ADAPTATION TO HEAT STRESS

1. ADAPTATION TO HEAT STRESS

2. TYPES OF ADAPTATIONS TO HEAT STRESS • ACCLIMATIZATION - ADAPTATIONS RESULTING FROM NATURAL CHANGES IN THE ENVIRONMENT. • ACCLIMATION - ADAPTATIONS INDUCED BY UNUSUAL ALTERATIONS IN THE ENVIRONMENT.

3. OTHER RELATED TERMS • HABITUATION - DESENITIZATION OR DAMPENING OF A NORMAL RESPONSE TO A STRESSOR (e.g., decreased effects of hidromeiosis after heat adaptation). • CONDUCTANCE - TENDENCY OF AN INDIVIDUAL TO LOSE BODY HEAT TO THE ENVIRONMENT. • INSULATION - TENDENCY OF AN INDIVIDUAL TO RESIST LOSS OF BODY HEAT TO THE ENVIRONMENT.

4. TYPES OF ADAPTATIONS TO HEAT STRESS • PERIPHERAL ADAPTATIONS. • CENTRAL ADAPTATIONS. • CIRCULATORY ADAPTATIONS. • METABOLIC ADAPTATIONS.

5. PERIPHERAL ADAPTATIONS

6. SWEATING BEGINS AT A LOWER CORE AND SKIN TEMPERATURE (I.E., LOWER THRESHOLD FOR THE ONSET OF SWEATING).

7. GREATER SWEAT RATE FOR A GIVEN INCREASE IN CORE TEMPERATURE (TC) AND SKIN TEMPERATURE DUE TO INCREASED SENSITIVITY OF THE SWEAT GLANDS TO SYMPATHETIC NERVOUS SYSTEM STIMULATION, INCREASED SWEATING CAPACITY OF THE GLANDS, AND/OR DECREASED EFFECTS OF HIDROMEIOSIS (e.g., habituation).

9. • REDUCED EFFECTS OF HIDROMEIOSIS (e.g., habituation) AS WELL AS REDUCED GLYCOGEN DEPLETION IN SWEAT GLANDS, WHICH RESULTS IN INCREASED SWEAT RATE. • MAINTENANCE OF HIGHER SWEAT RATES.

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11. INCREASED SELECTIVE REGIONAL SWEATING AS WELL AS INCREASED WETTED SKIN SURFACE AREA, WHICH ENHANCES THE POTENTIAL FOR EVAPORATIVE HEAT LOSS AND MINIMIZES THE WASTE OF SWEAT BY DRIPPING.

12. GREATER SWEATING CAPACITY, PARTICULARLY IN A HUMID ENVIRONMENT. SWEATING CAPACITY INCREASES FROM 1.5 L/HOUR UP TO 2-3 L/HOUR WITH MAXIMUM DAILY SWEAT RATE OF 10-15 L. • INCREASED VASODILATION OF CUTANEOUS VASCULATURE AND INCREASED SKIN BLOOD FLOW FOR A GIVEN INCREASE IN CORE TEMPERATURE, WHICH INCREASES CONVECTIVE HEAT TRANSFER TO THE SKIN AND EVAPORATIVE AND CONVECTIVE HEAT LOSS TO ENVIRONMET.

14. INCREASED SKIN BLOOD FLOW (I.E., TRANSFER OF HEAT) COMBINED WITH INCREASED SWEATING CAPACITY RESULTS IN PROPORTIONALLY GREATER RELIANCE ON EVAPORATIVE HEAT EXCHANGE, WHICH WILL DECREASE SKIN TEMPERATURE. NOTE: DECREASED SKIN TEMPERATUE INCREASES THE POTENTIAL FOR CONVECTVE AND CONDUCTIVE HEAT TRANSFER FROM THE CORE TO THE SKIN SURFACE AREA AND CONVECTIVE HEAT LOSS TO THE ENVIRONMENT IF NEEDED DUE TO AN INCREASE IN THE TEMPERATURE GRADIENT BETWEEN THE SKIN AND AIR.

15. DECREASE IN SODIUM LOST IN SWEAT DUE TO AN INCREASE IN SWEAT GLAND SENSITIVITY TO ALDOSTERONE. NOTE: SWEAT IS MORE DILUTE WHICH INCREASES THE POTENTIAL FOR EVAPORATIVE HEAT EXCHANGE.

17. CENTRAL ADAPTATIONS

18. DECREASE IN HEART RATE,WHICH DECREASES CARDIOVASCULAR STRAIN.

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20. LESS OF AN INCREASE IN CORE TEMPERATURE FOR A GIVEN HEAT STRESSOR, WHICH DECREASES THERMAL STRESS.

22. SYSTOLIC BLOOD PRESSURE IS BETTER MAINTAINED DUE TO INCREASED PLASMA VOLUME OR AT LEAST MAINTENANCE OF PLASMA VOLUME AT NORMAL BASELINE VALUE. • INCREASE IN SODIUM AND WATER REABSORPTION BY THE KIDNEYS.

25. CIRCULATORY ADAPTATIONS

26. INCREASE IN THE STABILITY OF THE CARDIOVASCULAR SYSTEM AND HENCE, A DECREASE IN CARDIOVASCULAR STRAIN. A. DECREASE IN HEART RATE. B. INCREASE IN STROKE VOLUME.

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28. INCREASE IN STROKE VOLUME • INCREASE IN END-DIASTOLIC VOLUME DUE TO AN INCREASE IN PLASMA VOLUME AND AN INCREASE IN THE RETURN OF BLOOD FLOW DUE TO INCREASED VENOCONSTRICTION. • DECREASE IN END-SYSTOLIC VOLUME DUE TO INCREASED CONTRACTILITY (STARLING LAW) AND DECREASED RESISTANCE TO BLOOD FLOW RESULTING FROM VASODILATION OF THE CUTANEOUS VASCULATURE AND DECREASED BLOOD VISCOSITY (OSMOLARITY).

29. PLASMA VOLUME INCREASES INITIALLY, BUT EVENTUALLY RETURNS BACK TO BASELINE VALUE (INCREASED PLASMA VOLUME DECREASES HEART RATE).

31. •INCREASE IN EXTRACELLUAR FLUID AND TOTAL BODY WATER, WHICH DECREASES TENDENCY OF HYPOHYDRATION AND INCREASES POTENTIAL SWEAT CAPACITY.

32. INCREASE IN HEMODILUTION RESULTING IN BETTER TRANSFER OF HEAT FROM THE CORE OF THE BODY TO THE SKIN. A. NOTE: BOTH TRAINING AND HEAT ADAPTATION INCREASE HEMODILUTION (INCREASE IN RELATIVE FLUID CONTENT OF THE BLOOD AS PROTEIN IS FLUSHED INTO THE VASCULAR SPACE). B. CONVERSELY, HEMOCONCENTRATION TENDS TO OCCUR AT ANAEROBIC THRESHOLD, WHICH TRAINING CAN INCREASE RESULTING IN THE TREND TOWARD GREATER HEMODILUTION.

33. INCREASE IN VASODILATION OF THE CUTANEOUS VASCULATURE FOR A GIVEN CORE TEMPERATURE.

35. DIASTOLIC BLOOD PRESSURE DECREASES DUE TO VASODILATION OF THE CUTANEOUS VASCULATURE.

36. SYSTOLIC BLOOD PRESSURE IS BETTER MAINTAINED DUE TO THE INCREASES PLASMA VOLUME OR AT LEAST MAINTENANCE OF PLASMA VOLUME AT NORMAL BASELINE VALUE. NOTE: Increased vasodilation and plasma volume decreases resistance to blood flow which decreases end-systolic volume (ESV) thereby increasing stroke volume (SV) and decreasing heart rate (HR) and cardiovascular (CV) strain.

37. METABOLIC ADAPTATIONS

38. DECREASE IN METABOLIC RATE TO PERFORM A GIVEN SUBMAXIMAL WORKLOAD. • DECREASE IN THE RELATIVE CONTRIBUTION OF ANAEROBIC METABOLISM AND AN INCREASE IN THE RELATIVE CONTRIBUTION OF AEROBIC METABOLISM TO PERFORM A GIVEN SUBMAXIMAL WORKLOAD.

40. DECREASE IN BASAL METABOLIC RATE DUE TO A DECREASE IN SYMPATHETIC NERVOUS SYSTEM STIMULATION OF THE THYROID GLAND. • DECREASE IN CHO UTILIZATION AND AN INCREASE IN FAT UTILIZATION AS AN ENERGY SOURCE. • DECREASE IN LACTIC ACID ACCUMULATION.

41. RETURN OF MAXIMAL OXYGEN UPTAKE RATE TO NORMOTHERMIC, BASELINE VALUE; ACUTE HEAT STRESS DECREASES MAXIMAL OXYGEN UPTAKE RATE. • THE AFFECT OF HYPOHYDRATION WTH THERMAL STRESS IN REDUCING MAXIMAL OXYGEN UPTAKE RATE IS NOT AFFECTED BY ADAPTATION TO HEAT.

42. TRAINING

43. MODERATE AEROBIC EXERCISE AT AN INTENSITY > 50% OF MAXIMAL OXYGEN UPTAKE RATE OR > 60% OF MAXIMAL HEART RATE) FOR 8-12 WEEKS HAS BEEN SHOWN TO PRODUCE ADAPTATIONS TO THE HEAT. • HIGH LEVELS OF FITNESS HAVE BEEN REPORTED TO ENHANCE THE HEAT ADAPTATION PROCESS. ALSO, PHYSICALLY FIT INDIVIDUALS HAVE BEEN SHOWN TO RETAIN HEAT ADAPTATION FOR LONGER PERIODS OF TIME.

44. OTHER COMMENTS REGARDING HEAT ADAPTATION

45. MOST PEOPLE BELIEVE THAT HEAT ADAPTATION IS A RELATIVE PHENONMENON. • ADAPTATIONS SUCH AS IMPROVEMENTS IN HEART RATE WHICH DEVELOP MOST RAPIDLY ARE ALSO LOST MORE RAPIDLY THAN SLOWER OCCURRING THERMOREGULATORY IMPROVEMENTS. GENERALLY TRUE FOR ALL ENVIRONMENTAL ADAPTATIONS.

46. SLEEP LOSS, INFECTIONS, ALCOHOL ABUSE, SODIUM DEPLETION, AND DEHYDRATION LESSEN THE BENEFITS OF HEAT ADAPTATION. • BECAUSE SODIUM MAY BE LOST DURING HYPERTHERMIC CONDITIONS, IT IS RECOMMENDED THAT A UNACCLIMATIZED PERSON TAKE 10 G/DAY SODIUM SUPPLEMENT, UNLESS WATER SUPPLY IS LIMITED. ACCLIMATIZED PERSON REQUIRES NO SODIUM SUPPLEMENTATION BEYOND THAT IN THE NORMAL DIET.

47. Physical Training, Cardiorespiratory Fitness, and Heat Tolerance

48. ADAPTATIONS FROM PHYSICAL TRAINING THAT IMPROVES HEAT TOLERANCE

49. PERIPHERAL RESPONSES TO TRAINING

50. •FOREARM BLOOD FLOW. LOWER THRESHOLD FOR ONSET OF INCREASED FOREARM BLOOD FLOW. NO CHANGE IN FOREARM BLOOD FLOW RATE FOR A GIVEN CHANGE IN CORE TEMPERATURE (I.E., NO CHANGE IN THE SENSITIVITY OR THE SLOPE OF THE LINE). INCREASED HEAT TRANSFER BY CONVECTION FROM CORETO SKIN. NOTE: ADDITIONAL HEAT ACCLIMATION LOWERS THRESHOLD FOR ONSET OF FOREARM BLOOD FLOW (I.E., SKIN BLOOD FLOW) AND INCREASES SENSITIVITY OF FLOW RATE (I.E., GREATER SKIN BLOOD FLOW FOR A GIVEN INCREASE IN CORE TEMPERATURE).