Week 8: Thermoregulation Flashcards
What conditions is the ironman world championships in Kona (Hawaii) completed in?
Athletes swim in waters of 26 degrees, the bike across lava fields where temperatures can get close to 40 degrees, various part of the race have 60% humidity and the marathon is completed in temperatures still high and fuel depleting
K2 is the …… highest mountain in the world (and more deadly than Everest) – only a few ….. have summited and fewer still without supplemental oxygen – at its peak climbers will have only ….. atmospheres of pressure or about …..% of what you experience at sea level resulting in a …..% equivalent oxygen level (compared to …..% normally) – add to that the freezing conditions (up to -….. degrees) – it’s one of the most inhospitable and extreme environments any human will find
Second
Hundred
0.3, 30%
7.2%
21%
-50
Other than exercise where else can extreme environments be an issue?
Occupational settings eg fieldworkers (extreme dehydration, heat illness, acute kidney injury) and military personnel/fireworks are exposed to dangerous settings eg live fire fields
What is acclimation
Process of adaptation to specific environmental stressors in a controlled/artificial environment
What is acclimatisation
Process of adaptation to various environmental stressors in a natural environment
What is core temperature
Measurement representing the true deep-body temp, typically measured from the gastrointestinal tract
What is dehydration
Process of body water losses that occurs when fluid loss > intake/replacement
What is euhydration
Normal hydration level/body water content – absence of hyper or hypohydration
Thermoregulation
- The body attempts to maintain thermal homeostasis within a fairly steady range of ….. to ….. degrees celsius. Although this might be slightly higher or lower in some individuals
- Our body temperature also naturally fluctuates across the day due to …… ……
- This ……. variation can be observed in …… temps during sleep/in the morning and ……. temperatures in the afternoon
- The safe operating range for humans is typically between …… and …../…..
- When our thermoregulatory systems fail we can lose control at both …. …. This represents medical emergencies – potentially leading to death
- When we get too hot we have increased risk of heat ….. and ….. …..
- When we get too cold we begin to lose ….. and ….. control (shivering, breathing and cardiac sinus rhythm)
- Now some individuals are able to tolerate these thermal extremes some endurance athletes have been observed at temperatures in …… of ….. degrees celsius but for most people this is an extreme danger
- 36-37 degrees
- Circadian rhythms
- Diurnal, lower, higher
- Safe operating range = 35 and 38/39
- Thermal extremes
- This represents medical emergencies – potentially leading to death
- Heat exhaustion & heat stroke
- Lose reflexive and autonomic control (shivering, breathing and ….. …… rhythm)
- Excess of 42 degrees celsius
Heat balance
* Thermoregulation modulated by ….. and …… temperature receptors
* Heat gain affected by ….., ……, …… and ……
* Exercise is a major driver of heat stress, with >……% of energy released internally as heat
* Demand on cardiovascular system creates …… and …… homeostasis conflict
Central & peripheral
Metabolism, hormones, behaviour and environment
>75%
Metabolic, thermal
4 mechanisms of heat loss
Conduction
Convection
Radiation
Evaporation
What is conduction
Direct contact between objects/materials
What is convection
movement of fluid (eg liquid or air) across body
What is radiation
from electromagnetic waves emitted from our bodies (eg sun, UV, infrared
What is evaporation
conversion of liquid (eg sweat), to gas (vapor).
Our most powerful mechanism which occurs with the conversion of sweat to vapour or gas. The relative contribution of these mechanisms changes with exercise intensity and various environmental factors
Mechanisms of heat loss: relative contributions
* Evaporation is our most ….. mechanism for heat loss
* Evaporative potential …… with rising ambient temperatures
* Dry heat exchange (….., ……) ……. as ambient temperatures reaches (and exceeds) skin temperature
Why does this occur?
Powerful
Increases
Convection/Radiation, decreases
Evaporative power increases because warmer air has a greater ability to hold moisture and therefore has a greater vapour pressure gradient to allow for evaporation of liquids and cooler air
Response to hyperthermia?
We have ….. and ….. receptors that sense changes in deep body or skin temperature. Deep body temperature might be altered by exercise and muscle activity, ….., …… or …… Superficial temperature might be altered by the …… environment and …… we are wearing.
These impulses go to our …… (the …. …… at the …… centre) which takes into account various other physiological factors such as …… variation, hormones, training and …… status and it might also consider a …… or …… nervous activity, ….. …… and …….
Signals are then sent out to affect the change at either the ….. body or ….. to manipulate core temperature. We can change core temperature directly through alterations in blood flow to various regions or behaviour and muscle activity such as shivering or exercise intensity. We can also change heat at the skin by …… superficial blood vessel to promote heat …… at the skin along with …… drive to stimulate our ……. sweat glands to increase …… heat loss. Through these responses we can minimise heat gain or reduce temperature altogether to bring it back within our preferred range
Central & peripheral
Metabolism, digestion or fever
Thermal, clothing
Hypothalamus
Master regulator
Integration
Diurnal, acclamation
Sympathetic or parasympathetic, blood pressure and osmolality
Deep, skin
Vasodilating
Exchange
neural
Eccrine
Evaporative
What factors affect thermal stress (Behavioural)
Factors affecting thermal stress
Behavioural
* Work intensity and duration
* Training status
* Acclimation/acclimatisation status
* Hydration & cooling
* Clothing
What factors affect thermal stress
(Environmental)
Environmental
* Ambient temperature (the temperature of the air itself which is measured in this shade or without light exposure)
* Relative humidity (amount of water held in the air which affects the ability of liquid to evaporate by vapour pressure gradients)
* Air velocity (speed of air travelling over the body which affects convective and evaporative heat loss)
* Radiation load (direct and indirect heat gained by sunlight in other electromagnetic sources)
Environmental conditions: Wet-Bulb Globe temperature
To estimate the total ..... ..... and ..... we need to consider the environmental conditions and how that alters the effectiveness of our heat loss mechanism * Temperature you see on the news or on your phone weather app is the ....... ..... - what we call the ..... ..... temperature and is measured in the shade * Wet bulb globe temperature or WBGT allows us to account for other ..... ..... ..... by using the wet bulb temperature which is a thermometer covered in .... ..... to simulate ...... * As well as the black globe temperature to account for heat gain by ...... load from the sun or other light sources * These measurements are used to calculate the WBGT index which is representation of .... ..... and overall ..... of heat related illnesses we can use this index to make decisions about the likelihood of risk and how we might ...... exercise .......and ...... as well as ...... needs based on the current conditions
- Heat load & tolerability
- Ambient temperature - what we call the dry bulb temperature
- Heat loss mechanisms, wet cloth to simulate evaporation
- Radiation load
- Heat stress, risk of heat related illness, modify, duration/intensity, hydration
Acute response to heat:
During exercise we have:
* Greatly increased demand for muscle blood flow to supply ……
* Increased demand for ….. blood flow to …… heat to prevent ……
* A need to maintain …… pressure and blood perfusion at tissues
* But heat stress adds to …… ……, making it more difficult to meet demands!
Oxygen
Skin, offload, hyperthermia
Arterial pressure & blood perfusion
Cardiovascular strain
Acute response
Exercise in the heat: cardiovascular responses
* Cardiovascular demand …… during exercise in the heat
* Due to competing ……. and ……. demand for blood supply
* Increased ….. and decreased ….. = decreased ……
* Decreased ……. due to ….. ….. ….., vascular responses contribute to decreased SV and CO
Increased
Metabolic & thermoregulatory
> HR & < SV = decreased CO
MAP, plasma volume losses
Metabolic responses to exercise in the heat
This increased …… demand can be observed in greater …….. (the breakdown of glycogen) to supply energy to perform work and that coupled with reduced oxygen supply to the muscle leads to an increase in ……. production. This lactate and ……. ion accumulation along with …… …… will hasten fatigue and that is if the heat stress doesn’t get to them first.
Anaerobic
Glycogenolysis
Lactate
Hydrogen ion
Glycogen depletion
Exercise in the heat - performance effects
We know that hypothermia …….. VO2 Max which results in increased relative exercise intensity.
We can see in the figure at bottom left the effects of wet bold globe temperature on marathon performance. …… performers are least affected by heat stress but can still see a ….. to ….% performance decrement in warmer conditions.
With greater exposure time, slower performance can experience a far greater reduction in performance upwards of ……% in the hottest conditions.
Hypothermia reduces …… ……. …..which results in decreased ……, power and large decrements in repeated sprint performance.
…… ……. ……. is enough to lead to a performance decrement.
In the four panel figure to the right we can see that despite an identical rectal or core temperature to cool conditions elite road cyclist perform at a lower absolute exercise intensity and have the typical increased heart rate when skin temperature is greatly elevated. This might suggest that there is a perceptual or behavioural response leading to a pacing strategy in these athletes to minimise additional heat gain.
Reduces
Faster
2-3%
10%
Voluntary muscle activation
Strength
Elevated skin temperature
Exercise in the heat integrated response?
- Increase in whole body temperature and this result in cardiovascular responses including increased ….. ….. and ….. …. …. difference along with decrease ….. …… and …… ……, …… …… …., ….. …… and ….. ….. to the brain and working muscle.
- As well as a reduction in the ….. to …… …… …….
- In the brain and nervous system we see changes in EEG activity resulting in reduced ……. processing, ……, lower ….. system drive with decrease …… muscle activation due to either an inability or unwillingness to activate the muscles and this results in increased perception of effort with the heat contributing to a reduced feeling of comfort.
- Peripherally within the working muscles we see increased metabolic demand leading to increased reliance on …… sources (so breakdown of glycogen) which results in increased accumulation of metabolites such as ……. ions and ……. phosphate. Neurally we also see an increase in grade …… or …… muscle …… as well as increase in …… feedback which results in a reduced …… production. All of these effects are interactive and all contributes together to our overall reduction and performance in the heat
Increased: Heart rate & arterial venous 02
Decreased: stroke volume and cardiac output, mean arterial pressure, V02 max and blood flow
Reduction in the core to skin temperature gradient.
Attentional, arousal, CNS system drive, voluntary
Carbohydrate sources, hydrogen ions & inorganic phosphates, grade 3 or 4 muscle afferents, inhibitory feedback = reduced force production.
Avoiding hyperthermia
* Individual’s baseline temperature is probably important in determining performance – warm muscle is essential to optimise force production but >…. degrees is detrimental
* Target is ….. muscles but ….. skin to reduce thermal and cardiovascular strain and dehydration
* Cooling improves performance when hyperthermic, especially when connective, …… ……. low (effect on VHIE minimal/negative – shorter warm-up more beneficial
*When is cooling successful?
* Applications in sport, military, industrial settings, as well as disabilities such as ….. ……
* Consider practical and theoretical issues – optimal temperature? Methods? GI Comfort? Cost?
39
Warm, Cool
Evaporative potential
Before, during (practical issues can occur though). note = rest periods = mixed results
Multiple sclerosis
Hydration & performance
What is dehydration?
Dynamic process of body water losses that occurs when fluid loss > intake/replacement
Hydration & performance
What is euhydration? Plasma osmolarity? Urine specific gravity?
Normal total body water
Plasma osmolarity 285-290 mOsmol/L, urine specific gravity 1 -1.026
Hydration & performance
What is hyperhydration?
state of excess total body water
What is hypohydration
state of depleted total body water
Hydration & performance
What is hypovolaemia?
state of reduced blood volume, typically from ECF/plasma losses due to dehydration
Hydration and performance
The power of sweat
* We rely on ….. and …… to minimise heat gain and maintain endurance performance/intensity
* Evaporating 1g of H20 removes ….. of heat
* Humans have >2.5-4 million eccrine sweat glands
* Sweat rates vary – typically …..-…../h but >…… possible in some individuals and conditions (exercise intensity/air temp)
* Depends mainly on absolute intensity and air temperature
* However, max gastric emptying ~……. -……L/hr so dehydration often inevitable during exercise in the heat
- You can see where this is problematic as this means that dehydration is typically inevitable during exercise in the heat because we can’t …… or ……. as much liquid as we’re losing
- Even if we were to replace fluid at the maximum rate that the GI system can handle, this dehydration compounds the effects of heat gain because the loss of both body water and sodium and sweat
*Note: max gastric emptying = the rate at which the stomach empties into the small intestine where ingested fluid is absorbed
Sweat & evaporation
2.4kJ
0.5-2.0L/hr, >3,5
max gastric emptying = 1.0-1.2L
Replace or absorb
Hydration and performance
Dehydration
* Initial body water losses, mainly from sweat, occur from …… …… …… (ECF), which consists of plasma & interstitial fluid
* The loss of ….. ….. (hypovolaemia) results in …… ECF …… (also from metabolites)
* Increased …… concentration results in increased …… and …..-….. ….. (vasopressin) release to …… body water
* ……. fluid shifts to extracellular fluid by ……, resulting in ICF dehydration
* If fluid replenishment doesn’t occur, ECF dehydration continues, subsequently drawing more from ICF
Extracellular fluid compartment
Plasma volume, increased, osmolarity
Electrolyte, aldosterone/anti-diuretic hormone, retain
Intracellular, osmosis
Hydration & performance
- Fluid losses from ECF/plasma volume results in …… in HR to maintain CO, but lower overall
- Decrease blood flow = decrease ….. & ….. supply, decrease heat loss and decrease metabolite accumulation
- Magnitude of performance decrement is proportional to dehydration level
- Endurance performance – decrease …… -…..% with 2-3% BM loss and …. -……% decrease if 5-6% body mass losses
Increases
02, nutrient supply
3-6%
10-20%
Hydration & performance
Hypohydration & hyperthermia – integrated response
Hypohydration seems to impair endurance performance through a combination of mechanisms primarily driven by ……. This dehydration and the hypovolemic state it induces results in greater …… which leads to a cascade of physiological and perceptual responses eg increased ….. and decreased …. and …… From here we get increased ……. strain (including increased ….. and reduce ….. ….) and …. cardiac output and mean arterial pressure. This results in reduced muscle and brain blood flow, increase in ……, lactate production and metabolite accumulation.
These factors result in increased perception of ….. and ……. Increased cardiovascular strained further drives temperature up we are unable to off load heat and this leads to increased perception of effort and discomfort. Together these factors impair endurance performance.
Hypovolemia
Osmolarity
Thirst, mood and motivation
Cardiovascular strain (increased HR, decreased SV) = Decrease CO
Glycogenolysis
Effort and discomfort
Rehydration
* Drinking to limit dehydration provides physiological and psychobiological benefits that can minimise decrements in aerobic performance – maintain …… output in time trial or …… fatigue
* However, thirst sensation is reduced during exercise, can be difficult to consume fluids without discomfort/GI distress, and possible to do harm by replenishing fluids the wrong way!
* …… balance is important for maintenance of blood volume, blood pressure and osmotic gradients
* Tightly regulated by ….. …… …. ….,
* Usually need to rehydrate with more fluid than volume lost - ~……% if water, ~…..% if electrolyte drink
* Excretion of large volume of pale urine doesn’t always indicate full rehydration! Why?
Power, defer
Electrolyte
Renin-angiotensin-aldosterone system, vasopressin, and atrial natriuretic peptide
150%, 120%
*Sodium in electrolyte drinks increases fluid retention)
Rehydration: Exercise-associated hyponatraemia
Sodium depletion is a major danger
Defined as NA <130mol/L
Common in ultra-endurance & novice athletes
Symptoms: loss of coordination, confusion, weakness, followed by convulsions, unconsciousness, pulmonary oedema
Adapting to heat stress
The effect of repeated exposure to heat stress?
- Decreased core temperature at rest and submaximal intensities
- Increased total body water and blood volume (hypervolaemia) improving haemodynamics & resilience to fluid loss
- Decreased heart rate at rest and during exercise, reducing cardiovascular strain at any workload
- Increased skin blood flow, sweat onset, rate, and hypotonicity, improving heat disposal and electrolyte retention
- Increased glycogen sparing due to reduced reliance on carbohydrate metabolism
- Increased heat shock protein production, providing cellular protection to various stressors
- Decreased perception of effort and discomfort
Heat-related illness
Heat cramps
* Move to ….. location
* Provide …… drink to speed hydration, reduce ……
Heat exhaustion
* Move to cooler location, ….. feet, remove ……/…… clothing
* Provide cold water for hand/foot ……, moisten/fan skin
* Give ….. if conscious or intravenous saline if unconscious
Heat stroke
* ….. ……
* …… cool body in cold water or with wet towels & ice placed at …., …., ….
* Place in …. ….. if unconscious, be ready to administer CPR/AED
Cooler
Electrolyte, cramps
Elevate, excessive/tight
Immersion
Saline
Medical emergency
Rapidly, neck, groin, armpits
Recovery position
Adapting to heat stress?
- After 8 day exercise in heat acclimation block we can see that before either training block individuals had a far steeper rise in heart rate and rectal temperature and reached exhaustion within an hour
- With aerobic exercise they were able to exercise an ….. longer and had a far ….. heart rate but were able to cope with a far greater core temperature
1.5hrs
Lower
Adapting to heat stress?
Time course of adaptation
* Plasma volume expansion (hypovolemia) – as a result we get improved ……. (reduced ….)
* Within one week thermal comfort and exercise capacity is greatly improved – we get approximately 2/3rds of our total adaptation after ~…. days of exposure and adaptations begin to plateau in the ….. week of exposure
* Responses are dependent on many factors (eg environmental, exercise stress imposed as well as the frequency and duration of exposures
* As seen on the right we ….. adaptations in plasma vol as quickly as we gain them when we stop exercise or heat exposure
* Additionally with the plasma volume expansion we see a reduction in …..
Haemodynamics, HR
4 days
2nd
Lose
Hematocrit
Adapting to heat stress
Heat shock proteins
* Heat shock proteins are a family of ….. ….. …. that are expressed in response to …., ….., exercise and heat/exercise combined
* They have been shown to improve …. and ….. tolerance to repeated heat hypoxia and exercise stress and these proteins are highly important at protecting cells, homeostasis and inflammatory immune responses
* Exposure to heat stress (environmental and/or exercise induced) results in …… of HSP
* Increase ……-…… function and fluid regulation hormone response to improve blood flow, plasma volume and sweat adaptations
* Contribute to improved ….. membrane integrity, reducing risk of …… commonly seen with hyperthermia
Stress inducible proteins, heat, hypoxia
Cellular & systemic
Upregulation
Vascular-endothelial
Gut, endotoxaemia
Performance effects
* 12 elite cyclists, V02max 67 +/- 2ml/kg/min completed 10 days training in either 38 or 13 degree (30% RH)
* V02 max +…..%, LT2 + …..%, Plasma volume +6.5%, HR -9%, Tc 0.5 degrees
* Time trial performance improved …..% in hot and …..% in cool conditions
* Heat training can provide performance benefits in hot and temperate conditions
8%
5%
8%
6%
