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
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
Feedback
Negative Feedback
– Slows down process to return
to equilibrium
Positive Feedback
– Accelerates process away
from equilibrium
Feedback (cont’d)
Stock Market
Aircraft autopilot
Computer modeling
Biological systems
Genes
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
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
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
Maintain homeostasis and deliver
oxygenated blood to cells
Negative Feedback
– Blood Pressure
– pH
– PCO2
Hyperventilation example
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)
Thyroid hormone
Endocrine System (cont’d)
Sex hormones
Endocrine System (cont’d)
Steroid hormones
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’d)
Positive Feedback Loop
–Name the other positive
system found in mammals
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
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