ACUTE PHYSIOLOGICAL RESPONSES TO HEAT STRESS

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ACUTE PHYSIOLOGICAL RESPONSES TO HEAT STRESS
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Basic Mechanisms of Thermoregulation

• Core temperature maintained between 35 to 41o C
  despite environmental extremes which fluctuate
  between -88 to 58o C via
• 1. Behavioral temperature regulation such as
  choice of clothing, shelter, ventilation, air
  conditioning, heating, humidifiers, and
  dehumidifiers.
• 2. Physiological temperature regulation
  controlled by rate of metabolic heat production,
  heat flow from core to skin, and sweating.

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Basic Mechanisms of Thermoregulation

• Physiological control systems operate on a graded
  or proportional response in which changes in
  controlled variables (e.g., sweating and skin
  blood flow) are proportional to displacements of
  the regulated variable (e.g., core temperature)
  from a threshold value.
• Note each physiological response has a core
  temperature at which the responses start to
  increase and the actual response is
• dependent on mean skin temperature the lower
  the skin temperature, the higher the increase in
  core temperature before the response is
  initiated.

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Basic Mechanisms of Thermoregulation

• Thus, thermoregulatory responses are related to
  both core and mean skin temperature and hence,
  (1) at any given skin temperature each response
  is proportional to core temperature and (2) an
  increase in skin temperature will decrease the
  core temperature threshold and increase the
  response at any given core temperature.

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CORE TEMPERATURE

• Temperatures within body regions are dependent
  on
• 1. The metabolic rate of surrounding tissues.
• 2. The source and magnitude of blood flow.
• 3. The temperature gradients between
  contiguous body regions.

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SKIN TEMPERATURE

• Determination of skin temperature is useful for
• 1. Estimating the input of skin temperature
  receptors into the hypothalamus for
  thermoregulatory control.
• 2. Predicting core temperature.
• 3. Calculating mean body temperature for
  heat storage determination.
• 4. Calculating sensible (radiation and
  convection) heat exchange.

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Exercise Intensity and Core Temperature

• At rest, 70 of metabolic heat comes from
  internal organs and viscera within the body core.
  During dynamic exercise, metabolic rate
  increases rapidly by 5-15 fold with 70-90 of
  metabolic rate released as heat (humans are least
  efficient at slow and fast speeds of movement).
  Thermoregulatory effectors for heat dissipation
  respond more slowly.
• Core temperature increases rapidly, then
  gradually leveling off at a steady-state value
  when heat production equals heat loss.

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Exercise Intensity and Core Temperature

• Magnitude of core temperature at steady-state is
  largely independent of environmental conditions
  within a fairly wide prescription zone.
  Increases in core temperature are proportional to
  increases in metabolic rate. 1 watt 6 kgm/min
  .01433 kcal/min.
• The prescription zone is smaller at higher
  exercise intensities.

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Exercise Intensity and Core Temperature

• If exercise intensity is expressed in absolute
  terms (L/min), large individual differences exist
  in steady-state core temperature however, if
  exercise intensity is expressed in relative terms
  (VO2max),the inter-individual differences
  disappear.

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Exercise Intensity and Core Temperature

• There is a curvilinear relation between
  steady-state core temperature and relative
  workload. The prescription zone is smaller at
  higher relative exercise intensities and it is
  more difficult to reach steady-state core
  temperature as core temperature and relative
  exercise intensity increases.
• Note An increase in VO2max from training would
  decrease relative workload and hence, reduce core
  temperature, particularly in a heat acclimated
  person.

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Exercise Intensity and Core Temperature

• An increase in ambient air temperature increases
  the metabolic rate (energy requirements) and
  hence relative workload of an absolute exercise
  task as well as the relative contribution of
  anaerobic energy metabolism to the total energy
  requirements.

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Effects of Mode of Exercise on Relative Workload

• Although VO2max of upper body exercise (e.g., arm
  cranking) is lower than lower body exercise
  (e.g., leg cycling) and consequently relative
  work intensity is much higher when performing the
  same absolute exercise task during arm cranking,
  core temperature responses appear to be quite
  similar when working at the same absolute
  metabolic workload.

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Acute Metabolic and Muscular Effects of Heat

• 1. Decreased VO2max due to diversion of blood
  to the skin (cutaneous) vasculature.
• - Decreased blood flow ( Q) to muscle.
• - Increased peripheral pooling of blood,
  which decreases central blood volume and hence
  end- diastolic volume (EDV).
• SV EDV - ESV, Q SV X HR,
• and VO2 Q X A - V O2 Diff

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Acute Metabolic and Muscular Effects of Heat

• 2. The decreased muscle blood flow increases
  tissue hypoxia and the
• relative contribution of anaerobic metabolism
  to total energy
• requirements.
• 3. Increased carbohydrate utilization, primarily
  from blood glucose.

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Acute Metabolic and Muscular Effects of Heat

• 4. Increased blood lactate levels, due to
  increased lactate production as
• well as reduced lactate removal due to
  splanchnic vasoconstriction and hence, decreased
  hepatic clearance of lactate.
• 5. Decreased free fatty acid utilization as an
  energy source.

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Acute Metabolic and Muscular Effects of Heat

• 6. Increased blood glucose levels, as glucose
  is released from the liver.
• 7. Decreased blood triglyceride levels, due to
  decreased mobilization of fat.

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Acute Metabolic and Muscular Effects of Heat

• Increased reliance on fast-twitch motor units.
• 9. Decreased efficiency of skeletal muscle
  contraction as fast-twitch motor units expend
  greater energy than ST motor units to develop the
  same tension.

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Acute Metabolic and Muscular Effects of Heat

• 10. Increased expired ventilation rate,
  primarily due to an increase respiratory rate
  (i.e.,breathing frequency) with minimal
  changes in respiratory (i.e.,tidal) volume.

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Effector Mechanisms of Acute Heat Exposure

• Vasodilation of Cutaneous Vasculature (Increased
  Skin Blood Flow)
• Increased Sweat Rate

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Vasodilation of Cutaneous Vasculature (Increased
Skin Blood Flow)

• There is threshold above which skin blood flow
  will increase. Skin blood flow carries heat by
  convection from the deep body tissues to the
  skin, which may lead to sensible and/or
  insensible heat loss.

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Vasodilation of Cutaneous Vasculature (Increased
Skin Blood Flow)

• Skin blood flow is dependent on both core
  temperature and skin temperature as
• - Increased skin blood flow is proportional to
  core temperature at any given skin temperature.
• - Increased skin temperature will decrease the
  core temperature threshold.

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Heat Transfer by Skin Blood Flow

• Dependent on the rate of blood flow and on the
  differences in temperature between arterial blood
  leaving the core on its way to the skin and
  venous blood returning to the core from the skin.
  Increases in the rate of blood flow, conduction
  of tissues, and the difference between core and
  skin temperatures will increase the heat
  transfer.

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• As ambient temperature increases, there is an
  increased dependence on insensible (evaporative)
  heat loss mechanisms and a decreased dependence
  on sensible (convection and radiation) heat loss
  mechanisms to minimize exercise-induced increases
  in core temperature.
• Evaporative heat loss is dependent on skin blood
  flow (provides latent heat for the evaporation of
  sweat) and on secretion of perspiration from the
  sweat glands.

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Types of Sweat Glands

• 1. Apocrine glands - nervous or emotional sweat
  due to neurochemical stimuli secretion is a
  watery substance that contains lipids, trace
  of color, and odor most commonly found on the
  palms of the hands, soles of
• the feet, arm pits, groin area, face, and
  upper lip.

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Types of Sweat Glands

• 2. Eccrine glands - respond to thermal stress
  by secreting a watery substance which contains
  electrolytes and is generally colorless and
  odorless there are 1.6 to 4 million eccrine
  glands and are most numerous on the sole of the
  feet and least numerous on the back, although
  the back eccrine glands are the first to
  respond to increases in core temperature.

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Sweat Rate

• People who sweat more (1) have larger sweat
  glands, (2) have a greater amount of sweating per
  gland, (3) have a higher sweating rate per unit
  of tubular length or unit volume of secretory
  coil, (4) have a reduced hidromeiotic effect, and
  (5) the sweat glands have a greater cholinergic
  (AcH) sensitivity.
• Hidromeiotic Effect - increased skin wettedness
  decreases sweating.

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Sweat Rate

• Sweat rate is dependent on both core temperature
  and skin temperature as
• - Increased sweat rate is proportional to core
  temperature at any given skin temperature.
• - Increased skin temperature will decrease the
  core temperature threshold.

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• Generally, the heat dissipating responses are
  sufficient to meet the rise in core temperature
  during exercise if not however, hyperthermia may
  occur leading to heat strain and/or other
• heat-related illnesses.

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Heat Strain

• Heat strain results in a decrease in
  end-diastolic volume as blood pools in the
  peripherally dilated veins and/or plasma volume
  decreases, which leads to a decrease in stroke
  volume and thus heart rate must increase to
  maintain cardiac output. Plasma volume decreases
  due to an increased movement of fluid from the
  plasma to tissue (affected by temperature,
  exercise intensity and mode, hydration level, and
  status of heat acclimation) and an increased
  fluid loss through sweating.

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Compensation of Heat Strain

• Decreased splanchnic (visceral) and renal blood
  flow, which is proportional to relative exercise
  intensity and skin temperature.

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Compensation of Heat Strain

• Decreased skin blood flow at high intensities as
  cardiovascular strain is increased (i.e., skin
  blood flow is proportionally lower than expected
  for a given skin or core temperature) this
  response occurs more quickly in the upright
  position as compared to the supine position and
  is probably controlled by cardiopulmonary
  baroreceptors.

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Compensation of Heat Strain

• 3. Venoconstriction of cutaneous (i.e., skin)
  veins during intense exercise.
• 4. Increased water and sodium retention by the
  kidneys.

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Heat Illnesses

• Heat Cramps
• Heat Syncope
• Water Depletion Heat Exhaustion
• Salt Depletion Leading to Heat Exhaustion
• Heat Hyperpyrexia Leading to Heat Stroke
• Skin Lesions

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HYPOHYDRATION AND HYPERHYDRATION
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Hypohydration and Body Fluids

• SWEAT LOSS OF 5 BODY WEIGHT WILL DECREASE TOTAL
  BODY WATER (TBW) BY AS MUCH AS 8.

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Hypohydration and Body Fluids

• Body Weight 75 kg
• TBW 45 kg (60 of BW)
• Sweat Loss of 5 BW 3.75 kg
• 3.75 kg/45 kg 8 Decrease in TBW

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Hypohydration and Body Fluids

• PROPORTIONATELY, WHEN TOTAL BODY WATER IS
  REDUCED
• INTRACELLULAR, INTERSTITIAL, AND PLASMA FLUID
  VOLUMES DECREASE.

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Hypohydration and Body Fluids

• IN GENERAL, THE GREATEST DECREASES TEND TO OCCUR
  IN THE INTERSTITIAL AND INTRACELLULAR FLUID
  VOLUMES.

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Hypohydration and Body Fluids

• PERCENT PLASMA VOLUME DECREASES STAY ABOUT THE
  SAME REGARDLESS OF THE DECREASE IN TOTAL BODY
  WATER.

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Effects of Heat Adaptation on Hypohydration

• 1. ADAPTED PERSON HAS A SMALLER PLASMA VOLUME
  REDUCTION AT A GIVEN BODY WEIGHT LOSS DUE TO
  HYPOHYDRATION.

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Effects of Heat Adaptation on Hypohydration

• 2. ADAPTED PERSON HAS MORE TBW THEREFORE,
  ABSOLUTE FLUID LOSS WOULD REPRESENT A
  SMALLER PERCENTAGE OF TBW.

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GENERAL EFFECTS OF HYPOHYDRATION

• 1. INCREASED CORE TEMPERATURE (INCREASE TENDS TO
  BE LINEAR).
• 2. INCREASED HEAT STORAGE DUE TO REDUCED HEAT
  LOSS THROUGH BOTH SENSIBLE AND INSENSIBLE
  MECHANISMS.
• 3. NO AFFECT ON RATE OF AEROBIC AND ANAEROBIC
  METABOLISM.

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GENERAL EFFECTS OF HYPOHYDRATION

• 4. DECREASED SWEAT RATE FOR A GIVEN CORE
  TEMPERATURE DUE TO HYPEROSMOLARITY
  (HIGH CONCENTRATION OF OSMOTICALLY ACTIVE
  PARTICLES IN A SOLUTION) AND/OR HYPOVOLEMIA
  (LOW PLASMA VOLUME) OF PLASMA.

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GENERAL EFFECTS OF HYPOHYDRATION

• NOTE HYPEROSMOLARITY OF PLASMA VOLUME HAS ALSO
  BEEN SHOWN TO INCREASE CORE TEMPERATURE DUE TO
  REDUCED CONVECTIVE HEAT TRANSFER (DECREASED SKIN
  BLOOD FLOW) AS WELL AS THE DECREASE IN SWEAT RATE
  THAT LEADS TO AN INCREASE IN CORE TEMPERATURE.

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HYPOHYDRATION AND EXERCISE

• 1. HYPOHYDRATION DURING SUBMAXIMAL EXERCISE
  WITHOUT THERMAL STRESS
• A. INCREASED HEART RATE.
• B. DECREASED STROKE VOLUME AS REDUCED
  PLASMA VOLUME DECREASES END-DIASTOLIC
  VOLUME.

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HYPOHYDRATION AND EXERCISE

• C. NO CHANGE IN CARDIAC OUTPUT.
• D. NO CHANGE IN VO2MAX.
• DECREASED PHYSICAL WORK CAPACITY DUE PRIMARILY TO
  THERMOREGULATORY STRESS.
• THERMOREGULATORY STRESS
• INCREASED CORE TEMPERATURE AND HEAT STORAGE AS
  INTERNAL CONVECTIVE HEAT TRANSFER AND SWEAT RATE
  IS DECREASED (I.E., DECREASED HEAT LOSS).

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HYPOHYDRATION AND EXERCISE

• 2. HYPOHYDRATION DURING SUBMAXIMAL EXERCISE
  WITH THERMAL STRESS.
• A. INCREASED HEART RATE
• B. DECREASED STROKE VOLUME
• C. DECREASED CARDIAC OUTPUT AS DECREASE IN
  STROKE VOLUME IS GREATER THAN INCREASE IN
  HEART RATE.

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HYPOHYDRATION AND EXERCISE

• D. DECREASED MAXIMAL OXYGEN OXYGEN UPTAKE RATE.
• E. DECREASED PHYSICAL WORK CAPACITY DUE TO
  THERMOREGULATORY STRESS AND CARDIOVASCULAR
  STRAIN.

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FITNESS LEVEL, ADAPTATION LEVEL, AND HYPOHYDRATION

• 1. IN A HYDRATED STATE, AN ADAPTED PERSON
  WITH A HIGH FITNESS LEVEL WILL HAVE LESS BODY
  HEAT STORAGE AND BETTER PERFORMANCE THAN
  UNADAPTED PERSON WITH LOW FITNESS LEVEL.

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FITNESS LEVEL, ADAPTATION LEVEL, AND HYPOHYDRATION

• 2. HOWEVER, HYPOHYDRATION MAY NEGATE THE
  THERMOREGULATORY ADVANTAGE OF THE HIGHLY
  TRAINED, ADAPTED PERSON, DESPITE THE FACT
  THAT THE HIGHLY FIT PERSON IS CAPABLE OF
  TOLERATING HIGHER CORE TEMPERATURES.

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FITNESS LEVEL, ADAPTATION LEVEL, AND HYPOHYDRATION

• 3. ADDITIONAL RESEARCH IS NEEDED IN THIS AREA.

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HYPERHYDRATION

• 1. HYPERHYDRATION CAN DELAY THE DEVELOPMENT OF
  DEHYDRATION DURING HEAT STRESS.
• 2. HOWEVER, EXCESS FLUIDS BY THEMSELVES MAY NOT
  PROVIDE A CLEAR EXERCISE ADVANTAGE.

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HYPERHYDRATION

• 3. ADDITIONAL RESEARCH IS NEEDED IN THIS AREA.

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GLYCEROLPERFORMANCE AIDOR FAD?
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HYPERHYDRATION USING GLYCEROL

• RAPIDLY ABSORBED WHEN INGESTED ORALLY AND EVENLY
  DISTRIBUTED TO ALL FLUID COMPARTMENTS.
• ATTRACTS AND HOLDS WATER LIKE A SPONGE.
• INCREASES PRE-EXERCISE HYDRATION LEVELS AS URINE
  PRODUCTION IS DECREASED.

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HYPERHYDRATION USING GLYCEROL

• DURING EXERCISE
• INCREASED SWEAT RATE AT A LOWER CORE
  TEMPERATURE.
• GREAT SWEAT CAPACITY.
• LOWER HEART RATE AND CARDIOVASCULAR
  STRAIN.
• IMPROVED ATHLETIC PERFORMANCE IN HOT, HUMID
  ENVIRONMENTS.

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QUESTIONS?THE END!

• AND THAT WINDS UP MY PRESENTATION TONIGHT AT
  SJSU WHERE THE WOMEN ARE STRONG, THE MEN ARE GOOD
  LOOKING, AND ALL OF THE STUDENTS ARE ABOVE
  AVERAGE!!