HOMEOSTATIS

OBJECTIVES
• INTRODUCTION OF HOMEOSTATIS
• ROLE OF VARIOUS SYSTEMS OF THE BODY IN
HOMEOSTASIS
• „ROLE OF HUMAN CONTROL SYSTEMS OF THE
BODY IN HOMEOSTASIS
• „COMPONENTS OF HOMEOSTATIC SYSTEM
• „MECHANISM OF ACTION OF HOMEOSTATIC
SYSTEM
• „ NEGATIVE FEEDBACK
• „ POSITIVE FEEDBACK
• EXAMPLES
INTRODUCTION
‘Homeostasis’ refers to the maintenance of constant
internal environment of the body (homeo = same;
stasis
= standing). Importance of internal environment was
notified by the great biologist of 19th century Claude
Bernard. He enlightened the fact that multicellular
organisms including man live in a perfectly organized
and
controlled internal environment, which he called
‘milieu
interieur’. The word ‘homeostasis’ was introduced by
Harvard Professor, Walter B Cannon in 1930.
Internal environment in the body is the
extracellular
fluid (ECF) in which the cells live. It is the
fluid outside the cell and it constantly
moves throughout the body. It includes
blood, which circulates in the vascular
system
and fluid present in between the cells
called interstitial fluid. ECF contains
nutrients, ions and all other substances
necessary for the survival of the cells.
Normal healthy living of large organisms including
human beings depends upon the constant maintenance
of internal environment within the physiological limits.
If
the internal environment deviates beyond the set
limits,
body suffers from malfunction or dysfunction.
Therefore,
the ultimate goal of an organism is to have a normal
healthy living, which is achieved by the maintenance of
internal environment within set limits.
The concept of homeostasis forms basis of
physiology
because it explains why various physiological
functions are to be maintained within a normal
range and
in case if any function deviates from this range
how it is brought back to normal.
Understanding the concept of
homeostasis also forms the basis for clinical diagnostic
procedures. For example, increased body temperature
beyond normal range as in the case of fever, indicates
that something is wrong in the heat production-heat
loss
mechanism in the body. It induces the physician to go
through the diagnostic proceedings and decide about
the treatment.
For the functioning of homeostatic mechanism, the
body must recognize the deviation of any physiological
activity from the normal limits. Fortunately, body is
provided with appropriate detectors or sensors, which
recognize the deviation. These detectors sense the
deviation and alert the integrating center. The
integrating
center immediately sends information to the concerned
effectors to either accelerate or inhibit the activity so
that the normalcy is restored.
„ ROLE OF VARIOUS SYSTEMS OF THE
BODY IN HOMEOSTASIS
One or more systems are involved in homeostatic
mechanism of each function. Some of the functions in
which the homeostatic mechanism is well established
are given below:
1. The pH of the ECF has to be maintained at the
critical value of 7.4. The tissues cannot survive if
it is altered. Thus, the decrease in pH (acidosis)
or increase in pH (alkalosis) affects the tissues
markedly. The respiratory system, blood and kidney
help in the regulation of pH.
2. Body temperature must be maintained at
37.5°C.
Increase or decrease in temperature alters the
metabolic activities of the cells. The skin,
respiratory
system, digestive system, excretory system,
skeletal muscles and nervous system are
involved
in maintaining the temperature within normal
limits.
3. Adequate amount of nutrients must be supplied
to the cells. Nutrients are essential for various
activities of the cell and growth of the tissues. These
substances also form the source of energy required
for various activities of the cells. Nutrients must be
digested, absorbed into the blood and supplied to
the cells. Digestive system and circulatory system
play major roles in the supply of nutrients.
4. Adequate amount of oxygen should be made
available to the cells for the metabolism of the
nutrients. Simultaneously, the carbon dioxide and
other metabolic end products must be removed.
Respiratory system is concerned with the supply of
oxygen and removal of carbon dioxide. Kidneys and
other excretory organs are involved in the excretion
of waste products.
5. Many hormones are essential for the metabolism of
nutrients and other substances necessary for the
cells. Hormones are to be synthesized and released
from the endocrine glands in appropriate quantities
and these hormones must act on the body cells
appropriately. Otherwise, it leads to abnormal signs
and symptoms.
6. Water and electrolyte balance should be
maintained
optimally. Otherwise it leads to dehydration or
water toxicity and alteration in the osmolality of
the body fluids. Kidneys, skin, salivary glands
and
gastrointestinal tract take care of this.
7. For all these functions, the blood, which forms the
major part of internal environment, must be normal.
It should contain required number of normal red
blood cells and adequate amount of plasma with
normal composition. Only then, it can transport the
nutritive substances, respiratory gases, metabolic
and other waste products.
8. Skeletal muscles are also involved in
homeostasis.
This system helps the organism to move around
in
search of food. It also helps to protect the
organism
from adverse surroundings, thus preventing
damage
or destruction.
9. Central nervous system, which includes brain
and spinal cord also, plays an important role in
homeostasis. Sensory system detects the state
of the body or surroundings. Brain integrates and
interprets the pros and cons of these information
and commands the body to act accordingly through
motor system so that, the body can avoid the
damage.
10. Autonomic nervous system regulates all the
vegetative functions of the body essential for
homeostasis.
„ COMPONENTS OF HOMEOSTATIC SYSTEM
Homeostatic system in the body acts through self regulating
devices, which operate in a cyclic manner
(Fig. 4.1). This cycle includes four components:
1. Sensors or detectors, which recognize the
deviation
2. Transmission of this message to a control center
3. Transmission of information from the control center
to the effectors for correcting the deviation
Transmission of the message or information may
be an electrical process in the form of impulses
through nerves or a chemical process mainly in the
form of hormones through blood and body fluids
4. Effectors, which correct the deviation.
MECHANISM OF ACTION OF HOMEOSTATIC SYSTEM
Homeostatic mechanism in the body is responsible
for maintaining the normalcy of various body systems.
Whenever there is any change in behavioral pattern
of any system, the effectors bring back the normalcy
either by inhibiting and reversing the change or by
supporting and accelerating the change depending
upon requirement of the situation. This is achieved by
means of feedback signals.
Feedback is a process in which some proportion of
the output signal of a system is fed (passed) back to
the input. This is done more often intentionally in
order to control the behavior pattern of the
system. Whenever any change occurs, system
receives and reacts to two types of feedback:
1. Negative feedback
2. Positive feedback.
 HUMAN CONTROL SYSTEM
The human body has thousands of control systems in it the most
intricate of these are the genetic control systems that operate in all cells
to help control intracellular function as well as extracellular function.
many other control systems operate within the organs to control
functions of the individual parts of the organs, others operate through
out the entire body to control the inter relations between the organs. for
instance, the respiratory system operating in association with the
nervous system regulates the concentration of carbon dioxide in the
extracellular fluid. The liver and pancreas regulate the concentration of
glucose in the extracellular fluid and the kidneys regulate
concentrations of hydrogen, sodium, potassium, phosphate, and other
ions in the extracellular fluid.
FUNCTION OF CONTROL SYSTEM
Control system maintain HOMEOSTATIS.

HOMEOSTATIS :

“Continuous process of adjusting and maintaining

physiological system within body is termed as
homeostatic regulation.”

“Maintains nearly constant conditions in the internal

environment of the body despite large swings in the
external environment static regulation.”
 CHARACTERISTICS OF
CONTROL SYSTEM
Control system is characterized by two
main control mechanisms.

• POSITIVE FEEDBACK MECHANISM.

• NEGETIVE FEEDBACK MECHANISM.

 NEGATIVE FEEDBACK
❑ Response in a direction opposite to the
change that is set
❑ e.g. Increased carbon dioxide excites
respiratoy centre that in turn decreases its
conc. In body
❑ Similarly increase in arterial pressure
stimulates vasomotor centre that in turn
decreases arterial pressure
POSITIVE FEEDBACK
❑ Mostly viscious
❑ Also called VISCIOUS CYCLE
❑ e.g.human body pumps 5L blood/min
❑ If a person bled 2L of blood
❑ Amount of blood decreased in body
❑ Insufficient blood for heart to pump
effectively
❑ Results in weakening of heart
❑ Further diminished pumping
❑ Cycle repeats itself until death occurs
❑ Therefore called VISCIOUS CYCLE
 USEFUL EFFECTS OF POSITIVE
FEEDBACK
❑ Sometimes useful
❑ e.g.BLOOD CLOTTING
❑ Rupture of blood vessels
❑ Leads to activation of clotting factors
❑ That then activate some other enzymes
❑ That help in further clotting until hole in
vessele is pluged
The positive feedbacks occurs during the
blood clotting. Blood clotting is necessary to arrest bleeding during injury and
it occurs in three stages.
The three stages are:
i. Formation of prothrombin activator
ii. Conversion of prothrombin into thrombin
iii. Conversion of fibrinogen into fibrin.
Thrombin formed in the second stage stimulates the formation of more
prothrombin activator in addition to converting fibrinogen into fibrin.
It causes formation of more and more amount of prothrombin activator so
that the blood clotting process is accelerated
and blood loss is prevented quickly (Chapter 20). Other processes where
positive feedback occurs are milk EJECTION reflex
2nd EXAMPLE:
❑ Generation of nerve signals
❑ Stimulation of nerve
❑ Opening of sodium channels
❑ Changes membrane potential
❑ Further opening of channels until
action potential develops
 SUMMARY-AUTOMATICITY OF BODY
❑ Cell –functional unit of body
❑ Aggregate to form organs
❑ Appropriate cells do appropriate
functions
❑ Organs are functional structres of body
❑ Each and every system has to perform
its function to maintain homeostasis
❑ If a system looses its ability to
contribute its share of function,all
cells of body suffer
❑ Moderate dysfunction leads to
sickness
❑ Extreme dysfunction leads to death
 Positive feedback
Positive feedback mechanisms are designed to
accelerate or enhance the output created by a
stimulus that has already been activated.

Also called Vicious cycle because the positive

feedback mechanisms are designed to push
levels out of normal ranges.

This process can be beneficial but is rarely used by

the body due to risks of the acceleration's
becoming uncontrollable.
 EXAMPLE OF POSITIVE FEEDBACK
1.BLOOD CLOTTING :
•Injured tissue releases chemicals signals to the brain
•Brain activates the clotting factors for example PROTHOMBIN
PROTEIN.
•It activates the white blood cells for example platelets
• An activated platelet releases chemicals to activate more
platelets.

RESULT:

•Clotting proceed until break is sealed by newly formed clot.

•Clotting of blood is harmful in some expects like in veins and
arteries causing myocardial infarction.
 2. Ferguson Reflex
• Contractions of smooth muscles for
example uterine walls during childbirth.
• Oxytocin and Prostaglandins is released.
• Uterine contractions are stimulated
(which will in turn increase oxytocin and
Prostaglandins production).
• Contractions increases in the walls of
uterus.

Result:

• CERVIS DILATE AND BABY COMES OUT

FROM THE BODY OF MOTHER.
POSITIVE FEEDBACK OF
OXYTOCIN
 Negative feedback :

• Negative feedback mechanisms consist of

reducing the output or activity of any organ or
system back to its normal range.

• It initiate or regulate physiological functions

within a set.

• It depresses stimulus to shut off or reduce the

effecter response and make the body system
stable.
 EXAMPLE OF NEGATIVE FEEDBACK
1.LEVEL OF GLUCOSE IN BLOODSTREAM
HORMONES:

• INSULIN and ADRENALINE

a. FACTOR:

• Glucose from the ingested lactose or sucrose is absorbed in the

intestine and the level of glucose in blood rises.

MECHANISM:

• Insulin hormones travel to target tissue and initiate a corrective

response.
• Insulin is secretion in the pancreas which converts the excess glucose
into glycogen.
 b. FACTOR:

• When the level of glucose

decreases.

MECHANISM:

• Adrenaline is released by the

adrenal gland.
• It helps in the synthesis of
glucose back from the glycogen.

RESULT:

• LEVEL OF SUGAR IN THE

BLOOSTREAM IS BALANCED AND
HOMEOSTATIS IS MAINTAINED.
BLOOD FLOW IN HUMAN BODY
 2. Regulation of TSH LEVEL.
Responsible for maintaining the levels of Thyroid Hormones
in the body.
HORMONES:

• Thyroxin (T4) and Triiodothyronine (T3).

MECHANISM:

• Neurons in the hypothalamus secrete thyroid releasing

hormone (TRH).

• TRH stimulates cells in the pituitary to secrete

thyroid-stimulating hormone (TSH).

• TSH synthesis and secrete THYROID HORMONE, which

affect probably all cells in the body.

RESULT :

• HOMEOSTATIS OF GROWTH,DEVELOPMENT AND

METABOLISM OF SALT AND WATER IS REGULATED.
 Conclusion:
These positive and negative feed backs of
human control system maintains the nearly
constant homeostatic conditions of the
human body.