thermoregulation

Question 1

Body temperature regulation

Answer 1

Humans are endotherms
They generate their own internal body heat and do not rely on the energy of
sunlight to warm the body

Question 2

Body temperature regulation

Answer 2

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

Question 3

Body temperature regulation

Answer 3

Humans maintain their
body temperatures within
very narrow limits despite
wide fluctuations in
ambient temperature and
are, therefore, also known
as homeotherms.

Question 4

Body core vs. body surface temperature

Answer 4

maintain their body
temperatures within very
narrow limits.
In contrast, body surface
temperature can vary
considerable depending
on the environmental
conditions

Question 5

Body temperature regulation

Answer 5

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

Question 6

Mechanisms of heat exchange

Answer 6

evaporation of sweat

radiation from sun

conduction to clothing

convection- environment

Question 7

sweating

Answer 7

Transformation of a liquid to gas
requires energy that is derived
from the skin, thereby cooling it

Question 8

radiation

Answer 8

Emission of heat in form of
electromagnetic waves.
The body can absorb or emit heat
depending on ambient conditions

Question 9

conduction

Answer 9

Direct conduct.
The body can gain or lose heat
depending on the temperature of
the adjacent surface

Question 10

convection

Answer 10

Movement of the layer
surrounding the skin (either air or water).
Convection is a form of
conduction

Question 11

Thermoregulation
Body temperature is maintained by balancing heat gain and heat loss

Answer 11

Heat
conservation/production
Shivering thermogenesis
Voluntary muscular activity
Non-shivering thermogenesis
Heat loss

Blood reaching the skin
Sweating

Question 12

Thermoregulatory responses to heat: vasodilation

Answer 12

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.

Question 13

Thermoregulatory responses to heat: sweating

Answer 13

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

Question 14

Thermoregulatory responses to cold: vasoconstriction

Answer 14

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.

Question 15

Thermoregulatory responses to cold: vasoconstriction

Answer 15

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.

Question 16

Thermoregulatory responses to cold: shivering

Answer 16

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

Question 17

heat conservation mechanisms

Answer 17

shivering

skin blood flow to the periphery

Question 18

What‘s the problem with getting too cold?

Answer 18

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

Question 19

heat loss mechanisms

Answer 19

sweating

skin blood flow to the periphery

Question 20

What‘s the problem with getting too hot?

Answer 20

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

Question 21

Exercise and thermoregulation

Answer 21

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.

Question 22

Acclimatization to heat during exercise

Answer 22

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.

Question 23

Acclimatization to heat during exercise

Answer 23

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

Question 24

summary

Answer 24

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.