Homeostasis
Homeostasis (homeo = similar + stasis = condition): ability to maintain a
relatively stable internal environment in response to an ever-changing
outside world
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1. Challenge (stimulus) produces a change in body status
that shifts it out of balance
2. Physiology responds to maintain return the variable to
balance (homeostasis)
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Homeostatic Control Mechanisms
• Challenge produces a change in body status
• Physiology responds to maintain homeostasis
• Components of a feedback system:
– Variable – Regulated factor/event (setpoint, operating range, "error
signal")
– Receptor/Detector – Monitors the environments and responds to a
challenge (stimulus). The afferent path.
– Control center – Determines the set point at which the “variable” is
maintained and compares the current level of the variable to the
regulated “setpoint”.
• Intrinsic (Local): cell or tissue autoregulates
• Extrinsic:
– nervous system sends electrical signals to target tissues (FAST)
– endocrine system secretes hormones into blood (SLOWER)
– Effector – Provides the means to respond to stimulus to return
variable to “setpoint”. The efferent path.
– Response
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EX: Maintain Room Temperature at 68 F?
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Negative Feedback System
Example: Household
temperature control
thermostat
1. Variable that is regulated
2. Stimulus
3. Receptor: monitors the
variable (receives and
transmits “stimulus”)
4. Control Center: integrates
information and compares it to
a “set point” (decides on
appropriate response) &
transmits command
5. Effector: receives & executes
response to stimulus
6. Response 6
Negative Feedback System
• In negative feedback systems the response of the
effector negates or opposes the stimulus (shuts off the
original stimulus)
• EX: Maintain Temperature
– Too hot > effector leads to less hot (sweat, increase skin blood flow,
decrease metabolism)
– Too cold > effector leads to less cold (shiver, behavioral changes, increase
metabolism)
• Responses are controlled by
• Extrinsic control systems
• Nervous system
• Endocrine systems
• Intrinsic control systems (also called autoregulation)
• Tissue or Cells
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Matinain Body Temperature @ ?
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Human thermostat
Body temperature (C°)
Time
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Fever?
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Changing the set-point: Pyrogens change the
temperature control center “set-point” to a new
higher level.
Fever is the result of a shift in the
temperature set-point (higher
level).
– Set-point shift often accompanies
bacterial or viral infections
Benefits of a higher temperature:
– Inhibits bacterial growth
– Speeds up metabolic rxns for repair
– Increases delivery of white blood cells
to infection sites
How is temperature increased?
•Blood flow shifted to core to
conserve heat, increased muscle
activity (shivering), increase in
metabolic rate.
•Chills stop when higher temp
reached 11
Negative Feedback
EX: Blood glucose regulation
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EX: Blood Glucose
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Negative Feedback
EX: Fluid/Electrolyte balance
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Fluid Electrolyte Balance
Body Fluids Compartments
1. Intracellular
2. Extracellular
a. Interstitial
b. Plasma
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Fluid Electrolyte Balance
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Positive Feedback System
Definition: enhances the
original stimulus causing a
greater deviation from the
set point.
– Activates infrequent
events that require
immediate action
– Most are not related to the
maintenance of
homeostasis.
– Positive feedback often
results in disease.
Examples:
Blood clot
Uterine contractions in childbirth
(stretch releases oxytocin
from posterior pituitary),
Disease: Cancer
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Positive Feedback
EX: Blood Clot Formation (prevent blood loss)
http://www.mhhe.com/biosci/esp/2002_general/Esp/folder_structure/tr
/m1/s7/trm1s7_3.htm
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Positive Feedback
EX: Childbirth
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Positive Feedback
EX: Disease (Melanoma)
Disorder or Disease
Disorder: is a general term for any
change or abnormality of function
Disease is a more specific term for an
illness characterized by a
recognizable set of signs and
symptoms.
– Signs are objective changes that
a clinician can observe and
measure; e.g., fever or rash.
– Symptoms are subjective
changes in body functions that
are not apparent to an observer;
e.g., headache or nausea
A local disease is one that affects one part or
a limited region of the body
A systemic disease affects either the entire
body or several parts
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