This lecture will provide an introduction
to anatomy and physiology.
This statement, Part 3 of 4, will cover the topics
of Homeostasis and negative & positive feedback loops.
Homeostasis refers to maintenance
of a relatively stable internal environment.
Homeostasis is necessary for survival.
If the internal environment was not kept stable,
there would be a large change and fluctuation
in things such as body temperature, blood PH,
ion concentrations, and so forth.
A large deviation from the normal range of
anyone of these things would create
catastrophic results in the body, including
the possibilities of organ failure and even death.
To prevent this, the organ systems of the human body
work together to maintain homeostasis
in a process known as homeostatic regulation.
In homeostatic regulation, there are
usually 3 components involved.
A receptor, a control center, and an effector.
The receptor's job is to be sensitive. To be able to
detect a specific change or stimulus in its surroundings.
Once a stimulus has been detected,
the receptor communicates this to a control center.
The control center's job is to process and integrate
the information it is receiving from the receptors
in order to formulate an appropriate response.
Once this has been completed, the control center will
communicate directions to an effector.
The effector's job is to respond to the original
stimulus by either opposing it
or enhancing it based on the instructions
from the control center. The end result of this process
is to bring about the necessary changes
in physiology systems
so that homeostasis can be maintained.
Let's explore the process of homeostatic regulation
in the context of room temperature.
Here, a constant, normal room temperature
provides a stable environment. A state of homeostasis.
If the temperature of the room begins to rise,
the normal condition is disturbed.
The receptor, in this case, a thermometer…
…will be sensitive to the increase in temperature
and relay this information to the
control center, the Thermostat in this case.
The thermostat will process and integrate the info
and send a command to the effector,
the air conditioner, to turn on.
As a result, the response will oppose
the original stimulus
of an increased temperature by utilizing an effector
that will bring about a decrease in temperature
to restore homeostasis…
…bringing the conditions back to normal.
This is an example of what is known as
a negative feedback loop.
Negative feedback corrects deviation
outside of the normal range by bringing
about a response
that opposes the original stimulus.
Variations from the norm
that are increases, are brought back down to normal levels and vice versa.
Negative feedback is the most common type
of feedback mechanism in the human body.
Let's consider how the human body responds
to the body temperature deviating from its normal
temperature of 37 degrees Celsius.
When the body temperature rises
to above 37.2 degrees…
…receptors responsible for monitoring the body's temp.
will pick up on the stimulus.
This info will be sent to the thermo-regulatory
center of the brain, the control center in this instance…
…where the info will be processed and integrated.
A command to effectors, in this case,
just the smooth muscles found in blood vessel walls,
and sweat glands will be sent.
These commands will instruct the
blood vessels to dilate, bring the blood
flow to the surface
and increase sweat gland activity.
Both of these responses will bring about
a lowering of body temperature, thus…
…opposing the original stimulus
and homeostasis will be restored.
On the flip side, when body temperature
decreases below 37.6 degrees Fahrenheit,
the body temperature's sensors will,
again, act as receptors
and relay the info to the
thermo-regulatory center of the brain.
To oppose falling temperature,
the control center will send out commands to
the effectors. In this case, smooth muscles
in the blood vessels,
skeletal muscles, and the sweat glands.
The effectors will respond by causing a decrease
in blood flow to the surface, shivering…
…and a decrease in sweating. All of these responses
will promote an increase of body temperature;
thus, providing negative feedback
that restores homeostasis.
In addition to negative feedback loops,
the body also utilizes positive feedback loops
in some instances.
Unlike negative feedback, positive feedback
will reinforce or exaggerate the initial stimulus.
Positive feedback loops are found in situations
were a process needs to be completed quickly,
such as blood clotting and child birth.
Because positive feedback builds upon the original
stimulus, it needs to have a built-in off switch
so it does not get out of control.
Let's examine the process of blood clotting to see how
positive feedback loops work.
The damage of a vessel wall results in bleeding
and the disruption of homeostasis.
When the wall of a blood vessel is damaged,
specific chemicals are released that
will trigger blood clotting to begin.
These chemicals attract platelets
and will start a series of change reactions leading to
the start of blood clot formation.
As this process proceeds,
more chemicals are released, causing the
clotting to accelerate
a positive feedback loop.
More clotting, leads to more chemicals being released,
resulting in more clotting and so on.
The positive feedback loop is turned off once
the clot has completely formed
and the damaged vessel has been repaired.
The bleeding is stopped and homeostasis is restored.
Let's review with a checkpoint. Can you explain the
function of negative feedback systems?
Feel free to pause this lecture while you
formulate your response.
Negative feedback systems maintain homeostasis
by counteracting stimuli.
Let's try another!
Why is positive feedback helpful in blood clotting,
but unsuitable for regulation of body temperature?
Blood clotting is needed to prevent too much blood loss
and requires fast response as can be
provided with positive feedback.
The case of accelerating the development of
a blood clot is useful.
If positive feedback was used to regulate
body temperature, the original stimulus
would be amplified or exaggerated. The result…
would be an increase of body temperature
and it would continue to increase as the positive
feedback loop continued.
This would lead to dangerously high body temperatures
and would not restore homeostasis,
rather it would make the change in
the internal environment
more severe and more dangerous.
This is way such mechanisms
use negative feedback loops as a means of control.
And finally, let's try one more.
What happens to the body when
homeostasis breaks down?
If the body is not in homeostasis,
the environment of the body changes
and the organs and organ systems
cannot function properly.
This results in disease and can lead to possible
organ damage and death.

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