hot & cold temp

8/9/2019 hot & cold temp

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EXERCISE IN HOT AND COLDENVIRONMENT: THERMOREGULATION

-Sampada A. Mahadik.- II MPT.


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I T TI

Physiological responses to acute and chronicexercise in both hot and cold environments..

Specific health risks associated with exercise inboth temperature extremes..

Prevention of temperature-related illness andinjuries during exercise..


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EXERC I E I T COLD ENV IRONMEN T:T ERMOREGULA TION

MECHANISM OF BODY TEMPERATURE REGULATION

Humans are homeothermic, meaning that theymaintain a constant internal body temperature,usually in the range of 36.1 to 37.8 0 C.


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TRANSFER OF BODY HEAT

Once heat nears the skin, it can be transferred to the environmentby any of the four mechanisms: conduction, convection, radiationand evaporation.

Conduction: involves the transfer of heat from one material toanother through direct molecular contact.

Convection: involves moving heat from one place to another by themotion of a gas or a liquid across the heated surface. When air and water temperature is greater than skin temperature, the bodygains heat through conduction and convection.

Radiation: is the primary method for discharging the bodys excessheat, when at rest. At normal room temperature (21-25 0 C), the

nude body loses about 60% of its excess heat by radiation.Evaporation: is the primary avenue for heat dissipation duringexercise. Evaporation of 1 L of sweat results in the loss of 580kcal. At rest, most heat is lost via radiation, but during exercise,evaporation becomes the most important avenue of heat loss.


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HUM I IT HEA T

When humidity is high, the air already contains many water molecules. This decreases its capacity to accept more water because the concentration gradient is decreased. Thus, highhumidity limits sweat evaporation and heat loss. Low humidityoffers an ideal opportunity for sweat evaporation and heat loss.

CONTROL OF HEAT EXCHANGE The Hypothalamus: The Bodys Thermostat Two sets of thermoreceptors provide temperature information to

thermoregulatory center. Peripheral receptors in the skin relay information about thetemperature of skin and the environment around it. Central receptors in hypothalamus transmit information about

internal body temperature.


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EFFECTORS THAT ALTER BODYTEMPERATURE Sweat glands: when either skin or blood is heated, hypothalamus sends impulses to sweat

glands, commanding them to actively secrete sweat that moistens the skin. The hotter theindividual, the more sweat is produced. The evaporation of this moisture, removes heatfrom skins surface.

Smooth muscles around arterioles: when the skin and the blood are heated, thehypothalamus sends signals to the smooth muscles in the walls of the arterioles thatsupply the skin, causing them to dilate. This increases blood flow to the skin. The bloodcarries heat from the deeper parts from the body to the skin, where the heat dissipates tothe environment through conduction, convection, radiation and evaporation.

Skeletal muscles: skeletal muscle is called into action when body needs to generate moreheat. In a cold environment, the thermoreceptors in the skin relay signal to thehypothalamus. Similarly, whenever blood temperature decreases, the change is noted bythe central receptors in the hypothalamus. In response to this neutral input, thehypothalamus activates the brain centers that control muscle tone. These centersstimulate shivering, which is a rapid involuntary cycle of contraction and relaxation of skeletal muscles. This increased muscle activity generates heat to either maintain or increase body temperature.

Endocrine glands: the effects of several hormones cause cells to increase their metabolicrates. Increased metabolism affects heat balance because it increases heat production.Cooling the body stimulates the release of thyroxine (elevates the metabolic ratethroughout the body by more than 100%) from the thyroid gland.


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PHYSIOLOGICAL RESPONSES TO EXERCISE IN THEHEAT

Cardiovascular Function Exercise increases the demands on the cardiovascular system. Exercising in hot environments sets up a competition between the active muscles and the skin for the limited

blood supply. The muscles need blood and the oxygen it delivers to sustain activity; the skin needs blood to facilitate heat

loss to keep the body cool. Although cardiac output may remain reasonably constant, stroke volume maydecline, resulting in a gradual upward drift in heart rate, this phenomenon is called as cardiovascular drift.

Energy Production Along with increasing body temperature and heart rate, exercise in the heat also increases oxygen uptake,

causing the working muscle to use more glycogen and to produce more lactate compared with exercise in thecold.

A warmer environment places greater stress on the cardiovascular system, which increases the heart rate. Also, increased sweat production and respiration demand more energy, which requires a higher oxygenuptake.

The compromised blood flow to the muscles during exercise in the heat leads to greater use of muscle

oxygen and production of more lactic acid. Thus, exercise in heat can hasten oxygen depletion and increasemuscle lactate, both of which are known to contribute to the sensations of fatigue and exhaustion.

Body Fluid Balance: Sweating Sweating increases during exercise in the heat, and this can quickly lead to dehydration and excessive

electrolyte loss. Loss of minerals and water by sweating triggers the release of aldosterone and antidiuretichormone (ADH).

Aldosterone is responsible for maintaining sodium levels and ADH maintains fluid balance. Aldosterone is released from adrenal cortex in response to stimuli such as decreased sodium content,

reduced blood volume, or reduced blood pressure.


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Heat Cramps They are characterized by severe cramping of the skeletal muscles. They are probably caused by losses of fluids and minerals that result from excessive

sweating. They are treated by moving the stricken individual to a cooler location and administrating

fluids or a saline solution.

Heat Exhaustion It is accompanied by symptoms like extreme fatigue, breathlessness, dizziness, vomiting,

fainting, cold and clammy or hot and dry skin, hypotension and a weak rapid pulse. It results from the inability of the cardiovascular system to adequately meet the needs of the

active muscles and the skin. It is brought on by reduced blood volume, typically caused by excessive loss of fluids and

minerals through prolonged heavy sweating, and by competition for the existing bloodvolume between the skin and the active muscles.

Treatment involves rest in a cooler environment with their feet elevated to avoid shock. Although, it is not in itself life threatening, it can deteriorate heat stroke if untreated.

Heat Stroke It is characterized by an increase in internal body temperature to a value exceeding 40 0 C,

cessation of sweating, hot and dry skin, rapid pulse and respiration, hypertension, andconfusion and unconsciousness.

It is caused by failure of the bodys thermoregulatory mechanisms. If untreated, it progresses and can be fatal.


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Preventing Hyperthermia

Several precautions must be taken when one is planning toexercise in the heat.

These include cancelling the event if the environmental stress istoo high (WBGT above 28 0 C), wearing proper clothing, being alertto the signs of hyperthermia, and ensuring adequate fluid intake,both before and during exercise.


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ACCLIMATIZATION TO EXERCISE IN THE HEAT

Effects of Heat Acclimatization Repeated exposure to heat stress gradually improves the ability to lose

excess heat. This process of adaptation is called heat acclimatization. With heat acclimatization, one starts to sweat earlier and the rate of

sweating increases, particularly in the areas that are well exposed andare the most efficient at promoting heat loss. This reduces skintemperature, which increases the thermal gradient from the internal toexternal body and promotes heat loss.

Core temperature and heart rate during exercise are reduced, whereasstroke volume increases with heat acclimatization. Plasma volumeincreases, contributing to an increase in stroke volume that aids thedelivery 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. Achieving Heat Acclimatization Heat acclimatization requires exercise in a hot environment, not merely

exposure to heat. The amount of heat acclimatization attained depends on the conditions to

the one is exposed during each session, the duration of the exposure,and the rate of internal heat production.


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EXERC I E IN T E COLD

The hypothalamus has a temperature set point of about 37 0 C, but daily fluctuations in the bodytemperature can be as much as 1 0 C. A decrease in

either skin or blood temperature provides feedbackto the thermoregulatory center (hypothalamus) toactivate the mechanisms that conserve body heatand increase heat production.

The primary means by which our body avoidexcessive cooling are shivering, nonshiveringthermogenesis, and peripheral vasoconstriction.


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Shivering a rapid, involuntary cycle of contraction and relaxationof skeletal muscles increase metabolic heat production to helpmaintain or increase body temperature in the cold. It can cause afour to five fold increase in the bodys resting rate of heatproduction.

Nonshivering thermogenesis involves stimulation of metabolismby the sympathetic nervous system. Increasing the metabolic rateincreases the amount of internal heat production.

Peripheral vasoconstriction occurs as a sympathetic stimulationto the smooth muscle surrounding the arterioles in the skin. The

stimulation causes the smooth muscles to contract, whichconstricts the arterioles, reduces the blood flow to the shell of thebody, and prevents unnecessary heat loss. The metabolic rate of the skin cells also decreases the skins temperature as the skinstemperature decreases, so the skin requires less oxygen.


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F AC TORS A FF EC TING OD Y EA T LOSS

The mechanisms of conduction, convection, radiation, and evaporation, usuallyperform effectively in dissipating metabolically produced heat during exercise inwarm conditions; can dissipate heat faster than the body produces it in a coldenvironment.

Thermal balance depends on a wide variety of factors that affect the gradientbetween body heat production and heat loss. The larger the difference betweenthe temperature of the skin and the cold environment, the greater the heat loss.

OD Y S IZE AND CO MP OS ITION S ubcutaneous fat is an excellent source of insulation. S kinfold measurements of

subcutaneous fat thickness are a good indicator of a individuals tolerance for cold exposure.

The thermal conductivity of fat is relatively low, so it impedes heat transfer fromdeep tissues to the body surface. People who have more fat mass conserveheat more efficiently in the cold.

The rate of heat loss also is affected by the ratio of body surface area to bodymass. Tall heavy individuals have a small surface area to the body mass ratio,which makes them less susceptible to hypothermia.

S mall children tend to have large area to mass ratio compared with adults, thismakes it more difficult for them to maintain normal body temperature in cold.

Women tend to have more body fat than men, but true sex differences in coldtolerance are minimal.


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WINDCHILL Wind creates a chill factor, known as the windchill, by increasing the rate

of heat loss via convection and conduction. Also the more humid air, thegreater the physiological stress.

HEAT LOSS IN COLD WATER Radiation and sweat evaporation are the primary mechanisms for the

heat loss in air, and conduction allows the greatest heat transfer duringimmersion in water.

Water has a thermal conductivity of about 26 times greater than air. Thismeans that heat loss by conduction is 26 times faster in water than inair.

When all heat-transfer mechanisms are considered, the body generallyloses heat four times faster in water than it does in air of the sametemperature.

Humans generally maintain a constant internal temperature when theyremain inactive in water at temperatures down to about 32 0 C. but whenthe water temperature decreases lower, they become hypothermic at arate proportional to either the duration of their exposure to the thermalgradient.

Because of the large drain of heat from the body immersed in coldwater, prolonged exposure or unusually cold conditions can lead to

extreme hypothermia and death.


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PHYSIOLOGICAL RESPONSES TO EXERCISE IN THECOLD

MUSCLE FUNCTION When muscle is cooled, it is weakened, and fatigue occurs more rapidly.

During prolonged exercise in the cold, as energy supplies diminish andexercise intensity declines, a person becomes increasingly susceptible tohypothermia.

METABOLIC RESPONSES Exercise triggers release of the catecholamines, which increase the

mobilization and use of free fatty acids for fuel. But in the cold, vasoconstriction impairs circulation to the subcutaneous

fat tissue, so this process is attenuated. Blood glucose plays an important role in cold tolerance and exercise

endurance. Hypoglycemia, suppress shivering and significantly reducesrectal temperature.

The blood glucose level is maintained reasonably well during coldexposure. Muscle glycogen is used somewhat at a higher rate in coldwater than in warmer conditions.


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HEALTH RISKS DURING EXERCISE IN THECOLD

HYPOTHERMIA It has been shown that individuals who are immersed in near-freezing

water will die within a few minutes when there rectal temperaturedecreases from a normal level of 37 0 C to 24 or 25 0 C.

Once the body temperature falls below 34.5 0 C, the hypothalamus beginsto lose its ability to regulate body temperature. This ability is completely

lost when the internal temperature decreases to 29.50

C. This loss of function is associated with slowing of metabolic reactions to one half their normal rates for each 10 0 C decline in cellular temperature. As a result,cooling the body can cause drowsiness and even coma.

CARDIORESPIRATORY EFFECTS The hypothalamus begins to lose its ability to regulate body temperature if

body temperature drops below 34.50

C. Hypothermia primarily affects the hearts sinoatrial node, decreasing theheart rate, which in turn reduces cardiac output.

Breathing cold air does not freeze the respiratory passages or the lungswhen ventilation is low.

Exposure to extreme cold decreases respiratory rate and volume.


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TREA TMEN T F OR Y O T ERM I A

M ild hypothermia can be treated by protecting the affected personfrom the cold and providing dry clothing and warm beverages.

M oderate to severe cases of hypothermia require gentle handlingto avoid initiating a cardiac arrhythmia. This requires slowlyrewarming the victim.

Severe cases of hypothermia require hospital facilities and medicalcare.

F RO STBIT E F rostbite occurs as a consequence of the bodys attempts to

prevent heat loss. Vasoconstriction in the skin reduces blood flow, so the skin rapidly

cools. This reduced blood flow, combined with the lack of oxygenand nutrients, causes the skin tissue to die.


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ACCL IMA TI ATION TO EXERC ISE IN T E COLD

C old acclimatization is not been well studied, so our knowledge is limited.

R epeated exposure to the cold may alter peripheral blood

flow and skin temperatures, allowing greater cold tolerance.


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hot & cold temp

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