugar for fuel and giving off heat in the process. Between that and the heat they absorbed from the sun, their body temperatures began to rise. That rise in temperature triggered their bodies to perspire, because as perspiration evaporates, it carries away heat, thereby cooling the body. Of course, some people found another way to cool off: swimming in the creek behind Farmer Melanie’s field.

Homeostasis – Maintaining Balance

Consider just a few of the things that are happening here:
  • hard work + hot sun ® increase in body temperature
  • increase in body temperature ® increased sweating
  • increased sweating ® decreased body temperature
    but...
  • increased sweating ® decrease in body fluids
  • decrease in body fluids ® increase in thirst
  • decrease in body fluids ® decrease in kidney output
  • increase in thirst ® increase in fluid consumption
  • increase in fluid consumption ® increase in body fluids
  • increase in body fluids ® decrease in thirst
  • increase in body fluids ® increase in kidney output
    and then in the evening as people sit, resting...
  • decrease in absorbed heat ® decrease in body temperature
  • decrease in body temperature ® shivering
  • shivering ® increase in body temperature
bonfire

Out of that list, there are several of these “reactions” that are rather interesting:  
increase in body temperature Þ increased sweating Þ decreased body temperature
decrease in body temperature Þ shivering Þ increase in body temperature
and, slightly more complex,
decrease in body fluids Þ increase in thirst Þ increase in fluid consumption Þ increase in body fluids  
Notice that in each case, a change in a particular body condition triggers a reaction which, in turn, brings about the opposite change in the body condition which triggered it. These are all examples of negative feedback loops, processes in which the “feedback” sends the process in the opposite direction from that in which it was initially headed. Our bodies use many types of negative feedback loops to maintain homeostasis, which is the ability of an animal’s body to maintain a constant internal environment.

There are many other examples of negative feedback loops in the world around us, many of which aren’t even biological. Consider what happens when you flush a toilet.
  • decreasing water level ® lower the float
  • lowered float ® increase in incoming water
  • increase in incoming water ® increasing water level
  • increasing water level ® raise the float
  • raised float ® decrease in incoming water
until the water reaches a normal level, again. Double-click the toilet tank to the left to see an animation of this process.



Background Information

Some links to other, related information on our Web server
Bio 104 Carbohydrates – Read the sections (near the middle of the page) on “Sugar Utilization by Our Bodies” and “Control of Blood Glucose Levels”
Bio 105 Tissues, Organs, and Systems – Read the second paragraph which discusses homeostasis
Bio 105 Excretory System – Pay special attention to the paragraph, about half-way down the page, which discusses antidiuretic hormone
Bio 105 Endocrine System – Many of these hormones are responsible for maintaining homeostasis in our bodies.

Your Assignment
Examples of Homeostasis

In this project, you will examine some negative feedback loops at work in your everyday life. Assuming your toilet is made like most here in America, the filling mechanism in its water tank is a good example of a negative feedback loop. The grading criteria for this assignment are given below, and you should also refer to those as you work on the assignment. A total of 10 points is possible.

  1. Carefully remove the lid from the tank and set it some place safe.
  2. Observe what you see inside the water tank. Probably there is an “arm” sticking out horizontally attached to an air-filled bulb which floats on the surface of the water. Also, somewhere there is a water source.
  3. With the lid off, flush the toilet and watch and take notes on what happens.
    • As the water level drops, what does the float do?
    • What happens because of that?
    • As the tank begins to refill, what does the float do?
    • What happens because of that?
  4. While the water level is low, what happens if you carefully lift up the float?
  5. While the tank is full, what happens if you carefully press down on the float?
  6. Negative feedback loops are used to maintain homeostasis within our bodies. For any given bodily process or condition, we have multiple feedback loops which help to avoid extremes in either direction from the desired norm. Is your toilet tank also an example of homeostasis? What happens if you add water so that the water level rises above normal (hopefully, it won’t run out on the floor)? Is there a negative feedback loop that works to bring the water level back down to normal?
  7. When you are done, carefully replace the lid.
  8. Now, look around your home to find other examples of negative feedback loops. Look for one biological example and one non-biological (not done by living organisms) example of negative feedback loops. Take notes on how each of those loops works.
  9. If you are a registered student, to receive credit for this assignment, you need to submit your work for this project.

Grading Criteria

1.   Overall:
2 — The student was very observant and took excellent notes on the functioning of the toilet
1 — The student observed and took adequate notes on the functioning of the toilet
0 — The student’s notes are sketchy and suggest that (s)he did not pay much attention to how the toilet works
2 — The student correctly identified and took excellent notes on a biological negative feedback loop
1 — The student correctly identified and took adequate notes on a biological negative feedback loop
0 — Either the process which the student observed was not a biological process or was not a negative feedback loop, and/or the student’s notes were too sketchy
2 — The student correctly identified and took excellent notes on a non-biological negative feedback loop
1 — The student correctly identified and took adequate notes on a non-biological negative feedback loop
0 — Either the process which the student observed was a biological process or was not a negative feedback loop, and/or the student’s notes were too sketchy
2 — The student, obviously, went beyond the minimum requirements of the assignment
1 — The student adequately completed the assignment
0 — The student completed considerably less of the assignment than what was required
2 — It is evident that the student used much insight, thoughtfulness, and critical thinking when completing this assignment
1 — The student adequately thought about the assignment – there was, perhaps, a bit of “fuzzy thinking” in a couple places
0 — The assignment gives the appearance of being “slapped together” just to get it done, with little evidence of thoughtfulness

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