HOMEOSTATIC REGULATION
• Objectives: This lecture will introduce the course by providing an
overview to the discipline of physiology, giving particular emphasis
to the concept of homeostasis and control theory. After completion
of this topic, students should be able to:
• 1. Explain the concept of a stable internal environment and
explain its importance.
• 2. Define homeostasis, and be able to give several examples of
homeostatic variables and how they are controlled.
• 3. Differentiate between negative and positive feedback systems,
being able to list the component parts of, and to give several
examples of each.
• 4. Distinguish between the conditions of equilibrium and steady
state.
• 5. Define redundancy and feed-forward regulation .
•
Organ Systems in Review
The integration between systems of the body
External and Internal Environments
“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
(1857)
• The Basis of
Physiological Regulation
• A Stable Internal
Environment Is Essential for
Normal Cell Function
Environments
Variable
Outside
Inside
Temperature
-10 and +40 °C 37 °C
PO2
160 mm Hg
PaO2 95 mm Hg
PCO2
0.23 mm Hg
40 mm Hg
pH
?/variable
pH 7.4
stability of the internal environment is the primary
condition for a free and independent existence-By
controlling its internal environment the organism is no longer
at the mercy of the environment
Body Fluid Compartments and
their Relationship
Blood Plasma
3L
Interstitial
11 L
Intracellular
28 L
Body
Transcellular
1L
•most cases substances within the plasma must pass through
the interstitial fluid before entering cells.
•Therefore the interrelationships between these 4
compartments are crucial in underlying whole body
homeostasis.
Body Fluid Constituents
Plasma
[Na+] = 142
[K+] = 4.4
[Cl-] = 102
[Protein] = 1
Osmolality
290 mOsm
Interstitial
[Na+] = 145
[K+] = 4.5
[Cl-] = 116
[Protein] = 0 mM
Osmolality
290 mOsm
Cellular
[Na+] = 15
[K+] = 120
[Cl-] = 20
[Protein] = 4
Osmolality
290 mOsm
•substances aren’t in equilibrium, but there is a balance
there is a difference between the basic constituents of the
body-fluid compartments. This means that homeostasis is not
about reaching equilibrium, but about maintaining a steadystate. Since the system is not necessarily in equilibrium energy
expenditure is required to maintain a steady state.
To summarize:
Homeostasis is the maintenance of a
steady state of the internal environment of
the body.
9
Homeostatic control
mechanisms
Feedback( flow of information along a closed loop )–
Negative or Positive
• Negative – change is sensed and action
taken to prevent further change e.gregulation of secretion of hormones.
• Positive – change is sensed and action
taken to amplify change (usually
associated with a discrete end point,
e.g. birth, ovulation)
Homeostatic Mechanisms
• Most homeostatic mechanisms are based
on negative feedback
• specific terms that are used to describe the processes
involvedControlled Variable
Sensor
Comparator, set point
Effectors
Blood Pressure Regulation
Blood Loss
Blood Pressure
(controlled variable)
Baroreceptor
(sensor)
Vasoconstriction
↑ Cardiac Output
(effectors)
Brain
(comparitor)
Cardiovascular
control
center – compares
BP to
set point and
adjusts
vascular tone and
cardiac output
accordingly
Blood Glucose -ve Feedback
b-cell
Insulin
secretion
Variable
Blood Glucose
Glucose
Cells
b-cell
“Gain” of a Control System
• The degree of effectiveness with which a
control system maintains constant, conditions
is determined by the gain of the negative
feedback.
• For instance, let us assume that a large volume of
blood is transfused into a person whose
baroreceptor pressure control system is not
functioning, and the arterial pressure rises from the
normal level of 100 mm Hg up to 175 mm Hg.
• Then, let us assume that the same volume of blood is
injected into the same person when the
baroreceptor system is functioning, and this time the
pressure increases only 25 mm Hg. Thus, the
feedback control system has caused a “correction” of
–50 mm Hg—that is, from 175 mm Hg to 125 mm
Hg. There remains an increase in pressure of +25 mm
Hg, called the “error,” which means that the control
system is not 100 per cent effective in preventing
change.
Q
Assume that excess blood is transfused into a patient
whose arterial baroreceptors are nonfunctional and
blood pressure increases from 100 to 150 mm Hg.
Then, assume that the same volume is blood is infused
into the same patient under conditions where his arterial
baroreceptors are functioning normally and blood
pressure increases from 100 to 125 mm Hg. What is
the approximate feedback “gain” of the arterial
baroreceptors in this patient when they are functioning
normally?
A) -1.0
B) -2.0
C) 0.0
D) +1.0
E) +2.0
Positive Feedback
• This is when instead of the comparator
causing the controlled variable to come back
to normal it potentiates the error signal and
the controlled variable moves further away.
• There are few normal physiological events
that are controlled by positive feedback.
Positive Feedback
All steps in this process produce an increase in the next step leading to a loop of
stimulation. The positive feedback loop is broken when the baby is expelled from
the uterus and hence the step involving pressure against the cervix has been
removed.
Contraction
oxytocin
Feed-forward Control
• Anticipation of change – gets the body
ready for change
• e.g. heart rate and ventilation can increase
even before exercise begins
• Or salivation and digestive enzyme
production begins before a meal is eaten
Redundancy
• Homeostatic mechanisms are important –
therefore often there is more than one
control mechanism
• If one mechanism fails – then there is a
backup system (e.g. control of cutaneous
blood vessels by both cardiovascular
control center and temperature control
center, or blood pressure)
Hypovolemic Shock
B.P. falls
Angiotensinogen
in blood
Renin
Kidney
Juxtaglomerular cells
Aortic arch
Carotid sinus
Activity drop
Angiotensin I
Hypothalamus
Posterior Pituitary
ACE
Medulla
oblongata
Sympathetic output
ADH
Angiotenin II
LUNG
Adrenal
Cortex
Kidney
Salt water conservation
Blood
Vessels
Heart rate
contractility
Aldosterone
Inc. vasc. resistance
Inc. volume
Inc.
B.P.
In response to a bacterial infection my body's
thermostat is raised. I start to shiver and produce more
body heat. When my body temperature reaches 101
degrees, I stop shivering and my body temperature stops
going up. This is an example of:
A) Negative feedback
B) A malfunctioning control system
C) Positive feedback
D) A negative impact
Which of the following is an example of a positive
feedback?
A) Shivering to warm up in a cold winter storm
B) A cruise control set on your car applies more gas when
going up a hill
C) You sweat on a hot summer's day and the blood vessels
in your skin vasodilate
D) You get cut and platelets form a clot. This in turn
activates the fibrin clotting system and more blood forms
clots
Where is the body's "thermostat" found?
A) Within the nervous system, in the
Hypothalamus
B) Within the integumentary system, in the skin
C) Within the brain, in the corpus callosum
D) Within the Urinary system, in the kidneys
What system has little to contribute to the
homeostasis of the organism?
A) Urinary System
B) Reproductive System
C) Respiratory System
D) Nervous System
Summary I
• Homeostasis – maintenance of a stable
internal environment
• Steady state – unchanging with time
• Equilibrium – when parameters are
maintained in an energetically favorable
situation
• Redundancy – more than 1 system to
control a variable (backup systems)
Summary II
• Negative feedback – feedback causes a
perturbation to be minimized or reversed
with view to keeping parameter at a set
point
• Positive feedback – amplification of a
deviation (usually defined end point)