Lecture 16 - Thermoregulation during exercise Flashcards
- what is thermoregulation?
- what is body’s resting core temp?
- what process is used to maintain thermoregulation?
- what is homeostasis?
- 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
what are 2 ways we metabolically produce heat?
- resting metabolic rate (RMR) –> energy expenditure at rest generates baseline heat
- exercise metabolism: during exercise, body’s metabolism increases up to 20x, generating significantly more heat
how does muscle contractions produce heat?
* does exercise intensity change heat production?
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
what are the 4 mechanisms of heat loss in the human body?
- which one is the primary method of cooling?
*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
describe hyperthermia vs hypothermia. At what temp
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
recap the things that cause heat loss (4) and heat gain (6) that affect body heat content
HEAT LOSS
- conduction, convection, radiation, evaporation
HEAT GAIN:
- BMR, postural changes, muscular activity, hormones, thermic effect of food, environment
explain figure of girl running
- what causes heat stress (5)
- what causes heat loss (4)
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
which part of brain is responsible for thermoregulation?
- which part is activated when high vs low core temp
- 2 roles ish
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
what happens when stimulus = increased body temp (ie during exercise) (4 steps ish)
- 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 - body temps decrease –> blood temp declines and hypothalamus heat loss center shuts off
what happens when stimulus = decreased body temp (due to cold env)? 4 steps ish
- 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 - body temp rises –> blood temps increase and hypothalamus heat production center shuts off
describe thermoregulation in cold loop schéma
- cold = what temps?
- 3 main steps with some substeps
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!
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?
- 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
what happens to oxygen uptake at low vs high intensity in the cold?
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
in the cold, what does peripheral vasoconstriction do to cardiovascular response? (3 steps)
- cold = peripheral vasoconstriction –> shift in blood volume toward central circulatory system = increasing central blood volume
- increase central blood volume results in higher preload (amount of blood returning to heart before it contracts) –> heart is filling with more blood
- increase stroke volume bc higher EDV!
*at similar exercise intensity, stroke volume is greater in cold environment compared to thermoneutral environment
what happens to SV, HR and CO when exercising in the cold?
- 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
how does blood flow and energy metabolism differ when exercising in the cold?
- 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
what happens to ventilation when exercising in the cold?
- low vs high intensity?
- cold sensitive thermoreceptors send signals to hypothalamus to regulate both thermoregulation and respiratory responses
- 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
explain the 3 reasons why shorter time to exhaustion while exercising in cold vs regular temperatures
- 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 - 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 - respiratory challenges
- exercising in cold stimulates rapid, shallow breathing (bc need more O2 bc shivering) –> decrease O2 intake per breath = increased breathlessness
why can we experience reduced manual dexterity and increased risk of cold-related injuries (frost bite) in extremities in extreme cold conditions?
- 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
what are 5 recommendations for exercising in cold
- dress in layers to trap heat close to your body
- protect extremities: wear gloves, hats, thick socks –> consider covering nose and mouth to warm up air you breathe
- warm up before exercising to increase body temp and improve blood circulation
- stay hydrated! body still loses fluid through sweat
- listen to your body: be mindful of how you are feeling –> fatigue, dizziness, excessive breathlessness are signs to slow down, decrease intensity or stop
describe thermoregulation in heat loop schéma
- heat = what temps?
- 3 main steps with some substeps
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!
exercising in very hot temperatures leads to significant increase/decrease in core body temp
- how does that affect exercise performance? give 2 concrete examples
- 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
explain cardiovascular response to exercise in heat (think of what are the mechanisms of thermoregulation when its too hot)
- 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
which cardiovascular measures decrease/increase during exercise in heat?
- HR increases
- SV, CO and MAP decreases!
*compared to thermoneutral
