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.
Cont...
• Mechanisms that raises the body temperature are:
1- stimulation of skeletal muscle causes shivering.
2- narrowing of the blood vessels in the skin reducing
blood flow to, and heat loss from the peripheries.
3- behavioural changes, e.g, we put on more clothes or
curl up.
the
• Most of the homeostatic controls in the body use
negative feedback mechanisms to prevent sudden and
serious changes in the internal environment.
• 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
• This arises when the fine control of a factor in the
internal environment is inadequate and the level
of the factor falls outside the normal range.
• If the control system cannot maintain
homeostasis, an abnormal state develops that
may threaten health, or even life.
• 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 againt dehydration.
Skin
Dermis
Epidermis
Beneath epidermis
Thicker layer
Connective tissue
Outermost layer
Epithelial tissue
• Most superficial layer of the skin.
• Stratified squamous epithelium.
• No blood vessels (avascular), no nerve supply, but its deeper layers
are bathed in interstitial fluid from the dermis, which provides
oxygen and nutrients, and is drained away as lymph.
• In palms of hand and soles of feet, epidermis is thicker.
• Composed of five layers:
[1] Stratum corneum
[2] Stratum lucidum
[3] Stratum granulosum
[4] Stratum spinosum
[5] Stratum basale
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Stratum spinosum and stratum basale are called as germinative layer.
• Deepest cell layer.
• Receives most adequate nutrients from blood in
the dermal vessels.
• Mitosis occurs in this layer.
• Melanocytes chiefly present in this layer, causes
skin pigmentation from yellow to brown to black
• Thick and outermost layer, accounts for 2/3rd of the epidermal
thickness.
• Cells on the surface are flat, thin, non-nucleated, dead cells, or
squames, in which the cytoplasm has been replaced by the
fibrous protein KERATIN.
• This layer continuously rubbed off and replace by new cells.
• Blisters develop when trauma causes separation of the dermis
and epidermis and serous fluid collects between the two layers.
• True skin, Highly elastic, tough and flexible
tissue.
• Meshwork of collagenous, reticular and
elastic fibres.
• this layer has blood vessels, lymph vessels,
sensory nerve endings, sweat glands, hairs,
arrector pili muscle and sebaceous glands.
• 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.
• Most heat loss occur through skin, small amount are lost by expired air,
urine, and faeces.
• Heat loss is affected by difference between body and environmental
temperature.
1- in evaporation:
• the body is cooled when heat is used to convert water in sweat to water
vapour.
2- in radiation:
• exposed parts of the body radiate heat away from the skin.
3-Inconduction:
• Clothes and other objects in contact with the skin take up heat.
4- In convection:
• Air passing over the exposed parts of the body is heated and rises, cool air
replaces it and convection currents are setup
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.
• Result of infections, caused by the release of chemical
known as pyrogen from the damage tissue.
• That activate heat-promoting mechanisms e.g, shivering,
vasoconstriction.
• This occur when core temperature is below 35oC.at 32oC
shivering is replaced by muscle rigidity and cramps.
Vasoconstriction fails and blood pressure, pulse and
respiration rates fall. Mental confusion occur.
• Death occur when temperature falls below 25oC.
• Can be caused by, heat, cold, electricity, and chemicals.
• Damage occurs disrupting the structure and functions of the skin.
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infection is its common complication.
• Burns are classified according to their depth:
1- partial thickness (superficial):
• only epidermis is involve, sign of inflammation and sometimes blisters.
2- full thickness (full):
• Epidermis and dermis are destroyed.
• Is painless because nerve endings destroy with dermis and epidermis.
• Destroyed tissues coagulate and form eschars
1- dehydration and hypovolaemia:
• May occur in extensive burn due to excessive leakage of
water and plasma protein from the surface of damage skin.
2- shock:
• This may be due to severe hypovolaemia.
3- hypothermia:
• This develop when excessive heat is lost.
4- infection:
• This may result in septicaemia: