homeostasis dr robert ryan buenaventura medical group april 24, 2006
TRANSCRIPT
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HOMEOSTASIS
DR ROBERT RYANBUENAVENTURA MEDICAL GROUP
APRIL 24, 2006
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Homeostasis
From the Greek rootsFrom the Greek roots– Homo: alike or the sameHomo: alike or the same– Stasis: to stand or postureStasis: to stand or posture
Term coined by Walter Cannon in Term coined by Walter Cannon in 1932 1932
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HomeostasisAbility or tendency of an organism/cell to maintain internal equilibrium by adjusting its physiological processes
– e.g., temp, pH, blood pressure, PCO2, etc.
Negative or Positive Feedback
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Feedback
Negative Feedback– Slows down process to return to
equilibrium
Positive Feedback – Accelerates process away from
equilibrium
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Feedback (cont’d)
Stock Market
Aircraft autopilot
Computer modeling
Biological systems
Genes
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Feedback (cont’d)
Control triangle preprogrammed with desired rangesInput arrives and acted upon in control triangle to provide an outputOutput sampled and fed back to control triangle Output modified to make sure output stays within programmed range for center
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Types of Feedback
Negative Feedback: change in an environment causes response that returns conditions to original state– ThermostatThermostat
Positive Feedback: response of the system is to change that variable even more in the same direction– ““Sim City” Sim City” – Two in human bodyTwo in human body
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Nervous SystemNervous SystemBody’s surveillance system– Blood pressure,
temperature, pH, etc. Sensory centers – Brain or spinal cord– Barrage of input
integrated and response generated
Organs convert signal into some form of action– E.g., movement,
changes in heart rate, release of hormones, etc.
Reflex arc
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Cardio-Respiratory System Maintain homeostasis and deliver oxygenated blood to cells
Negative Feedback
– Blood Pressure
– pH
– PCO2
Hyperventilation example
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Endocrine SystemElegant feedback networks Affects activities of cells, tissues, and organs throughout the body Controls slower, longer lasting responses to internal stimuliSelf-Regulating Negative Feedback Mechanism– Hypothalamus – Anterior Pituitary – Other Endocrine End Organs
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Endocrine System (cont’d)
Thyroid hormone
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Endocrine System (cont’d)
Sex hormones
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Endocrine System (cont’d)
Steroid hormones
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Endocrine System (cont’d)
Positive Feedback Loop– Childbirth
Increased rate of contractions causes increased rate of stretch receptor stimulus (feedback)Continues positive feedback loop until baby is delivered
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Endocrine System (cont’d)
Positive Feedback Loop– Name the other positive
system found in mammals
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Endocrine System (cont’d)
Blood clotting – Self-catalytic, self-accelerating
effect – Clotting process begins
Runs faster and fasterUntil bleeding stops
– Positive feedback loop is part of a larger negative feedback loop
Activated by bleeding Ultimately works to stop the bleeding
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Blood Clotting
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SummaryHomeostasis is achieved by– Mainly negative feedback– Infrequent cases positive feedback
Negative feedback’s goal is to maintain equilibrium
Positive feedback’s goal is to accelerate away from equilibrium
Endocrine, Nervous and Cardio-Respiratory systems all use feedback to maintain life
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