APK3110 Ch12 Temperature Regulation Fall 2024 PDF

TEMPERATURE REGULATION Chapter 12  Temperature Homeostasis Heat balance…production must match loss Heat conservation and generation is very efficient for human but we have a limited cooling capacity Heat loss > heat production → ↓body temp Heat loss < heat production → ↑body temp...

TEMPERATURE REGULATION Chapter 12  Temperature Homeostasis Heat balance…production must match loss Heat conservation and generation is very efficient for human but we have a limited cooling capacity Heat loss > heat production → ↓body temp Heat loss < heat production → ↑body temp Long-term exercise poses a serious challenge to temp homeostasis because contracting skeletal muscles generate large amounts of heat.  Heat Production: Metabolic Heat Body can increase heat production by 5x with maximal shivering Release of thyroxine and catecholamines increases whole body cellular metabolism. Humans are only about 20 to 30% efficient which means that the other 70% to 80% appears as heat. During heavy exercise, this can result in large heat load. Heat Loss: Four Processes to earth... heat Radiation how sun transfers - contact necessary no Conduction Convection Evaporation Radiation Radiation – transfer of heat in the form of infrared rays No contact necessary This is how the sun delivers heat to the earth Radiation can result in either heat loss or heat gain depending on environmental conditions: At rest, ~60% of heat loss occurs through radiation Skin temp > surrounding surface temps (walls, floor, etc.) On a hot day, however, heat gain occurs Surface temps > skin temp  Conduction Conduction – heat lost to a cooler object in physical contact with the skin Plays an insignificant role in heat loss during exercise Cooler objects must be touching the skin directly…AND… The heat only transfers as long as the other object is still cooler  Convection Convection – a form of conductive heat loss in which heat is transferred to air or water molecules in contact with the body Heat is transferred to cooler air/water molecules, forcing them away from the skin…to allow more cooler molecules nearby Effectiveness of convective air cooling depends on air flow Use of a fan or riding a bike at high speeds increases convective cooling Effectiveness of convective water cooling depends on water temp Cooler water more effective Water 25X more effective at cooling than air at same temp Evaporation Evaporation – heat from skin heats water (sweat) to water vapor (gas). When sweat evaporates, it takes the heat away from the body. Accounts for ~25% of heat lost at rest Most important means of heat loss during exercise When the body temp rises, the nervous system activates sweat glands Sweat is secreted onto the skin’s surface...when it evaporates, it cools the skin Evaporation Rate of evaporative cooling depends on three factors: 1. Air temperature and relative humidity 2. Convective currents around the body 3. Surface area of skin exposed to the environment At high temps, relative humidity is the most important factor determining rate of evaporation Evaporation works via a vapor pressure gradient Humidity decreases the vapor pressure gradient between the skin’s surface and the environment…inhibiting the ability of the sweat to evaporate Exercising in hot/humid climates can result in “useless” water loss because sweating per se does not cool the skin; it is evaporation that cools the skin Vapor pressure: pressure exerted by water molecules that have been converted to gas (water vapor). Temperature and humidity affect environmental vapor pressure In summary Radiation Conduction Require a temperature gradient between the skin and the environment Convection Evaporation When exercise is performed in a hot environmental (air temperature is hotter than skin temp, evaporation is the ONLY mechanism of heat loss. Most heat loss during exercise (other than swimming) in a cool/moderate environment occurs primarily due to evaporation  I Check your understanding  Body heat can be lost by ______________, radiation _______________, conduction ______________, evaporation and ______________. convection 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) temp/humidity around (2) covertive currents body (3) amount of skin exposed end Of exam into  The Body’s Thermostat = Preoptic-Anterior Hypothalamus (POAH) Thermoreceptors detect ↑ core temperature and relay info to POAH which induces : 1) sweating → ↑ evaporative heat loss 2) ↑ skin blood flow → ↑ heat loss Thermoreceptors detect ↓ core temperature and relay info to POAH which induces: 1) Shivering 2) ↑ norepinephrine ↑ heat production 3) ↑ thyroxine release 4) ↓ skin blood flow → ↓ heat loss Check your understanding  The body’s thermostat is located in the ________________________ preoptic anterior ____________________________________ hypothalamus (POAH).  ___________________ thermoreceptors are sensory receptors that detect changes in body temperature and relay this information to the body’s thermostat (i.e., POAH).  An increase in core temperature results in the POAH promoting heat loss by: (1) _________________________________________ sweating and (2) ________________________________________________. increasing blood flow to skin ) cutaneous ↳ vasodilation  Cold exposure results in the POAH promoting heat production and decrease in heat loss:(1) ___________________________________ shivering and (2) ________________________________________________. cutaneous valoconstriction decreases blood flow - Thermal Events during Exercise: Key Points - As exercise intensity increases: intensity &... Heat production ↑ Linear ↑ in body temp Core temp ↑ proportional to active muscle mass ↑ reliance on evaporative heat loss  Exercising in the Heat Increased water loss + high core temps can lead to hyperthermia! Heat Index - how hot it FEELS when we consider relative humidity and air temp aeludratdooa n When humidity is high, it FEELS ↳ hotter than it actually is because evaporative cooling is reduced ↓ heat loss ↑ heat storage ↑ core temperature ↑ heart rate Risk of hyperthermia and heat injury Higher sweat rate May be as high as 4-5 L/hr Risk of dehydration  Check your understanding  Body heat production increases in proportion to ___________________. intensity  During prolonged exercise in a moderate environment, core temperature will increase gradually above the normal resting value and will reach _________________ Steady State at approximately thirty to forty-five minutes.  During exercise in a hot/humid environment, core temperature does not reach a ___________________, Steady state but will continue to rise. Long-term exercise in this type of environment increases the risk of heat injury.  Exercise-Related Heat Injury is Preventable Clinical Applications 12.1 >3°C increase in body temp leads to a continuum of problems Muscle cramping might be the first sign of heat-related illness. Preventative actions: Exercise during coolest part of the day Minimize intensity and duration of exercise Expose maximal skin surface area Frequent breaks Hydration Removal from direct sunlight Exposure to circulating cool air Measure body weight before and after exercise session to determine amount of fluid replacement required. Prevention of Dehydration During Exercise 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 loss 1 kg body weight = 1.5 L fluid replacement Monitor urine color (should be clear or color of lemonade if well-hydrated). Some sports drinks are superior to water for rehydration  Exercise Performance is Impaired in a Hot Environment Due to: Accelerated muscle fatigue ↑ glycogen breakdown and depletion (↑ lactic acid production and ↓ muscle pH) ↑ free radical production (damage to muscle contractile proteins) Afferent feedback from metaboreceptors in fatiguing muscles to the brain ↓ central motor drive Cardiovascular dysfunction Cardiovascular strain (↑ heart rate and ↓ stroke volume) ) ↳ drift cardiovascular ↓ muscle blood flow (competition for blood flow between muscle and skin) during high-intensity exercise in hot environment Central Nervous System dysfunction Hyperthermia can impair brain function Dehydration can also impair brain function Afferent feedback from metaboreceptors in the fatiguing muscles to the brain ↓ central motor drive Heat Acclimation Acclimation - short Rapid adaptation (days to weeks) to environmental change Acclimatization long - Adaptation over a long time period (weeks to months) Requires exercise in hot environment - need to be exposed to those conditions Elevated core temperature promotes adaptations Acclimation lost within a few days of inactivity or no heat exposure Significant decline in 7 days Complete loss in 28 days Physiological Adaptations during Heat Acclimation Increased plasma volume (+10-12%). Maintains blood volume, stroke volume, and sweating capacity. Earlier onset of sweating and higher sweat rate. Less heat storage, maintain lower body temperature. Reduced sodium chloride loss in sweat. Reduced risk of electrolyte disturbance. Reduced skin blood flow. Increased cellular heat shock proteins. Prevent cellular damage due to heat. ↳ allow repair Impact of Heat acclimation on Heart rate and Core temperature during Exercise  Precooling the Body Improves Exercise Performance in the Heat Precooling body before exercise in the heat ↓ body temperature and improves exercise performance in a hot environment Various techniques exist Cold water immersion Cooling ice vest Cooling packs Ingestion of cold drinks A combination of cooling techniques (e.g., combining cold water immersion and cooling packs) improves performance more than a single technique  Exercise in the Cold Exercise in a cold environment enhances an athlete’s ability to lose heat and reduces risk of heat injury Hypothermia – large ↓ in core body temp Prolonged cold exposure or swimming in COLD water Cold Acclimation adaptations begin in 1 week: Increased non-shivering thermogenesis Higher intermittent blood flow (heat flow) to the hands and feet resulting in higher Goal: increased heat production and hand/foot temp maintenance of core temp Improved ability to sleep in cold environments Because less shivering  Check your understanding Heat acclimation results in: (1) ↑ plasma volume (2) earlier onset of sweat (3) higher sweat rate (4) reduction in electrolytes lost in sweat (5) reduction skin blood flow (6) ↑ neat shock protein level in tissues Cold acclimation results in: (1) ↑ non slivering thermogenesis (2) ↑ blood flow to extremities (3) improved sleep  Physiological Response to Exercise in the Cold Combination of proper clothing and increased metabolic heat production during exercise reduces risk of hypothermia. Swimming in cold water can overpower the body’s ability to prevent heat loss and hypothermia can occur. Subcutaneous fat improves cold tolerance and reduces heat loss. Age can impact cold tolerance during exercise children often have a large surface-to-mass ratio →↑ heat loss. Older adults with sarcopenia are at ↑ risk for hypothermia ↳ loss muscie reduced shivering-induced thermogenesis. Cold Stress and Performance Cold temperature can negatively impact sports performance. Hands exposed to cold temperatures often become numb due to ↓ blood flow to hands and ↓ neural transmission →loss of dexterity and ↓ motor skills. Cold temperatures can ↓ muscle strength and power. Cold water immersion ↓ muscle temperature by 8°C → ↓ muscle strength and power by 31 to 44%. During exercise in cold air, muscle temperature does not decrease → cold air exposure rarely decreases muscle force production. Impact of cold air on aerobic exercise performance unclear. Health Risks during Exercise in the Cold Cold water immersion (15°C or lower) ↑ hypothermia risk When body temperature ↓ to 25°C or lower →life-threatening cardiac arrhythmias. Compared to cold water, exercise in cold air→ ↓ hypothermia risk frostbite when air temperatures are below freezing. breathing cold air during exercise does not pose a risk air is rapidly warmed before entering the lungs.