EXPH 387 WVU Exam 5 (exercise & thermal stress)
Average cutaneous blood flow rate is approximately ______ mL/min in a thermo-neutral environment; this can fall to approximately ______ mL/min in extremely cold environments
- 250 mL/min - 0 mL/min
recommendations for hyperhydration
- 500 mL the night before exercise - 500 mL in the morning - 400-600 mL of cold water before exercise
Hypothalamus: _________ receptors in the skin provide input to the central control center. The more numerous _______ _______ _______ generally exists near the skin surface and evokes ________ ______ mechanisms. Changes in the _________ of the blood that perfuses the _______ portion of the hypothalamus directly stimulates this area, causing a coordinated response for heat conservation from the _______ hypothalamus and heat dissipation from the _______ hypothalamus
- Thermal - cutaneous cold receptors - heat-conserving - temperature - anterior - posterior - anterior
5 factors that modify heat tolerance
- acclimatization - training status - age - gender - body fat level
age effects on heat tolerance
- age delays onset of sweating and blunts magnitude of sweat response - modifies sensitivity of thermoreceptors - limits sweat gland output - dehydration limited sweat output with insufficient fluid replacement - decreased peripheral vascular sensitivity impairs cutaneous vasodilation
pre-exercise suggestion to optimize hydration
- approx 17-20 oz, 2-3 hrs before - 7-10 ounces after warm-up (10-15 min before exercise)
during exercise suggestion to optimize hydration
- approx 28-40 oz every hr (7-10 oz every 10-15 min) - rapidly replace lost fluids within 2 hrs after activity to enhance recovery
acclimatized individuals can lose water at what rate?
- approx. 3 L per hr through sweating
what can drinking a cold beverage before exercising in the heat do?
- attenuated increased rectal temp. - reduce physiological strain - 23% improved endurance capacity
compensatory mechanisms in children & cold stress
- augmented energy metabolism - more effective peripheral vasocontriction
gender and heat tolerance
- both genders acclimatize to the same degree - women tolerate thermal stress of exercise at least as well as men of comparable aerobic fitness and level of acclimatization - men make greater use of evaporative cooling
trained/fit and untrained individuals and core temp.
- both individuals would have same core temperature and any given % VO2 - trained generates more total heat - extra heat production in trained individuals is lost via larger sweat output
temperature of the blood
- changes in blood temperature detected by anterior hypothalamus - heightens activity and stimulates hypothalamus to initiate coordinated responses
the ________ system is the main physiological system maintaining thermal balance, and with extreme heat, ______ - ______ % of cardiac output passes through skin
- circulatory - 15-25%
a heat stroke can rapidly progress
- circulatory system collapse - death ensues
physiological consequences of fluid loss
- decreased plasma volume - reduced skin blood flow for a given core temp. - reduced stroke volume - increased near compensatory HR - general deterioration in circulatory and thermoregulatory efficiency in exercise
why is there increased accumulation of lactate while exercising in heat?
- decreased uptake @ the liver because of vasoconstriction - reduced muscle catabolism because of increased lactate diverting to periphery
hyperhydration
- delays hypohydration from inadequate fluid replacement - increases sweating during exercise - produces smaller rise in core temp. - decreased plasma osmolality - pure water dilutes plasma sodium - cold beverage before exercise in the heat
conduction
- direct heat transfer requiring molecular contact - heat moves through deep tissues to the cooler skin surface
evaporation and sweating
- during heat stress, eccrine sweat glads secrete large quantities of hypotonic saline solution - evaporation of sweat from the skin creates a cooling effect - the cooled skin in turn cools the blood diverted from interior tissues to the surface
peripheral thermal receptors
- early warning system - free nerve endings in the skin - responsive to rapid changes in heat and cold
what happens when ambient temperature exceeds body temperature?
- effectiveness of conduction, convection, and radiation is reduced - in turn, evaporative cooling is the only effective method
radiation
- electromagnetic heat waves - does not require molecular contact - the body is usually warmer than the environment
shivering
- exercise metabolism that can produce and oxygen consumption of approx 1200 mL/min
Children with large SA:mass
- facilitates heat loss in warm environment - detrimental in cold
fluid replacement consists of:
- focus on maintaining plasma vol. - circulating and sweating progress @ optimal levels - voluntarily replace 1/2 the water lost in exercise
excess body fat does what to heat tolerance
- forms insulatory layer - retards heat conduction to periphery - increased metabolic cost of weight bearing activities
exertional heat stress
- greater dependence on anaerobic metabolism - earlier and increased accumulation of lactate - encroachment on glycogen reserves - premature fatigue during prolonged moderate exercise
heat stress complications
- heat cramps - heat exhaustion - heat stroke
evaporation
- heat loss via water vaporizing from respiratory passages and skin surface - 580 kcal of heat are lost per liter of water vaporized
heart rate at submax levels of exercise in the heat
- higher HR at all submaximal levels of exercise in heat - does not offset decreased SV - decreased max CO
why is fatigue often associated with temperatures between 38-40 degrees Celsius?
- higher brain temperatures decreasing central drive to exercise - impaired muscle activation/depressed neuromuscular drive - depressed GI blood flow - increased permeability of toxins
acclimatization to the cold
- humans possess much less capacity for adaptation to long term cold exposure than to prolonged heat exposure - must rely on external relief
sweat remains ____________ to other body fluids
- hypotonic - hypovolemia (a decreased volume of circulating blood in the body) from sweating increases plasma osmolality
physiological adjustments during heat acclimatization
- improved cutaneous blood flow - effective distribution of CO - lowered threshold for start of sweating - more effective distribution of sweat over skin surface - increased sweat output - lowered salt concentrations of sweat - lowered skin and core temp. - less reliance on CHO metabolism
body fat, exercise, and cold stress in cold environments
- increased body fat slows heat loss - athletes with greater thermal insulation swim in cool ocean water with almost no decline in core temp - heat lost to water is offset by increased heat production during exercise - cold stress is highly relative-
exertion heat stroke is a condition of extreme hyperthermia because of
- increased metabolic heat load during exercise - challenge for heat dissipation imposed by a non-humid environment
heat exhaustion
- ineffective circulatory adjustments - blood pools in the dilated vessels - decrease central blood volume
ACSM recommendations for electrolyte replacement
- ingested fluid following exercise must exceed by 25-50% the exercise sweat loss - kidneys continually form some urine regardless of hydration status
heat cramps
- involuntary muscle spasms - fluid/electrolyte imbalance - body temp does not necessarily increase
convection
- loss of heat due to movement of air or water adjacent to the skin - air passing from riding a bike accelerates heat loss - little air movement or convection creates a zone of insulation and minimized further conductive heat loss
children and heat tolerance
- lower sweat rate - higher core temp - exercise intensity should be decreased when exposed to heat - require longer time frame for heat acclimatization
elevated resting metabolism may allow Ama to tolerate colder temperatures due to
- mean skin and body temp. remain lower during the winter - exhibit a higher shivering threshold - RMR 25% greater than controls
during exercise in the heat, a competition exists between 2 mechanisms
- mechanisms that maintain larger muscle blood flow - thermoregulatory mechanisms
amount of clo units needed are influenced by
- metabolic rate - insulatory layers of adiposity
clo unit
- military grade - index of thermal resistance - indicates the insulating capacity of any layer of air trapped between skin and clothing
ideal clothing for heat
- moist clothing is more effective in allowing evaporative heat loss - cottons and linens
cold receptors
- more numerous than thermal - exist near the skin surface - initiate regulatory response to cold
two competing demands when one exercises in the heat
- muscle requires blood to sustain energy metabolism - arterial blood diverts to the periphery to transport metabolic heat for cooling at the skins surface
fluid balance
- prevent dehydration - > 2% body weight loss from water deficit
wind chill temperature index
- provides a useful way to understand the dangers from winter winds and freezing temperature - air currents on a windy day magnify heat loss - warmer insulating air layer surrounding the body continually exchanges with cooler ambient air
four physical processes that facilitate heat loss
- radiation - conduction - convection - evaporation
fluid deficits
- reduce the ability to dissipate heat - increase the rate of heat storage and CV strain
heat stroke
- requires immediate medical attention - produces alterations in the immune system such as leukocyte adhesion and activation
ideal clothing for cold
- several layers of light clothing lined with various materials - blocks air movement - allows water vapor from sweating to escape - saturation of a garment affects heat exchange
muscular activity in cold stress
- shivering (involuntary muscle contractions)
muscular activity during exercise in the cold
- shivering occurs during strenuous exercise if core temp remains low - when exercise stops & the metabolic rate of the individual falls, shivering may not be able to prevent a decline in core temp - muscle fatigue will not depress the shivering response
body fat level and heat tolerance
- smaller SA:BM for effective sweat evaporation - fatal heat stroke occurs 3.5 times more frequently in obese young adults
conduction is influenced by
- state of matter - temperature gradient - thermal qualities of the surfaces
decrease plasma osmolality does what to the body?
- stimulates urine production - blunts the normal sodium dependent stimulation of the thirst mechanism
evaporation from the skin and pulmonary surfaces are influenced by
- surface area exposed to the environment - temperature & relative humidity - convective air currents about the body
temperature gradient can flow from
- the body to the environment - in extreme cold, increase heat production and minimize heat loss to prevent a decline in core temperature
the hypothalamus regulates body temperature in two ways:
- thermal receptors in the skin provide input to the central control center - changes in the temp of blood that perfuses the hypothalamus directly stimulate this area
heat dissipation in women
- use circulatory mechanisms - sweat less prolifically - start to sweat at higher skin & core temperature - less sweat for a comparable heat exercise load
conduction in aquatic environments
- water absorbs more heat than air - water conducts heat away from the body - lose more heat in water than air of same temp.
characteristics of heat exhaustion
- weak rapid pulse - headache - low bp when upright - dizziness - great weakness - sweating may decrease
what does maintaining a relatively high plasma sodium concentration do?
-sustains the thirst drive - promotes retention of ingested fluids - restores lost plasma volume more rapidly
Sweat loss for moderate-intensity exercise lasting more than an hour is between:
.5 and 1.0 L
every pound of weight lost through sweating needs how many oz of fluid replacement
20-24 oz
shivering alone can increase increase whole body metabolism by
3 to 5 fold
elite marathon runners can lose how many liters of water
5 L ~ 6-10% of body mass
hypothalamus
contains specialized neurons that coordinate and control temperature regulation of the body
stroke volume during max exercise in the heat
decreased SV
CV function and exercise capacity in hot environment are compromised due to
dehydration
added sodium or potassium will ________ water retention and electrolyte balance
enhance
what is the only method of heat loss?
evaporation
what provides the body with the best defense against overheating?
evaporation
for each liter of fluid lost through sweating what happens to the HR
exercise HR increases approximately 8 bpm
the collective physiological adaptive changes that improve heat tolerance:
heat acclimatization
exercising while dehydrated
increases the risk of heat illness
sweat rate at an increased metabolic rate
increases to 2.0 L/hr
which part of the hypothalamus controls heat conservation
posterior hypothalamus
what is the most important factor determining the effectiveness of heat loss and represents the ratio of water in ambient air at a particular temperature to the total quantity of moisture that air contains?
relative humidity
what determines changes in core temperature with exercise?
relative workload
cardiac output during submax in heat and cold
similar cardiac output in both
vascular adjustments to cold stress
stimulation of cutaneous cold receptors: - constricts peripheral blood vessels - reduces heat loss - redirects flow of warm blood from cooler surface to warmer core - average cutaneous blood flow rate is approx. 250 mL/min; this can fall to approximately 0 in environments that are extremely cold
which system protects primarily against overheating?
thermoregulation system
prolonged cold exposure causes the release what hormone?
thyroxine, which increases resting metabolism
increases permeability of the collecting tubules of the kidneys to facilitate fluid retention
vasopressin (anti-diuretic hormone)
all major acclimatization occurs in the first _________
week
the hypothalamus is usually set and regulated at approx what temperature?
~ 37 +/- 1 degrees C
what is the best defense against heat stress?
adequate hydration
where does epinephrine and nor-epinephrine come from
adrenal medulla
increased heat production occurs because of what two calorigenic hormones?
adrenal medulla hormones; epinephrine and nor-epinephrine
which individuals perform better in environments where thermoregulatory mechanisms are inadequate?
aerobically fit individuals perform longer and tolerate higher levels of hyperthermia
which hormone acts on renal tubes to increase sodium retention
aldosterone
hormonal adjustments during sweating are stimulated by
aldosterone and vasopressin
which part of the hypothalamus controls heat loss
anterior hypothalamus
what system is the main physiological system maintaining thermal balance?
circulatory
