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Homeostasis and Feedback
Homeo = Similar
Stasis = State
• Maintaining a stable internal environment
within a narrow range
• Necessary for proper function of enzymes that
carry out all metabolic processes
• Involves all organ systems
What needs to be regulated?
• Nutrients – oxygen, glucose, proteins etc.
• Electrolytes – salts (muscle contractions nerve
• Waste concentrations – CO2, urea (nitrogen
wastes), bile
• pH
• Temperature
• Pressure/volume – gases in blood, lungs
How to regulate
• Set point – what is the desired range?
• Sensors – structures including receptors on
target cells that will monitor levels relative to
set point
• Controller – determine if levels are within set
point range and what response is necessary to
return to range
• Effector – parts of body that physically make
the change to return to set point range
Negative Feedback
• Any change from the normal range is opposed
(thus negative)
• This brings it back within optimal range and
decreases the chaos in the body
• Most systems in body are negative feedback
as they maintain the balance
• Examples – body temperature, blood sugar
Thermostat Analogy Sensors = sensory
Controller =
Hypothalamus in brain
send signals to muscles
via nerves
Effector = muscles
Long term: endocrine
system increases thyroid
activity to raise metabolic
rate to increase heat
production from cellular
Body Temperature Effectors
• Muscles/circulatory constrict peripheral (arms
and legs) blood flow,
• Muscles at follicles raise hairs,
• Skeletal muscles contract (shiver) generate
• Heart rate slows to keep blood concentrated
in core body
• Skeletal muscle adjust body curls up
Positive feedback
• Reinforces the original stimulus – pushing
conditions further from the normal
• Less common as it can get out of control and
will not stop until stimulus is removed
• Increases chaos in body systems
• Examples – blood clotting and child birth
Clotting signals continue until clot
is complete