kin 4385 - chapter 12

human core temperature is…

37 °C

Humans are…

• homeotherms

• Maintain constant body core temperature

• Heat loss must match heat gain

temperatures above 45 C

• damage proteins and enzymes

• lead to death

temperatures below 34 C

• decreased metabolism

• cardiac arrhythmias

Thermal gradient exists between

• body core to skin surface

• typical = 4 C

During exercise body temperature is regulated by adjustments in…

• heat loss

  • circulatory system is central

  • blood flow increased to skin to lose heat

  • blood flow reduced to skin to retain heat

heat production at rest

Metabolic heat production is small

voluntary - heat production

Exercise

• 70 to 80% energy expenditure

released as heat

involuntary - heat production

Shivering

• Increases heat production by

approximately 5x

Action of hormones

• Thyroxine

• Catecholamines

• Called non-shivering thermogenesis

heat exchange during exercise

• radiation

• conduction

• convection

• evaporation

radiation

• Transfer of heat via infrared rays

• 60% heat loss at rest

• Can be a method of heat gain

conduction

Body heat loss due to contact with another surface (for

example, sitting on metal chair)

convection

Heat transferred from body to air or water

• Example: a fan moving air over skin

evaporation

When water (sweat) gains sufficient heat, water converted to gas (water vapor)

• Requires vapor pressure gradient between skin and air

• Higher relative humidity & temperature raise the vapor pressure, lowering the evaporative capacity

preoptic anterior hypothalamus (POAH)

• Operates much like a thermostat in your home-attempts to maintain a relatively constant core temperature around a “set point

POAH’s Response to increase in core temperature

• Stimulation of sweat glands-evaporative heat loss

• Cutaneous vasodilation, allowing increased heat loss

POAH’s Response to decrease in core temperature

• Shivering and increased norepinephrine release

  • Increased heat production

• Decreased skin blood flow via vasoconstriction

  • Decreased heat loss

As exercise intensity increases:

• Heat production increases

• Linear increase in body temperature

  • Core temperature increases proportional to active muscle mass

  • Increased reliance on evaporative heat loss

As ambient temperature increases:

• Exercise-induced heat production remains constant

• Lower convective and radiant heat loss

• Higher evaporative heat loss

Results in reduced ability to lose body heat:

• Higher core temperature

• Higher heart rate

• Risk of hyperthermia and heat injury

Higher sweat rate

• Sweat rate can be as high as 4 to 5 L/hour

• Increases risk of dehydration

Humans dissipate heat via sweat produced by sweat glands under:

sympathetic cholinergic control

3 types of sweat glands contribute to cooling

1. apocrine

2. apoecrine

3. eccrine

apocrine

Open to hair follicles rather than skin surface, limiting effectiveness

apoeccrine

Located in few body regions, limiting effectiveness

eccrine

• Most abundant, most responsible for heat loss

• Clothing can limit cooling by inhibiting evaporation

impaired exercise performance in heat are:

• central nervous system dysfunction

• accelerated muscle fatigue

• cardiovascular dysfunction

central nervous system dysfunction

• decreased motivation

• reduced voluntary activation of motor units

cardiovascular dysfunction

• reduced stroke volume

• decreased cardiac output during high intensity exercise

• decreased blood flow

accelerated muscle fatigue

• increased radical production

• decreased muscle pH

• muscle glycogen depletion

3 major factors are increased and contribute to muscle fatigue:

• The rate of muscle glycogen breakdown

• Muscle lactate production

• The rate of free radical production in muscles

accelerated muscle fatigue factors are elevate chemical feedback, elevating CNS dysfunction via…

type III/IV afferents

Hyperthermia

• directly impacts the CNS

• Reduces mental drive

• Response bolstered by type III/IV afferent feedback from hyperthermic muscle

• Results in inhibitory effect on the central motor drive, decreasing recruitment of motor units

Does aging impair the ability to thermoregulate during exercise in heat?

Yes-skin blood flow is reduced in older individuals (that is, >65 years)

• Therefore, aging results in reduced ability to lose heat during exercise

Acclimatization

Adaptation to heat over a period of 1-2 weeks

Heat acclimation

• Requires exercise in hot environment

  • Elevated core temperature promotes adaptations

• Acclimation lost within a few days of inactivity or no heat exposure

  • Significant decline in 7 days

  • Complete loss in 28 days

Physiological adaptations during heat acclimation

• Increased plasma volume (+10 to

  12%)

  • Maintains blood volume, stroke

    volume, and sweating capacity

• Earlier onset of sweating and higher sweat rate

  • Less heat storage, maintain lower

    body temperature

• Reduced sodium chloride loss in

  sweat

  • Reduced risk of electrolyte

    disturbance

• Reduced skin blood flow

Cold stress

environmental condition that threatens temperature homeostasis by decreasing core temperature

hypothermia

large decrease in body core temperature (for example, >3°C)

Cold temperature can negatively impact sports performance

• hands become numb due to reduced blood flow and depressed rate of neural transmission

• loss of dexterity and negatively impacts motor skills

Cold temperatures can reduce muscle strength and power

• decrease in muscle temperature by -8°C reduces muscle strength and power by 31 to 44%

• temperature of the exercising muscle does not typically decrease and therefore, cold air exposure rarely decreases muscle force production

Acclimation to cold environments

• Results in lower skin temperature at which shivering

  begins (increased non-shivering thermogenesis)

• Improved ability to maintain a higher hand and foot

  temperature during cold (improved peripheral blood flow

• Improved ability to sleep in the cold (reduced shivering)

• Cold adaptations occur rapidly (first 7 days)