3. Learning Objectives:
At the end of this session, the students should
be able to:
• Define Homeostasis
• Explain control systems in the body including Feed
forward and feedback mechanisms.
7. The internal environment is dynamic and
equilibrium state
– dynamic?
– equilibrium (steady) state?
8. Factors homeostatically maintained
• Concentration of nutrient molecules.
• Concentration of oxygen and carbon dioxide.
• Concentration of waste products.
• Concentration of water salts and electrolytes.
• Volume and pressure
• Temperature.
9. Contribution of the body systems to
homeostasis
• Circulatory system
• Digestive system
• Respiratory system
• Urinary system
• Skeletal system
• Integumantory system
• Nervous system
• Immune system
• Endocrine system
• Reproductive system
10. Homeostatic control system
• Functionally interconnected network of body
components to maintain a given factor in the
internal environment at a relatively constant
optimal level.
• To maintain homeostasis, the control system
must be able to
– detect deviations from normal sensor
– integrate this information with any other relevant
information Integrator
– make appropriate adjustments Effectors
12. Types of control system
• Negative feedback control system
• Positive feedback control system
• Feed forward control system
13. Negative feedback control system
• Negative feedback opposes an initial change.
• The commonest control system.
• E.g:
– Temperature control
– BP control
– Hormonal secretion .etc
14. Compensatory response
Deviation in
controlled variable
Controlled variable
restored to normal
Negative
feedback
Integrator
(sends instructions to)
Effector(s)
Negative feedback to
shut off the system
responsible for the
response
(detected by)
Sensor
relieves
15. Set point
Is a critical point around which
variables are controlled
16. 39oC
37oC
37oC
Set point normal Set point raised Set point decrease
Back to normal
What would happen here? What would happen here?
17. Case scenario: Fever
A 70 years old lady presented to
hospital complaining of on
and off fever and cough for the
last three days. She noted that
she used to feel chilly, shivers
and her skin turns pale and dry
before the attack of fever. She
used aspirin to treat the fever
and few minutes after
ingesting the drug, she starts
sweating copiously and her
skin flush red before the fever
wanes off.
18. Rise in
temperature
Above set point
decreases heat production
Negative
feedback
Temperature
monitoring
nerve cells
Temperature
control center
Negative feedback
mechanism
Rise in the
temperature above
set point
relieves
19. Fall in Blood
Pressure
below set point
Increase in Blood
Pressure to normal
Negative
feedback
Cardiovascular
control centers
Heart and
blood
vessels
Negative feedback to
shut off the system
responsible for the
response
Pressure
monitoring
nerve cells
relieves
20. Positive feedback
• Positive feedback amplifies an initial change.
• An output is enhanced.
• A controlled variable moves in the direction of an
initial change. Moves the controlled factor away
from a steady state
• Less frequent
• Examples:
– During the birth of a baby.
– Depolarization beyond threshold during an action
potential
– Clotting mechanism
23. Positive feedback may leads to
vicious circle
Initial
stimulus
Response
Positive feedback
cycle
↑ stimulus
Further increase
temperature
+
Increase in
body
temperature
24. Feed-forward control
• Allows the body to anticipate
change and maintain stability
• Examples:
• Watering of mouth (Salivation) as a
result of sight, smell or thought of
food
• Rise in respiration before exercise
• Increased secretion of Insulin as a
result of presence of food in digestive
tract avoids excessive rise of glucose
in blood after meal
25. Summary
• Homeostasis: To maintenance of nearly constant
conditions in the internal environment
• Negative feedback: a change in a controlled variable
triggers a response that opposes the change – driving the
variable in opposite direction of the initial change.
• Positive feedback: a change in a controlled variable
triggers a response that reinforces the change – driving
the variable in the same direction as of the initial change.
• Feed forward: brings about a response in anticipation of
a change.
