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Thermoregulation Homeostasis Temperature Extremes • Humans are subjected to vast changes in environmental temperatures • Enzymes operate over a very narrow range of temperatures • Failure to control body temperature can result in physiological changes & damage • The 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 note increase in temperaturemessate sent to preoptic nucleus heat loss center (also in the hypothalamus) stimulatedsets off series of events to produce heat loss • inhibition of vasomotor centerperipheral vasodilationwarm blood flows to skin’s surface • as skin temperatures rise, radiation & convection loses increase • sweat glands are stimulated increase outputevaporative loss increases • respiratory centers are stimulateddepth of respiration increases Mechanisms for Heat Gain • Skin receptors notice that temperature is droppingpreoptic nucleus is notificed • heat loss center is inhibited • heat gain center is 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 which increases energy consumption of skeletal muscle throughout the 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 is releasedincreases rate of glycogenolysis (break down of glycogen) in liver & skeletal musclemetabolic 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