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

2. Homeostasis From the Greek roots Term coined by Walter Cannon in 1932
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 Positive 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