Lecture 16 - Thermoregulation during exercise

Question 1

• what is thermoregulation?
• what is body’s resting core temp?
• what process is used to maintain thermoregulation?
• what is homeostasis?

Answer 1

• thermoregulation = process by which body maintains its internal temp in a narrow optimal range, despite outdoor env.
• critical bc avg resting core temp = around 37°C
• thermal gradient = difference btw core and skin temperature –> enables heat flow from core to periphery to help lose heat to the environement to maintain thermoregulation
• homeostasis = keeling everything in balance –> maintain stable environment for organs and enzymes to function effectively

Question 2

what are 2 ways we metabolically produce heat?

Answer 2

1. resting metabolic rate (RMR) –> energy expenditure at rest generates baseline heat
2. exercise metabolism: during exercise, body’s metabolism increases up to 20x, generating significantly more heat

Question 3

how does muscle contractions produce heat?
* does exercise intensity change heat production?

Answer 3

through ATP breakdown!
- muscle contraction requires ATP –> ATP breakdown is exothermic = produces heat as a byproduct
- 75-80% of energy from ATP breakdown dissipates as heat –> contributes to increasing core temp!

*yes! at any given ambient temp, core temperature increases as exercise intensity (% of VO2max) increases

Question 4

what are the 4 mechanisms of heat loss in the human body?
- which one is the primary method of cooling?

Answer 4

*heat moves from hot to cold
1. RADIATION: heat emitted from body to cooler surroundings –> ie running on a cool day
2. CONDUCTION: direct heat transfer through contact with cooler surface –> ie sit on cold bench or place cold towel on face
3. CONVECTION: heat loss through air or water moving across skin –> ie swimming or running in wind
4. EVAPORATION: heat loss via sweat evaporating from skin = PRIMARY METHOD OF COOLING!
*evaporation not effective in humid environment bc air already saturated with H2O

Question 5

describe hyperthermia vs hypothermia. At what temp

Answer 5

HYPERTHERMIA:
- increase in body temp > 39.5°C
- may cause death due to complications associated with heat stroke (ie organ failure, seizure)
HYPOTHERMIA:
- decrease in body temp <37°C
- may cause death due to complications associated with cold shock

Question 6

recap the things that cause heat loss (4) and heat gain (6) that affect body heat content

Answer 6

HEAT LOSS
- conduction, convection, radiation, evaporation
HEAT GAIN:
- BMR, postural changes, muscular activity, hormones, thermic effect of food, environment

Question 7

explain figure of girl running
- what causes heat stress (5)
- what causes heat loss (4)

Answer 7

HEAT STRESS
- solar radiation (body absorbs heat from sun)
- hot air temp
- high relative humidity
- metabolic heat production
- ground heat (friction)
HEAT LOSS
- respiratory evaporation (through water vapor)
- sweat evaporation
- radiation from body (if env. is colder than body)
- convection from wind currents

Question 8

which part of brain is responsible for thermoregulation?
- which part is activated when high vs low core temp
- 2 roles ish

Answer 8

HYPOTHALAMUS! acts as thermostat!
- high core temp = activate heat loss = anterior hypothalamus
- low core temp = activate heat gain = posterior hypothalamus
1. integrates information from central and peripheral neurons
2. maintains body temp at around 37°C

Question 9

what happens when stimulus = increased body temp (ie during exercise) (4 steps ish)

Answer 9

1. increase body temp activates anterior hypothalamus which activates 2 pathways
   a. peripheral dilation of skin’s blood vessels –> leads to increased perfusion and increased heat dissipation –> more warm blood at skin surface –> los heat through radiation and convection
   b. increased sweating leads to heat dissipation through evaporation
2. body temps decrease –> blood temp declines and hypothalamus heat loss center shuts off

Question 10

what happens when stimulus = decreased body temp (due to cold env)? 4 steps ish

Answer 10

1. decreased temp activates posterior hypothalamus –> activates heat production:
   a) stimulated skeletal muscle leading to contraction = shivering –> involuntary muscle contraction that uses ATP –> ATP breakdown = exothermic
   b) constriction of skin’s blood vessels = less blood to skin, warm blood stays at core –> decreased perfusion and decreased heat dissipation
2. body temp rises –> blood temps increase and hypothalamus heat production center shuts off

Question 11

describe thermoregulation in cold loop schéma
- cold = what temps?
- 3 main steps with some substeps

Answer 11

cold = 0-10°C …..
1. cold stress = decrease body temp –> decrease skin and blood temp –> activates thermal receptors in skin, spinal cord and hypothalamus –> afferent nervous pathways –> activate posterior hypothalamus
2. through the efferent nervous pathway, 3 things happen:
a) increase vasoconstrictor tone to the skin –> cutaneous vasoconstriction –> decrease skin BF –> redistribution of blood flow from cool skin to warm core
b) increase release of catecholamines (E, NE) & thyroxine) –> increase cellular metabolism –> increase ATP breakdown = increase heat prod
c) skeletal muscle –> shivering = increase heat prod
3. all lead to decreased heat loss –> which increases body temp!

Question 12

oxygen uptake (VO2) in response to exercise in cold
- O2 uptake is proportional to what? –> this relationship is influenced by what?
- high vs low intensity = increase/decrease VO2 = increase/decrease heat prod?

Answer 12

• O2 uptake = proportional to rate of energy expenditure! –> influenced by exercise intensity!
• high intensity = increase VO2 = increase heat prod
• low intensity = decrease VO2 = decrease heat prod

Question 13

what happens to oxygen uptake at low vs high intensity in the cold?

Answer 13

HIGH INTENSITIES (70% VO2)
- exercise can produce enough heat to keep core temp stable, potentially preventing or reducing need for shivering
- VO2/oxygen uptake in cold is same as VO2 in thermoneutral env.
LOW INTENSITY (30-50% VO2):
- not enough heat produced –> body initiates shivering –> contribute additional metabolic heat production to maintain temperature
- need more O2 uptake compared to thermoneutral bc of extra muscle contraction from shivering

Question 14

in the cold, what does peripheral vasoconstriction do to cardiovascular response? (3 steps)

Answer 14

1. cold = peripheral vasoconstriction –> shift in blood volume toward central circulatory system = increasing central blood volume
2. increase central blood volume results in higher preload (amount of blood returning to heart before it contracts) –> heart is filling with more blood
3. increase stroke volume bc higher EDV!
   *at similar exercise intensity, stroke volume is greater in cold environment compared to thermoneutral environment

Question 15

what happens to SV, HR and CO when exercising in the cold?

Answer 15

• SV increases! –> body is able to meet metabolic demands of exercise with fewer heartbeats –> can pump same amount of blood with fewer contractions bc pumping larger volume of blood per heart beat
• HR decreases! compensation for higher SV to avoid overfatiguing the heart
• CO remains similar to thermoneutral conditions!
  *for a given level of VO2, heart pumps same volume of blood per minute during exercise in cold vs warm –> BUT mechanisms differ bc SV increases and HR decreases

Question 16

how does blood flow and energy metabolism differ when exercising in the cold?

Answer 16

• vasoconstriction = less muscle blood flow and less muscle oxygenation while exercising in cold –> blood moves more slowly = RBC aggregate together –> increases blood viscosity –> decreases oxygenation = harder to rely on aerobic mechanisms!
  SO increase glycolysis rate = increase anaerobic metabolism –> body relies more on glycogen as fuel source, even during low-intensity exercise
  SO lactate accumulation is greater in cold weather at similar exercise intensity

Question 17

what happens to ventilation when exercising in the cold?
- low vs high intensity?

Answer 17

1. cold sensitive thermoreceptors send signals to hypothalamus to regulate both thermoregulation and respiratory responses
2. respiratory center will increase ventilation to increase O2 supply to maintain muscle activity and heat production (through shivering)

LOW intensities: minute ventilation is higher (vs thermoneutral) bc shivering makes you need more oxygen
- increase in ventilation is facilitated through increased respiratory rate (more breath per minute) rather than tidal volume –> leads to shallow breathing = increase breathlessness = limited O2 intake = bad

HIGH intensity: as exercise intensity increases, difference in minute ventilation bs warm and cold conditions diminish, eventually reaching similar levels during steady-state exercise

Question 18

explain the 3 reasons why shorter time to exhaustion while exercising in cold vs regular temperatures

Answer 18

1. increased metabolic demand
   - increase shivering = more energy demand = use glycogen = increase glycolysis and lactate production + faster depletion of energy stores
   - leads to muscle fatigue and lower endurance
2. impaired oxygen transport and muscle efficiency:
   - vasoconstriction = restricts blood flow to working muscle and increase blood viscosity –> decreases O2 delivery and aerobic E prod –> recruit more type II fiber for anaerobic met
   - limited sustained performance
3. respiratory challenges
   - exercising in cold stimulates rapid, shallow breathing (bc need more O2 bc shivering) –> decrease O2 intake per breath = increased breathlessness

Question 19

why can we experience reduced manual dexterity and increased risk of cold-related injuries (frost bite) in extremities in extreme cold conditions?

Answer 19

• bc peripheral thermoreceptors may signal intense vasoconstriction and initiate shivering even when exercise-induced heat production is already elevated
• response occurs bc body prioritizes protecting core temps over extremities, which leads to decreased blood flow to hand and feet

Question 20

what are 5 recommendations for exercising in cold

Answer 20

1. dress in layers to trap heat close to your body
2. protect extremities: wear gloves, hats, thick socks –> consider covering nose and mouth to warm up air you breathe
3. warm up before exercising to increase body temp and improve blood circulation
4. stay hydrated! body still loses fluid through sweat
5. listen to your body: be mindful of how you are feeling –> fatigue, dizziness, excessive breathlessness are signs to slow down, decrease intensity or stop

Question 21

describe thermoregulation in heat loop schéma
- heat = what temps?
- 3 main steps with some substeps

Answer 21

heat= >30°
1. heat load = increase body temp –> increase skin and blood temp –> activates thermal receptors in skin, spinal cord and hypothalamus –> afferent nervous pathways –> activate anterior hypothalamus
2. through the efferent nervous pathway, 2 things happen:
a) decrease vasoconstrictor tone to the skin –> cutaneous vasodilation –> increase skin BF –> decrease flow of warm blood to bodies core + increase evaporation of sweat
b) decrease activity of erector hair muscles + increase activity of sweat glands –> increase sweating –> increase evaporation of sweat –> decrease peripheral blood temp that flows to core tissues to absorb heat
3. all lead to increased heat loss –> which decreases body temp!

Question 22

exercising in very hot temperatures leads to significant increase/decrease in core body temp
- how does that affect exercise performance? give 2 concrete examples

Answer 22

• increase in core body temp in all body parts!
• body hyperthermia is associated with impaired exercise performance!
• decrease CO2 max at body temp increases + decrease time to exhaustion

Question 23

explain cardiovascular response to exercise in heat (think of what are the mechanisms of thermoregulation when its too hot)

Answer 23

1. peripheral vasodilation –> directs blood towards skin to facilitate heat loss VS exercising muscle –> directs blood towards working muscles
   2; competition for blood flow btw skin and muscles –> cardiovascular strain! tricky situation for heart!

• SO to meet increased demand of skin and muscles –> increase heart rate!
• ALSO, vasodilation to skin creates blood pooling in skin –> decrease venous return = decrease SV = decrease CO = decrease MAP

*even if HR increases, decrease in SV is more severe so CO decreases

Question 24

which cardiovascular measures decrease/increase during exercise in heat?

Answer 24

• HR increases
• SV, CO and MAP decreases!
  *compared to thermoneutral

Question 25

explain muscle metabolism in response to exercise in heat

Answer 25

• at high exercise intensity –> decrease blood flow to muscle (bc body prioritizes thermoregulation over exercise) –> decrease muscle O2 –> decrease aerobic –> increase anaerobic metabolism –> increase type II fibers –> increase muscle glycogen use (more rapid depletion) + lactate production

Question 26

explain cardiopulmonary response to exercise in heat

Answer 26

• increase metabolite accumulation (bc more anaerobic metabolism bc less blood flow to muscles) –> increase stimulation of group IV afferents –> metaboreceptors! –> send signal to respiratory centers –> increase minute ventilation!

Question 27

does sweating rate increase in heat?

Answer 27

yes! with more heat gain, more sweating is required to maintain heat balance!

Question 28

what are the consequences of increased sweating rates in heat? 2 + sub

Answer 28

1) electrolyte loss
- higher sweat rates = higher electrolyte losses –> electrolytes are needed for nerve conduction and muscle contraction
2) dehydration:
- performance impairments seen with as little as 2% loss in body water
- sweat rates decrease with progressive dehydration (dangerous cause lose ability to lose heat via evaporation)
- reduced plasma volume (extreme cases) can cause greater rises in HR bc low venous return

Question 29

overall, what are 3 reasons why exercising in heat decreases performance?

Answer 29

1) decrease muscle efficiency –> due to reduced blood and oxygen flow (due to CV response of decreasing CO)
2) accelerated fatigue, from accumulation of metabolic byproducts (bc more anaerobic met and glycolysis)
3) cardiovascular strain: due to elevated HR at low intensity

Question 30

recommendations for exercising in heat (5)

Answer 30

1) dress appropirately: protect yourself from sun and lightweight clothes
2) lower intensity: try to lower exercise intensity so you don’t generate lots of heat
3) drink electrolytes to replenish losses –> important for nerve conduction and muscle function
4) stay hydrated: 150-300mL every 10-20min
5) listen to your body: be mindful of how you’re feeling –> fatigue, dizziness, rapid HR are signs to slow down or stop

Question 31

what happens to thermoregulation when swimming in
COLD WATER
HOT WATER

Answer 31

COLD WATER
- vasoconstriction
- shivering
- reduced sweating –> sweat production is usually minimal bc body is working to conserve heat rather than release it
- lose heat through convection
HOT WATER
- vasodilation
- sweat doesn’t evaporate
- rely on convection to lose heat –> but no convection if water is hotter than body temp (if that’s the case, heat from water goes into you through convection)