Humans are homeotherms
Maintain constant body core temperature, Heat loss must match heat gain
Normal core temperature is 37�C (Fahrenheit not used)
Above 45�C: May destroy proteins and enzymes and lead to death, Below 34�C: May cause slowed metabolism and arrhythmias
Thermal gradient from body core to skin surface
Ideal gradient is ~4�C, In extreme cold, may be 20�C
Homeotherms are
animals that maintain a rather constant body core temperature. To maintain a constant core temperature, heat loss must match heat gain.
Temperature varies markedly within the body. In general, there is a thermal gradient from
the deep body temperature (core temperature) to the shell (skin) temperature.
Deep-body (core) temperature
Measured at rectum, ear, or esophagus, Usually in laboratory
Ingestible temperature sensor telemetry system (GI Pill)
In athletes during practice sessions
Skin temperature
Thermistors at various locations, Calculate mean skin temperature
Voluntary heat production
Exercise, 70-80% energy expenditure appears as heat
Involuntary heat production
Shivering, Increases heat production by ~5x
Action of hormones for involuntary heat production (non shivering thermogenesis)
Thyroxine, Catecholamines
Obligatory heat production
Heat production through anabolic and catabolic system, Energy produced - heat produced, expending energy - heat produced
Total energy expenditure
Exercise, Non-exercise, Diet, Resting Energy Expenditure
Dry Heat Exchange
Radiation, conduction, convection
Radiation
Transfer of heat via infrared rays, 60% heat loss at rest, Can be a method of heat gain
Conduction
Heat loss due to contact with another surface
Convection
Heat transferred to air or water, Example: a fan pushing air past skin
Note clothing and adipose tissue can influence
heat loss
Evaporation (wet heat exchange)
Heat from skin converts water (sweat) to water vapor, Requires vapor pressure gradient between skin and air, Strongly influenced by sweat rate
Evaporation rate depends on
Temperature and relative humidity, Convective currents around the body, Amount of skin surface exposed
Body loses how many kcal heat/L sweat evaporated
0.58 kcal heat/L sweat evaporated
1 L sweat results in heat loss of
580 kcal
25% heat loss at
rest
Evaporation most important means of
heat loss during exercise
Heat produced that is not lost is stored in body tissues will
raise body temperature
Amount of heat required to raise body temperature
Specific heat of human body is 0.83 kcal/kg
Anabolic and catabolic processes
exercise activates catabolic processes
Muscular exercise can result in large amounts of heat production. Because the body is at most 20% to 30% efficient
70% to 80% of the energy expended during exercise is released as heat.
Body heat can be lost by
evaporation, convection, conduction, and radiation. During exercise in a cool environment - evaporation is the primary avenue for heat loss.
The rate of evaporation from the skin is dependent upon three factors
(1) temperature and relative humidity, (2) convective currents around the body, and (3) the amount of skin exposed to the environment.
Body heat storage is the difference between
heat production and heat loss.
The amount of heat required to elevate body temperature by 1�C is termed
the specific heat of the body.
Anterior hypothalamus
Responds to increased core temperature, Commencement of sweating: Increased evaporative heat loss, Increased skin blood flow: Allows increased heat loss
Posterior hypothalamus
Responds to decreased core temperature, Shivering and increased norepinephrine release: Increased heat production, Decreased skin blood flow: Decreased heat loss
Fever
Increased body temperature above normal, Due to pyrogens: Proteins or toxins from bacteria, Change in set point of hypothalamus, IL - 1, IL - 6, TNF
The body's thermostat is located in the
hypothalamus.
The anterior hypothalamus is responsible for
reacting to increases in core temperature
posterior hypothalamus governs the body's responses to
a decrease in temperature.
An increase in core temperature results in the
anterior hypothalamus initiating a series of physiological actions aimed at increasing heat loss. These actions include the commencement of sweating and an increase in skin blood flow.
Cold exposure results in the
posterior hypothalamus promoting physiological changes that increase body heat production (shivering) and reduce heat loss (cutaneous vasoconstriction).
Increase in body temperature is directly related to
exercise intensity
Body heat load increases with
intensity
Measure of body's perception of how hot it feels
Relative humidity added to air temperature, Example: Air temperature = 80�F, relative humidity = 80% Heat index = 89�F
High relative humidity reduces
evaporative heat loss, Lowers heat loss (vapor gradient), Increases body temperature
Mechanisms of heat loss during exercise
Evaporation: Most important means of heat loss, Convection: Small contribution, Radiation: Small role in total heat loss
As exercise intensity increases: (Fig. 2,4)
Heat production increases, Linear increase in body temperature, Core temperature proportional to active muscle mass, Higher net heat loss: Lower convective and radiant heat loss, Higher evaporative heat loss
As ambient temperature increases: (Fig. 3)
Heat production remains constant, Lower convective and radiant heat loss (indoor vs out), Higher evaporative heat loss
Inability to lose heat
Higher core temperature (thermal gradient), Risk of hyperthermia and heat injury
Higher sweat rate
May be as high as 4-5 L/hour, Risk of dehydration (Alberto Salazar
Sweat rate factors
Body weight difference, drink volume, urine, and time of exercise
Exercise-Related Heat Injuries Can Be Prevented
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 w
Dehydration of 1-2% body weight can impair performance
Guidelines, Hydrate prior to performance: 400-800 ml fluid within three hours prior to exercise, Consume 150-300 ml fluid every 15-20 min, Volume adjusted based on environmental conditions: Ensure adequate rehydration, Consume equivalent of 150% weight lo
Exercise Performance is Impaired in a Hot Environment due to
(1) Heat-related muscle fatigue: During prolonged and intermittent exercise, (2) Accelerated glycogen metabolism: Increased lactate accumulation, (3) Increased free radical production: Damage to muscle contractile proteins, (4) Reduced muscle blood flow:
Gender differences in heat tolerance are small
When matched for level of acclimation and body composition
Age itself does not limit ability to thermoregulate; Decreased thermotolerance with age due to
Deconditioning with age, Lack of heat acclimatization
Acclimation
physiological, biochemical, or anatomical modifications within an individual organism that result from exposure to an environmental challenge in a laboratory or field setting.
Acclimatization
physiological, biochemical, or anatomical modifications occurring within the lifetime of an individual organism that result from exposure to a naturally occurring environmental challenge
Heat Acclimatization
requires 8-14 days, should involve exercise, Environment should match the expected conditions of the competition or labor, The early physiological adaptations [initial 3-5 days]: plasma volume increase, heart rate decrease, rating of perceived exertion (R
Benefits of heat acclimatization
Improves exercise performance, Reduces the risk of heat illnesses
Maintenance
Rapid adaptation (days to weeks) to environmental change, Requires only one or two exposures per week, this preserves physiological adaptations.
Decay
Loss of adaptation can occur within a week of non-exposure
Adaptions During Heat Acclimation
(1) 10-12% increased plasma volume: Maintains blood volume, stroke volume, and sweating capacity, (2) Earlier onset of sweating and higher sweat rate, (3) Less heat storage: maintain lower body temperature, (4) Reduced sodium chloride loss in sweat: Reduc
Heat acclimation reduces the risk of heat injury
In response to exposure of heat stress
Related to synthesis of heat shock proteins
Protect cells from thermal injury, Stabilizing and refolding damaged proteins
During prolonged exercise in a moderate environment, core temperature will increase gradually above the normal resting value and will reach a plateau at approximately
30 - 45 minutes
During exercise in a hot/humid environment, core temperature does not reach a plateau, but will
continue to rise. long term exercise in this type of environment increases the risk of heat injury
Heat acclimatization results in
(1) an increase in plasma volume, (2) an earlier onset of sweating, (3) a higher sweat rate, (4) a reduction in the amount of electrolytes lost in sweat, (5) a reduction in skin blood flow, and (6) increased levels of heat shock protein in tissues
Heat Illnesses
Mild: Heat rash, Major: Heat cramps, Heat syncope (fainting), Heat exhaustion, Emergency!!!!: Heat stroke
Heat Rash
Prickly Heat, Cause:Clogged sweat glands from damp skin and clothing from unevaporated sweat, Symptoms: Red itchy bumps at skin folds or where clothing rubs skin
Heat Cramps
Cause:Temporary fluid electrolyte imbalance. Low salt with heavy physical exertion, Symptoms:Painful muscle spasms in the arms, legs, and abdomen, Consequences:With heavy sweating and thirst may signal heat exhaustion, Rx: Rest in cool environment; replac
Heat Syncope
Cause:Blood pooling in legs, standing a long time, not used to heat., Symptoms: Brief loss of consciousness., Consequences:Injury from falling.
Heat Stroke
Cause:Temperature regulation has failed. No sweat available for cooling., Symptoms: same as exhaustion + Very high body temperature (> 40 C), Hot, dry, reddish skin; dry mucosa, Central nervous system: irrational behavior, incoherent, confused, psychosis,
Exercise in a Cold Environment
Enhanced heat loss: Reduces chance of heat injury, May result in hypothermia: Loss of judgment and risk of further cold injury, Subcutaneous fat improves cold tolerance, Reduced dexterity in extremities (hands): Due to reduced blood flow and slower nerve
Cold Acclimation
Results in lower skin temperature at which shivering begins: increased non shivering thermogenesis
Maintain higher hand and foot temperature: Improved peripheral blood flow
Improved ability to sleep in the cold: Due to reduced shivering
Adaptations begin
Exercise in a cold environment enhances an athlete's ability to
lose heat and therefore greatly reduces the chance of heat injury
Cold acclimatization results in three physiological adaptations
(1) improved ability to sleep in cold environments
(2) increase non shivering thermogenesis
(3) a higher intermittent blood flow to the hands and feet
The overall goal of these adaptations is to
increase heat production and maintain core temperature, which will make the individual more comfortable during cold exposure
Frostnip
the mildest form of a freezing cold injury. Only the very outer layers of the skin free, usually on the cheeks earlobes, fingers, and toes, and also nose and chin
Frostnip usually occurs at about
29�F, Generally reversible, no tissue injury or permanent damage skin turns white, top layer of skin feels hard but deeper tissue still feels normal, may feel tingling or numbness
Frostnip treatment
Gently re-warm affected area by blowing warm air on it or placing it against a warm body part (e.g., another person's stomach or armpit)., Do not use very hot objects such as hot water bottles to rewarm the area or person., Do not rub the affected part ic
Frostnip can be prevented by wearing
warm clothing, gloves and insulated footwear
Frostbite
In cold conditions, your body reduces heat loss and increases heat production in order to maintain an internal body temperature of 98.6�F. Over time, your body will decrease blood flow to your extremities and outer skin and shift it to the body core to ke
Superficial Frostbite
includes all layers of the skin, Initially redness in light skin, grayish in dark skin, Burning, tingling, itching, or cold sensations in the affected areas, followed by numbness, Skin turns white, waxy; some resistance when pressed (feels firm or "wooden
Deep Frostbite
Involves skin, muscle, tendons, nerves, blood vessels; may include bone, White or yellowish waxy skin that turns purplish blue as it thaws, Underlying tissue hard, no resistance when pressed, may appear blackened and dead, Blood-filled blisters and swelli
Hypothermia ("low heat")
is a potentially serious and fatal health condition resulting from the body's failure to maintain its normal core temperature of 98.6�F.
When exposed to cold conditions, your body begins to lose heat faster than it can be produced. Prolonged exposure to cold eventually uses up your body's stored energy and your core body temperature drops to
95�F or below. The result is hypothermia.