Homeostasis
A fundamental
paradigm in physiology
Homeostasis: Definition
• maintenance of an internal state
that is distinct from the external
environment and is defended
against perturbation
Homeostasis - Origin
• “It is the fixity of the milieu interieur
which is the condition of free and
independent life, all the vital mechanisms,
however varied they may be, have only one
object, that of preserving constant the
conditions of life in the internal
environment.” Claude Bernard
1
• Walter Cannon
emphasized
that
homeostasis is
dynamic and
“admits some
variation”
• Slight
variations
around a “set
point” value are
normal and
tolerable
Homeostasis
What are some homeostatically
controlled variables in vertebrates?
What are some NOT homeostatically
controlled variables?
Regulated Physiological Parameters
Parameter
Normal Range Non-lethal limits
Blood Glucose 75-95 mg/dl
20-1500 mg/dl
Body Temp.
98.0-98.8°F
65-110°F
pH or [H+]
7.3-7.4
6.9-8.0
[Na+]
138-145
O2 tension
95-105 mmHg 10-1000mmHg
CO2 tension
35-45 mmHg
mmol/L
115-175 mmol/L
5-80mmHg
Body weight??
2
A tale of two Freds
• Fred 1
• Fred’s body weight
changes according
to an annual cycle.
From October
through March he
typically weighs
about 85% of what
he does in the
spring and summer.
• Fred 2
• Fred’s body weight
changes according
to an annual cycle.
From October
through March he
typically weighs
about 60% of what
he does in the
spring and summer.
Which Fred is ‘less homeostatic’?
A Tale of Two Freds
Some more information
• Fred 1
• Fred’s heart rate
does not change
significantly over
the year
• Fred’s body
temperature does
not change
significantly over
the year
• Fred 2
• Fred’s heart rate is
normally about 110
bpm. But Oct-March
his HR is about 15 bpm
• Fred’s body
temperature is
normally about 35oC.
But Oct-March his
body temp is closer to
8oC
Which Fred is ‘less homeostatic’?
Fred 1 and Fred 2
3
Homeostasis: Dynamic Control
“Homeostasis is not a single optimal
control condition but rather a variety
or continuum that varies with the
animal’s circumstances.” Donald C.
Jackson
Mechanism: The negative feedback loop
Compenents:
Sensor
Integrator
Effector
“A series of interconnected components that serve to
maintain a physical or chemical parameter of the body at
a near constant value”
Fig 2.4
4
Other Control Systems?
• Feed-forward Control – a detected
change in state leads to an
anticipatory effector response
• Respiration at the onset of exercise?
Conceptual Aspects of Control Systems
• Set point or set point range
• Threshold
• Gain
5
Homeostasis vs. Steady-State
Homeostasis
Steady-State
describes the
process whereby
the value of a
variable is
defended against
perturbation
a variable may be
described as
steady-state if it
remains fairly
constant in value
over the time
period being
considered
Steady-State
39
Cool Environment
Core Temp oC
38.5
Hot, humid
environment
38
37.5
37
36.5
36
-15
0
15
30
45
60
Time (min)
Figs. 2.1; 12.10
Homeostasis: Dynamic
• “…set points or regulated values are
not fixed, but may…change depending
on ambient conditions…or because of
changing physiological conditions or
demands.” D. C. Jackson, 1987
6
Homeostasis vs. SS – Powers and
Howley - Oversimplified
• “…the term homeostasis is generally
reserved for describing normal
resting conditions, and the term
steady state is often applied to
exercise where the physiological
variable in question…is unchanging but
may not equal the “true” resting
value.”
Exercise-Induced Disturbance to Homeostasis
Temperature regulation
(10-15 kcal/min)
Glucose
(↑uptake 7-20 fold)
PO2
(↑ VO2 )
Exercise
PCO2
(↑ VCO2)
↓pH
(↑lactate)
Mean Arterial Blood Pressure
(vasodilation of muscle)
Fig 13.1
ALTITUDE
MENTAL
STRESS
HEAT
Cellular
Environment
INJURY
EXERCISE
7
Stress Proteins
• Stress can damage cellular proteins (i.e.,
enzymes, transporters, structural)
–
–
–
–
pH
High temperatures
Hypoxia
Free radicals (i.e., O-)
• Stress proteins repair damaged proteins
to restore normal function and maintain
homeostasis
– Heat-shock proteins (HSP70)
8