Ep. 27 - Thermoregulation: The Physics and Physiology of Body Temperature Regulation

In this episode of Wilderness Medicine Updates, host Patrick Fink delves into the topic of thermoregulation, explaining how our bodies regulate temperature in response to environmental changes. Listeners will learn about methods of heat exchange (conduction, convection, radiation, and evaporation), how to measure core temperature, and the body's physiological responses—such as shivering, sweating, and blood flow control. 

The episode also touches on the effects of age, metabolic stressors, and pregnancy on thermoregulation, as well as the importance of understanding these principles in the context of treating conditions like hypothermia and heat-related illnesses.

00:00 Introduction and Episode Overview
01:29 Importance of Thermoregulation
03:12 Human Thermoregulation Mechanisms
04:20 Measuring Core Temperature
07:55 Heat Exchange Methods
14:52 Behavioral and Physiological Responses
24:48 Sex and Age Differences in Thermoregulation
27:14 Conclusion and Next Steps

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Episode Transcript

Patrick Fink (00:13):
Hi everyone.
Welcome back to another episodeof Wilderness Medicine Updates.
I'm your host, Patrick Fink.
Today we're gonna talk about anerdy topic.
It is the topic of thermoregulation or how the body
regulates its temperature inresponse to the environment.
You can expect to learn abouthow we exchange heat or cold

(00:34):
with the environment, whatresponses the body has to try to
regulate its temperature, how weshould be measuring.
Core temperature and how thiscan inform our next steps, our
next few episodes on frostbite,hypothermia, and then moving
into next season into heatrelated illness.
Before we start, I wanna say abig thank you to Ana Kino

(00:57):
Medical Director up in Seattleof Seattle Mountain Rescue.
Thanks for reaching out withkind words and give a shout out
to all the members of SeattleMountain Rescue who are perhaps
listening to this podcast.
I'm very glad that you gotpointed this way.
Happy to have you here.
if there's anything that youwant me to address on the show,
topics that are unclear to you.

(01:18):
No issue.
Too small, no question, toomenial.
Send those questions toWilderness Medicine
[e-mail] or just reachout and say, Hey, I love to hear
from you guys.
You might be thinking, could Iskip this episode?
I'm really a practical person.
I don't know if I need thisinformation.
In short, you probably can skipthe episode.

(01:41):
But I, I recommend that youdon't, because I'm gonna be
spending a fair amount of timeon environmental illness.
That's gonna be the next seriesof educational topics that I
wanna focus on, and I think it'simportant to have a strong
grounding in the backgroundbecause if you just learn, you
know, the stages of hypothermia,you learn your protocols for

(02:04):
treating hypothermia that works90% of the time.
But then when you encounter aunique situation where you're
starting to move outside of yourprotocols, having to make
judgment instead of justfollowing an algorithm, it's
helpful to know a little bitmore than you otherwise might

(02:25):
have to know, knowing morebackground, knowing the reason
for those protocols, knowing thereason why they say this and not
that gives you the basis formaking decisions in unique
situations.
Still, if you'd rather not spendthe time, I will give some
background and some rehashing ofthe basic concepts when we get
to those environmental sections,but I do hope you'll stick with

(02:47):
me here.
This has been a particularlytroublesome episode two record.
This is my fourth time recordingmy way through this episode
because making it useful to youand concise is challenging.
So today I am ditching thenotes.
I'm ditching all the details,and I'm just gonna give you the
fireside chat version.

(03:08):
Let's dive in.
humans are what are called homeoTHMs, and that means that we
maintain our core bodytemperature across a range of
environments.
This is best contrasted withsomething like a lizard or an
iguana whose core bodytemperature is subject to the

(03:28):
environment.
They cannot get up early in themorning and record a podcast
because they have to wait forthe sun to come up to warm them
and start driving theirmetabolism.
Humans in contrast are more likea house with a thermostat.
We have a central coretemperature that has a certain
set point, and we can regulatethat core temperature across a

(03:52):
range of environments.
What determines our set point isa default temperature, and then
things can shift it.
Metabolic stresses or fever canshift that set point.
It can also reduce the range oftemperatures across which we can
effectively regulate, and thathappens more as we get older or
when we're ill or if you'repregnant.

(04:13):
The range of temperatures acrosswhich you can safely regulate
your own core temperature startsto decrease.
So how do you measure coretemperature?
Why do we care about coretemperature?
The reason that we care aboutcore temperature is that the
skin temperature can varywidely.
In cold temperatures, forexample, we pull blood away from

(04:35):
our skin and our coretemperature.
The temperature of us on theinside can vary quite
significantly in comparison tothe outside.
if we're thinking about definingheat or cold related illness, we
need to define those on thebasis of core temperature and
not skin temperature.
There isn't actually a consensusplace to measure a core

(04:55):
temperature.
If you were to ask an intensivecare doctor, it would probably
be in the pulmonary artery, inthe central circulation, right
in the middle of thebloodstream.
But that's pretty impracticalfor anyone outside of an
intensive care unit because youneed a big IV line in the neck,
and a probe that sits withinthat circulation.
So that's not an effective meansof measuring core temperature in

(05:18):
the field.
There are other invasive meansthat leave an indwelling
temperature sensor.
So I'm thinking here of a Foleycatheter that's a urinary
catheter that has a temperatureprobe or an esophageal
temperature probe, something youwould slide down into the
esophagus.
The Foley catheter has obviousdownsides.

(05:41):
You can't really walk with aFoley catheter and the
esophageal probe.
Likewise, not very comfortablein an awake patient, but both of
those are fine in a patient whois not with it because you can
then have a constant measurementof core temperature that is less
affected by the environment.
A third orifice that is morecommonly used would be a rectal

(06:02):
temperature.
You can leave an indwellingrectal temperature probe in
someone who is unconscious.
That obviously doesn't work insomeone who's up and walking
around, but the most common waysthat we might measure core
temperature are the mostconvenient ways would be the
tympanic membranes.
So in the ear, The tympanicmembrane is a super fast way to

(06:22):
measure core temperature, and itclosely approximates that
central circulation coretemperature.
It is however variable betweendevices.
There are crummy devices outthere and there are good
devices, and technique isimportant in that you have to
actually completely block theexternal ear canal when you take
this measurement, or it can beaffected by wind or other

(06:44):
environmental conditions.
The temperature under the tongueis a good place to take a
temperature, provided that themouth has been closed for a
period of time.
You can't just suck on ice cubesand mouth breathe for the 10
minutes before you take thattemperature and expect it to be
accurate.
The axilla or the armpit doesalso closely approximate the
core temperature, however.

(07:06):
It is very subject toenvironmental conditions,
patient positioning, sweating,et cetera.
So this is a less accurate way,which I wouldn't recommend.
in general, for minimallyinvasive means of measuring core
temperature, using a typamembrane thermometer, or under
the tongue is an entirelyreasonable way to do it.
in patients who are unconscious,an esophageal temperature probe

(07:29):
is convenient as something thatyou can slide into the esophagus
and leave in place tocontinuously monitor core body
temperature.
In general, all of these methodsare gonna be within about 0.4
degrees Celsius of thatpulmonary artery.
Temperature in the centralcirculation, and you can just
pick one and go with it as themeans of measuring temperature

(07:51):
in a given patient.
You don't have to worry abouthow they differ one from the
other.
Now, let's think about how weactually exchange heat with our
environment.
There's four ways that weexchange heat with the
environment.
conduction, convection,radiation.
And evaporation.

(08:11):
Conduction is pretty basic andeasy to understand.
If two objects are in contact,like my butt and this chair,
they will exchange heat throughthat direct contact.
Heat moves from whatever iswarmer to whatever's cooler.
energy's always trying to moveto lower energy states.

(08:33):
That's why in the finalaccounting, the entire universe
ends up this thin spread ofextremely cold nothingness.
But for right now, we areconcentrations of energy,
dumping energy into ourenvironment because humans
constantly produce energy.
The most common context forconduction to happen is between

(08:53):
someone who is sitting or lyingdown in whatever surface they're
on.
How much heat can be exchangedthis way depends on the
conductivity of the material, sosome things are very poorly
conductive.
Error specifically is poorlyconductive, while other things
are very highly conductive,think metal.

(09:13):
Water and actually granite Wateris 25 times more conductive than
air, and granite is abouthalfway in between.
So if we're looking forsomething to be an insulator, we
want it to have lowconductivity.
We wanna have, you know, a puffyjacket.
What's the puff?
It's air.
And if.

(09:33):
You're wondering what is gonna,cause most conductive heat loss
water is, is a good touchstone.
We don't encounter a lot oflying flat on cold metal.
Convection is basicallyconduction.
That happens faster because of amoving liquid or gas.

(09:54):
So think here about a convectionoven.
So you have a hot oven and youthe air moves around and it, and
that causes faster heatexchange.
Think about, sitting in a coldplunge.
If you get into a very cold tubof water, and you stay very
still, it feels a lot less coldthan if that cold water is
circulating around you.

(10:14):
blowing wind, blowing air willcool a body much faster than
still air, and moving water isgonna cool a body faster than
still water.
In basically all situationswhere there is air or water
involved, there's some degree ofconvection happening because our
body will warm the air or waterat the interface with the skin,

(10:39):
and that causes it to move awayfrom us.
the usefulness of this conceptis that if you want to limit
convective heat loss, you haveto limit the movement of the
water or air at the skin barrierif I'm wearing a puffy jacket,
it limits wind movement next tomy skin.
Or if I'm in the water, I mightwear a wetsuit, which is gonna

(11:02):
trap a layer of water next to myskin.
I only have to warm that water,and then the heat loss is much
less.
It's conductive.
Heat loss instead of convectiveradiation is the third way that
we can gain or lose heat in theenvironment.
All objects in the universe thatare not at the temperature,

(11:22):
absolute zero are radiatingenergy into the environment.
It's called black bodyradiation.
The hotter they are, the higherthe energy state, the more
energy is lost throughradiation.
The two biggest radiativeobjects we need to think about
in terms of heat loss or heatgain are our own bodies and the

(11:45):
sun.
When we think about heat lossfrom radiation, the enemy here
is outer space, which isessentially a cold void of
energy.
So the biggest source ofradiative heat gain is solar
energy.
the best way we can loseradiative heat is we radiate it

(12:05):
out into the universe on a cold,clear night.
Reducing radiative heat gain isabout increasing reflectivity,
so we want to reflect radiativeenergy away from ourselves.
Clothing is minimally effectivein this respect.
dark clothing is more absorptiveof, the heat of the sun than

(12:26):
light clothing, but both arestill pretty absorptive.
In contrast, a metal surface,think Mylar space blanket is
very effective at reflectingaway radiative heat.
So while your clothing mightreflect 10% of radiative heat
away from you, a Mylar spaceblanket can reflect 98% of that.

(12:50):
that's a very.
Effective tool.
Likewise, it can help you retainyour radiative heat.
If your snowmobile broke down inthe middle of winter, in the
middle of nowhere, and you haveto survive the night.
A Mylar space blanket can helpkeep you from losing your
radiative heat out into space.

(13:11):
The fourth way that we exchangeheat with our environment is
evaporation, and that's takingwater in the liquid phase,
putting energy into it andturning it into a gas, into
steam or water vapor.
This is essentially a one waystreet.
no practical situation out thereis taking steam and condensing

(13:35):
it into water to warm the body.
The body uses this tactic ofevaporation to cool the body, so
you're producing sweat.
That sweat evaporates, and whenit evaporates, that is a
reaction that pulls energy fromthe body to make that phase
change.
This is a highly effective meansof cooling the body in hot

(13:57):
situations because it takes aremarkable amount of energy to
create this phase change inwater.
If you evaporate just a hundredmilliliters of water, it can
reduce the core temperature ofsomeone who's 70 kilos or about
160 pounds by about 0.6 degreesCelsius, which is a substantial

(14:17):
change when you think about themass of the body.
So those are the four ways thatwe exchange heat with the
environment.
conduction, objects in contact,convection, swirling water or
gas, radiation.
We're all glowing and losing ourheat to the universe and the sun
is trying to fry us.
And evaporation, which you canconceptualize as sweating, but

(14:40):
also we can do that for someone.
We'll talk about that when weget to heat illness, but misting
people with water and thenblowing fans helps that water
evaporate and wick heat out ofthat person.
Now let's talk about how weregulate our temperature in
response to the environment.
The first thing to know is thatthe brain is in charge.

(15:01):
There are some reflexive.
Reactions to our environmentthat don't involve the brain.
For example, if your hand isplaced in a very cold
environment, just the contact ofcold with the hand will cause
blanching of the capillariesthere.
or a sweat gland actuallyproduces less sweat if the skin

(15:24):
on top of it is wet.
But for all intents andpurposes, the central nervous
system, the brain and the spinalcord are regulating the core
body temperature.
The majority of the temperaturesensors of the body are out in
the periphery on the skin.
There are a few core temperaturesensors closely related to the
central circulation and yourgut, but the majority of the

(15:46):
sensing that we're doing inresponse to our environment is
at the interface with theenvironment, and we have
separate cold and heat sensorsin the skin.
Those can sense a wide range oftemperatures.
If you get outside of thetemperatures that you can sense,
you just perceive that as aburning pain.
If you've ever stuck your handin a cold bucket of water when

(16:06):
your skin becomes too cold, youcan't sense the temperature
anymore, and it just feels likeburning those sensors, the hot
and cold sensors areconcentrated on.
The hands on the face, on theareas that are commonly exposed,
they're less common in the legs.
In the feet.
They give information about theenvironment back to the brain,

(16:28):
and the brain uses what arecalled effector responses to
regulate the core temperature inresponse to those signals.
The most effective effectorresponse is behavioral change,
and this is actually driven bythe central nervous system.
It's not all free will here.
Modifying the environment,choosing where the body is, or

(16:51):
trying to change thatenvironment is the strongest
means of.
Protecting yourself fromenvironmental stresses.
So either finding shelter orassuming a different posture is
the single biggest mover interms of managing core body
temperature.
And this is intuitive, right?
If, if you're cold becauseyou're outside, the easiest way
to get warm is to go inside.

(17:13):
the body also regulates what youfind.
Pleasurable.
So on a hot day, cool things arepleasurable.
On a cold day, warm things arepleasurable, and that drives
behavioral adjustment.
It also drives changes inposture if you're feeling super
hot, you're more likely toassume a sprawled posture

(17:34):
spreading out arms and.
Versus if you're cold, you tendto curl up to reduce your body
surface area, protect yourgroin, your armpits, areas where
the circulation is exposed, andtry to retain your body heat.
The second most effective andthe most energetically cheap way
to regulate our temperature isto control blood flow to the

(17:57):
surface of the skin.
You can think of this as aradiator.
We have heat.
We're producing heat all of thetime.
It's a metabolic byproduct We'reactually trying to get rid of
some of that heat to ourenvironment.
Most of us don't live in anenvironment that is warmer than
36 degrees Celsius or 98 degreesFahrenheit, so we are constantly

(18:20):
losing that heat to theenvironment.
We can control how much of thathappens by where we drive our
blood flow.
the body will direct blood flowto the surface of the skin.
If we need to lose heat to theenvironment, and if we need to
protect our core bodytemperature from cold, it will
direct that blood flow away fromthe surface of the skin.

(18:41):
Below the skin, we have a layerof subcutaneous fat that's an
insulating layer.
if you keep the blood below thatlayer, you're gonna help
preserve your core bodytemperature.
Whereas if you direct it abovethe layer, it will cause you to
lose heat to the environment,assuming the environment is
colder than you are.
This can be a very effectiveresponse in very cold

(19:03):
environments, bringing thatblood below the subcutaneous fat
layer is highly insulating Bloodflow up towards the surface can
actually approach zero in verycold conditions.
So the difference between thatskin temperature and the core
body temperature can becomepretty stark in a hot
environment.
You'll notice people become veryflush.

(19:26):
They get red in the face.
Children, particularly becausethey're, they're sweating,
sweating, responses are lessthan in adults.
And that's blood coming up tothe surface to try to increase
heat loss to the environment.
That leads us into the idea ofsweating or evaporative cooling.
the cardiovascular response, thedriving of blood circulation is

(19:48):
limited in very hot situations.
You can only lose so much heatthat way'cause we're talking
about just kind of convectiveheat loss.
So anytime the temperaturebecomes much higher where you
have a higher activity level,such as exercise, we have to
start engaging in evaporativeheat loss, which is using

(20:09):
sweating to put water on thesurface of the skin that can
then evaporate and wick heataway from the body.
This is going to be moreeffective in drier climates,
less effective in humid climatesbecause.
The tendency of water toevaporate is gonna be less in
those settings.
You can develop an adaptation.
If you live in a hot environmentfor a long time.

(20:31):
People who are adapted to heatwill actually sweat more,
produce more liquid, and thatcan be something like one to two
liters of sweat per hour in apeak setting.
This, is a very effective way toreduce core body temperature
because water takes a lot ofenergy to undergo that phase
transition.
There are impairments of thiseffect in people who are quite

(20:56):
young or quite old, kids whohave not yet gone through
puberty.
Don't sweat as much as adultsdo.
folks over the age of about 60have less sweating less
shivering and other suchresponses.
they are less able to regulatetheir temperature in significant
heat.
That leads us to the idea ofshivering, which I think is the

(21:19):
conceptual opposite of sweatingshivering, is using our
metabolic energy to try to warmourselves in response to a cold
environment.
What actually is shivering.
Shivering is muscle contractionthat isn't doing any work.
the central nervous systemtriggers your muscles to fire in

(21:40):
opposition to one another.
So it would fire my bicep and mytricep at the same time and fire
over, over, over, over, overagain.
The purpose of those repeatedmuscle contractions is that
using our metabolic energyproduces heat.
So if you burn.
Glucose to produce a TP, whichis the energy currency that the

(22:00):
cell uses 75% of that energy isactually wasted as heat into the
environment.
As soon as you do work, theremaining 25% is also released
as heat into the environment,with the exception of any energy
that turns into kinetic energy,movement of the limbs.
shivering is a way that you canuse calories to burn energy,

(22:21):
start a fire to warm you up.
It's worth knowing that if youget up and move around, activity
can help warm you.
but right away, as soon as youstart moving, that actually
suppresses shivering becauseobviously if your muscles are
contracting involuntarily, youcan't move effectively.
you need a rigorous amount ofactivity to offset the gains

(22:44):
that are lost when you suppressyour shivering.
you can also produce heatwithout shivering.
That's called non shiveringthermogenesis.
And this is using what's calledbrown fat.
Brown fat is a ki.
Most of our fat is white,meaning that if you look at it
under a microscope, the cellslook clear and white, but brown
fat looks.
Huh Brown.

(23:04):
And that's because it's full ofmitochondria, typically referred
to as the powerhouse of thecell.
These are the metabolicfactories of the cell.
brown fat can take the energythat it stores as fat and burn
it in these mitochondria toproduce heat.
It uncouples the burning ofcalories from the capture of

(23:25):
energy.
It says, yeah, we're not gonnacapture any of that energy and
just starts a bonfire to produceheat.
This is most common in infants.
Infants are not able to shiver,so they have a higher amount of
brown fat, but it's preserved inadults across the back of the
neck and across the shoulders,it's more common in individuals
who spend a lot of time in thecold, particularly in cold

(23:46):
water, you will increase theamount of brown fat that you
have.
Finally, the last main responsethat we have to regulate our
temperature in the environmentis using a hormonal response.
So cold stimulus in particular,but any stress in general is
going to increase levels ofepinephrine.
That's adrenaline for myEuropean colleagues.

(24:07):
Epinephrine increasespredominantly during cold
exposure, and it stimulates bothincreased baseline metabolism
and also increases, nonsshivering thermogenesis from our
brown adipose tissue from ourbrown fat.
There are other hormones whichdo contribute to the regulation
of temperature.
The main one there is going tobe thyroid hormone, which.

(24:30):
You know, it is not somethingthat is going to change
significantly in response to ourenvironment.
However, if someone has lowthyroid hormone, they have
hypothyroidism and they're nottaking their thyroid medication,
it will cause them to have alower resting body temperature
and impaired heat generation.
The last thing that I want totouch on before we wrap up with

(24:51):
this little fireside chat onthermo regulation is the idea of
sex differences.
I've mentioned that there aresome differences across the
lifespan in that infants andchildren don't sweat, infants in
particular, cannot shiver, andthat those responses get better
as you get into childhood andinto the teenage years when

(25:13):
people start.
Reaching peak ability toregulate their temperature.
As you get older, you have lessability to shiver, less ability
to regulate your vasodilationblood flow to the surface,
reduced nons shiveringthermogenesis, so less ability
to regulate the temperatures atextremes of age.
One might wonder whether thereare sex differences.

(25:35):
We have an intuitive thoughtthat there might be sex
differences.
For example, my wife alwayscomplains that she's cold.
When I feel warm, does that meanthat we have different abilities
to regulate our core bodytemperature?
the answer is essentially no.
women have a smaller bloodvolume thinner extremities,
higher body fat, and lower leanbody mass.

(25:57):
However, in well-controlledtrials, there is no difference
in their ability to regulatecore body temperature except at
very extreme levels of exercisein heat.
So at that level, we see aslight impairment in comparison
to males.
In contrast, pregnant womenmaintain a different core body

(26:18):
temperature.
over the course of pregnancy,the set point, the thermostat
temperature of the pregnantwoman slowly declines so that
they have a lower core bodytemperature by the end of
pregnancy.
the purpose of that is toprotect the fetus from heat
stress.
heat does bad things to a fetusin early pregnancy, fever or

(26:39):
hyperthermia can induce birthdefects because in early
pregnancy, that's when you'reforming your neurological
system.
Heat stress in the later phasesof pregnancy can lead to low
birth weights, but not birthdefects.
So when we come back to heatillness, we'll touch on this
again, in women who are ofreproductive age.

(27:00):
Heat stress is something that wehave to address effectively and
early because women who are inthe very early stages of
pregnancy when their fetusesmost vulnerable, might actually
not know that they're pregnant.
So something just to bookmarkthere.
So that wraps up a quick runthrough on the principles of
Thermo regulation to just touchback and review what we've gone

(27:21):
over.
We are homeo THMs.
We're regulating our bodytemperature against the
environment.
We have a set point which we'retargeting and our ability to
regulate our temperature can beaffected by metabolic stressors.
Medications, age, many things.
Our best ways to measure ourcore body temperature are using
a tympanic thermometer or underthe tongue in someone who's

(27:44):
awake a rectal temperature's notbad.
And in people who are.
Completely obtunded.
We can use an esophagealtemperature or a Foley catheter
temperature probe to measure thecore temperature on an ongoing
basis.
When we think about exchangingheat with the environment, we're
talking about conduction,touching things, convection,

(28:05):
swirling liquids or gases,radiation, the glowing sun, and
the glowing body andevaporation, using water to cool
our body temperature.
when we are creating a responseto our environment, the brain is
the one that's in charge thatregulates the set point and uses
different effector responses toprotect itself from the

(28:26):
environment.
The best protection being changeyour environment, move, change
your body position, go somewherewarm if you're cold.
If you can't address that, thencardiovascular responses.
Blood flow to the surface isvery energetically cheap and is
our main way of regulating corebody temperature.
But that doesn't work when itgets really hot.
so then we're talking aboutsweating or if we're getting too

(28:49):
cold, we're talking aboutshivering or non shivering
thermogenesis.
That's our brown fat bonfire.
there are some minor changesthat we get from hormonal
responses as well, namelyepinephrine, and we can have
impaired temperature responsesif we have hypothyroidism.
there aren't big sex differencesbetween men and women, except in

(29:09):
the pregnant person who is goingto have a progressively lower
core temperature set point overthe course of pregnancy and heat
stress is particularlyproblematic in that setting.

(29:31):
So I hope this was useful toyou.
Maybe you learned something.
This will provide a goodfoundation moving forward as we
talk about environmental illnessand heat and cold stress.
If anything was unclear to you,let me know.
Reach out by email and I willclarify.
This might be a good episode tobookmark and revisit when we get
back to those.

(29:52):
Episodes, but I will also rehashthe pertinent physiology.
When we get to hypothermia, forexample, we'll talk about
shivering, and we'll also talkabout how as you reach the
extremes of temperature as youbecome more hypothermic or my
more hyperthermic,unfortunately, those conditions
also cause impairment of theresponses that are supposed to

(30:14):
protect us from them.
So if you get too hypothermic,you can't shiver, and that's a
problem.
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and quickly give it a five starrating.

(31:20):
That helps us game the algorithmand move up the search rankings
and get in front of more people.
You don't have to type anything.
It takes about two seconds and Ireally appreciate it.
Thank you all for listening.
Until next time, this is Dr.
Patrick Fink.
Stay fit, stay focused, and havefun.