Homeostasis
August 10, 11 2015
Homeostasis
 The maintenance of relatively stable internal
conditions
 Dynamic process – the body is constantly regulating
to keep conditions within acceptable ranges
Examples of Homeostasis
Many variables are maintained by homeostasis.
What examples can
you think of?
Examples of Homeostasis
Many variables are maintained by homeostasis.
Examples include:
 Temperature
 Blood pH
 Blood sugar
 Water balance
 Blood pressure
 Ion balance
Homeostatic Control Mechanisms
Homeostatic Control Mechanisms
Analogies
Thermostat
Cruise control
others?
What is the ...
Variable?
Stimulus?
Receptor?
Control
center?
Effector?
(some may
not be listed
on chart)
What is the ...
Variable?
Stimulus?
Receptor?
Receptors aren’t shown on this
picture, but there are two sets of
thermoreceptors: some in the
hypothalamus to measure internal
temperature , and some in the skin
to measure external temperature
Control
center?
Effector?
You do: What is the ...
Variable? Stimulus? Receptor? Control center? Effector?
Receptors aren’t shown again, but
there are two types of calcium
receptors scattered throughout the
body
You do: What is the ...
Variable? Stimulus? Receptor? Control center? Effector?
Negative Feedback Mechanisms
Most homeostatic mechanisms are examples of negative
feedback.
In negative feedback, the output acts to change the direction of
the stimulus.
Example: If the body is hot, the hypothalamus will activate the
sweat glands and dilate the blood vessels (the two outputs) …
both of which will act to reduce the body temperature.
Positive Feedback Mechanisms
Some processes in the body are positive feedback mechanisms.
In positive
feedback, the
output
enhances the
stimulus
Positive or Negative Feedback?
Blood Clotting
Blood Pressure
Positive or Negative Feedback?
Blood Clotting
Positive –
The output (platelet
adhering) increases the
original stimulus
(platelet-attracting
chemical)
Blood Pressure
Negative–
The output (decreased
heart rate / increased
diameter) decreases the
original stimulus (high
blood pressure)
Positive and Negative Feedback
Come up with examples and non-examples of negative
and positive feedback.
(For non-examples, I want something that at first glance
might seem like positive or negative feedback, but
isn’t).
What is it that distinguishes the true examples from the
non-examples?
Homeostatic Imbalance
Most disease is caused by a disturbance of homeostatic
mechanisms known as homeostatic imbalance.
Homeostatic imbalance can result from




Aging
Genetic mutations
Pathogens
Environmental factors
Congo Line review
- 10 minutes -
Homework
Chapter 1 outline due next class
First quiz Aug 14 / 17!
Closure
What are your key takeaways from our objectives?
What was our LP and how did we use it?
How does what we did today relate to our significant
concept?
Exit Ticket
Study this
diagram.
1) What is the
stimulus?
2) What are the
effectors?
3) Is this + or –
feedback, and
how can you
tell?
Exit Ticket
Identify the body system to which each organ belongs
4) Pancreas
5) Liver
Ex) Which system cleans and returns body fluids to the
blood stream?