II. Physiology: Nervous system

  1. Hypothalamus controls Thermoregulation
  2. Parasympathetic Nervous System controls sweating
  3. Sympathetic Nervous System controls skin Blood Flow
    1. Vasodilation results in heat dissipation

III. Physiology: Temperature variation

  1. Ideal Body Temperature for physiologic function: 98.6 (37 C)
  2. Heat
    1. Core Temperature rises 1 C per 30 C Ambient Temperature
    2. Core temp. rise 0.6 C, raises basal metabolic rate 10%
    3. Cellular damage occurs at core Body Temperature above 104 F (40 C)
    4. Cellular death occurs at core Body Temperature above 107.6 F (42 C)
      1. Known as Thermal Maximum
      2. Cell death occurs at variable duration of exposure: 42 C (107.6 F) for 45 minutes to 8 hours
  3. Cold
    1. Mild Hypothermia at 95 to 90 F (35 to 32.2 C) presents with shivering, vasconstriction, Tachycardia, cold diuresis
    2. Moderate Hypothermia 90 to 82.4 F (32 to 28 C) presents with confusion to stupor, Bradycardia, Mydriasis
    3. Severe Hypothermia 82.4 to 75 degrees F (28 to 24 C) presents with coma, apnea, VT/VF, DIC
    4. Very Severe Hypothermia < 75 degrees F (24 C) absent Brainstem reflexes, Vital Signs absent and Asystole

IV. Physiology: Heat Dissipation Mechanisms

  1. Sweat evaporation (primary mechanism at >68F or 20C)
    1. Optimally dissipates 600 kcal/hour
    2. Sweat loss maximum 3 Liters/hour
      1. Endurance athletes lose up to 1.5 Liters/hour
    3. Rate of perspiration in hot environments is almost exclusively responsible for heat loss
    4. Perspiration becomes less effective in humid environments
  2. Heat Conduction
    1. Cold packs applied to skin
    2. Cold water emersion is the most expedient way to dissipate heat
    3. Conductive heat loss is typically minor, but increases 5 fold with wet clothing, and 25 fold with cold water immersion
  3. Heat Convection (Air or wind flows over skin)
    1. Body can dissipate heat with greater air circulation
    2. Cooler air also dissipates heat by providing a larger gradient between Body Temperature and air Temperature
    3. Permeable clothing allows for greater heat dissipation
  4. Heat Radiation (Infrared dissipation)
    1. Radiant energy is resorbed or reflected
    2. Light-colored clothing reflects away heat
    3. Half of heat loss is radiative (of which 60% is via the head)

V. Physiology: Heat Adaptation

  1. Athletes may require 10-14 days to acclimitize to Exercise in hot environments
  2. Mechanisms
    1. Improved Sodium retention
    2. Increased Glomerular Filtration Rate
    3. Increased cardiovascular performance
    4. Heat shock Proteins

VI. Physiology: Children and increased propensity for Heat Illness

  1. Greater surface area to body mass
  2. Slower sweat rates
  3. Sweating initiated at a higher Temperature set point
  4. Sweat is more dilute
  5. Slower accilimitization to hot environments than adults
  6. Decreased thirst response

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