1. HOMEOSTASIS DR ROBERT RYAN BUENAVENTURA MEDICAL GROUP APRIL 24, 2006

2. Homeostasis • From the Greek roots • Homo: alike or the same • Stasis: to stand or posture • Term coined by Walter Cannon in 1932

3. Homeostasis • 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

5. Feedback • Negative Feedback • Slows down process to return to equilibrium • Positive Feedback • Accelerates process away from equilibrium

6. Feedback (cont’d) • Stock Market • Aircraft autopilot • Computer modeling • Biological systems • Genes

7. 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

8. 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

9. 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

10. Cardio-Respiratory System • Maintain homeostasis and deliver oxygenated blood to cells • Negative Feedback • Blood Pressure • pH • PCO2 • Hyperventilation example

11. 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

12. Endocrine System (cont’d) • Thyroid hormone

13. Endocrine System (cont’d) • Sex hormones

14. Endocrine System (cont’d) • Steroid hormones

15. 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

16. Endocrine System (cont’d) • Positive Feedback Loop • Name the other positive system found in mammals

17. 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

18. Blood Clotting

19. Summary • 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