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					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?




                                   5
           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 @ ?




                                8
Human thermostat




          Body temperature (C°)




                                  Time
                                         9
Fever?




         10
     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




                               12
EX: Blood Glucose




                    13
     Negative Feedback
EX: Fluid/Electrolyte balance




                                14
                Fluid Electrolyte Balance

Body Fluids Compartments
1. Intracellular
2. Extracellular
    a. Interstitial
    b. Plasma




                                            15
Fluid Electrolyte Balance




                            16
                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




                                           17
             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
                                                                  18
Positive Feedback
  EX: Childbirth




                    19
                         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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posted:11/16/2011
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