thermal physiology III

metabolic heat production (?) during exercise

increases

body temperature increases with increasing (?) intensity

exercise

body temperature will eventually (?) depending on work rate

plateau

body temp rises until heat loss is (?) to heat production

equal

why is thermoregulation negative feedback control and not positive?

counteracts changes and reverses the direction of a stimulus to maintain the body's internal temperature

exercising in heat creates trade-off between (a) and need of (b)

(a) exercise performance

(b) thermoregulation

target of blood flow (prioritising exercise performance vs prioritising thermoregulation)

exercise performance → blood flow to muscles

thermoregulation → blood flow to skin → heat loss

wet-bulb globe temperature index

index of how easy it is to lose heat to the environment

wet bulb calculation

natural wet bulb-temperature

• Measures lowest temp possible achieved by evaporative cooling and windspeed

• Bulb of thermometer covered in cloth kept wet

• Therefore evaporation from the wet bulb will lower temp reading of the wet bulb thermometer

• If wet bulb reads lower than the dry bulb -> evaporation occurring from the bulb of the wet bulb thermometer

• Difference between dry and wet bulb is called the wet bulb depression

• The drier it is, the lower the reading on the wet bulb

• The more humid it is the higher the reading on the wet bulb

ventilated wet-bulb temperature

• Temperature is independent of the wind speed

• Function of just the humidity

• Measures lowest temp possible achieved by evaporative cooling

stevenson screen

• Shields dry bulb thermometer from environmental radiation

• As wet-bulb test is performed in sunlight

• So the dry bulb thermometer is registering just the temperature of the air and not the radiation temp

black globe

• Purposely captures environmental radiation

• Registers increase in temperature when the globe is exposed to radiation in the environment

globe temperature

• measured by a black-bulb thermometer

• accounts for air temperature, radiation temperature, and wind speed

recommendations based on wet bulb globe temperature

• 26.5 - 28.8 → use discretion especially if unacclimatised

• 29.5 - 30.5 → avoid strenuous activity in the sun

• >31.2 → avoid any exercise

why is there a decrease in exercise performance on hotter days?

due to compromise cardiovascular system reaches to satisfying the demands of blood flow between the muscle and skin

explain the difference in sweat rate between the different relative humidity %

• When exercising at the same work rate

• Heart rate is higher when it's hotter

• VO2 max (maximum O2 your body can use per minute during high-intensity exercise) is limited by your maximum heart rate

• VO2 max is lower in the heat -> exercise performance compromised

• 50% humidity vs 90% humidity at 35 degrees dry bulb temp

• Exercising at 60% VO2 max at 35 degrees under 90% humidity requires a 60% increase in sweat rate compared to the same exercise at 50% humidity

• Demonstrates physiological strain caused by humidity

dehydration (before exercise or during exercise) compromises (?) performance

exercise

effect of dehydration on % change in blood volume

• replacing fluids during exercise will result in minimal % change or even a % increase in blood plasma volume

• no fluid replenishment during exercise will result in a % decrease in blood plasma volume

dehydration will (?) stroke volume during exercise

decrease

dehydration will (?) heart rate to a greater extent by the end of exercise

increase → heart rate highest in subjects that did not replenish fluids at the end of exercise

muscle and skin blood flow compromised due to loss of fluids (smaller blood volume thus heart workload increases)

skin blood flow (?) as a result of dehydration

decrease

esophageal and rectal temp (?) with dehydration

increase

the sweating threshold (?) with progressive dehydration

increases

thirst is a poor indicator of dehydration:

you don’t feel thirsty until you have lost about 2% body mass

the sweating threshold is (?) in acclimatised individuals

decreased

vital signs of heatstroke (list 3)

1. high body temperature (more than 40 degrees celsius)

2. significantly low blood pressure

3. deteriorating mental status → confusion, coma)

summary

• metabolic rate increases → from increase in heart/muscle workload

• increase from 100 watts at rest to 1500 watts during exercise

• activation of heat loss mechanisms

• thermoregulation relies on negative feedback → counteracts change and reverses the direction of a stimulus to maintain the body’s internal temperature

• many challenges to assessing thermal environment → multiple variables, human variability in thermal sensation

• wet bulb globe temperature index → how easy it is to lose heat to the environment

• natural wet bulb test → wet bulb covered in damp cloth

• if temp reading is lower than dry bulb thermometer → evaporation occurring

• the lower the reading on the wet bulb thermometer → the lower the humidity of the environment

• natural vs ventilated wet bulb → measurement of wind speed and evaporative cooling vs only evaporative cooling

• stevenson screen → protects dry bulb → dry bulb only measures air temp rather than radiation temp

• black globe → intentionally measures increase in temp due to environmental radiation

• WBGT used to measure environmental temp and conditions

• exercise performance compromised in humidity due to increase in sweating → increase in fluid loss

• cardiovascular compromise between distribution of blood flow to muscle (for exercise performance) vs skin (for heat loss via vasoconstriction and sweating)

• dehydration compromises exercise performance → (decreases blood plasma volume and stroke volume, increases heart rate, decreases skin blood flow, increased sweating threshold) → elevated body temp from greater heat storage due to compromised heat loss mechanisms