EP Chapter 12

What is a homeotherm?

Humans are homeotherms, meaning they maintain a constant body core temperature. Heat loss must match heat gain.

What is the normal body core temperature for humans?

The normal core temperature is 37°C.

What are the risks of body temperatures above 45°C?

Temperatures above 45°C can damage proteins and enzymes and lead to death.

What are the risks of body temperatures below 34°C?

Temperatures below 34°C can result in decreased metabolism and cardiac arrhythmias.

Describe the thermal gradient in the body.

A thermal gradient exists between the body core and the skin surface. A typical gradient is approximately 4°C, but in extreme cold, it may be 20°C.

How is deep-body (core) temperature measured?

Deep-body temperature can be measured at the rectum, ear, or esophagus, usually in a laboratory. Ingestible temperature sensors can also be used.

How is mean skin temperature calculated?

Mean skin temperature is calculated using the formula: (Tforehead + Tchest + Tforearm + Tthigh + Tcalf + Tabdomen + Tback) ÷ 7.

How much metabolic heat is produced at rest?

Metabolic heat production at rest is small.

How much energy expenditure is released as heat during voluntary exercise?

70 to 80% of energy expenditure during exercise is released as heat.

What are involuntary ways the body produces heat?

Involuntary heat production includes shivering, which increases heat production by approximately 5 times, and the action of hormones like Thyroxine and Catecholamines, called non-shivering thermogenesis.

What is Radiation as a mechanism of heat loss?

Radiation is the transfer of heat via infrared rays. It accounts for 60% of heat loss at rest and can also be a method of heat gain.

What is Conduction as a mechanism of heat loss?

Conduction is body heat loss due to contact with another surface, like sitting on a metal chair.

What is Convection as a mechanism of heat loss?

heat transferred from the body to air or water, such as a fan moving air over the skin.

What is Evaporation as a mechanism of heat loss?

Evaporation occurs when water (sweat) gains enough heat to convert to gas (water vapor). It requires a vapor pressure gradient between the skin and air.

What factors affect the rate of evaporation?

temperature and relative humidity (vapor pressure), convective currents around the body, and the amount of skin surface exposed to the environment.

How much heat loss occurs due to evaporation at rest?

25% of heat loss at rest is due to evaporation.

Why is evaporation important during exercise?

key means of heat loss during exercise.

How much heat is lost per milliliter of sweat evaporated?

The body loses 0.58 kcal of heat per ml of sweat evaporated. 1 Liter of sweat evaporation equals a loss of 580 kcal of heat.

What happens to heat produced during exercise that is not lost?

Heat produced that is not lost is stored in body tissues, increasing body temperature. Heat gain during exercise = (heat produced − heat loss).

What is the specific heat of the human body?

0.83 kcal/kg.

How much heat is required to raise body temperature by 1°C?

Heat required to raise body temp 1°C = (specific heat × body mass).

What part of the brain acts as the body's thermostat?

The Preoptic Anterior Hypothalamus (POAH) acts much like a thermostat to maintain a constant core temperature around a 'set point'.

How does the POAH respond to an increase in core temperature?

The POAH stimulates sweat glands for evaporative heat loss and causes cutaneous vasodilation, allowing increased heat loss.

How does the POAH respond to a decrease in core temperature?

The POAH triggers shivering and increased norepinephrine release to increase heat production. It also decreases skin blood flow via vasoconstriction to decrease heat loss.

What is a fever and what causes it?

Fever is an increased body temperature above normal. It is caused by pyrogens (proteins or toxins from bacteria) which change the set point of the hypothalamus thermostat.

How does increasing exercise intensity affect heat production and body temperature?

As exercise intensity increases, heat production increases, leading to a linear increase in body temperature. Core temperature increases proportionally to the active muscle mass.

How does increasing ambient temperature affect heat loss during exercise?

As ambient temperature increases, exercise-induced heat production remains constant. There is lower convective and radiant heat loss and higher evaporative heat loss.

What is the Heat Index?

The Heat Index is a measure of the body’s perception of how hot it feels. It is influenced by the interaction between relative humidity and air temperature.

How does high relative humidity affect how hot it feels and the body's ability to cool?

High relative humidity and high ambient temperature make it feel hotter. This lowers the body's ability to lose heat through evaporation and increases body temperature. The air is saturated with water vapor, making it harder for sweat to evaporate.

What are some consequences of exercising in a hot environment?

Exercising in a hot environment reduces the ability to lose body heat, resulting in a higher core temperature, higher heart rate, and risk of hyperthermia and heat injury.

How high can sweat rates be during exercise in the heat?

Sweat rate can be as high as 4 to 5 L/hour in a hot environment, increasing the risk of dehydration.

What are general guidelines to prevent exercise-related heat injuries?

Exercise during the coolest part of the day, minimize intensity/duration on hot/humid days, expose maximal skin surface area, provide frequent rests/cool-down breaks (in shade, with circulating cool air, removing equipment), avoid dehydration with frequent water breaks, and measure body weight for fluid replacement needs.

How much dehydration can impair exercise performance?

Dehydration resulting in 1 to 2% of body weight loss can impair exercise performance.

What are hydration guidelines before exercise?

Consume adequate fluid daily several days prior to an event. Consume 400 to 800 ml of fluid within three hours prior to an event.

What are hydration guidelines during exercise?

Consume 150 to 300 ml of fluid every 15 to 20 minutes. The volume should be adjusted based on environmental conditions.

What are rehydration guidelines following exercise?

Consume fluid equivalent of 150% of weight loss. 1 kg of body weight loss equals 1.5 L of fluid replacement. Monitor urine color (light color indicates proper hydration).

When are sports drinks superior to water for rehydration?

Some sports drinks are superior to water for rehydration, particularly in events longer than an hour. For activities less than an hour, water is generally sufficient.

What are the main reasons for impaired endurance exercise performance in a hot and humid environment?

Impaired performance is due to accelerated muscle fatigue, cardiovascular dysfunction, and Central Nervous System Dysfunction.

What contributes to cardiovascular dysfunction in the heat?

Cardiovascular dysfunction is caused by a reduction in stroke volume (amount of blood ejected per beat), a decrease in cardiac output (heart rate x stroke volume), and a decrease in blood flow to the muscles.

What contributes to accelerated muscle fatigue in the heat?

Accelerated muscle fatigue is caused by an increase in free radical production, a decrease in muscle pH, and depletion of muscle glycogen.

What contributes to central nervous system dysfunction in the heat?

CNS dysfunction is linked to cardiovascular dysfunction and accelerated muscle fatigue, resulting in less motivation and fewer motor units activating muscle fibers.

Are there significant gender differences in thermoregulation during exercise?

Recent studies matching men and women for body composition and heat acclimation conclude that gender differences in thermoregulation and heat tolerance are relatively small.

Does aging impair the ability to thermoregulate and exercise in the heat?

Yes, aging impairs this ability. Skin blood flow is reduced in older individuals (>60 years), resulting in a reduced ability to lose heat. Exercise training in the heat can improve heat tolerance in older individuals.

How do children and elderly individuals differ from adults in cold tolerance?

Children often have a large surface-to-mass ratio, resulting in increased heat loss compared to adults. Elderly individuals who have lost muscle mass (sarcopenia) are at greater risk for hypothermia due to reduced shivering-induced thermogenesis.

What is the difference between Acclimation and Acclimatization?

Acclimation is rapid adaptation (days to weeks) to environmental change. Acclimatization is adaptation over a long time period (weeks-months).

How quickly can heat acclimation be lost?

Acclimation is lost within a few days of inactivity or no heat exposure. There is a significant decline in 7 days, and complete loss in 28 days.

What are some physiological adaptations during heat acclimation?

Increased plasma volume (+10-12%), earlier onset and higher sweat rate, reduced sodium chloride loss in sweat, reduced skin blood flow, and increased cellular heat shock proteins.

How does increased plasma volume help during heat acclimation?

Increased plasma volume (10-12%) maintains blood volume, stroke volume, and sweating capacity.

How do earlier onset and higher sweat rates help during heat acclimation?

Earlier onset and higher sweat rates result in less heat storage and help maintain a lower body temperature.

What is the role of cellular heat shock proteins in heat acclimation?

Heat shock proteins are synthesized in cells upon repeated heat exposure. They protect cells from thermal injury by stabilizing and refolding damaged proteins.

Approximately how long does it take for key heat acclimation adaptations to occur?

Heart rate decreases within 3-7 days. Plasma volume expands within 3-6 days. Perceived exertion decreases within 5-9 days. Sweat rate increases within 8-14 days.

How can precooling the body affect exercise performance in the heat?

lowers body temperature and has been shown to improve exercise performance.

What are techniques for precooling the body?

Techniques include cold water immersion, cooling ice vests, cooling packs, and ingestion of cold drinks. A combination of techniques is often more effective than a single one.

What is cold stress?

Cold stress is an environmental condition that threatens temperature homeostasis by decreasing core temperature.

What is hypothermia?

Hypothermia is a large decrease in body core temperature, specifically more than 3°C.

How does exercising in a cold environment affect body heat loss?

Exercise in a cold environment results in enhanced body heat loss.

Why is swimming in cold water riskier than exercising in cold air?

Swimming in cold water can overpower the body’s ability to prevent heat loss. The body cools itself 25 times faster in water than in air at the same temperature.

How does subcutaneous fat affect cold tolerance?

Subcutaneous fat improves cold tolerance and reduces heat loss.

How do cold temperatures negatively impact sports performance?

Cold temperatures can cause numbness in hands due to reduced blood flow and depressed neural transmission, leading to loss of dexterity and negatively impacting motor skills.

How much can cold water immersion reduce muscle strength and power?

Cold water immersion causing an 8°C decrease in muscle temperature can reduce muscle strength and power by 31 to 44%. Cold air exposure rarely decreases muscle force production as muscle temperature doesn't typically decrease.

What are the risks of severe hypothermia?

When body temperature declines from 37°C to 25°C (77°F) or lower, this level of hypothermia is associated with life-threatening cardiac arrhythmias.

What are health risks during exercise in cold air?

Exposed skin is at risk for frostbite when air temperatures are below freezing. Breathing cold air does not harm the respiratory tract or lungs as it is warmed, but it can trigger exercise-induced asthma in some individuals due to airway cooling/drying.

What are some adaptations seen with acclimation to cold environments?

Lower skin temperature at which shivering begins (increased non-shivering thermogenesis), improved ability to maintain higher hand and foot temperature (improved peripheral blood flow), and improved ability to sleep in the cold (due to reduced shivering).

How quickly do cold adaptations occur?

Cold adaptations occur rapidly, within the first 7 days of chronic cold exposure.

What type of activity is most affected by altitude?

Endurance activity is affected the most at altitude due to reliance on O2 transport and the aerobic energy system.

What type of activity is least affected by altitude?

Anaerobic sprint activities that last 2 minutes or less are the least affected by altitude.

How does thinner air at altitude affect sprinting, jumping, and throwing events?

The thinner air at altitude provides less aerodynamic resistance and less gravitational pull, potentially improving sprinting, jumping, and throwing events.

What happens to atmospheric pressure and the pressure of oxygen at altitude?

The percentage of oxygen in the air stays the same (21%), but the atmospheric pressure decreases. This causes the pressure of oxygen to drop (e.g., from 159 at sea level to 110 at 10,000 ft).

What are some physiological adaptations seen with acclimatization to altitude?

Increase in number of red blood cells, short-term decrease/later reversal in plasma volume, increase in hemoglobin and blood viscosity, decrease in muscle fiber areas and total muscle area, increase in capillary density, and increase in pulmonary ventilation.

What altitude is typically required to see significant physiological changes during acclimatization?

Getting above 7,000 feet is needed to see significant physiological changes.

How does the body increase red blood cell production at altitude?

The kidneys are activated to produce erythropoietin (EPO). EPO sends a signal to the bone marrow to start producing new red blood cells. This process starts immediately upon exposure to altitude.

Does altitude training always improve sea-level performance?

It is not proven that altitude training improves sea-level performance. It's difficult to study because intensity and volume are often reduced at altitude. The benefit (1-5%) can be significant for elite athletes.

What is the 'living high/training low' strategy?

This strategy involves living at high altitude to stimulate physiological adaptations (like increased RBCs) and training at lower altitudes to maintain exercise intensity and volume.

What is a training recommendation for competing at altitude if you've only trained at sea level?

Compete within 24 hours of arrival to altitude.

What is a training recommendation for acclimatizing before competing at altitude?

Train at 1,500 to 3,000 m above sea level (approx 5,000 to 10,000 ft) for at least 2 weeks before competing.

How can you better prepare for competing at altitude if you primarily train at sea level?

Increase your VO2max at sea level so you can compete at a lower relative intensity while at altitude.