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HOMEOSTASIS. DR ROBERT RYAN BUENAVENTURA MEDICAL GROUP APRIL 24, 2006. Homeostasis. From the Greek roots Homo: alike or the same Stasis: to stand or posture Term coined by Walter Cannon in 1932. Homeostasis.

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APRIL 24, 2006



  • From the Greek roots

    • Homo: alike or the same

    • Stasis: to stand or posture

  • Term coined by Walter Cannon in 1932



  • Ability 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



  • Negative Feedback

    • Slows down process to return to equilibrium

  • Positive Feedback

    • Accelerates process away from equilibrium

Feedback cont d

Feedback (cont’d)

  • Stock Market

  • Aircraft autopilot

  • Computer modeling

  • Biological systems

  • Genes

Feedback cont d1

Feedback (cont’d)

  • Control triangle preprogrammed with desired ranges

  • Input arrives and acted upon in control triangle to provide an output

  • Output sampled and fed back to control triangle

  • Output modified to make sure output stays within programmed range for center

Types of feedback

Types of Feedback

  • Negative Feedback: change in an environment causes response that returns conditions to original state

    • Thermostat

  • Positive Feedback: response of the system is to change that variable even more in the same direction

    • “Sim City”

    • Two in human body

Nervous system

Nervous System

  • Body’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

Cardio respiratory system

Cardio-Respiratory System

  • Maintain homeostasis and deliver oxygenated blood to cells

  • Negative Feedback

    • Blood Pressure

    • pH

    • PCO2

  • Hyperventilation example

Endocrine system

Endocrine System

  • Elegant feedback networks

  • Affects activities of cells, tissues, and organs throughout the body

  • Controls slower, longer lasting responses to internal stimuli

  • Self-Regulating Negative Feedback Mechanism

    • Hypothalamus

    • Anterior Pituitary

    • Other Endocrine End Organs

Endocrine system cont d

Endocrine System (cont’d)

  • Thyroid hormone

Endocrine system cont d1

Endocrine System (cont’d)

  • Sex hormones

Endocrine system cont d2

Endocrine System (cont’d)

  • Steroid hormones

Endocrine system cont d3

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

Endocrine system cont d4

Endocrine System (cont’d)

  • Positive Feedback Loop

    • Name the other positive system found in mammals

Endocrine system cont d5

Endocrine System (cont’d)

  • Blood clotting

    • Self-catalytic, self-accelerating effect

    • Clotting process begins

      • Runs faster and faster

      • Until bleeding stops

    • Positive feedback loop is part of a larger negative feedback loop

      • Activated by bleeding

      • Ultimately works to stop the bleeding

Blood clotting

Blood Clotting



  • Homeostasis 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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