ADAPTATIONS TO THE COLD

ACCLIMATIZATION – ADAPTATIONS RESULTING FROM NATURAL CHANGES IN THE ENVIRONMENT (REVIEW FIGURES) ACCLIMATION - ADAPTATIONS INDUCED BY AN UNUSUAL ALTERATION OF THE ENVIRONMENT HABITUATION - DESENSITIZATION OR DAMPENING OF A NORMAL RESPONSE TO A STRESSOR (REVIEW FIGURE)

DECREASE IN SKIN TEMPERATURE IS GREATER THAN DECREASE IN CORE TEMPERATURE WHICH REFLECTS DECREASED HEAT CONDUCTANCE DUE TO COUNTER-CURRENT HEAT EXCHANGE AND/OR VASOCONSTRICTION

INSULATION - TENDENCY OF AN INDIVIDUAL TO RESIST LOSS OF BODY HEAT TO THE ENVIRONMENT; TISSUE INSULATION IS DIRECTLY RELATED TO FAT MASS AND LEAN BODY MASS INSULATION WHICH INCREASES WITH COLD ADAPTATION (REVIEW FIGURE) TOTAL BODY INSULATION • SUPERFICIAL SHELL - SKIN AND SUBCUTANEOUS FAT • MUSCLE LAYER AND DEEPER BODY FAT

THE GREATER NON-FATTY (I. E THE GREATER NON-FATTY (I.E., MUSCLE) SHELL INSULATION IS DUE TO INCREASED COUNTER-CURRENT HEAT EXCHANGE AND/OR DECREASED CORE TEMPERATURE THRESHOLD FOR ONSET OF SHIVERING AND/OR INCREASED VASOCONSTRICTION

CONDUCTANCE - TENDENCY OF AN INDIVIDUAL TO LOSE BODY HEAT TO THE ENVIRONMENT; THERMAL CONDUCTANCE IS INVERSELY RELATED TO FAT MASS (REVIEW FIGURE)

ADAPTATIONS TO THE COLD COLD ADAPTIVE PROCESSES TEND TO OCCUR IN THE FOLLOWING ORDER: 1. METABOLIC 2. HYPOTHERMIC 3. INSULATIVE

GENERAL: 1. ADAPTATIONS TO THE COLD TEND TO BE A FUNCTION OF THE PHYSIOLOGICAL STRESS INDUCED BY THE DURATION, FREQUENCY, AND INTENSITY OF THE COLD EXPOSURE 2. CHANGES IN SYMPATHETIC NERVOUS SYSTEM ACTIVITY MAY, IN PART, BE UNDERLYING THE OBSERVED ADAPTATION PROCESS REMEMBER: ACUTE COLD EXPOSURE STIMULATES THE SNS RESULTING IN AN INCREASE IN THE CIRCULATING LEVELS OF NOREPINEPHRINE

METABOLIC ADAPTATIONS INCREASED BASAL METABOLIC RATE AND METABOLIC HEAT PRODUCTION DUE TO: 1. INCREASED SHIVERING (?) IF IN FACT INCREASED SHIVERING OCCURS, IT APPEARS TO BE RELATED TO INCREASED SHIVERING CAPACITY AS THRESHOLD FOR THE ONSET OF SHIVERING IS DECREASED RATHER THAN INCREASED AFTER HYPOTHERMIC AND INSULATIVE ADAPTATIONS (REVIEW FIGURE)

2. INCREASED NON-SHIVERING THERMOGENESIS A. INCREASED METABOLISM IN BROWN ADIPOSE TISSUE B. INCREASED SNS ACTIVITY (INCREASED NOREPINEPHRINE RELEASED) C. INCREASED THYROXIN RELEASED NOTE: CHEMICAL THERMOGENSIS (B & C) INCREASES HEAT PRODUCTION BY 10-15%

HYPOTHERMIC ADAPTATIONS 1. DECREASED METABOLIC HEAT PRODUCTION AND CORE TEMPERATURE 2. SKIN TEMPERATURE DOES NOT CHANGE OR INCREASES SLIGHTLY

THESE ADAPTATIONS MAY BE DUE TO: 1. ACCLIMATION A. CIRCULATORY ADJUSTMENT THAT DIRECTED BLOOD FLOW FROM CORE TO PERIPHERAL VESSELS THEREBY KEEPING EXTREMITIES WARM WITHOUT INCREASED METABOLIC HEAT PRODUCTION (REVIEW FIGURE) NOTE: ESKIMOS ALSO SHOW A HABITUATION RESPONSE (DAMPENING OF INITIAL RESPONSE) AS THEY HAVE LESS OF AN INITIAL VASOCONSTRICTION RESPONSE TO COLD EXPOSURE

AND/OR 2. HABITUATION A. DECREASED SET POINT FOR THE ONSET OF SHIVERING; THEREFORE, MAINTENANCE OF SKIN TEMPERATURE WITH DECREASED METABOLIC HEAT PRODUCTION RESULTED IN THE OBSERVED DECREASE IN CORE TEMPERATURE (REVIEW FIGURE) B. DECREASED SNS AND NOREPINEPHRINE RELEASE IN RESPONSE TO COLD EXPOSURE: • DECREASED NON-SHIVERING THERMOGENESIS • DECREASED VASOCONSTRICTION OF SUBCUTANEOUS VASCULATURE AND HENCE INCREASED VASODILATION

INSULATIVE ADAPTATIONS 1. BOTH CORE AND SKIN TEMPERATURES DECREASE 2. DECREASE IN CORE TEMPERATURE DUE TO A DECREASE IN SHIVERING THERMOGENESIS; LOWER CORE TEMPEATURE THRESHOLD FOR THE ONSET OF SHIVERING

3. DECREASED SKIN TEMPERATURE (WHICH DECREASES HEAT LOSS GRADIENT FROM SKIN SURFACE TO ENVIRONMEMT) DUE TO: A. LOWER THERMAL CONDUCTIVITY OF METABOLIC HEAT AS SHIVERING THERMOGENESIS IS REDUCED, WHICH INCREASES INSULATIVE EFFECT OF LBW B. INCREASED VASOCONSTRICTION AS INCREASED SNS ACTIVITY INCREASES CIRCULATING LEVELS OF NOREPINEPHRINE WHICH REDUCES SKIN BLOOD FLOW DUE VASOCONSTRICTION C. DEVELOPMENT OF COUNTER-CURRENT HEAT EXCHANGE MECHANISMS

ABORIGINES COMPARED TO EUROPEAN CONTROL GROUP HAD: - LOWER RESTING METABOLIC RATE - LOWER SKN TEMPERATURE (Tsk) - LOWER CORE TEMPERATURE (Tc) - LOWER CONDUCTANCE AS DECREASE IN Tc IS LESS THAN DECREASE IN Tsk; MAY BE DUE TO ENHANCED VASOCONSTRICTION OR COUNTER- CURRENT HEAT EXCHANGE REVIEW FIGURE

DECREASE IN SKIN TEMPERATURE WAS GREATER THAN DECREASE IN CORE TEMPERATURE; THEREFORE, THERMAL GRADIENT BETWEEN THE CORE AND SUBCUTANEOUS MUSCLE SHELL WAS INCREASED

HOWEVER, ENHANCED INSULATION OF THE SUPERFICIAL SHELL DUE TO GREATER CUTANEOUS VASOCONSTRICTION, WHICH WOULD LIMIT HEAT LOSS FROM THE MUSCLE SHELL RESULTING IN A WARMER AND BETTER PERFUSED MUSCLE SHELL IN THE COLD, OR INCREASED COUNTER-CURRENT HEAT EXCHANGE NOTE: DECREASED SKIN TEMPERATURE MAKES SKIN TEMPERATURE CLOSER TO AIR OR WATER TEMPERATURE THEREBY INCREASING INSULATION AND DECREASING HEAT LOSS TO THE ENVIRONMENT REVIEW FIGURES

SUMMARY COMMENTS REGARDING INSULATIVE ADAPTATIONS • DECREASE IN SKIN TEMPERATURE IS GREATER THAN DECREASE IN CORE TEMPERATURE WHICH INCREASES THERMAL GRADIENT BETWEEN CORE AND MUSCLE SHELL PROVIDING PROTECTION TO MUSCLE SHELL

SUMMARY COMMENTS REGARDING INSULATIVE ADAPTATIONS • INCREASED VASOCONSTRICTION AND/OR INCREASED COUNTER-CURRENT HEAT EXCHANGE WILL DECREASE HEAT TRANSFER FROM MUSCLE SHELL TO SKIN SURFACE AND ENVIRONMENTAL SURROUNDINGS

COLD ADAPTATIONS AND BODY FLUID REGULATION MINIMAL EFFECTS ON BODY FLUID REGULATION AS THE MAGNITUDE OF PLASMA VOLUME REDUCTION INDUCED BY COLD EXPOSURE APPEARS TO BE UNAFFECTED BY COLD ACCLIMATION REVIEW FIGURE

COLD ADAPTATIONS AND THE CARDIOVASCULAR AND RESPIRATORY SYSTEMS

1. TENDS TO DECREASE THE PRESS. RESPONSE (BLOOD PRESSURE 1. TENDS TO DECREASE THE PRESS RESPONSE (BLOOD PRESSURE INCREASES WITH ACUTE COLD EXPOSURE) DUE TO HABITUATION OR INCREASED PERFUSION OF VASCULAR BEDS IN SKELETAL MUSCLE (ASSOCIATED WITH INSULATIVE ADAPTATIONS) REMEMBER: DECREASED PERFUSION IS INITIALLY INDUCED WITH ACUTE COLD EXPOSURE

2. NO EFFECT ON HEART RATE AND CARDIAC OUTPUT REMEMBER: MAXIMAL HEART RATE AND CARDIAC OUTPUT ARE REDUCED IN THE COLD; SUBMAXIMAL HEART RATE IS ELEVATED DUE TO SHIVERING THERMOGENESIS AND/OR INCREASED SNS ACTIVITY

3. NO EFFECT ON EXPIRED. VENTILATION RATE,. RESPIRATION RATE, AND 3. NO EFFECT ON EXPIRED VENTILATION RATE, RESPIRATION RATE, AND TIDAL VOLUME REMEMBER: EXPIRED VENTILATION RATE IS INCREASED IN THE COLD DUE INCREASED RESPIRATION RATE AND/OR TIDAL VOLUME

SUMMARY OF METABOLIC, HYPOTHERMIC, AND INSULATIVE ADAPTATIONS

METABOLIC • INCREASED BASAL OR RESTING METABOLIC RATE • INCREASED HEAT PRODUCTION

HYPOTHERMIC • DECREASED BASAL OR RESTING METABOLIC RATE • DECREASED HEAT PRODUCTION • DECREASED CORE TEMPERATURE • NO CHANGE OR SLIGHT INCREASE IN SKIN TEMPERATURE

INSULATIVE • DECREASED BASAL OR RESTING METABOLIC RATE • DECREASD HEAT PRODUCTION • DECREASED CORE TEMPERATURE • DECREASED SKIN TEMPERATURE • DECREASED HEAT LOSS TO THE ENVIRONMENT

PHYSICAL TRAINING AND COLD TOLERANCE

THRESHOLD FOR ONSET OF SHIVERING DURING COLD EXPOSURE IS DECREASED BY PHYSICAL TRAINING (I.E., LOWER CORE TEMPERATURE BEFORE ONSET OF SHIVERING) DECREASED METABOLIC RATE AND RELATIVE EXERCISE INTENSIY (%VO2max) DECREASED GLYCOGEN DEPLETION AND LACTATE PRODUCTION INCREASED WORK TIME TO EXHAUSTION

5. PHYSICAL TRAINING MAY. INCREASE OR DECREASE SKIN 5. PHYSICAL TRAINING MAY INCREASE OR DECREASE SKIN TEMPERATURE DURING COLD EXPOSURE A. IF SKIN BLOOD FLOW AND SKIN TEMPERATURE INCREASE AFTER TRAINING, METABOLIC RATE WOULD HAVE TO INCREASE TO MAINTAIN CORE TEMPERATURE

B. IF SKIN TEMPERATUR DECREASES. AFTER TRAINING, THERMAL B. IF SKIN TEMPERATUR DECREASES AFTER TRAINING, THERMAL GRADIENT BETWEEN SKIN SURFACE AND ENVIRONMENT WOULD DECREASE AND THERMAL GRADIENT BETWEEN CORE AND SKIN WOULD INCREASE THEREBY FACILITATING THE TRANSFER OF HEAT FROM THE CORE TO THE SUBCUTANEOUS MUSCLE SHELL (INSULATIVE EFFECT) C. MORE RESEARCH IS NEEDED TO CLARIFY ISSUE

6. TRAINING IN COLD WATER. IMPROVES COLD TOLERANCE 6. TRAINING IN COLD WATER IMPROVES COLD TOLERANCE MORE THAN TRAINING IN COLD AIR A. HEAT LOSS TO COLD WATER IS FOUR TIMES GREATER THAN HEAT LOSS TO AIR; THEREFORE, INTENSITY OF COLD EXPOSURE IS MUCH GREATER IN COLD WATER COMPARED TO COLD AIR

B. IN ADDITION TO INSULATIVE. ADAPTATIONS, COLD WATER B. IN ADDITION TO INSULATIVE ADAPTATIONS, COLD WATER TRAINING MAY PREVENT THE LOSS OF SUBCUTANEOUS FAT TYPICALLY OBSERVED WITH TRAINING (I.E., SWIMMERS TEND TO RETAIN SUBCUTANEOUS FAT TO A GREATER EXTENT THAN OTHER ATHLETES TRAINING AT A SIMILAR LEVEL IN THE AIR)

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