thermoregulation Flashcards
Body temperature regulation
Humans are endotherms
They generate their own internal body heat and do not rely on the energy of
sunlight to warm the body
Body temperature regulation
Humans are endotherms
They generate their own internal body heat and do not rely on the energy of
sunlight to warm the body
Heat is generated during metabolism
» At least 75-80% of energy released in
physical activity “wasted” as heat,
while only 20-25% yields external work
Body temperature regulation
Humans maintain their
body temperatures within
very narrow limits despite
wide fluctuations in
ambient temperature and
are, therefore, also known
as homeotherms.
Body core vs. body surface temperature
maintain their body
temperatures within very
narrow limits.
In contrast, body surface
temperature can vary
considerable depending
on the environmental
conditions
Body temperature regulation
There is a characteristic
circadian fluctuation in body
core temperature of about
1°C
Body temperature is lowest
during the night and highest
during the day
Mechanisms of heat exchange
evaporation of sweat
radiation from sun
conduction to clothing
convection- environment
sweating
Transformation of a liquid to gas
requires energy that is derived
from the skin, thereby cooling it
radiation
Emission of heat in form of
electromagnetic waves.
The body can absorb or emit heat
depending on ambient conditions
conduction
Direct conduct.
The body can gain or lose heat
depending on the temperature of
the adjacent surface
convection
Movement of the layer
surrounding the skin (either air or water).
Convection is a form of
conduction
Thermoregulation
Body temperature is maintained by balancing heat gain and heat loss
Heat
conservation/production
Shivering thermogenesis
Voluntary muscular activity
Non-shivering thermogenesis
Heat loss
Blood reaching the skin
Sweating
Thermoregulatory responses to heat: vasodilation
Vasodilation of blood vessels in the skin near the
extremities happens due to a reduction of
sympathetic stimulation.
In skin away from the extremities vasodilation also
happens due to the release of acetylcholine from
specialised sympathetic neurons.
Thermoregulatory responses to heat: sweating
Sweat production rates affect the
composition of sweat with a larger amount
of Na+ and Cl- being lost at high sweat rates
Sweat glands are innervated by cholinergic
nerve fibers via stimulation of muscarinic
receptors.
Circulating catecholamines can also stimulate
sweat production
Thermoregulatory responses to cold: vasoconstriction
The blood vessels in the skin are
innervated by sympathetic
adrenergic fibers.
Vasoconstriction happens as a
result of norepinephrine acting
on αadrenergic receptors.
Blood flow can be reduced to
zero at extreme temperatures.
Thermoregulatory responses to cold: vasoconstriction
Paradoxically, during long exposure to
cold the skin circulation will often
show intermittent periods of
vasodilation.
This is called the “Hunting reaction”
and it is considered to be a protective
mechanism against ischaemic injury
(frostbite).
The mechanism is unclear but it may
be due to a periodic insensitivity to
norepinephrine.
Thermoregulatory responses to cold: shivering
Muscular contraction releases heat because ATP is
broken down.
In response to cold, voluntary movements increase and
this prevents core temperature to drop.
Shivering is a specialised form of muscular contraction
in which the muscles perform no external work and
virtually all the energy of contraction is converted into
heat. Shivering is a result of stimulation by somatic
motor neurons.
Non-shivering thermogenesis from brown fat metabolism but this is not important
in adult humans
heat conservation mechanisms
shivering
skin blood flow to the periphery
What‘s the problem with getting too cold?
37 °C: Resting zone
36 °C: Moderate Shivering; Reduced strength, power & dexterity
35 °C: Maximal Shivering; physical performance
<29 °C: Severe cardiac arrhythmias & unconsciousness common
<33 °C: Cardiac arrhythmias; highly variable inter-individual responses
Hypothermia
heat loss mechanisms
sweating
skin blood flow to the periphery
What‘s the problem with getting too hot?
Hyperthermia
When core body temperature becomes dangerously high (38 - 40.5 °C)
Mild hyperthermia = heat exhaustion
Severe hyperthermia = heatstroke
Heatstroke defined as a failure of thermoregulation with a body temperature ≥ 40.6 °C
Signs of hyperthermia
Dizziness
Disorientation
Skin can appear dry
Heart rate is elevated
Collapse
Exercise and thermoregulation
During exercise body
temperature is maintained
higher than at rest.
This is an example of set-point
resetting and it has functional
implications for muscle
contraction.
Acclimatization to heat during exercise
Acclimating to heat means that an individual can regulate body
temperature more efficiently.
This is crucial for exercise performance in a hot environment.
E.g., Tokyo 2020
Acclimatization/Acclimation involves the repeated exposure to heat
either by habitation in a hot climate or the use of environmental
chambers.
Acclimatization is considered to begin after 5-10 days of prolonged
exercise in the heat. This happens via adjustments in blood flow and
sweating.
Acclimatization to heat during exercise
An acclimated individual will:
Sweat more and sooner
Increased blood flow to skin
Have more dilute sweat (conserve electrolytes)
Increased blood volume and stroke volume
Have a lower body core temperature,
heart rate, glycogen use and, thus, fatigue
summary
Humans are endotherms - heat is generated during metabolism
Humans are homeotherms - maintain body temperatures within very narrow limits
Body core temperature is maintain within very narrow limits vs. Body surface temperature
can vary considerable
But, body core temperature does fluctuate (due to eating, exercise, circadian rhythm etc.)
The 4 modes of heat exchange are: conduction, convection, radiation, and evaporation
Thermoregulation = maintaining a constant core body temperature
Physiological heat loss mechanisms are: blood reaching the skin & sweating
Physiological heat production mechanisms are: shivering thermogenesis, voluntary muscular
activity, Non-shivering thermogenesis
Summary
Responses to cold include peripheral vasoconstriction and shivering and
responses to heat include peripheral vasodilation and sweating
The excessive amount of heat that is produced during exercise means that
core body temperature is elevated.
In a very hot environment exercise performance is compromised but
acclimatization to these conditions can improve the way that the body
thermoregulates core body temperature.
