FEEDBACK MECHANISM OF HOMEOSTASIS
Feedback Mechanism of Homeostasis
dr. Soewono
Block 3-International Programme
FEEDBACK MECHANISM OF HOMEOSTASIS
1. Homeostasis is a term introduced by Cannon as a doctrine that
the organisms freedom from disturbance in spite of extensive
changes in the outer world, has been brought about by mechanisms,
which maintain uniformity of the fluid matrix. This doctrine of the
steady states maintained in the internal environment, or fluid
matrix, and of the importance of constancy of this matrix for
continuous efficient action of the organism.
2. Nowadays physiologists use the term to mean maintenance of
[relative] static, or constant, conditions in the internal
environment. Furthermore, this means that the internal environment
in a dynamic equilibrium. Why? The internal environment baths the
cells, and provides the cells with gases and nutrients, but gets
the waste product from the cells.
3. Providing the food and eliminating the waste products the
internal environment must be helped by all the organs and tissues
in the body. This is the challenge faced by the multi cellular
organisms. The action of the organs and tissues must be under
controlled.
4. The body has many control systems, but all of them can be
divided into three groups of control system. The first is genetic
control system that operates in all cells to control intracellular
and extra cellular functions. The second is control system operate
within the organ to control functions of the individual parts of
the organ. The third is control system operate through out the
entire body to control the interrelationship between the
organs.
5. The genes control heredity from parents to children, but the
same genes also control day-by-day cells functions by determining
substances will be synthesized within the cells such as structures,
enzymes and chemicals. The genes (DNA) automatically control the
formation of RNA, which spread s through out the cell and controls
the formation of a specific protein, such as structural proteins
and enzymes.
6. The heart has a control system that regulates the heart rate
and rhythm. The control system begins with the pacemaker (SA node)
and the electrical currents spread in sequence through the
conductor system to the all parts of the heart. If some disorders
occur the heart will beat asynchronously. The phenomena also occur
in the gastrointestinal tract. There is a control system in the
wall of the gastrointestinal tract that regulates the peristalsis
movement.
7. The nervous and hormonal systems control all the body
activities; control the relationship between the organs. The
nervous system controls through generating electrical currents and
the hormonal system through releasing the chemical substances
called hormones. Both will affect the organs activities, so organs
have synchronous activity.
8. Each important extra cellular fluid constituent has normal
value, normal range and approximate non-lethal limits. It means
that all the control systems should operate to maintain the values,
because maintaining the values never cause death for short periods
of time. Value outside the range is usually the cause of or the
result of illness.
9. The constancy of the internal environment is not absolute;
under normal condition slight variations occur as with all
so-called physiological constants. If the stresses imposed become
too great the composition of the internal environment may alter
significantly, often with disastrous effects.
10. All the control systems have certain characteristics and
therefore can be divided based on their characteristics into
negative feedback, positive feedback and adaptive control system.
Most of the control systems are negative feedback, because it leads
to stability of the internal environment. The positive feedback, in
other hand, will lead to instability and often to death. The
adaptive feedback control system is delayed negative feedback.
11. The negative feedback system means that the response is
negative to the stimulus. If some factor becomes excessive or too
little, a control system initiates negative feedback, which
consists of a series of changes that return the factor toward a
certain value, thus maintaining homeostasis. For example in the
regulation of carbon dioxide and arterial pressure. The results are
the carbon dioxide still low and the arterial pressure still
normal.
12. The positive feedback system means that the initiating
stimulus causes more of the same. For example the heart of the
normal human being pumps about 5 liters of blood per minute, but if
suddenly bled 2 liters, the heart fails to pump effectively. The
arterial pressure decreases; the blood to the heart muscle also
decreases, and weakened the heart. The cycle repeats itself again
and again until death.
13. Positive feedback system is known as a vicious circle, but
actually a mild degree of positive feedback control system can be
overcome by the negative feedback control mechanisms of the body,
and a vicious circle will fail to develop. For instance if the
aforementioned person was bled only 1 liter, the normal negative
feedback mechanisms for controlling cardiac output and arterial
pressure would overbalance the positive feedback, and the person
would recover.
14. The positive feedback can sometimes be useful. The body has
learned to use positive feedback to its advantage. The positive
feedback itself is part of an overall negative feedback process.
For instance, in the case of blood clotting, the positive feedback
clotting process is a negative feedback process for maintenance of
normal blood volume. The positive feedback that causes nerve
signals allows the nerves to participate in thousands of negative
feedback nervous control systems.
15. The adaptive feedback control system aims to correct the
imprecise response of the body. For example if the body needs the
rapid movements, the brain uses a principle called feed-forward
control to cause the required muscle contractions. The sensory
signals from the moving parts apprise the brain in retrospect
whether or not the appropriate movement as envisaged by the brain
actually has been performed correctly. If not, the brain corrects
the next time the movement is required. Once again if still further
correction needs to be made, this too will be done for subsequent
movements.
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