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Thermoregulation
Homeostasis
Homeostasis
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organ systems are interdependent
share same environment
composition effects all inhabitants
internal environment must be kept stable
maintaining stable internal environmentshomeostasis
–dynamic equilibrium
Homeostasis
• varies around a Set Point
–average value for a variable
• specific for each individual
–determined by genetics
• normal ranges for a species
–temperature 36.7 – 37.2
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Parts of Homeostatic
Regulation
Receptor
– sensitive to environmental change or
stimuli
• Control or Integration Center
– receives & processes information
supplied by receptor
– determines set point
• Effector
– cell or organ which responds to
commands of control center
HOMEOSTATIC REGULATION
• Autoregulation
– cells, tissues, organs adjust
automatically to environmental changes
• Extrinsic Regulation
– Nervous System
• Fast
• Short lasting
• Crisis management
– Endocrine System
• Longer to react
• Longer lasting
FEEDBACK LOOPS
• Negative Feedback
– output of system shuts off or reduces intensity
of initiating stimulus
– most often seen in the body
• Positive Feedback
– initial stimulus produces a response that
exaggerates or enhances its effect
– blood clotting & child birth
Negative Feedback
Temperature Extremes
• Humans are subjected to vast
changes in environmental
temperatures
• Enzymes operate over very
narrow range of temperatures
• Failure to control body
temperature can result in
physiological changes &
damage
• body has several
mechanisms to maintain body
temperature
• Thermoregulation
Temperature
• Core temperature
– most important body temperature
– temperature of organs in major
cavities
– rectal temperature gives best
estimation
• Shell temperature
– temperature closer to surface
– skin & oral temperatures
Mechanisms of Heat Transfer
• Radiation
• Conduction
• Convection
• Evaporation
Evaporation
• water changes
from liquid to
vapor
Thermoregulation
Homeostasis
• Control Center
– preoptic area of
hypothalamus
• Receptors
– in skin
• Effectors
– eccrine sweat
glands & blood
vessels
Mechanisms for Heat Loss
• skin receptors detect
increase in
temperaturemessage
sent to preoptic nucleus
heat loss center (also in
hypothalamus)stimulated
sets off series of events
heat loss
• inhibition of vasomotor
centerperipheral
vasodilationwarm blood
flows to skin’s surface
• as skin temperatures rise,
radiation & convection
loses increase
• sweat glands stimulated
increase output
evaporative loss increases
• respiratory centers
stimulateddepth of
respiration increases
Mechanisms for Heat Gain
• skin receptors notice temperature is
droppingpreoptic nucleus notificed
• heat loss center inhibited
• heat gain center activated
• sympathetic vasomotor center decreases blood
flow to dermis of skin
• vasoconstriction reduces heat loss by
radiation, convection & conduction
• blood returning from limbs is shunted into deep
veins
• Piloerector muscles are stimulatedhair
stands on endtraps air near the skin
Heat Gain
• if vasoconstriction cannot restore or maintain
core temperatureshivering thermogenesis
begins
– gradual increase in muscle tone
– increases energy consumption by skeletal muscle
throughout body
– increases work load of muscles & elevates O2
& energy consumptionproduces heat which
warms deep vessels to which blood has been
shunted by sympathetic vasomotor center
• can increase rate of heat generation by
400%
Heat Gain
• Non shivering thermogenesis
– long term mechanism for heat production
• sympathetic nervous system & thyroid hormone
produce an increase in metabolism
• Heat gain center stimulates adrenal medulla via
sympathetic ANSepinephrine released increases
rate of glycogenolysis (break down of glycogen) in
liver & skeletal musclemetabolic rate increases
• preoptic nucleus regulates production of TRHthyrotropin releasing hormone by the hypothalamus
• TRH increases production of thyroxin by thyroid gland
• Thyroxin is a key hormone in control of metabolism