Thermoregulation

Homeostatic Regulation of Body Temperature during Exercise

 

I.  An Overview of Heat Balance

1.  In order to maintain a constant core temperature, heat loss must match heat gain

2.  If heat loss exceeds heat gain, then heat is removed from the body

- Heat lost is manifest as a decrease in body temperature

    3.   If heat gain exceeds heat loss, then heat is stored in the body

- Heat storage is manifest as an increase in body temperature

 

Scenario 1:  Heat gain = Heat loss – Tbody stable

Scenario 2:  Heat gain + Heat removed = heat loss – Tbody decreases

Scenario 3:  Heat gain = Heat loss + Heat Stored – Tbody increases

 

II.  Temperature Measurement

1.  Ideally, the mean body temperature would be measured

–   Basically impossible to do

2.  Deep-body (core) temperature index

–   Rectum, ear, esophagus are common

3.  Skin temperature

–   Measured at several points

–   Weighted average to calculate mean skin temperature

4.  MBT is approximated as 0.7(core temp) + 0.3(skin temp) – though fractions will vary

 

III.  Heat Production

     A.  Sources of heat production

1.  Basal metabolism or basal metabolic rate

2.  Additional heat production:

–  Voluntary = Exercise – 70-80% of energy released as heat

–  Involuntary

Digestion (“thermic effect of food”)

Shivering

Uncoupling proteins (non shivering thermogenesis) e.g., brown fat

Action of hormones such as T3/T4 and catecholamines on cellular metabolism

 

B.  Measuring Heat Production

1.  Measure metabolic rate – ml O2/min

2.  Multiply by 21J/ml O2

–    more specific value may be used if RER is known

3.  For exercise, multiply by 0.8 (assumes ~20% of the energy went into external work)

4.  Yields heat production in J/min or J/sec (W)

5.  If the environmental temperature is greater than body temperature, heat may also be gained by conduction and convection

6.  If solar radiation is strong, (net) heat gains from radiation are also possible

 

 

IV.  Heat Loss

A.  Non-Evaporative Heat Loss (NEHL)

1.  Radiation

–   Transfer of heat via infrared rays

–   No physical contact between surfaces

–   60% heat loss at rest

         2.  Conduction

–   Heat loss due to contact with another surface

          3.  Convection

–   Really a form of conduction

–   Transfer of heat to air or water where air or water surface is continuously moving

–   Within the body, heat is moved by convection – particularly blood circulation

 

B.  Evaporative Heat Loss (EHL)

1.  Evaporation

–   Heat transferred via evaporation of water from the body (from skin, nasal or buccal mucosa, etc.)

–   Primary forms: sweating (humans, horses), panting (dogs, sheep, birds, reptiles), saliva spreading (small mammals, kangaroos)

–   Evaporation rate depends on:

•    Temperature and relative humidity gradient (vapor pressure gradient mmHg)

•    Convective currents around the body

•    Surface area available for evaporation

–   25% heat loss at rest

•    Most important means of increasing heat loss during exercise

        2.  Measuring Evaporative Heat Loss

   a.  Measure the sweat rate – ml/min/cm2 of body surface

   b.  Do not count sweat that doesn’t evaporate – sweat that drips off does not significantly contribute to heat loss

   c.  Measure the rate of respiratory water loss – ml/min

   d.  Drool doesn’t count

  e.  Latent heat of vaporization: 2.43 Kjoules per ml water evaporated

   f.  Multiply the total evaporative water loss by the latent heat of vaporization to get J/min or J/sec (W) of heat loss

 

C.  Measuring Heat Storage

1.  Calculate the increase in mean body temperature (T2-T1)

2.  Multiply by 4.186 J/ml/oC (specific heat of water)

3.  Multiply by weight converted to vol (body density is near 1 g/ml) to obtain J

4.  Divide by the time over which the temperature increase took place to get J/min or J/sec (W)

 

D.  Computing Heat Balance

Under normal circumstances during exercise:

 

Heat Pro = EHL + Heat Storage + NEHL

 

EHL = evaporative heat loss (measurable)

NEHL = non-evaporative heat loss (hard to measure)

 

V.  Control of thermoregulation

The Hypothalamus

A.  Increased core temperature

–   Anterior hypothalamus

–   Behavioral thermoregulation

–   Commencement of sweating

–   Increased skin blood flow

–   Hyperventilation

–   Panting:  Increased blood flow to upper airways, tongue, and respiratory muscles

 

B.  Cold exposure

–   Posterior hypothalamus

–   Behavioral thermoregulation

–   Shivering

–   Non-shivering thermogenesis (UP’s)

–   Decreased skin blood flow – vasoconstriction

–   Piloerection

–   Hypoventilation

 

VI.  Heat Exchange During Exercise

A.  Effect of Exercise Intensity

     1.  Body Temperature Increase  (Heat Storage) During Exercise

     2.  Increase in body temperature with work rate

–  Linear across wide range of temperatures

–  Linear for both arm and leg exercise

- Temperature rise proportional to active muscle mass

 

B.  Effect of Ambient Temperature

     1.  As ambient temperature increases:

–  Heat production (metabolic rate) is only slightly affected by temperature in mammals (Q10 effect)

–  Lower convective and radiant heat loss

–  Higher evaporative heat loss due to greater sweating rate

     2.  Exercise in Hot/Humid Environments

- Reduced NEHL

–   If environmental temperature is greater than body temperature, heat flow direction is reversed (body gains heat from the environment)

-  Reduced gradient for evaporative loss (water vapor pressure gradient – mmHg)

–   If the air is hot but DRY, the gradient for evaporation is large

–   If the air is saturated at body/skin temperature there is NO gradient evaporation and sweat drips off

     3.  Consequences of Hyperthermia

l     High heart rate

l     Redistribution of blood flow to skin, respiratory system

l     Higher lactate production

l     Impaired performance

l     Possibly heat exhaustion or heat stroke

 

C.  Heat Acclimatization

            1.  Characteristics

l      Doesn’t happen overnight – requires a few days

l      Increased plasma volume

l      Earlier onset of sweating (threshold)

l      Higher sweat rate (gain or slope)

l      Reduced sodium chloride loss in sweat

l      Reduced skin blood flow

l      Increased synthesis of heat shock proteins

2.  Hydration is the key to exercise in heat

l     Dehydration leads to shorter endurance time in the heat

l     Hyperhydration leads to improved endurance time in the heat

l     Eu- or hyperhydration will help prevent reduction in stroke volume

l     If severe, dehydration can lead to cessation of sweating and heat shock

 

D.  Exercise in a Cold Environment

1.  Higher rates of NEHL

–   Reduces chance of heat injury

–   May result in hypothermia

2.  Cold acclimatization

–   Improved ability to sleep in the cold

–   Increased nonshivering thermogenesis

–   Higher intermittent blood flow to hands and feet

–   Results in ability to maintain core temperature