Chapter 12/23: Temperature Regulation & Altitude Flashcards
describe the term homeotherms
maintain constant body temperature, heat loss must match heat gain
normal core temp
37 degrees celsius
temperatures above 45 degrees celsius might
damage proteins and enzymes, leading to death
temperatures below 34 degrees celsius might
result in decreased metabolism and cardiac arrhythmias
describe the thermal gradient. typical value?
thermal gradient between deep body core temp and skin surface (typically 4 degrees celsius but can be up to 20 degrees celsius in extreme cold)
how is deep body (core) temp measured?
measured at rectum, ear or esophagus (in the lab) or ingestible pill (athletes can use during practice sessions)
how is skin (shell) temperature measured?
using thermistors at various skin locations, calculate the mean skin temp
3 methods of heat production (both voluntary and involuntary)
exercise (voluntary): 70-80% of energy expenditure released as heat
shivering & non-shivering thermogenesis (involuntary)
hormones that cause non-shivering thermogenesis
thyroxine (thyroid hormone) and catecholamines
4 mechanisms of heat loss
1) evaporation: via sweat
2) radiation: body heat lost to nearby objects
3) conduction: body heat lost by physically touching something (ice cubes)
4) convection: movement of air molecules (fan)
3 factors influencing evaporation rate
1) temperature and relative humidity
2) convective currents around the body
3) amount of skin surface exposed
how does high relative humidity affect evaporation rate?
high relative humidity decrease the vapor pressure gradient between the skin and the environment, which leads to decreased rate of evaporation
what is the heat index? how does high relative humidity affect the heat index?
the heat index is a measure of the body’s perception of how hot it feels (high relative humidity reduces evaporative heat loss which leads to increased perception of how hot it feels)
how does the body respond to increased core temperature?
the preoptic anterior hypothalamus stimulates sweat glands (evaporative heat loss) and cutaneous vasodilation via sympathetic cholinergic control of sweat glands and cutaneous vasculature
what ligand and receptor are responsible for stimulating sweat in eccrine glands?
Ach binds to mAchR
what ligand and receptor are responsible for vasodilation of blood vessels in the skin?
Ach on mAchR
what happens to heat production as exercise intensity increases?
heat production increases due to muscular contraction, there is a linear increase in body temp
core temp increase is proportional to:
active muscle mass
what determines heat production during steady state exercise? what is not responsible?
exercise intensity, not environmental temperature
submaximal exercise in a hot/humid environment leads to:
higher core temperature, which leads to risk of hyperthermia and heat injury
what happens to oxygen uptake during prolonged exercise in a hot and humid environment?
upward drift in oxygen uptake (VO2)
how is cardiac output affected during exercise in hot and humid environments?
to compensate for the decrease in stroke volume, heart rate gradually creeps up to maintain cardiac output
how is blood flow affected during exercise in hot and humid conditions?
blood flow is shunted away from working muscle and nonessential areas (gut, liver, and kidneys) to the skin
how does exercise in hot humid environments affect sweat rate?
higher sweat rate, up to 4-5 L / hour
larger individuals will have higher _____ compared to smaller individuals
sweat rates
what endocrine responses occur due to exercise in the heat?
increased release of vasopressin and aldosterone (helps retain water in the kidneys, but isn’t as helpful during exercise because decreased blood flow to kidneys, so more helpful after exercise)
factors the contribute to impaired exercise performance in the heat
1) central nervous system dysfunction: decreased motivation
2) cardiovascular dysfunction: reduced stroke volume and muscle blood flow
3) accelerated muscle fatigue: increased radical production, decreased muscle pH, muscle glycogen depletion
describe acclimation
rapid physiological adaptation that occurs within days to a few weeks, or is artificially induced in a climatic chamber
describe acclimatization
gradual, long-term adaptation that occurs within months to years of exposure to the environment stress (i.e. climate)
acclimation and acclimatization both require
exercise in hot environment, because elevated core temp promotes adaptations
end result of heat acclimation
lower heart rate and core temperature during submaximal exercise
5 adaptations during heat acclimation
1) increased plasma volume
2) earlier onset of sweating and higher sweat rate
3) reduced sodium chloride loss in sweat
4) reduced skin blood flow (at same exercise intensity)
5) increased cellular heat shock proteins
role of having increased plasma volume for heat acclimation?
maintains blood volume, stroke volume, and sweating capacity
role of earlier onset of sweating and higher sweat rate for heat acclimation
less heat storage, maintain lower body temp
role of reduced sodium loss in sweat for heat acclimation
reduced risk of electrolyte disturbance
how do we decrease sodium loss in sweat?
aldosterone acts at both the eccrine glands and the kidney to reabsorb Na+
role of reduced skin blood flow for heat acclimation
occurs because we are able to maintain core body temperature better, so we don’t need to lose as much heat via evaporative heat loss
role of increased cellular heat shock proteins for heat acclimation
prevent cellular damage due to heat, protects cells from thermal injury by stabilizing and refolding damaged proteins
which adaptations occurs first in heat acclimation?
heart rate decrease and plasma volume increase occur first, then perceived exertion decreases, then sweat rate increases
sex differences in thermoregulation and heat tolerance
minimal
aging influence on thermoregulation and heat tolerance
aging results in reduced ability to lose heat during exercise (e.g. skin blood flow is reduced in older individuals)
how quickly does loss of acclimation occur? why?
lost within a few days of inactivity (no heat exposure), significant decline in 7 days, complete loss in 28; maintaining a high blood volume takes a lot of energy
how does the body respond to decreased core temperature?
the preoptic anterior hypothalamus (POAH) stimulates shivering and decreased skin blood flow
mechanism of involuntary shivering
somatic motor neurons stimulate skeletal muscle contraction, leading to heat production
mechanism of nonshivering thermogenesis
the POAH initiates the release of NE and thyroxine, which increases the rate of cellular metabolism by binding to receptors on adipocytes
mechanism of cutaneous vasoconstriction
NE acts on alpha 1-ADR on blood vessels in the skin to cause vasoconstriction
exercise in a cold environment leads to:
enhanced heat loss, which may result in hypothermia, loss of judgment and risk of further cold injury
how do insulating factors affect exercise in a cold environment?
subcutaneous fat is the primary fuel for shivering in well-fed individuals
how does the wind speed affect heat loss during exercise?
rate of heat loss increased with higher wind
how does water immersion affect heat loss during exercise?
rate of heat loss is 25x greater than of air of the same temp
cardiovascular responses to exercise in the cold
blood flow is shunted away from the skin to the core (cutaneous vasoconstriction)
muscular responses to exercise in the cold
hands exposed to cold temps often become numb due to reduced blood flow and depressed rate of neural transmission, resulting in loss of dexterity and negatively impacts motor skills
endocrine response to exercise in the cold
increase release of norepinephrine, epinephrine, and thyroid hormone for metabolic heat production (non-shivering thermogenesis)
what water temps present risk for hypothermia?
cold water 15 degrees celsius or cooler
when body temp declines from 37 to 25 degrees celsius, what is the risk?
this level of hypothermia is associated with life-threatening cardiac arrhythmias
affect of breathing cold air during exercise?
breathing cold air does not pose a risk to respiratory tract or lungs because the air is rapidly warmed before entering the lungs, however it can trigger exercise-induced asthma in some individuals because of cooling and drying of airways
3 adaptations during cold acclimation
1) lower skin temp at which shivering begins
2) maintain higher hand and foot temperature
3) improved ability to sleep in the cold
why does shivering start at a lower skin temp when acclimated to the cold?
cold-acclimatized people maintain heat production with less shivering by increasing non-shivering thermogenesis
how are cold-acclimatized people able to maintain higher hand and foot temp?
improved peripheral blood flow
why are cold-acclimatized people better at sleeping in the cold?
due to reduced shivering
how does sex affect responses to cold exposure? at rest? in cold water? and why?
at rest, women show faster reduction in body temp than men (mainly due to surface area differences)
in cold water, decrease in body temp about the same for men and women (so rapid, surface area doesn’t matter)
how does age affect responses to cold exposure?
older individuals (>60) are less tolerant to cold, also children experience faster fall in body temp
does percentage of oxygen in the air change as you move up in altitude?
no, always 20.93% oxygen
problem posed by lower partial pressures of oxygen at higher altitudes?
partial pressure of O2 in venous blood is around 40 mmHg, partial pressure of O2 on Mt Everest is 53, maybe 42 mmHg by the time it reaches the alveoli
how does hypoxia affect the oxygen-hemoglobin dissociation curve?
shifts the curve left
3 factors determining arterial oxygen content
1) saturation
2) hemoglobin concentration
3) partial pressure of oxygen in the arteries (diffused O2 - minor contribution)
how does altitude affect VO2 max?
decreased VO2 max at higher altitude
how does the percent decline in VO2 max of trained individuals compare to untrained individuals? why?
trained individuals have a larger decline; because of reduced pulmonary capillary transit time
mechanism of effect of altitude on heart rate?
acute hypoxia —> decreased PiO2 —> decreased PaO2 —> epinephrine release —> increased HR
effect of altitude on ventilation response during submaximal exercise
peripheral chemoreceptors sense lower PaO2 (switches to sensing O2 at high altitude) —> ventilation increases
how does altitude affect short-term anaerobic performance?
lower PO2 at altitude should have no effect on perforation, because we are relying heavily on non-oxidative energy sources, in fact lower air resistance may improve performance
how does altitude affect long-term aerobic performance?
lower PO2 results in poorer aerobic performance because it is dependent on oxygen delivery to the muscle, mostly oxidative energy sources
acute cardiovascular responses to altitude
decrease plasma volume upon initial arrival —> decreased stroke volume due to respiratory water loss and increase urine production —> increased hematocrit
how does plasma volume return to normal after a few weeks at altitude?
if adequate fluid is ingested
3 longer term adaptations to high altitude
1) production of more red blood cells
2) greater oxygen saturation
3) hyperventilation
how is a higher red blood cell concentration achieved?
erythropoietin (EPO)
how is greater oxygen saturation achieved with acclimatization to high altitude?
increasing blood flow to the lungs due to high nitric oxide levels
how does hyperventilation assist with acclimatizing to high altitude?
increases the sensitivity of the carotid chemoreceptor so that it is more sensitive to changes in PO2 —> increased ventilation
describe the strategy of “live high, train low”
living at high altitude elicits and increase in red blood cell mass via EPO which leads to an increase in VO2 max
training at low altitude, maintain high interval training velocity
