aep 5 pt 1

Human thermoregulation

Internal body temperature is kept constant (usually) despite environmental temperature changes; heat sensors in brain, skin, and spinal cord

Thermoneutral zone

Minor adjustments to skin blood flow minimize heat loss or gain

Exercise in the heat responses

↑ core temp, ↑ heart rate, ↓ stroke volume (blood sent to skin for cooling), ↓ max cardiac output

Cardiovascular drift

Blood volume decreases from sweat → stroke volume cannot increase → heart rate increases to compensate

Critical temperature limit

Brain shuts down exercise around 40–41°C to prevent damage

ATP breakdown and heat

~1/4 used for cellular work; remainder produces metabolic heat

Heat balance requirement

Heat gained must equal heat lost to maintain core temperature

Conduction (K)

Heat transfer through direct molecular contact (negligible)

Convection (C)

Heat transfer via movement of air/liquid across skin; major thermoregulatory factor

Radiation (R)

Heat loss via infrared rays; major thermoregulatory factor

Dry heat exchange

C + K + R

Insulation (I)

Resistance to dry heat exchange; still air is ideal insulator

Evaporation (E)

Heat loss via liquid to gas; primary heat loss during exercise (~80%)

Clothing effect on evaporation

Clothing increases resistance to evaporation

Heat balance equation

M − W ± R ± C ± K − E = 0 → balance;

POAH (pre-optic anterior) hypothalamus function

Body thermostat; senses temperature and activates responses

Heat response

Vasodilation and sweating decrease body temperature

Eccrine sweat glands

Critical when air temperature exceeds skin temperature

evaporation (sweating) and humidity

High humidity reduces evaporation

Sex differences in sweating

Females sweat less output per gland but have more active glands

Sweat loss effect

Leads to dehydration

Sweat gland stimulation

Sympathetic cholinergic (acetylcholine)

Sweat gland sensitivity

More responsive to core temperature than skin temperature

Sweating in hot environments

C, K, R cause heat gain; evaporation is only heat loss mechanism

Sweat adaptation with training

More Na+ and Cl− reabsorbed; electrolyte conservation

Sweat losses during exercise

1.6–2.0 L/hour (2.5–3.2% body weight)

Sweating effect on circulation

Increased sweating decreases blood volume and cardiac output

Dehydration effects

Less sweating, less skin blood flow

Cardiovascular effects of dehydration

↑ heart rate, ↓ stroke volume, ↓ blood flow to muscles/brain

Fluid intake effect

Reduces body heat storage after prolonged exercise (~45 min+)

Heat acclimation

Timeframe: 9–14 days

Heat acclimatization

Months to years

Heat acclimation benefit

Plasma volume expansion

Acclimation effects

Improved cardiovascular function and sweating responses

Core temperature with acclimation

Lower during exercise

Heart rate with acclimation

Reduced during exercise

Sweating changes with acclimation

Earlier onset and higher rate

Sweat electrolyte changes with training

Lower Na+ and Cl− loss in trained individuals

WBGT importance

Better than heat index for measuring thermal stress

WBGT components

Dry-bulb (air temp), wet-bulb (evaporation), globe temp (radiation)

Heat index limitation

Does not reflect physiological stress

Heat cramps

Painful large muscle cramps from Na+ loss and dehydration

Heat cramps prevention

Adequate Na+ and water intake

Heat exhaustion

Symptoms: fatigue, dizziness, nausea, weak/rapid pulse

Heat exhaustion cause

Severe dehydration and low blood volume

Heatstroke

Failure of thermoregulation; core temp >40°C or CNS dysfunction

Heatstroke treatment

Immediate whole-body cooling (ice bath)

Heat stress factors

Metabolic heat, air temp, humidity, wind, radiation, clothing

Preventing hyperthermia

Avoid WBGT >28°C; hydrate frequently; minimize clothing

Fluid replacement guideline

1 kg body weight lost = 1 L fluid intake. via frequent breaks every 15 to 30 min

Preventing High Body Temperature (Hyperthermia)

1. Avoid outdoor activities when WBGT >28° C (82 ° F).

 2. Never restrict fluid intake

3. Minimize clothing impeding sweating

Precooling methods

Cold water, cold air, cooling vest, icy drink

Precooling effect

Improves long-duration performance; may impair short events

Cold exercise muscle effects

↓ contractile force, ↓ shortening velocity, ↓ power

Cold fatigue effect

↓ metabolic heat production → ↑ hypothermia risk

Cold metabolism

Catecholamines (adrenaline) increase but no free fatty acid mobilization

Cold glucose response

Blood glucose maintained; ↑ glycogen use

Hypoglycemia (low blood sugar) effect

Shivering suppressed

Windchill definition

Enhanced heat loss due to wind; not just air temperature

Windchill effect

↑ convective heat loss; ↑ freezing risk

Cold response

Vasoconstriction and shivering increase body temperature

Cold hormones

Thyroxine and catecholamines released

Behavioral response to cold

Controlled by cerebral cortex

pre-optic anterior) hypothalamus

controls body temperature

Shivering thermogenesis fuels

Shift from carbohydrates to fats over time

Body composition and heat loss

More fat and less surface area reduce heat loss

Water vs air heat loss

Heat loss is 4× faster in water

Core temp in water

Stable until water <32°C 90 F

Cold adaptation stages

Habituation, metabolic acclimation, insulative acclimation

Cold habituation

Reduced shivering and vasoconstriction core temp drop more

Metabolic acclimation

Enhanced heat production, shivering heat production

Insulative acclimation

Increased peripheral vasoconstriction

Exercise-induced asthma

Caused by airway drying in cold air; treated with inhalers affects up to 50% of winter sport athletes . treated by beta antagonist and steroid inhalers

Hypothermia mild

Core 34.5–29.5°C; impaired hypothalamus func

Hypothermia severe

Core <29.5°C; loss of thermoregulation metabolism slowed; drowsiness, lethargy, coma possible

Cardiorespiratory effects of cold

–Low core temperature slows heart rate (via SA node)

–Cold may lower ventilation

Hypothermia treatment mild

Core temp 34.5 to 29.5 ° C

- Remove from cold + provide dry clothing, blankets, warm beverages

Hypothermia treatment severe

Core temp <29.5 ° C

- Rewarm gradually to avoid arrhythmias; May require hospital facilities/medical care

Frostbite

Peripheral tissue freezing

- From excess vasoconstriction limiting O₂ supply

- Gradually rewarm only when no risk of refreezing

- Untreated frostbite à gangrene, tissue loss

Frostbite treatment

Rewarm gradually only if no refreezing risk

Frostbite complication

Gangrene and tissue loss if untreated

Thermoregulation summary POAH (pre-optic anterior hypothalamus)

Acts as body thermostat controlling vasodilation and sweating

Thermoregulation summary evaporation

Most important cooling method; limited by humidity

Thermoregulation summary heat exercise

↓ stroke volume → compensatory ↑ heart rate

Thermoregulation summary heatstroke

Core temp >40°C; WBGT best risk measure

Thermoregulation summary cold

Excess vasoconstriction can lead to frostbite

The _________ acts as the body's thermostat, regulating internal temperature by initiating skin ______ and ________

pre-optic anterior hypothalamus,vasodilation, sweating.

While the body can lose heat through conduction, convection, and radiation, _______from sweat is the most critical cooling mechanism, though its effectiveness is heavily restricted in environments with ________

evaporation, high humidity.

Exercising in the heat causes a decrease in __________ because blood must be redirected to the _____ for cooling, which forces a compensatory increase in ______ to maintain cardiac output.

stroke volume, skin, heart rate

_______ is a life-threatening emergency characterized by a thermoregulatory failure and a core temperature ______, and the environmental risk for such illnesses is most accurately measured using the _____________________

Heatstroke, >40° C, Wet-Bulb Globe Temperature (WBGT).

In extreme cold environments, excessive peripheral _________ limits oxygen and nutrient delivery to the skin, which can lead to ________and___________

vasoconstriction, tissue death and frostbite.