Transcript Positive feedback system

LEC: CONTROL OF INTERNAL
ENVIRONMENT
BY DR FARIHA RIZWAN
Homeostasis:
 homeo = same;
 stasis = standing
 Homeostasis =constancy of the internal fluid
environment.
Definition:
 Maintenance of internal environment
Homeostasis
 Maintenance of nearly
constant conditions in
the internal
environment during
unstressed conditions
Hemostasis
 Stoppage of bleeding
when blood vessel is
injured
 The term homeostasis is defined as the maintenance
of a constant internal environment during unstressed
conditions.
 A similar term, steady state , is often used to denote
a steady and unchanging level of some physiological
variable (e.g., heart rate).
 The term steady state is also defined as a constant
internal environment, but this does not necessarily
mean that the internal environment is at rest and
normal.
 When the body is in a steady state, a balance has been
achieved between the demands placed on the body
and the body's response to those demands
 So the term homeostasis is generally reserved
for describing normal resting conditions
 and the term steady state is often applied to
exercise where in the physiological variable in
question (i.e., body temperature) is
unchanging but may not equal the
“homeostatic” resting value
CONTROL SYSTEMS OF THE BODY
 The body has literally hundreds of different control systems, and the
overall goal of most is to regulate some physiological variable at or
near a constant value
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The most intricate of these control systems reside inside the cell
itself. These cellular control systems regulate cell activities such as
protein breakdown and synthesis, energy production, and
maintenance of the appropriate amounts of stored nutrients
 Almost all organ systems of the body work to help maintain
homeostasis
 For example, the lungs (pulmonary system) and heart (circulatory
system) work together to replenish oxygen and to remove carbon
dioxide from the extracellular fluid.
 The fact that the cardiopulmonary system is
usually able to maintain normal levels of oxygen
and carbon dioxide even during periods of
strenuous exercise is not an accident but the end
result of a good control system.
 Although much is known about how specific
control systems of the body operate, the details of
how many control systems work to maintain
homeostasis remain a mystery.
 This remains an active area of research in exercise
physiology
NATURE OF CONTROL SYSTEMS
 To develop a better understanding of how the body
maintains a stable internal environment, let's begin with
the analogy of a simple, nonbiological control system such
as a thermostat-regulated heating and cooling system in a
home.
 Suppose the thermostat is set at 20° C. Any change in
room temperature away from the 20° C “set point” results
in the appropriate response by either the furnace or the air
conditioner to return the room temperature to 20° C.
 If the room temperature rises above the set point, the
thermostat signals the air conditioner to start, which
returns the room temperature to 20° C.
 In contrast, a decrease in temperature below
the set point results in the thermostat
signaling the heating system to begin
operation
 In both cases the response by the heating and
cooling system was to correct the condition,
low or high temperature, that initially turned
it on.
COMPONENTS OF CONTROL
SYSTEMS
 Similar to the example of a mechanical control
system, a biological control system is a series
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of interconnected components that maintain a
chemical or physical parameter of the body near
a constant value .
Biological control systems are composed of
three elements:
(1) a sensor (or receptor);
(2) a control center (i.e., center to integrate
response)
(3)effectors (i.e., organs that produce the
desired effect)
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The signal to begin the operation of a control system is the stimulus that
represents a change in the internal environment (i.e., too much or too
little of a regulated variable).
The stimulus excites a sensor that is a receptor in the body capable of
detecting change in the variable in question.
The excited sensor then sends a message to the control center.
The control center integrates the strength of the incoming signal
from the sensor and sends an appropriate message to the effectors to
bring about the appropriate response to correct the disturbance (i.e.,
desired effect).
The return of the internal environment to normal results in a decrease in
the original stimulus that triggered the control system into action.
This type of feedback loop is termed negative feedback and is the primary
method responsible for maintaining homeostasis in the body
NEGATIVE FEEDBACK
 Most control systems of the body operate via negative feedback
 An example of negative feedback can be seen in the respiratory
system's regulation of the CO 2 concentration in extracellular
fluid.
 In this case, an increase in extracellular CO 2 above normal levels
triggers a receptor, which sends information to the respiratory
control center (integrating center) to increase breathing.
 The effectors in this example are the respiratory muscles. This
increase in breathing will reduce extracellular CO 2
concentrations back to normal, thus reestablishing homeostasis.
 The reason that this type of feedback is termed negative is that
the response of the control system is negative (opposite) to the
stimulus.
POSITIVE FEEDBACK
 Although negative feedback is the primary type
of feedback used to maintain homeostasis in the
body, positive feedback control loops also exist.
 Positive feedback control mechanisms act to
increase the original stimulus.
 This type of feedback is termed positive because
the response is in the same direction as the
stimulus.
 A classic example of a positive feedback
mechanism is child birth.
Extracellular Fluid
 60 % of human body is fluid.
(Total fluid =42 L,ICF=28 L)
 One third of it is present in the
spaces outside the cell and called
extracellular fluid (ECF). (Blood +
interstitial fluid+trans cellular
fluid) = (3 + 11 = 14L)
 ECF is in constant motion in exchange with
blood and body cells.
 Contains ions and nutrients needed to
maintain cell life and receive cell wastes.
 Provide internal environment (milieu interior)
for body cells.
 The body cells are capable of living, growing
and performing special functions as long as
internal environment remain constant.
 The internal environment in the body is the
extracellular fluid in which the cells live.
 It’s the fluid outside the cell and it constantly
moves throughout the body.
 It includes the blood which circulates in vascular
system and fluid present b/w the cells called
interstitial fluid.
 ECF contains nutrients ,ions and all other
substances necessary for the survival of the cells.
Differences between intracellular
and extracellular fluids
 Extracellular fluid
 14 L
 Large amount of
Sodium, chloride and
bicabonate ions.
 Oxygen and carbon
dioxide.
 Nutrients: Glucose,
fatty acids and amino
acids.
 Intracellular fluid
 28 L
 Potassium, magnesium
and phosphate
ECF
 Ions & nutrients form
constant internal
environment / milieu
(Bernard 19th cent.)
 Sodium, chloride,
bicarbonate, oxygen,
glucose, fatty acid, amino
acid, carbon-dioxide (cell
 lungs), wastes (cell 
kidneys).
ICF
 Potassium ion.
 Magnesium ion.
 Phosphate ion.
Why ECF is called the
internal environment?
 ECF has ions & nutrients needed by the cells to maintain
cell life.
 ECF = internal environment / milieu interieur
 Cell growth & functions depend on proper concentration
of components of internal environment (oxygen, glucose,
different ions, amino acids, fatty substances etc).
Examples of homeostatic
control
 Regulation of body temperature (37.5 degree C)
 Regulation of blood glucose
 Stress proteins assists in regulation of cellular
homeostasis
 The pH of the extracellular fluid has to be
maintained at the critical value of 7.4
 The respiratory system and the kidney help in the
regulation of pH.
 The supply of nutrients must be adequate
 Nutrients must be digested absorbed into the blood
and supplied to the cells.(digestive system)
THERMOREGULATION
REGULATION OF BLOOD GLUCOSE
 Homeostasis is also a function of the endocrine system.
 The body contains eight major endocrine glands, which synthesize
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and secrete blood borne chemical substances called hormones.
Hormones are transported via the circulatory system throughout the
body as an aid to regulate circulatory and metabolic functions .
An example of the endocrine system’s role in the maintenance of
homeostasis is the control of blood glucose levels.
For example, the hormone insulin regulates cellular uptake and the
metabolism of glucose and is therefore important in the regulation
of the blood glucose concentration.
After a large carbohydrate meal ,the blood glucose level increases
above normal
The rise in blood glucose signals the pancreas to release insulin,
which then lowers blood glucose by increasing cellular uptake.
Failure of the blood glucose control system results in disease
Diabetes
REGULATION OF BLOOD GLUCOSE
TO MAINTAIN HOMEOSTATSIS
STRESS PROTEINS ASSIST IN
REGULATION OF CELLULAR HOMEOSTASIS
 A disturbance in cellular homeostasis occurs when a
cell is faced with a “stress” that surpasses its ability
to defend against this particular type of disturbance.
 ” The cellular stress response is a biological control
system in cells that battles homeostatic disturbances
by manufacturing proteins designed to defend
against stress
 A brief overview of the cellular stress response
control system and how it protects cells against
homeostatic disturbances follows.
STRESS PROTEINS ASSIST IN REGULATION OF
CELLULAR HOMEOSTASIS
 At the cellular level, proteins are important in
maintaining homeostasis.
 For example, proteins play critical roles in normal
cell function by serving as intracellular transporters
or as enzymes that catalyze chemical reactions.
 Damage to cellular proteins by stress (e.g., high
temperature) can result in a disturbance in
homeostasis.
 To combat this type of disruption in homeostasis,
cells respond by rapidly manufacturing protective
proteins called stress proteins
 After synthesis, these stress proteins go to work to protect the cell
by repairing damaged proteins and restoring homeostasis.
 The above mentioned figure provides an overview of how this
control system regulates protein homeostasis in cells.
 The process starts with a stressor that results in protein damage.
 Stresses associated with exercise that are known to produce cellular
protein damage include high temperatures, reduced cellular oxygen,
low pH, and the production of free radicals.
 Damaged proteins become signals for the cell to produce stress
proteins.
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After synthesis, these stress proteins work to repair damaged
proteins and restore homeostasis
Digestive system-GASTROINTESTINAL TRACT:
Origin of Nutrients in ECF:
 Absorption of dissolved nutrients
(carbohydrates, fatty acids, amino acids)
from ingested food  ECF.
Skin and temperature regulation
Sensation of cold Shivering,
We look for warmth
Excretory system: Removal of
Metabolic End Products
 Removal of CO2 by the
lungs:
Simultaneous processes:
 O2 from lungs  blood
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 CO2 from blood lung
alveoli  atmosphere
(expiration).
Note: Most abundant end
product of
metabolism
is CO2
 Removal of other end products
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of cellular metabolism (besides
CO2) Kidneys:
Urea
Uric acid
Excess of ions
Excess of water
Filtration of plasma followed by
reabsorption of useful
substances (glucose, amino
acids, water, ions)
Un-useful substances (urea) are
poorly reabsorbed  renal
tubules  urine
Nervous system
 Fit to survive under varying conditions
 Hunger  Hunger center in hypothalamus we seek
food.
Maintains homeostasis by generating new
beings to take the place of those that are
dying.
REPRODUCTION: ROLE IN
HOMEOSTASIS
 The autonomic nervous system regulates all
the vegetative functions of the body essential
for homeostasis.
 Water and electrolyte balance should be
maintained . Otherwise it may lead to
dehydration or water toxicity .
 Kidneys ,skin , salivary glands and GIT take
care of this.
NATURE OF CONTROL SYSTEMS
 A feedback system or feedback loop is a cycle of
events in which the status of a body condition is
monitored, evaluated, changed, re-monitored,
reevaluated, and so on.
 Each monitored variable, such as body temperature,
blood pressure, or blood glucose level, is termed a
controlled condition.
 Any disruption that changes a controlled condition is
called a stimulus.
 A feedback system includes three basic
components—a receptor, a control center, and an
effector.
Normal
feedback
system
Negative
feedback
system
A negative feedback
system reverses a
change in a controlled
condition.
 When thyroxine secretion is increased ,it
inhibits the secretion of TSH from pituitary so
that secretion of thyroxine from thyroid
decreses and vice versa.
 Another example is maintenance of water
balance in the body
Positive feedback
system
A positive
feedback system
tends to
strengthen or
reinforce a change
in one of the
body’s controlled
conditions.
Examples of positive
feedback system
• 1) BLOOD CLOTTING
• 2) CHILDBIRTH
 Local disease affects one part or a limited region of
the body.
 Systemic disease affects either the entire body or
several parts of it.
 A person with a disease may experience symptoms,
Subjective changes in body functions that are not
apparent to an observer, e.g. headache, nausea, and
anxiety.
 Objective changes that a clinician can observe and
measure are called signs. Signs of disease can be
either anatomical, such as swelling or a rash, or
physiological, such as fever, high blood pressure, or
paralysis.
CONCLUSION:
 All body structures are so organized by
nature that they help to maintain the
automaticity & continuity of life.
Gain of a Control System
 The precision with which a control system
maintains homeostasis is called the gain of
the system.
 Gain can be thought of as the “capability” of
the control system.
 This means that a control system with a large gain is
more capable of correcting a disturbance in
homeostasis than a control system with a low gain.
 As you might predict, the most important control
systems of the body have large gains.
 For example, control systems that regulate body
temperature, breathing (i.e., pulmonary system),
and delivery of blood (i.e., cardiovascular system) all
have large gains.
 The fact that these systems have large gains is not
surprising, given that these control systems all deal
with life-and-death
EXERCISE: A TEST OF
HOMEOSTATIC CONTROL
 Muscular exercise can be considered a
dramatic test of the body's homeostatic
control systems
 Because exercise has the potential to disrupt
many homeostatic variables.
• For example, during heavy exercise, skeletal
muscle produces large amounts of lactic acid,
which causes an increase in intracellular and
extracellular acidity .
 This increase in acidity represents a serious
challenge to the body's acid-base control system .
 Additionally, heavy exercise results in large increases
in muscle O 2 requirements, and large amounts of
CO 2 are produced.
These changes must be countered by
 increases in breathing (pulmonary ventilation) and
 blood flow to increase O 2 delivery to the exercising
muscle and remove metabolically produced CO 2 .
 Further, during heavy exercise the working
muscles produce large amounts of heat that
must be removed to prevent overheating.
 The body's control systems must respond
rapidly to prevent drastic alterations in the
internal environment.
SUMMARY
 Exercise represents a challenge to the body's
control systems to maintain homeostasis.
 In general, the body's many control systems are
capable of maintaining a steady state during
most types of sub maximal exercise in a cool
environment.
 However, intense exercise or prolonged work in
a hostile environment (i.e., high
temperature/humidity) may exceed the ability of
a control system to maintain a steady state, and
severe disturbances in homeostasis may occur.
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