Lecture 14 - Exercise In High/Low Body Temperature Flashcards
Humans are * meaning we fluctuate between 1 degree
Homeotherms
Heat is gained through ** (75% converted to heat) and ** (C, K, R)
ATP breakdown
Environmental heat
What are the ways heat is lost or gained?
Radiation (R)
Convection (C)
Conduction (K)
Evaporation (E) - Loss only
BMR (M) Muscular activity (W)
Hormones
Food thermic effect
Postural changes
- of heat * by core must match flow of heat from * to * to *
Define Pskin
Rate; production; core; skin; environment
Water vapour pressure of H2O at the skin
What does convection do?
Removes metabolic heat when air temperature < skin temperature
What type of receptors sense heat?
What do they do when it’s warm or cold?
Thermoreceptors
Warm - increasing firing rate with increase in local temp up to 44/46 degrees
Cold - increasing firing rate with decrease in local temp down to 24/28 degrees
What does clothing do in terms of heat loss?
Insulates body and limits heat transfer to environment
How does heat move from core to shell?
When skin temp is greater than the environment, how can heat to skin be dissipated?
What about when environment temp is greater than skin?
Via blood and conduction through subcutaneous tissue
By conduction, convection, radiation or evaporation
Evaporation only
In a graph of energy flow (Y) vs time, what do the lines of the following look like:
Metabolic heat production
Evaporative loss
Convective loss
Radiative loss
Highest to lowest:
Metabolic heat production - straight, then vertical increase, then steady state and vertical decrease
Evaporative loss - similar to metabolic production but more slope and not as high
Convective loss - same shape as metabolic but very low max
Radiative loss - lowest straight line
Where are the body core thermoreceptors?
When do they fire?
Hypothalamus
Fire when temperature bakes over 2 degree to 4 degrees about the mean
How does information from the skin thermoreceptors travel?
Sustained temperature * may cause stable * in sensor’s ** e.g tonic or static response, or temp change e.g * or * response
Via thalamic pathways to cortex
Change; change; firing rate; phasic; dynamic
What are three other thermoregulatory effectors?
Skin arterioles
Sweat glands
Skeletal muscle
Exercise * metabolic heat load and disturbs *
What is exercise’s effect on cardiovascular function?
Increases; homeostasis
1) Vasodilation to increase convection heat loss (requires more blood flow compared to exercise in the cold)
2) Pre optic anterior hypothalamus (POAH) triggers sympa system and increases cardiac output via heart rate and contractility, and increases vasoconstriction to nonessential tissues
3) Blood volume decreases due to sweat, stroke volume can’t increase due to blood pooling so heart rate increases to compensate (causing CV drift)
What can cholinergic sympathetic nerve fibres do?
Can cause dilation to regulate blood flow in extreme circumstances
What is the process of vasodilation?
1) decrease in vasoconstrictor tone due to decrease in sodium in cleft and binding to adrenergic receptors
2) activation of sympa cholinergic nerves (NOT using ACh) - instead use vasoactive intestinal peptide or pituitary adenylate cyclase activating peptide (both act via Gs and AC pathways)
Define CV drift
Increase in heart rate after 10 mins of prolonged moderate intensity exercise in neutral/warm environment. Cardiac output is maintained
What are the two theories of CV drift?
As blood moves to skin, there’s a decrease in venous return, stroke volume and increase in heart rate
Decrease in ventricle filling time due to increased heart rate which could be due to hyperthermia and/or dehydration
What are the limits to exercise in the heat?
1) CV overload - can’t supply enough blood, bigger risk of overheating, impaired performance, CV drift associated with decreased VO2 max during heat stress
2) critical temp theory - brain shutdown when exercising at 40-41 degrees m, explains limits in trained athletes
In hot conditions, there is a difference between ** and RPE (**)
VO2 max; rating of perceived exertion
When does sweat occur?
What does it and what controls it?
What happens during light sweating and heavy sweating?
Occurs when environment hotter than core and skin temp
Done by eccrine sweat glands which are controlled by POAH
Light sweating - slow travel through duct and very dilute sweat and reabsorption of sodium and chloride
Heavy sweating - fast travel through duct and less dilute sweat, more loss of sodium and chloride
Ideally, sweat electrolyte content will be ** plasma due to * of sodium and chloride ions
Less than; reabsorption
What is the process of sweating? (6)
1) release of ACh
2) muscarinic GPCR activation
3) activation of phospholipase C
4) stimulation of PKC
5) increase in calcium concentration
6) triggers primary secretion
Training affects **. Exercise of body water loss * adrenal cortex and ***
- control of * balance ratio to / release due to * of /
Sweat composition;
Stimulate; posterior pituitary gland
Hormonal; fluid; aldosterone/ADH; loss; H2O/electrolytes
What are ADH and aldosterone and what do they do?
ADH - anti diuretic hormone; retains water at kidneys
Aldosterone - retains sodium at kidneys
What are the effects of acclimation to exercise in the heat? (5)
1) CV function optimised
2) Sweat rate, distribution and content change (earlier, more dilute and widespread as glands more aldosterone sensitive - prevents bad sodium loss and optimises E heat loss)
3) Increase in oncotic/osmotic pressure as muscles contract protein into blood, meaning fluid moves in and plasma expands (plasma vol increase). Returns to normal, supports stroke volume so body maintains CO
4) increase in heart rate and CO
5) supports increase in skin blood flow, greater heat loss and lower core temp
What factors modify heat tolerance?
Explain one of them
Body fat level Age Training status Acclimatisation Gender - lower sweat rates and production but more active glands in girls - disadvantage in hot/dry climates but advantage in humid climates
What are the health risks during exercise in heat?
Heatstroke due to failure of heat regulating mechanisms from excessively high core temperature
Heat cramps causing excessive sodium losses - treated with IV drip
Heat exhaustion due to ineffective circulatory adjustments, made worse by depletion of extracellular fluid (principally plasma volume from excessive sweating)
Hyperthermia can be made worse by * due to high -induced * losses. It’s mitigated by pre exercise * and * ingestion but can’t * too much or you’ll get **
Dehydration; sweat; fluid
Cooling; fluid
Drink; exercise induced hyponatraemia
How does POAH regulate temperature in the cold? What does the cerebral cortex do? What else happens?
1) Triggers vasoconstriction through sympathetic activity of adrenaline and noradrenaline via alpha-1 adrenoceptors on smooth-muscle
2) Triggers nonshivering thermogenesis (NST) and skeletal muscle shivering to increase metabolism and generate heat
Cerebral cortex triggers behavioural adaptations
Body temperature increases
What is non shivering thermogenesis? (6)
Heat production mechanism at temps close to thermoneutral zone; induced by adaptation to cold; regulated by hypothalamus; based on actions of NA released from sympa nerve endings; localised in skeletal muscles and brown adipose tissue; occurs when peripheral vasoconstriction doesn’t prevent heat loss enough
How is non shivering thermogenesis achieved?
1) Brown adipose tissue has uncoupling protein called thermogenin.
2) FFA’s remove purine inhibition of thermogenin, causing influx of H+ into mito matrix, bypassing ATP synthase channel
3) This uncouples oxidative phosphorylation and energy from proton motive force dissipated as heat rather than producing ATP from ADP
4) Leakage of Na/K and Ca pumps makes thermogenesis and ACh stimulates muscle to raise metabolic rate
5) Low demands of thermogenesis mean FFA’s draw on lipolysis as method of energy production
What regulates non shivering thermogenesis?
Describe the schematic of NST until mitochondria
What is PKA’s role?
Thyroid hormones and sympathetic nervous system
NA at B adrenergic receptor > G protein > AC > ATP into cAMP > PKA > hormone sensitive lipase > triglycerides > FFA’s > mitochondria
PKA inhibits a thermogenin transcription suppressor leading to uncoupling
How does cold habituation occur?
What happens/how does it happen?
Occurs post repeated cold exposure
How it happens/what happens: vasoconstrictions, blunted shivering, core temperature allowed to decrease more, small areas of skin exposed
When does metabolic acclimation occur? What is enhanced?
What is insulative acclimation?
After repeated exposure with heat loss; enhanced metabolic shivering heat production
Enhanced vasoconstriction that occurs when increased metabolism doesn’t prevent heat loss
What happens if you overdress in the cold?
Subsequent sweating lowers body temperature
How does body composition affect heat loss?
How does this relate to boys vs girls?
More inactive peripheral muscle = more insulation
More subcutaneous fat = more insulation
Lower body area:mass ratio = less heat loss
Girls have more subcutaneous fat (advantage) but less active muscle (disadvantage)
How does wind chill affect heat loss? (Air movement, not air temperature)
Increases convective loss
Increases risk of freezing tissues
- water causes faster ** than in air
* loss * in moving water and * with exercise
Cold; heat loss
Heat; increases; decreases
What happens to muscle in response to exercise in the cold?
As fatigue *, metabolic heat production *
Altered fibre recruitment means less force
There’s a shortening velocity and decrease in power
Increases; decreases
What happens to FFA metabolic response when exercising in the cold?
Normally, increase in carecholamines causes increase in FFA oxidation, but in cold, while there’s an increase in catecholamine secretion, there’s no increase in FFA mobilisation
What happens to glucose metabolic response when exercising in the cold?
Blood glucose maintained well, more muscle glycogen used, hypoglycaemia suppressed shivering
What are the health risks of acute exposure to cold?
Hypothermia (at 29.5-34.5 degrees, POAH compromised; at <29.5 degrees, POAH lost)
Frostbite
Cardiorespiratory effects (decrease in core temp = decrease in HR due to SA node)
Exercise induced asthma
What is the heat balance equation?
If <0 = *
If >0 = *
M - W +/- R +/- C +/- K - E = 0
Loss
Gain
