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Homeostasis
State of steady internal, physical and chemical conditions maintained by living
systems
...

Factors homeostatically regulated: concentration of nutrient molecules/
concentration of CO2 and O2/ concentration of waste products/ concentration
of water, salt and other electrolytes/ pH/ temperature/ volume and pressure

Cellular Homeostasis
Intracellular fluid: contained within all cells of the body
Extracellular fluid: outside the cells of the body, the internal environment in
which cells live, composed of plasma and interstitial fluid

Homeostatic Control System
Functionally interconnected network of body components that operate to
maintain a given physical or chemical factor in the internal environment
relatively constant around an optimal level
Intrinsic controls: inherently compensatory responses of an organ to a
change
Extrinsic controls: responses of an organ that are triggered by factors
external to the organ e
...
by the nervous & endocrine systems

Negative Feedback
Change in a homeostatically control factor triggers a response that seeks
to restore the factor to normal by moving the factor in the opposite
direction of its initial change, or it is a pathway where the response
opposes or removes the signal
Stimulus → Receptor → Control Centre → Effector → → → Stimulus

Homeostasis

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Positive Feedback
Output is continually enhanced or amplified so that the controlled variable
continues to be moved in the direction of the initial change or pathway in
which the response reinforces the stimulus
Stimulus → Response → Feedback cycle → Stimulus → Response →
Stimulus
Can be dangerous due to self-amplifying capabilities
Induction of labour, Blood Clotting

Homeostasis

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Feedforward mechanism
Brings about a compensatory response in anticipation of a change in a
regulated variable

Cellular Homeostasis
Intracellular and extracelluar environments consists of ions at differing
concentrations
Cell membrane separates intracellular and extracellular environments
Membrane is selectively permeable; allows the movement of some
materials and not others, thus regulating the cell's content
e
...
Oxygen & Carbon Dioxide diffuse readily, water and Cl- diffuse through
membrane pores, Na+ and K pass through specific ion channels

Intracellular: fluid contained within all the cells of the body
Extracellular: fluid outside the cells of the body; the internal environment in
which cells live; composed of plasma and interstitial fluid

Homeostatic Control System
Instrinsic local controls are inherent compensatory responses of an organ
to a change
Extrinsic controls are responses of an organ triggered by factors external
to the organ e
...
by nervous and endocrine systems

Thermoregulation
A negative feedback loop
Hypothalamus (endocrine) triggers involuntary responses

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Cerebral cortex (neural) triggers voluntary responses
Constant Core Temperature
36
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3 97
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1
Body temperature regulated by the hypothalamus; recieves input from
peripheral and central thermoreceptors, constantly monitors
temperature conditions and signals appropriate change to maintain our
internal core temperature

Ongoing balance of heat production, heat gain and heat loss keeps
core temperature relatively constant
Range provides optimal conditions for physiology reactions to occur (if
cooled, enzymatic reactions slow, if heated, enzymes at risk of
denaturing)
Thermoneutral Zone
27
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g
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contraction of the muscle pulls on hair follicles to create a
small depression, making surrounding area protrude
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Free nerve endings (thermoreceptors): sense changes in external
temperature and signal to hypothalamus
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Subcutaneous fat: provides layers of insulation to retain body heat
...
process of vaporization removes heat from the body
Free nerve endings (thermoreceptors): sense changes in external
temperature and signal to hypothalamus
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Physiological mechanisms of heat loss
Vasodilation
Shunt vessels dilate which diverts the flow of heated blood to the
skin
Arterioles supplying superficial capillaries also dilate
Once at the surface, heat can be lost from the body to the external
environment through radiation, conduction, convection and
evaporation of surface water

Sweating
Stimulation by the sympathetic nervous system causes sweat
glands to secrete isosmotic salt solution that moves towards
surface of body
...

Results in hypotonic solution that is delivered to the body surface
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Homeostasis

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Evaporation
When water vaporises, requires significant amounts of energy
...
Higher the temperature, most risk of damage to cells,
with denaturing of critical proteins and consequent organ failure
...
5
Severe hyperthermia: heatstroke 40
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In pyrexia, hypothalamic set point is reset,
hence cold feeling at the beginning; this does not occur in
hyperthermia

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Homeostasis

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