Maintenance of internal environment constant
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The composition of internal environment is tightly controlled, and this constant
state is called Homeostasis.
This term means UNCHANGING.
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Homeostasis is maintained by control systems that detect and respond to changes
in the internal environment.
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A control system has three basic components:
A- detector (sensor).
B- control centre.
C- effector.
The control centre determines the limits within which the variable factor should be
maintained. It receive input from detector (sensor), and integrates the incoming
information.
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When the incoming signal indicates that an adjustment is needed, the control
centre responds and its output to the effector is changed.
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This is dynamic process that allows constant readjustment of many physiological
variable.
• In systems, controlled by negative feedback, the effector
response decreases or negates the effect of the original
stimulus, maintaining or restoring homeostasis.
• Body temperature is a physiological variable controlled by
negative feedback.
• A decrease in blood glucose that causes the inhibition of
insulin is an example of Negative feedback system.
• In positive feedback mechanism, the stimulus progressively
increases the response, so that as long as the stimulus is
continued the response is progressively amplified.
• E.g, blood clotting and uterine contractions during labour.
• During labour, contraction of uterus are stimulated by the hormone
oxytocin (OT).
• Afferent fibre stimulate hypothalamus, OT released from posterior
pituitary upon stimulation from hypothalamus.
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OT causes uterine contraction so move the head of baby towards
cervix, stimulating stretch receptors to open birth canal.
Example of Positive Feedback Mechanism
• Also known as integument.
• Skin completely covers the body and is continuous
with the membranes lining the body, it:
1- protects the structure from injury and from invasion by
microbes.
2- contain sensory nerve endings of pain,
temperature
and
touch.
3- is involved in the regulation of body temperature.
4- it act as a barrier against dehydration.
Skin
Dermis
Epidermis
Beneath epidermis
Thicker layer
Connective tissue
Outermost layer
Epithelial tissue
• Body temperature fairly remains constant 36.80C. It raised slightly in the evening, during
exercise, and in women just after ovulation.
• When the rate of metabolism rises temperature increases.
• To ensure this constant temperature, a balance is maintained between heat production in the
body and heat lost to the environment.
• Some of the energy released in the cells during metabolic
activity is in the form of heat and the most activity organs
produces the most heat:
1- constriction of blood vessels.
2- inactive sweat glands.
3- wearing extra clothes.
4- increase muscle contraction.
• Organs involve in heat production:
1- muscle.
2- liver.
3- digestive system.
1- Nervous Control:
• Part of hypothalamus control the temperature of
circulating blood, vasomotor centre in the medulla
oblongata regulate the diameter of small arteries and
arterioles to regulate the flow of blood in the dermis.
2- Activity of sweat glands:
• Sweating increase when body temp rises.
3- Effect of vasoconstriction and vasodilation:
• When heat production increases vessels become
vasodilated, when heat decreases vasoconstriction occurs.