Exercise

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CHAPTER

10

EXERCISE IN HOT AND COLD ENVIRONMENTS: THERMOREGULATION

Learning Objectives
Find out how the body gets rid of excess body heat to maintain homeostasis at rest and during exercise. Discover how the body adapts to exercise in a hot environment. Learn why humidity, wind, and cloud cover are important factors when exercising in the heat. Differentiate heat cramps from heat exhaustion from heat stroke. (continued)

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Learning Objectives
Learn how the body minimizes excessive heat loss during exposure to cold. Find out the dangers of cold-water immersion. Discover how to exercise safely in the cold.

BODY HEAT GAINED AND LOST

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Modes of Heat Transfer
Conduction—direct molecular contact with an object Convection—motion of gas or liquid across heated surface Radiation—infrared rays Evaporation—as fluid evaporates, heat is lost (580 kcal/L)

HEAT REMOVAL FROM THE SKIN

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THERMOGRAMS
Before

Front

After

Before

Back

After

Before running outside at 30° C (75% humidity)

After running outside at 30° C (75% humidity)

Evaporation
As body temperature rises, sweat production increases. Sweat reaches the skin and evaporates. Evaporation accounts for 80% of heat lost during exercise, but only for about 20% at rest. Insensible water loss removes about 10% of heat. Dehydration is a potential problem with sweating.

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Estimated Caloric Heat Loss at Rest and During Prolonged Exercise Rest Mechanism of heat loss Conduction and convection Radiation Evaporation Total % total 20 60 20 100 Exercise kcal/min 2.2 0.8 12.0 15.0

kcal/min % total 0.3 0.9 0.3 1.5 15 5 80 100

MECHANISMS FOR HEAT BALANCE

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Humidity
Plays a major role in heat loss Affects our 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 °C (97.0 to 100.0 °F). 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.

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Internal Body Temperature
Can exceed 40 °C (104 °F) during exercise May be 42 °C (107.6 °F) in active muscles Small increases can make muscles' energy systems more efficient Above 40 °C can affect the nervous system and reduce the ability to unload excess heat

Regulators of Heat Exchange
Hypothalamus Central and peripheral thermoreceptors Effectors Sweat glands Smooth muscle around arterioles Skeletal muscles Endocrine glands

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HYPOTHALAMUS AND HYPERTHERMIA

HYPOTHALAMUS AND HYPOTHERMIA

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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
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.

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Cardiovascular Response to Exercise in the Heat

Active muscles and skin compete for blood supply. Stroke volume decreases. Heart rate gradually increases to compensate for lower SV (cardiovascular drift).

Metabolic Responses to Exercise in the Heat
Body temperature increases. Oxygen uptake increases. Glycogen depletion is hastened. Muscle lactate levels increase.

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Body Fluid and Exercise in the Heat
Sweating increases. High volumes of sweat cause – Blood volume to decrease, – Loss of minerals and electrolytes, and – Release of aldosterone...
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