k - 31 environmental influences on exercise
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Deparment of PhysiologyFaculty of Medicine
Universitas Sumatera Utara
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During exercise in the heat, the heat loss mechanisms
compete with the active muscle for more of the limited
blood volume. Thus, neither area is adequately supplied
under extreme conditions.Though cardiac output may remain reasonably constant,
stroke volume may decline, resulting in gradual upward
drift in heart rate.
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Stroke volume increases with the acclimatization. This aids
delivery of more blood to the active muscles and skin when
necessary.
Heat acclimatization reduces the rate of muscle glycogen
use, delaying the onset of fatigue.
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Heat acclimatization requires exercise in a hot
environment, not merely exposure to heat.
The amount of heat acclimatization attaineddepends on the conditions to which you are
exposed during each session, the duration of
exposure, and your rate of internal heatproduction.
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Shivering (involuntary muscle contractions)increases metabolic heat production to helpmaintain or increase body temperature in the cold.
Nonshivering thermogenesis accomplishes thesame goal, but through stimulation of thesympathetic nervous system and by the action of
hormones such as thyroxin and the catecholamines.
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Peripheral vasoconstriction decreases the transfer of coreheat to the skin, thus decreasing heat loss to theenvironment.
Body size is an important consideration for heat loss. Bothincreased surface area and reduced subcutaneous fatfacilitate the loss of body heat to the environment. So thosewho have a small surface area-to-body mass ratio and thosewith more fat are less susceptible to hypothermia.
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Exercise triggers release of catecholamines,which increase the mobilization and use of
free fatty acid for fuel. But in the cold,vasoconstriction impairs circulation to thesubcutaneous fat tissue, so this process isattenuated.
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Wind increases heat loss by convection and conduction, so
this effect, known as wind chill, must be considered along
with air temperature during cold exposure.
Immersion in cold water tremendously increases heat loss
through conduction. Exercise generates metabolic heat to
offset some of this loss.
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When muscle is cooled, it is weakened, andfatigue occurs more rapidly.
During prolonged exercise in the cold, asenergy supplies diminish and exercise intensitydeclines, a person becomes increasinglysusceptible to hypothermia.
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Cold acclimatization results in three physiologicaladaptations: 1. increased nonshivering thermogenesis 2. a higher intermittent blood flow to hands and feet
3. improved ability to sleep in cold environments
The overall goal of these adaptations is to increaseheat production and maintain core temperature,
which will make the individual more comfortableduring cold exposure.
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