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Physiology 3120 Study Guide - Homeostasis, Endocrine System

Course Code
Anita Woods

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Weekly Thoughts
From now until the end of the course I will be posting “weekly thoughts” about the topics
covered during that week. I will usually post these towards the beginning of each
module, although this week I wanted to give everyone time to acquire the CD and
become familiar with the Discussion area. Please let me know if you are having
Below are some "Thoughts" to consider after completing the first module. These
“thoughts” point out some of the key concepts and explain things using everyday
examples. We will also post some "thought provoking questions" that we hope will
stimulate lots of good discussion. Feel free to copy and paste these “thoughts” to any
word processor you are using. It's a good way to "take notes". As I mentioned, I will do
this on a regular basis (usually once a week at the start of a new module).
Here we go …
Internal and External Environments
You should be able to distinguish between the internal and external environments. Be
careful! - there are parts of the body that seem to be inside us when in fact they are
considered external environments! These include the entire "inside" of your digestive
system, your lungs and parts of your kidneys and bladder.
As you have read, homeostasis is defined as the maintenance of relatively constant
conditions in the internal environment (despite a variable external environment).
Maintaining this internal environment requires almost all the organ systems in the body. I
say "relatively constant conditions" because certain levels within the body will change
slightly throughout the day and from time to time. For example, your body temperature,
which averages 37 degrees Celsius, will fluctuate slightly during the day.
Negative Feedback (and positive feedback)
Almost all the organs within the body are involved in maintaining homeostasis. They are
controlled by two systems - the endocrine system (hormones) and the nervous system.
These two systems perform their wondrous job by using negative feedback (and
sometimes, to a lesser extent, positive feedback). The reason it's called negative
feedback is because the final product or "the controlled variable" (eg. heat) ends up
shutting off the effector - which, in the example on the CD, is the furnace. In a positive
feedback system, the controlled variable actually keeps the effector on.
The body's structural hierarchy
Atoms forms the basic component of everything. Different atoms combine to form
molecules, which in turn group together to form larger Macromolecules. These
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