· Homeostasis is defined as maintenance of a constant internal environment. The term homeostasis was coined by W B Cannon.
·
Internal
environment refers to Extracellular Fluid and all the tissues are dependent on
extracellular fluid for their survival. Hence maintenance of internal
environment / extracellular fluid is very much essential for survival of life.
The extracellular fluid includes blood and interstitial fluid.
·
The
term internal environment or mileu interior was coined by Claude Bernard.
·
Homeostasis
is maintained in the body by Feedback mechanism and Adaptive control system.
·
Feedback
mechanisms
·
Feedback
mechanism of is of two types
o
Negative
feedback mechanism
o
Positive
feedback mechanism
·
Negative
feedback mechanism
o
If
the activity of a particular system is increased or decreased, a control system
initiates a negative process which brings about the change towards normal. The
action is negative to the initial response. Most of the regulatory mechanism in
the body is by negative feedback. Eg:
§ Regulation of
blood pressure
§ Secretion of
thyroid hormone
o
Regulation
of blood pressure
§ When the blood
pressure suddenly rises or lowers, it initiates a series of reactions that
tries to bring the blood pressure to normal levels.
o
Regulation
of thyroid hormone secretion
§ When thyroid
hormone secretion is more, it inhibits the secretion of thyroid stimulating
hormone from pituitary so that thyroid hormone is not secreted from the thyroid
gland.
·
Positive
feedback mechanism
o
In
this mechanism the system reacts in such a way to increase the intensity of
change in the same direction. Usually it is harmful. Eg:
§ Formation of
blood clot
§ Uterine
contractions during labour
o
Formation
of blood clot
§ Injury to the
blood vessel and bleeding causes formation of prothrombin activator which
converts prothrombin into thrombin. Thrombin accelerates formation of
prothrombin activator as well as converts fibrinogen to fibrin which arrests
bleeding
o
Uterine
contractions during labour
§ During onset of
labor contraction of uterus cause movement of fetus into cervix leading to its
dilation. Movement of fetus through cervix stimulates the receptors present
there and the impulse generated stimulates hypothalamus to release oxytocin
which further contracts the uterus.
·
Adaptive
control system
o
It
is also known as feed-forward control or servo mechanism. It is a delayed type
of negative feedback mechanism. In a
feed-forward system, the output of one stage of the processing of the control
system is sent to a later stage of the process to affect later activity. Eg:
§ Preadaptation for exercise: changing the activity of postural
muscles and of the vascular system in order to ready the body for the movement
when it occurs. Moving the arm laterally shifts the center-of-gravity
laterally, and the person would be in danger of falling over.
