Heat

What is heat - kinetic energy of molecules

Introduction to Body Heat:

• Humans are homeotherms

• Heat loss must match the heat gain

Normal Core temperature: 37 degree C

• temperatures about 44 degree C (111.2 F) is almost certain death

• 42 degree C (107.6 F) is medical emergency

• 35 degree C (95 F) begins hypothermia

  • Main cause of deaths by hypothermia is cardiac arrest

• Certain death at 24-26 degree (74.2-78.8F)

• C to F conversion. = (F-32)/.1.8

Thermal gradient exists between body core to skin surface

• typical gradient is approximately 4 degree C

• In extreme cold, may be 20 degree C

• Why does this gradient exists?

  • The external environment always changes, so that’s why your skin can be colder than internal body

Rest

• metabolic heat production is small

Voluntary

• exercise 70-80% energy released as heat

Involuntary

• Shivering

  • increases heat production by approximately 5 times

• Action of hormones

  • Thyroxine

  • Catecholamines

  • Called non-shivering thermogenesis

Body temperature: Preoptic Anterior Hypothalamus (POAH)

• Hypothalamus control body temperature, specifically the preoptic anterior hypothalamus

• Responsible for shivering, starting sweating (vasodilation)

POAH

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

Response to increase in core temperature

• stimulation of sweat glands-evaporate heat loss

• Cutaneous vasodilation, allowing increased heat loss

Response to decrease in core temperature

• Shivering and increases norepinephrine release

  • Increased heat production

• Decreased skin blood flow via vasoconstriction

  • Decreased heat loss

Fever

• increased body temperature above normal

• Duke to pyrogens

  • Proteins or toxins from bacteria

• Pyrogens result change in set point of hypothalamus thermostat

Measurement of Body Temperature during exercise

Deep-body (core) temperature

• Measured a rectum, ear, or esophagus

  • Usually in laboratory

• Ingestible temperature sensor telemetry system

  • Can be used in athletes during practice sessions

Skin Temperature

• thermistors at various locations

• Calculate mean skin temperature

  • Takin = (Tforehead + Tchest + Tforearm + Tthigh + Tcalf + Tabdomen + Tback) / 7

Four major ways to lose body heat

1. Radiation

   1. Transfer of heat via infrared rays

   2. 60% heat loss at rest

   3. Can be a method of heat gain

2. Evaporation

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

3. Convection

   1. Heat transferred from body to air or water

   2. Example: A fan moving air over skin

4. Conduction

   1. Body heat loss due to contact with another surface (example: sitting on a metal chair)

• Radiation the the most important when your just sitting here

• What about during exercise?

  • Evaporation

• Evaporation rate depends on:

  • Temperature and relative humidity

  • convection currents around the body

  • Amount of skin surface exposed to the environment

    • 25 % heat loss at rest due to evaporation

If we are exercising in an environment that is cooler than our body temperature but has a very high relative humidity?

In what way can we lose heat efficiency?

• Radiation

We are in an environment that is cooler than body temp and low relative humidity convection humidity?

• Evaporation

Biking athlete and we are biking across Florida?

• Convection

In direct sunlight on metal park bench on a hot day with high relative humidity

In what ways are we Gaining thermal energy?

• Radiation

• Conduction

Exercise in a Hot environment

Results in reduced ability to lose body heat

• Higher core temperature

• Higher heart rate

• Risk of hyperthermia and heat injury

Higher sweat rates

• sweat rates can be as high as 4-5 L/hr

• Increases risk of dehydration