Principles of Homeostasis

Principles of Homeostasis
Homeostasis is about staying the same ...
but ... things don’t stay the same ... They
Conditions change here constantly ...
Where they cannot change is here ...
The Internal Environment
Homeostasis is about staying alive!
•For the body’s cells to survive and function properly, the
composition and temperature of the fluids around the cells
(“interstitial fluid”) must remain much the same.
•An organism is said to be in homeostasis when the internal
environment contains:
The optimal concentration of gases
The optimal concentration of nutrients
The optimal concentration of ions and water
At the optimal temperature
Which increases the amount of
CO2 in our blood
such as
change the
rate at which
we breathe ...
Which changes
the pH of the
•Which is
•Potentially fatal,
unless ...
The body responds homeostatically by changing the
volumes of air we breathe and adjusting blood pH with
buffers (HCO3 , Hb and others)
Homeostasis operates by means
of control systems
•Sensory receptors
•Control centre
Homeostasis is characterized by
Negative Feedback
...response is negative to the initiating stimulus ...the control system
initiates a series of changes that return the factor toward a certain
mean value ... thus maintaining homeostasis.
Maintaining Homeostasis
The various organ systems of the body act to maintain
homeostasis through a combination of hormonal and
nervous mechanisms. In everyday life, the body must regulate
respiratory gases, protect itself against agents of disease
(pathogens), maintain fluid and salt balance, regulate energy
and nutrient supply, and maintain a constant body
temperature. All these must be coordinated and appropriate
responses made to incoming stimuli. In addition, the body
must be able to repair itself when injured and be capable of
reproducing (leaving offspring).
Oxygen must be delivered
to all cells and carbon
dioxide (a waste product
of cellular respiration)
must be removed.
Breathing (inhalation and
exhalation) brings in
oxygen and expels CO2.
The rate of breathing is
varied according to the
oxygen requirement. Both
gases are transported
around the body in the
blood; the oxygen mostly
bound to hemoglobin.
All of us are under
constant attack from
pathogens (disease causing
organisms). The body has a
number of mechanisms
that help to prevent the
entry of pathogens and
limit the damage they
cause if they do enter the
body. The skin, the
digestive system and the
immune system are all
involved in limiting
Food and drink must be
taken in to maintain the
body's energy supplies.
Steady levels of energy (as
glucose) is available to
cells through hormonal
regulation of blood sugar
Insulin, released by the
endocrine cells
of the pancreas, causes
cells to take up glucose
after a meal.
Glucagon causes the
release of glucose from
the liver.
Damage to body
tissues triggers the
response. There is
pain, swelling, redness,
and heat. Phagocytes
and other white blood
cells move to the
injury site. The
inflammatory response
is started (and ended)
by chemical signals
(e.g. from histamine
and prostaglandins)
released when tissue
is damaged.
The levels of water and
ions in the body are
maintained mainly by
thekidneys, although the
skin is also important.
Osmoreceptors monitor
the fluid and ion levels of
the blood and bring about
the release of regulatory
hormones; the kidneys
regulate reabsorption of
water and sodium from
blood in response to
levels of the hormones
ADH and aldosterone.
The body is constantly
bombarded by stimuli
from the environment.
The brain sorts these
stimuli into those that
require a response and
those that do not.
Responses are
coordinated via nervous
or hormonal controls.
Simple nervous responses
(reflexes) act quickly.
Hormonal responses take
longer to produce a
response and the
response is more
Nervous System Regulation of Homeostasis?
Endocrine System Coordination of Homeostasis?
See package ...
What matters?
See .ppt “Thermoregulation”
See .ppt “Regulating Blood Sugar
Levels” as well as the notes
What about “Positive Feedback”?
Generally --- but not always --- this is bad news for the
body as it generally leads to a vicious cycle. Unlike
negative feedback which tends to dampen the original
stimulus, positive feedback acts to intensify that stimulus.
Positive Feedback?
A system exhibiting positive feedback, in response to perturbation, acts to
increase the magnitude of the perturbation. That is, "A produces more of B
which in turn produces more of A". A vicious cycle!
In contrast, a system that responds to a perturbation in a way that reduces its
effect is said to exhibit negative feedback.
The Tacoma Narrows
Bridge collapsed in 1940,
due to a design flaw that
allowed positive feedback
to dominate.
Alarm or panic can
spread by positive
feedback among a
herd of animals to
cause a stampede
In sociology a network
effect can quickly create
the positive feedback of a
bank run.
Some examples ...
Example # 1
•A panic attack ... or stress of ANY kind ... causes high levels
of adrenaline to be released from the adrenal medulla.
• The body responds by preparing to either run away or
fight for its life (“fight or flight response”)
•If no actual confrontation occurs then more anxiety
results and the body responds by releasing ACTH from the
anterior pituitary gland which acts on its target gland the
adrenal medulla to produce yet more adrenaline .. And
hence the vicious cycle continues ... Positive feedback for
Example # 2
... Insert your own story
about 5
L of
of blood
With the withdrawal of 2L
Positive Feedback
Negative Feedback
of blood the heart becomes
progressively less effective in
terms of its pumping
effectiveness. This leads to
less blood being delivered to
tissues which in turn further
weakens the heart making it
even less able to effectively
pump ... A vicious cycle or
positive feedback that
without intervention quickly
leads to eternity.
Note that with only 1L of
0 hours
1 hour
2 hours
3 hours
4 hours
blood loss the heart is
capable of returning (via
negative feedback) to
normal function.
Example # 3
Childbirth ... oxytocin
See explanation
on next page
a (usually) non-fatal example
Now this is somewhat of an exception in our examples of positive
feedback. Depending on your point of view, you see, positive feedback in
this case results in nothing more disastrous than
a wee crying baby.
What happens is that labour is set in motion by contractions of the walls
of the uterus. These contractions are stimulated by the hormone oxytocin
(a posterior pituitary hormone). More contractions result in more
oxytocin release causing more and deeper contractions until the wee
bundle is finally born. Bravo for positive feedback!
If the uterus is not contracting sufficiently to expel a fully developed fetus,
commercial preparations of oxytocin are sometimes used to stimulate
uterine contractions, thus inducing labour. Also, such preparations are
often administered to the mother following childbirth to ensure that the
uterine muscles will contract enough to squeeze broken blood vessels
closed, minimizing the danger of haemorrhage.
Example # 4
... Once again insert your own story
... 2 hours in the hot – tub? (especially after consuming alcohol)
... Running or exercising in the hot sun (in poorly ventilated clothing)?
... Obesity?
... Heavy exertion?
Sample Problem:
Explain how homeostasis is involved in regulating temperature when the
body becomes too hot as a result of heavy exertion. Your explanation
should include a discussion of regulating mechanisms, body responses,
and the role of negative feedback. (8 Marks)
See .ppt “Thermoregulation” for the normal (negative feedback) maintenance
of body temperature.
Increase in body temperature leads to increase in blood temperature.
Increase in blood temperature detected by receptors in the hypothalamus
... it is the hypothalamus that causes various responses in the body designed
to lower temperature.
Blood vessels carrying blood to the skin dilate, carrying more blood to the
Heat loss from the blood by radiation
Sweat glands become active and produce sweat
Loss of heat by radiational cooling and through evaporation of sweat lead
to a decrease in blood temperature (negative feedback)
Decrease in blood temperature will be detected by the receptors in the
hypothalamus which will decrease dilation of blood vessels in the skin and
stop sweat production.
What would be the consequences if this negative
feedback mechanism failed?
• This would result in positive feedback
• A positive feedback mechanism reinforces the
existing state, i.e. would cause a further increase in
• A further increase in temperature would cause
enzymes to work faster, increasing heat production
and leading to denaturation of enzymes and
ultimately death.
For the best summary of negative feedback and
homeostasis, see handout by the same name. Keep in mind
that the two parameters you are required to become an
expert on are the control of blood glucose level
(glucagon/insulin) as well as control of body temperature.

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