Thermoregulation

Homeostasis

Homeostasis• organ systems are interdependent

• share same environment

• composition effects all inhabitants

• internal environment must be kept stable

• maintaining stable internal environments-homeostasis

–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

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 temperaturemessage sent to preoptic nucleus heat loss center (also in hypothalamus)stimulated sets off series of events heat loss

• inhibition of vasomotor centerperipheral vasodilationwarm blood flows to skin’s surface

• as skin temperatures rise, radiation & convection loses increase

• sweat glands stimulated increase output evaporative loss increases

• respiratory centers stimulateddepth of respiration increases

Mechanisms for Heat Gain• skin receptors notice temperature is

droppingpreoptic 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 stimulatedhair

stands on endtraps air near the skin

Heat Gain• if vasoconstriction cannot restore or maintain

core temperatureshivering thermogenesis begins– gradual increase in muscle tone– increases energy consumption by skeletal muscle

throughout body– increases work load of muscles & elevates O2

& energy consumptionproduces 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 ANSepinephrine released increases rate of glycogenolysis (break down of glycogen) in liver & skeletal musclemetabolic rate increases

• preoptic nucleus regulates production of TRH-thyrotropin releasing hormone by the hypothalamus

• TRH increases production of thyroxin by thyroid gland• Thyroxin is a key hormone in control of metabolism