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Temperature Regulation

Terms in this set (40)

Homeotherms
Animals that maintain a fairly constant internal body core temperature (regulate their own body temperature to maintain a constant core body temperature)
- The maintenance of a constant body temperature (temperature homeostasis) requires that heat loss = rate of heat production (heat gain)

The body is equipped with both nervous and hormonal mechanisms to regulate metabolic rate and the amount of heat loss in response to body temperature changes to maintain a constant body temperature
Body Core Temperature Regulation
Body core temperature regulation is critical because cellular structures and metabolic pathways are affected by temperature

The goal of temperature regulation is to maintain a constant core temperature and thus prevent overheating or cooling
- To maintain a constant temperature, heat loss must equal heat gain
- Regulation is therefore achieved by controlling the rate of heat production and heat loss

Normal core temperature is 37 degrees C
- Above 45 degrees C may destroy proteins and enzymes, causing inability to produce cellular energy and causing cell death
- Below 34 degrees C may cause slowed metabolism and arrhythmias (can also lead to death
Temperature Homeostasis
In order to maintain temperature homeostasis, homeotherms use a furnace to maintain body temperature at a constant level
- The body temperature is set near the high end of the survival range and is held constant by continuous metabolic heat production coupled with a small but continual heat loss

Temperature regulation by heat generation and conservation is very efficient, whereas cooling capacity is much more limited
Temperature Regulation During Exercise
During exercise, body temperature is regulated by making adjustments in the amount of heat that is lost and minimize the increase in body temperature
- Circulatory system transports heat
- Blood has high capacity to store heat
- When the body is attempting to lose heat, blood flow is increased to the skin to promote heat loss to the environment
- The nervous system also increases sweat production via eccrine sweat glands
Preventing Heat Loss
When the goal of temperature regulation is to prevent heat loss, blood is directed away from the skin towards the interior of the body to prevent additional heat loss
Thermal Gradient
Within the body, temperature varies a great deal;
- In general, there is a gradient between core temperature (i.e. deep central areas including the heart, lungs, and abdominal organs) and the "shell" (skin) temperature

The ideal thermal gradient from body core to skin surface is 4 degrees C
- In extreme cold however, this gradient may be as much as 20 degrees C

Even within the core, temperature varies from one organ to another
- Why we must be precise when recording where the temperature measurements were obtained (i.e. specify skin or core temperature)
Temperature Measurement During Exercise: Deep-Body (Core) Temperature
Provides more information about how body temperature is being regulated (the temperature that we are more concerned about)

Deep-body (core) temperature can be accomplished using
- Mercury thermometers
- Thermocouples or thermistors
- Ingestible core temperature pills

Measured at the rectum, ear, or esophagus
- Usually in the laboratory (has limitations in a field setting)
- Most common site is the rectum to measure deep-body core temperature during exercise (can also do this by measuring at esophagus)
- Eardrum is good estimate of actual brain temperature

Ingestible temperature sensor telemetry system
- Useful for taking deep-body temperature measurements during practice sessions
- Uses a low-peer radio frequency transmission to communicate with a temperature monitor
Temperature Measurement During Exercise: Skin Temperature
Skin temperature can be measured by placing temperature sensors (thermistors) on the skin at various locations
- Then, mean skin temperature is calculated by averaging the sum of temperatures

Tskin = (T forehead + T chest + T forearm + T thigh + T calf + T abdomen + T back)/7

Skin temperature can provide useful information about the temperature gradient between the deep body (core) temperature and skin
- Magnitude of this gradient is useful for body heat loss
Hypothalamus
In order to regulate body temperature and thus maintain a constant core temperature, the hypothalamus works as a thermostat
- Initiates an increase in the production of heat when body temperature falls
- Initiates increase in rate of heat loss when body temperature rises

Temperature regulation is controlled by both physical and chemical processes
Heat Production
The body produces internal heat due to normal metabolic processes
- At rest or during sleep, metabolic heat production is small
- During intense exercise, heat production is large

Heat production in the body can be classified as voluntary or involuntary
- Voluntary: exercise
- Involuntary: shivering or biochemical heat production by the the action of hormones (non shivering thermogenesis)
Voluntary Heat Production
Accomplished through exercise

Because the body is only 20-30% efficient at most, 70-80% of energy expended during exercise appears as heat
- Can result in a large heat load during intense exercise
- Prolonged exercise in hot/humd environment serves as series test of body's ability to lose heat
Involuntary Heat Production
Accomplished through shivering and non shivering thermogenesis

Shivering: maximal shivering can increase the body's heat production by 5x the resting value

Nonshivering thermogenesis: biochemical heat production caused by secretion of thyroxine and catecholamine hormones (involuntary heat production caused by action of hormones)
- Thyroxine: released by thyroid gland; increases metabolic rate of all cells in the body
- Catecholamines (epinephrine and norepinephrine): causes increase in rate of cellular metabolism as well
- Increase in heat production due tot he combined influences of thyroxine and catecholamines is called non shivering thermogenesis
Heat Loss
Can occur by four processes:
- Radiation
- Conduction
- Convection
- Evaporation

First three require a temperature gradient to exist between the skin and the environment
- Not very effective in Texas heat as evaporation
- In many cases, evaporation can be the only way to lose heat during exercise

High heat reduces the body's ability to lose heat by radiation/conduction/convection

High humidity reduces the body's ability to lose heat by evaporation
Radiation
Transfer of heat via infared rays
- Involves the transfer of heat from the surface of one object to the surface of another with no physical contact (i.e. sun transferring het tot the earth via radiation)

Accounts for 60% of heat loss at rest
- Occurs because skin temperature is greater than the temperature of surrounding objects (walls, floor, etc) and a net loss of body heat occurs due to the thermal gradient

Can also be a method of heat gain
- i.e. on hot sunny days when surface temperature is greater than skin temperature

Thus, radiation an result in either heat loss or gain
Conduction
Heat loss due to contact with another surface; the transfer of heat from the body into the molecules of cooler objects in contact with its surface

In general, the body loses only small amounts of heat due to this process

i.e. transfer of heat from the body to a metal chair while a person is sitting
- Heat loss occurs as long as the chair is cooler than the body surface in contact with it
Convection
Form of conductive heat loss in which heat transferred to air or water molecules in contact with the body
- Air or water molecules are warmed and move away from the source of the heat (the body) and are replaced by cooler molecules

i.e. a fan moving large quantities of air past the skin
- Increases the number of air molecules coming in contact with the skin and thus promotes heat loss

The amount of heat loss that occurs due to convection is dependent on the magnitude of the airflow over the skin
- Under calm wind conditions, cycling at high speeds would improve convective cooling when compared to cycling at slow speeds

i.e. Swimming in cool water (water temperature less than skin temperature) also results in convective heat loss
- Water's effectiveness in cooling is about 25x greater than that of air at the same temperature
Evaporation
Heat is transferred from the body to water (sweat) on the surface of the skin , which is then converted to water vapor once the water gains sufficient heat (energy); heat from the skin converts water to water vapor
- Evaporation occurs due to the vapor pressure gradient between skin and air

When body temperature rises above normal, as during exercise, the nervous system stimulates sweat glands to secrete sweat onto the surface of the skin
- As sweat evaporates, heat is lost to the environment, which in turn lowers skin temperature

Body loses 0.58 kcal heat/ml sweat evaporated
- 1 L sweat results in heat loss of 580 kcal

25% heat loss at rest
- Most important means of heat loss during exercise (in cool/moderate environment)
- When exercise is performed in a hot environment, evaporation is the only means of losing body heat
Vapor Pressure
The pressure exerted by water molecules that have been converted to gas (water vapor)

Dependent upon air and relative humidity
- At any given temperature, a rise in humidity causes increased vapor pressure
Evaporation Rate
Evaporation of sweat from the skin depends on:
- Temperature and relative humidity (ambient conditions)
- Convective currents around the body
- Amount of skin surface exposed to the environment

At high environmental temperatures, relative humidity is the most important factor influencing the rate of evaporative heat loss
- High relative humidity reduces the rate of evaporation by reducing the vapor pressure gradient
- Cooling via evaporation is therefore most effective under conditions of low humidity

High relative humidity reduces the vapor pressure gradient between the skin and the environment
- On a hot/humid day, (RH = 80-90%) the vapor pressure int the air is close to the vapor pressure on moist skin
- Skin vapor pressure = 32 mmHg

High sweating rate in a hot/humid environment results in useless water loss
- Sweating per se does not cool the skin, the evaporation does
- For evaporative cooling to occur, the vapor pressure on the skin must be greater than the vapor pressure in the air
Heat Storage in the Body During Exercise
Heat produced that is not lost is stored in body tissues
- Will raise body temperature

Body heat gain during exercise = heat produced - heat loss

Amount of heat required to raise body temperature:
- specific heat of human body is 0.83 kcal/kg

Heat required to raise body temp 1 degree C (specific heat of the body) = specific heat x body mass
The Body's Thermostat: Hypothalamus
The body's temperature regulatory center; responds to increases or decreases in body core temperature to maintain a set point temperature around 37 degrees C

Input to the hypothalamus comes from receptors in both the skin and core

Change in environmental temperature is first detected by thermal receptors in the skin
- Transmits impulses to the hypothalamus, which initiates the appropriate response to maintain the set-point temperature

Temperature sensitive neurons also located in the spinal cord and hypothalamus to sense changes in core temperature
Anterior Hypothalamus
Responds to increased core temperature by increasing heat loss

1) Sweating: stimulates the sweat glands to commence sweating and increase evaporative heat loss

2) Cutaneous vasodilation: vasomotor control center withdraws normal vasoconstrictor tone to the skin to promote cutaneous vasodilation
- Increases skin blood flow and allows for increased heat loss

When core temperature returns to normal, the stimulus to promote sweating and vasodilation are removed
- i.e. example of negative feedback
Posterior Hypothalamus
Responds to decreased core temperature by minimizing heat loss and increasing heat production

1) Cutaneous vasoconstriction: vasomotor control center directs peripheral blood vessels to vasoconstrictor (decreased skin blood flow prevents heat loss)

2) Involuntary shivering: produces heat under significant core temperature drops

3) Catecholamine release: initiates release of norepinehrine to increase the rate of cellular metabolism (nonshivering thermogenesis)

4) Thyroxin release: indirectly increases thyroxine production and release, which increases cellular heat production (nonshivering thermogenesis)

Promotes pilorection (goosebumps); increases insulation space for fur-bearing animals
Thermal Events During Exercise
As exercise intensity increases:
- Energy output increases
- Heat production (metabolic rate) increases
- Total heat loss increases
- Body core temeprature incrases
- Heat loss via evaporation increases
- Convective and radiative heat loss decreases

1) Heat production increases
- This occurs due to muscular contraction and is directly proportional to the exercise intensity
- Venous blood draining the exercising muscle distributes the excess heat throughout the body core
- As core temperature increases, thermal sensors in the hypothalamus sense the increase in blood temperature
- Hypothalamus integration sensor detects difference in the thermal set point and the stimuli received
- Responds by directing the nervous system to commence sweating and to increase blood flow to the skin to increase body heat loss and minimize the increase in body temperature
- Causes new internal body core temperature to reach a new elevated steady-state level (does not result in a change in the set-point)

2) Linear increase in body core temperature
- Core temperature proportional to the active muscle mass (due to increased metabolic rate from higher intensities)
- Higher net heat loss

3) Lower convective and radiant heat loss
- Higher evaporative heat loss

As ambient temperature increases:
- Heat production remains constant
- Lower convective and radiant heat loss
- Higher evaporative heat loss
Thermal Events During Exercise Continued
Increase in body temperature is directly related to exercise intensity (not the environmental temperature)
- Body heat load increases with intensity
- Linear increase in energy output, heat production, and total heat loss as a function of work rate

Mechanisms of heat loss during exercise
- Evaporation (most important means of heat loss)
- Convection (small contribution)
- Radiation (small role in total heat loss)
Changing the Temperature
As temperature increases at constant intensity:
- Total energy output and heat production remain constant
- The contribution of heat loss from convection and radiation decreases
- At around 20 degrees C, evaporation becomes the primary mechanism for evaporative heat loss and heat loss from evaporation increases

Method of heat loss during continuous exercise is modified according to ambient conditions
Heat Index
Measure of the body's perception of how hot it feels
- Relative humidity added to air temperature

ex:
- air temperature = 80 degrees F
- relative humidity = 80%
- heat index = 89 degrees F

High relative humidity reduces evaporative heat loss
- Lowers heat loss (heat is lost at a slower rate)
- Increases body temperature
Exercise in the Heat
Inability to lose heat
- Higher core temperature
- Risk of hyperthemia and heat injury

Higher sweat rate (eccrine sweat glands stimulated by sympathetic cholinergic control at a higher rate)
- May be as high as 4-5 L/hr
- Risk of dehydration

Sweat rate depends on body mass and training level of working out in the heat
- Better training stimulates earlier onset of sweating and higher sweat rate throughout the exercise
Preventing Exercise-Related Heat Injuries
Guidelines:
- Exercise during the coolest part of the day
- Minimize exercise intensity and duration on hot/humid days
- Expose a maximal surface area of skin for evaporation
- Provide frequent rests/cool-down breaks with equipment removal
- Avoid dehydration with frequent water breaks
- Rest/cool down breaks should be in the shade and offer circulating, cool air
- Measure body weight at the beginning and end of training session to determine the amount of fluid replacement required
Prevention of Dehydration During Exercise
Dehydration of 1-2% of body weight can impair performance

Guidelines:
1) Hydrate prior to performance
- 400-800 ml fluid within 3 hours prior to exercise
2) Consume 150-300 ml fluid every 15-20 min
- Volume adjusted based on environmental conditions
3) Ensure adequate dehydration
- Consume equivalent of 150% weight loss
- 1 kg body weight = 1.5 L fluid replacement
- Monitor urine color

Sports drinks are superior to water for rehydration
Exercise Performance is Impaired in a Hot Environment
Exercise performance is impaired in a hot environment
- Performance during prolonged, sub maximal exercise (i.e. marathon) is impaired in a hot/humid environment
- Performance during intermittent, high-intensity exercise (i.e. rugby, soccer, 15 min on cycle) is also compromised

Exercise in the heat accelerates muscle fatigue and impairs exercise performance due to a combination of metabolic events:
1) Heat-related muscle fatigue
- During long and intermittent exercise

2) Accelerated glycogen metabolism
- Increased lactate and accumulation and carbohydrate metabolism
- Muscle glycogen depletion and hypoglycemia (low blood glucose) are associated with muscle fatigue during prolonged exercise
- Lower pH caused by increased lactate levels also causes muscle fatigue

3) Increased free radical production
- Damage to muscle contractile proteins (actin and myosin)

4) Reduced muscle blood flow
- During high-intensity exercise
- Caused by cardiovascular strain and a progressive decline in muscle blood flow (due to competition for blood between the working muscles and skin)

5) High brain temperature reduces neuromuscular drive
- Caused by both hyperthermia and dehydration, results in central nervous system impairment
- Hyperthermia acts upon the CNS to reduce the mental drive for motor performance
- Reduction in motor unit recruitment

Key factors that contribute to heat-related muscle fatigue are alterations ink muscle metabolism impaired cardiovascular function/fluid balance, and CNS dysfunction resulting in impaired neuromuscular function
Gender and Age Differences in Thermoregulation
Gender differences in heat tolerance are small
- Used to be believed that women were less tolerant of exercise in hot environment than men
- When matched for level of acclimation and body composition (i.e. equal percent of body fat)

Exercise-conditioned old and young men show little difference in thermoregulation during exercise
- Heat tolerance is not severely compromised by age in healthy an physically active older subjects

Age itself does not limit ability to thermoregulate
- Decreased thermotolerance with age due to:
1) Deconditioning with age (i.e. lower VO2max)
2) Lack of heat acclimitization

Aging is thus associated with reduced ability to regulate body temperature in sedentary individuals
Acclimation
Rapid adaptation (days to weeks) to environmental change
Acclimitization
Adaptation over a long time period (weeks to months)
Heat Acclimation
Requires exercise in hot environment
- Elevated core body temperature is the primary factor that promotes adaptations associated with heat acclimation
- Recommended that an athlete perform strenuous interval training or continuous exercise at intensity exceeding 50% of VO2max to promote higher core temperature

Minimizes disturbances in homeostasis:
End result is a lower heart rate during sub-maximal exercise in the heat and decreased body core temperature
- Also causes decreased psychological rating of perceived exertion, and improved exercise performance in a hot environment

Heat adaptation occurs rapidly as a result of regular bouts of exercise in a hot environment, with almost complete acclimation being achieved by 7-14 ays after the first exposure
- Acclimation lost within a few days of inactivity; complete loss of heat tolerance lost after 28 days
- Repeated exposure to heat is therefore necessary to maintain heat acclimation
Adaptations During Heat Acclimation
10-12% increased plasma volume
- Maintains blood volume, stroke volume, and sweating capacity
- Allows body to store more heat with a smaller temperature gain

Earlier onset of sweating and higher sweat rate
- Less heat storage, maintain lower core body temperature
- Can increase sweating capacity a lost threefold prior to heat adaptation

Reduced sodium chloride loss in sweat
- Reduced risk of electrolyte disturbance

Reduced skin blood flow

Increased synthesis of heat shock proteins

Adaptations are sport specific and occur quickly
Heat Acclimation and Heat Shock Proteins
Heat acclimation reduces the risk of heat injury
- In response to exposure of heat stress

Related to synthesis of heat shock proteins
- Protects cells from thermal injury
- Stabilizing and refolding damaged proteins to protect cells against thermal injury
Number of Days Required for Heat Acclimation
Heart rate decreases: 3-7 days of acclimation
Plasma volume expansion: 3-6 days of acclimation
Perceived exertion decrease: 5-9 days of acclimation
Sweat rate: 8-14 days of acclimation
Exercising in Cool Conditions and Heat Acclimation
Training in cool conditions can promote heat acclimation, but the magnitude of the adaptation is less than training in hot/humid environment

"Artificial" heat may help
- i.e. training in heavy clothes

Exercise in rubberized vinyl suits:
- Raises body temperature and may promote acclimation
- Risk of hyperthermia
- Not an effective method of weight loss
Lebron James
Should have taken off compression garments to reduce heat injuries
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