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Exercise in Hotand Cold Environments: Thermoregulation

chapter

To Maintain a Steady-State Core Temperature, the Body Must Balance Heat

Gain With Heat Loss

Modes of Heat Transfer

Conduction involves direct molecular contact.

Convection involves motion of gas or liquid across heated surface.

Radiation involves infrared rays. *

Evaporation involves loss of heat as fluid evaporates.

* Primary means of heat loss at rest

Removal of Heat From the Skin

MECHANISMS FOR HEAT BALANCE

Evaporation

• As body temperature rises, sweat production increases.• Sweat reaches the skin and evaporates. (up to 3-4 L/h)• Evaporation accounts for 80% of heat lost during

exercise.

Humidity

• Plays a major role in heat loss• Affects perception of thermal stress• When high (regardless of temperature), limits

evaporation of sweat

Key Points

Heat Balance• Humans maintain a constant internal

temperature of 36.1 to 37.8 ºC (97.0 to 100.0 ºF).

• Body heat is transferred by conduction, convection, radiation, and evaporation.

• During exercise, evaporation is the main means of heat loss; during rest, radiation is.

• Higher humidity reduces potential evaporation and thus affects heat loss.

Internal Body Temperature

• Can exceed 40 ºC (104 ºF) during exercise• May be 42 ºC (107.6 ºF) in active muscles• Small increases can make muscles’ energy

systems more efficient• Above 40 ºC can affect the nervous system and

reduce the ability to unload excess heat

Regulators of Heat Exchange

HypothalamusCentral and peripheral thermoreceptorsEffectors• Sweat glands• Smooth muscle around arterioles• Skeletal muscles• Endocrine glands

ECCRINE SWEAT GLAND

Rate of Heat Exchange

• Heat produced by an average body at rest is 1.25 to 1.5 kcal per minute.

• Heat produced during exercise can exceed 15 kcal per minute.

• This heat must be dissipated by the body’s thermoregulatory systems.

Key Points

Control of Heat Exchange• The hypothalamus monitors the body’s temperature

and speeds up heat loss or heat production as needed.• Peripheral thermoreceptors in the skin relay

information back to the hypothalamus.• Central thermoreceptors in the hypothalamus transmit

information about internal body temperature.• Sweat gland activity increases to lower body

temperature by evaporative heat loss.

(continued)

Key Points (continued)

Control of Heat Exchange• Smooth muscles in the arterioles dilate or constrict

to allow the blood to dissipate or retain heat.• Skeletal muscle activity increases temperature by

increasing metabolic heat.• Metabolic heat production can also be increased

by actions of hormones.• Mean body temperature is a weighted average of

skin and core temperatures.

Cardiorespiratory Exercise in the Heat

• Active muscles and skin compete for blood supply.• Stroke volume decreases.• Heart rate gradually increases to compensate for lower

SV (cardiac drift).

Metabolic Responses to Exercise in the Heat

• Body temperature increases.• Oxygen uptake increases.• Glycogen depletion is hastened.• Muscle lactate levels increase.

Body Fluid and Exercise in the Heat

• Sweating increases (up to 3-4 L/hr or 10-15 L/d)• High volumes of sweat cause

blood volume to decrease, loss of minerals and electrolytes, and release of aldosterone and ADH and water

reabsorption in kidneys.

Variables of Environmental Heat Load

• Air temperature• Humidity• Air velocity• Amount of thermal radiation

Warning Signs and Symptoms of Heat Cramps, Heat Exhaustion, and Heatstroke

Adapted by permission of All Sport, Inc.

Treatment of Heat Disorders

Heat cramps—Move to cooler location and administer fluids or saline solution.

Heat exhaustion—Move to cooler environment, elevate feet; give saline if conscious or intravenous saline if unconscious.

Heatstroke—Rapidly cool body in cold water or ice bath or with wet towels; seek medical attention.

Preventing Hyperthermia

• Avoid exercising in humid conditions above a WBGT index of 28 °C (82.4 °F).

• Schedule practices or events in early morning or at night.

• Wear lightweight, light-colored, loosely woven clothing.• Drink plenty of fluids.• Know the symptoms of heat stress.

Key Points

Heat Stress• WBGT measures air temperature and accounts

for heat exchange via conduction, convection, evaporation, and radiation.

• Heat cramps appear to be caused by loss of fluids and minerals due to sweating.

• Heat exhaustion results from the cardiovascular system being unable to meet the needs of muscles and skin due to lower blood volume (from sweating).

• Heatstroke is caused by failure of the body’s thermoregulatory system.

Heat Acclimation

• Ability to get rid of excess heat improves.• Sweating becomes more efficient.• Blood flow to skin is reduced; more blood is available to

muscles.• Blood volume increases.• Heart rate increase is lower.• Stroke volume increases.• Muscle glycogen usage decreases.

Effects of Fluid Intake on Core Body Temperature During 2-Hour Run

Archives of Environmental Health "Fluid ingestion during distance running," D.L. Costill, 1: 520-525, 1970. Adapted with permission of the Helen Dwight Reid Educational Foundation. Published by Heldref Publications, 1319 18th Street, NW, Washington, DC 20036-1802. www.heldref.org. Copyright © 1970.

Rectal Temperature and Heart Rate Responses to Exercise Before and After

Heat Acclimation

Adapted, by permission, from D.S. King et al., 1984, "Muscle metabolism during exercise in the heat in unacclimatized and acclimatized humans," Journal of Applied Physiology 59: 1350-1354.

Did You Know . . . ?

You can achieve heat acclimation by exercising in the heat for 1 hour or more each day for 5 to 10 days. Cardiovascular adaptations occur within the first 3 to 5 days, but changes in sweating mechanisms may take up to 10 days. Reduce exercise intensity to 60% to 70% the first few days before resuming more intense workouts.

Key Points

Heat Acclimation• Repeated exposure to heat stress during exercise

improves your ability to get rid of excess heat.• Sweating increases in well-exposed areas to

promote heat loss.• Stroke volume increases to aid in delivery of more

blood to the muscles and skin.• Muscle glycogen use is reduced to delay onset of

fatigue.• Amount of heat acclimation depends on

environmental conditions and duration of exposure and intensity of exercise.

How Does the Body Conserve Heat?

Shivering involves rapid involuntary cycle of contraction and relaxation of muscles.

Nonshivering thermogenesis is stimulation of metabolism.

Peripheral vasoconstriction reduces blood flow to skin.

WARMING OF INSPIRED AIR

Factors That Affect Body Heat Loss

• Body size and composition• Air temperature• Windchill• Water immersion

Windchill Equivalent Temperature Chart

Responses to Exercise in the Cold

• Muscles weaken and fatigue occurs more rapidly.• Susceptibility to hypothermia increases.• Exercise-induced FFA mobilization is impaired due to

vasoconstriction of subcutaneous blood vessels.

Health Risks of Exercise in the Cold

• Ability to regulate body temperature is lost if Tbody drops below 34.5 °C (94.1 °F).

• Hypothermia causes heart rate to drop, which reduces cardiac output.

• Vasoconstriction in the skin reduces blood flow to skin, eventually causing frostbite.