Chapter 74: Body Temperature Regulation and Fever

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You know, usually when we think about a survival situation, we picture something entirely external and like highly dramatic.

Oh, for sure.

Like running from a bear or something.

Right, exactly.

You're braving a blizzard or trying to find water in a desert.

The threat is outside of you and your response is, well, it's obvious.

It's visible.

You perceive the threat and you react to the environment around you.

But the truth is, the most intense, high -stakes survival situation you will ever face is actually happening inside you right now.

Yeah, every single second.

Without you even realizing it, your body is performing this incredibly precise, almost balancing act between the heat you're constantly producing and the heat you're losing to the room around you.

And if that balance tips too far either way, I mean, the results are catastrophic.

It's crazy.

It is the absolute definition of a biological high -wire act.

It's this beautifully designed, continuous physiological negotiation that your brain is just managing constantly.

And that biological high -wire act is exactly what we're getting into today.

Welcome to the Deep Dive.

Glad to be here.

Our mission today is for you to master the central physiological concept of body temperature regulation.

And we're going to do that using a very specific lens.

Right.

We are relying on one source and one source only.

Exactly.

We are using only Chapter 74 of the Guidance and Hall textbook of medical physiology, 15th edition.

No outside fluff, just pure, targeted medical physiology translated so you can totally grasp it.

The way this specific chapter breaks down the logic is it's just perfect for really understanding how your body works.

Because to understand how your body prevents you from overheating or freezing, we first have to look at the internal engine producing all that heat.

OK.

So let's unpack this.

We're going to figure out how that engine is insulated.

And then we'll discover how the body vents that heat to the outside world.

From there, we'll track down the master thermostat in your brain that controls the whole operation.

And finally, we'll see what happens when that system completely fails, like in a high fever or severe heat stroke.

We definitely have to establish the baseline of normal first, though.

Because well, normal body temperature is a bit of a myth, at least in the way we usually think about it.

Yeah.

The textbook divides the body into two totally different temperature zones, right?

The core and the shell.

Exactly.

The core, meaning your deep tissues and organs, like your brain, heart, and liver, is rigorously defended.

The body will do almost anything to keep the core constant, usually within plus or minus one degree Fahrenheit or 0 .6 degrees Celsius.

But the shell, which is your skin and the tissues just beneath it, is a totally different story.

Oh, completely.

Your skin temperature fluctuates wildly depending on whether you're, I don't know, standing in the snow or sitting in a sauna.

And even when we talk about a normal core temperature, it isn't just one magic number.

No, not at all.

If you took thousands of healthy people and measured their temperatures, you wouldn't get a single dot on a graph.

You'd get a pretty wide curve.

The average core is generally considered 98 .0 to 98 .6 degrees Fahrenheit orally and about a degree higher rectally.

But depending on the time of day, your emotional state, or if you're exercising, a perfectly healthy person can have a core temperature ranging from below 97 degrees all the way up to over 99 .5 degrees.

So normal is really a flexible range, not some fixed point.

OK, so we have this core temperature that we absolutely must defend.

How do we do it?

It comes down to a constant battle between heat production and heat loss.

Let's start with production.

Heat is essentially just a byproduct of metabolism.

Your deep organs are like a furnace that is always running.

They're constantly churning out heat through their basal metabolic rate, any muscle activity, sympathetic nerve stimulation, and hormones like thyroxine.

And all that heat generated deep inside by the liver, brain, and muscles has to be transferred out to the skin so it can be released, right?

Otherwise, you'd basically cook from the inside out.

Exactly.

This is where the physical anatomical structures of your body come into play, starting with our insulation.

The fat.

Yes.

The skin, your subcutaneous tissues, and especially your subcutaneous fat are just terrible conductors of heat.

Which is exactly what you want from an insulator.

Yeah.

Fat conducts heat only one third as readily as other tissues.

It perfectly traps the heat inside the core.

The textbook notes something here that totally blew my mind.

A male body's natural fat insulation is roughly equal to wearing three quarters of a suit of clothes.

It's wild.

And that natural insulation is even more effective in women.

So think about it this way.

Your body is basically a car engine wrapped in a really thick winter coat.

I like that analogy.

The deep organs are the engine generating all the heat, and the fat is the heavy coat keeping it trapped.

But if you keep a car engine running inside a thick coat, it's going to overheat quickly.

You need a way to vent it.

And that's where the body's radiator system comes in.

Oh, this part is so cool.

Literally.

So beneath your skin there is this incredible continuous venous plexus.

It's a vast network of blood vessels.

In highly exposed areas like your hands, your feet, and your ears blood, from the deep core is supplied directly to this surface network.

By those highly muscular blood vessels, right?

The arteriovenous anastomoses.

Yes, exactly.

So it's quite literally a radiator.

The hot coolant, which is your warm blood from the core,

is pumped away from the deep organs past that thick layer of insulating fat right up to the surface of the skin where the heat can radiate away.

And the sheer capacity of this radiator is astonishing to me.

Oh, it's massive.

The rate of blood flow into the skin network can be dialed down to almost zero when you're

or cranked up to as much as 30 % of your total cardiac output when you're hot.

30 %?

Yeah, if you could look at the blood vessels under your skin when you're overheating, you'd see your sympathetic nervous system opening those vessels up so wide that your body can suddenly dump heat from the core to the skin eight times faster than its resting rate.

An eight -fold increase in heat conductance.

That is insane.

Okay, so that explains how the heat travels from the deep core to the surface of the skin.

Right.

But we have to move from internal anatomy to external physics now.

How does that heat physically leave the skin and enter the room around you?

The physics of heat loss basically happen in four ways.

In a normal room temperature, a nude person loses about 60 % of their heat through radiation.

Meaning literal infrared heat rays.

Yes, infrared rays radiating from your warmer body out to the cooler surrounding objects like the walls of the room.

Okay, got it.

That you have conduction.

Right.

About 3 % is lost by direct physical contact with solid objects.

Like if you sit on a cold metal chair, your body heat conducts directly into the metal.

But another 15 % is lost via conduction to the air around you.

Your body warms the tiny layer of air physically touching your skin.

And that ties directly into convection, right?

Exactly.

Once your body warms that microscopic layer of air, that warm air rises and moves away and new cold air replaces it.

Which is why wind makes you feel so cold.

Yes.

The cooling effect of wind is proportional to the square root of the wind velocity.

It's constantly stripping away that layer of warmed air.

And as a quick side note, if you fall into cold water, watch out.

Water absorbs heat thousands of times better than air.

So conduction and convection will drain your body heat incredibly fast in water.

Right, that makes sense.

Which leaves us with the final piece of the pie evaporation.

About 22 % of your baseline heat loss is just moisture evaporating off your skin and from your lungs.

Yeah, and every single gram of water that evaporates from your body surface pulls ball point five eight calories of heat away with it.

And that physical reality introduces one of our most critical physiological functions.

The specific mechanism of sweating.

Oh, the sweat glands are brilliantly engineered.

They really are.

A sweat gland essentially has two parts.

First, there's the deep coiled portion down in the lower layers of the skin.

When stimulated, this coil secretes a fluid that is basically your blood plasma just without the heavy proteins.

But it's packed with high amounts of sodium and chloride, so it's incredibly salty.

Extremely salty.

But you know, the body doesn't want to lose all that precious salt.

So that salty fluid travels up the second part of the gland, which is a long duct leading to the surface of your skin.

And as it flows up, the cells lining the duct frantically work to reabsorb the salt back into the body.

Right.

If you're only sweating a little bit, the fluid moves slowly.

The duct has plenty of time to work and almost all the salt is reabsorbed.

You sweat out watery fluid and you save your electrolytes.

But if you're exercising heavily or you're in a sauna, the deep coil is pumping out massive amounts of fluid.

It rushes up the duct so fast that the reabsorption pumps just, well, they can't keep up.

Exactly.

The fluid hits the surface of your skin, still completely loaded with sodium and chloride.

Which brings us to the fascinating process of acclimatization.

Yes, this is so cool.

If you take someone who isn't used to the heat and put them in a hot environment, their sweat glands aren't prepared.

They might waste 15 to 30 grams of vital salt a day through their sweat and they'll max out at producing about one liter of sweat per hour.

But if you leave them in that hot environment for one to six weeks,

the body adapts.

Their adrenal glands start pumping out a hormone called aldosterone.

And aldosterone acts directly on that sweat duct.

It supercharges the duct's ability to reabsorb sodium and chloride.

Wow.

So after a few weeks of acclimatization, that massive salt loss drops down to just three to five grams a day.

Meanwhile, the actual volume of sweat they can produce doubles or triples up to two to three liters an hour.

They become highly efficient salt -saving cooling machines.

That's an incredible adaptation.

It really is.

The text also briefly covers how animals handle this, which is panting.

Right, because dogs and cats are covered in fur and lack extensive sweat glands.

So their pneumotaxic respiratory center triggers rapid, very shallow breathing.

This moves tiny amounts of air rapidly over the saliva on their tongue to evaporate it.

It cools them down, but because the breasts are so shallow, they don't blow off too much carbon dioxide and cause hyperventilation.

It's a very clever workaround for having fur.

Now wait a minute.

I actually have a question about the physics of our human cooling system here.

Okay, shoot.

If radiation relies on heat rays moving from a hotter object to a colder object, what happens when I walk outside and it's 105 degrees?

The air and the pavement are hotter than my skin.

Won't the physics just reverse?

Won't I just absorb radiation and convective heat from the environment?

Oh, this raises a vital point, and your logic is entirely correct.

In high environmental temperatures, the physics completely flip.

You start gaining heat through radiation and conduction.

So in a 105 degree desert, your radiator system actually becomes a liability.

Yes.

In that scenario, evaporation is your absolute only cooling mechanism.

Your body's only defense is sweating.

Just sweating.

Yeah.

The text actually notes that people born with a rare congenital absence of sweat glands can survive in cold climates just fine.

But if you put them in the tropics, they can rapidly die of heat stroke because their one only avenue for heat loss is physically missing.

That is terrifying.

Evaporation really is a life or death mechanism.

So since the stakes are that high, what is acting as the commander in chief?

What part of the body is deciding when to deploy the sweat glands or when to clamp down the blood vessels?

We looked to the brain's thermostat for that.

To prove how good this thermostat is, Guyton and Hall describe a stability experiment.

Oh yeah, the graph.

Right.

So if you expose a nude person to completely dry air and wildly swing the temperature of the room from 55 degrees all the way up to 130 degrees Fahrenheit,

their internal core temperature remains essentially flat.

Which is mind blowing.

It is.

Despite extreme external swings, the core hovers securely right around 97 to 100 degrees.

And that perfectly flat line is courtesy of the anterior hypothalamic preoptic area of the brain.

That tiny region is your core thermostat.

Exactly.

It's packed with specialized heat -sensitive neurons.

As your core temperature rises, these neurons physically fire faster.

Specifically, 2 to 10 times faster for every 10 degrees Celsius increase.

But the brain's thermostat doesn't operate in a vacuum, right?

It needs intelligence from the field.

Yes, it relies heavily on peripheral receptors out in your skin and deep tissues.

These receptors belong to the TRP, or transient receptor potential family, of cation channels.

They act like highly specific thermal sensors.

Precisely.

For example, some sensors only fire if the temperature drops below 27 degrees Celsius.

Others only fire if it rises above 42 degrees.

But when you look at the breakdown of these thermal sensors, there is a massive asymmetry in the body.

Your skin has 10 times as many cold receptors as warmth receptors.

Huge difference.

And the deep receptors located in your spinal cord and around your abdominal organs also detect cold.

Why the massive bias toward detecting cold?

The bias exists because the peripheral system's primary mission is early warning.

It's trying to prevent hypothermia before the core even realizes there's a threat.

OK, that makes sense.

I mean, think about it.

If you step into a freezing room, your skin temperature drops immediately.

But your deep core temperature hasn't changed yet.

Because you have so many cold receptors, your skin immediately screams at the brain to start and conserving blood before your deep organs ever drop a fraction of a degree.

Early detection.

I love that.

So all these signals from the skin, the deep tissues, and the anterior preoptic area are all sent to a central integration hub, the posterior hypothalamus.

Yes, that is where all this sensory data is combined to launch a coordinated physical response.

So the posterior hypothalamus processes the danger.

Let's look at the specific biochemical and physiological orders at issues.

If it decides your core is too hot, what happens?

It issues three main orders.

First, massive vasodilation of the skin blood vessels to pump the hot core blood to the surface radiator.

Second, a strict detreason in any heat production.

It inhibits shivering entirely.

And third, it triggers the sweat glands.

And the trigger point is incredibly precise.

Extremely.

The moment the internal core temperature hits 37 .1 degrees Celsius, which is 98 .8 degrees Fahrenheit,

there is a sharp vertical spike in evaporative heat loss.

Sweating kicks in aggressively.

37 .1 degrees Celsius.

That is the magic number at the body set point.

And what about when you're too cold?

The orders reverse.

You get massive skin vasoconstriction clamping down those surface blood vessels to trap heat inside.

You get piloerection, which we call goose bumps.

Ah, goose bumps.

Yeah.

The tiny muscles at the base of your hairs contract to make the hairs stand on end, attempting to entrap a thick layer of insulator air next to the skin.

Which for humans doesn't really work.

Goose bumps are basically a vestigial reflex where our bodies are trying to fluff up a thick fur coat that we evolved out of thousands of years ago.

Exactly.

We lack the fur to make it effective.

So the most important human response to cold is increased thermogenesis, actively manufacturing more heat.

How does it do that?

The hypothalamus starts increasing skeletal muscle tone.

You feel physically tight.

This tone builds up until it hits a critical level that triggers the stretch reflex in the muscles, resulting in shivering.

And shivering is incredibly powerful.

Oh yeah.

It can cause your heat production to jump four to five times above your resting normal.

Then there is chemical thermogenesis, which might actually be my favorite mechanism in the whole chapter.

Your sympathetic nerves release norepinephrine, which targets brown fat.

Right.

Inside the cells of brown fat, there is a specialized protein called UCP -1, or thermogenin.

This protein deliberately uncouples oxidative phosphorylation.

To explain what that actually means,

normally your cells burn fuel to create ATP, the energy currency of the body, which allows muscles to do work, but UCP -1 short circuits that whole process.

It's just like putting a car in neutral and slamming your foot on the gas pedal.

You're burning massive amounts of fuel.

The engine block is getting incredibly hot, but the car isn't actually moving forward.

That's a great way to put it.

No work is being done.

The body literally wastes energy for the sole purpose of generating raw heat.

It's a huge factor in infants increasing their heat production by 100%, but it only provides about a 10 -15 % bump in adults since we have much less brown fat.

And if you are exposed to a freezing environment for weeks at a time, the hypothalamus issues a long -term order.

It releases a hormone called TRH, which prompts your pituitary gland to release TSH, which finally tells your thyroid gland to release more thyroxine.

And that takes a while to adapt.

Yeah, over time this physically enlarges the thyroid gland to chronically ramp up cellular metabolism across your entire body.

This physical adaptation has actually been documented in Arctic military personnel and the Inuit people.

Amazing.

All of these incredible effectors, sweating, shivering, uncoupling fat cells, they're all working endlessly to defend that magic set point 37 .1 degrees Celsius, and the text points out that the feedback gain for the system is 27.

Yes, the feedback gain is a measure of how fiercely a biological system corrects an error.

For context, the feedback gain for the system controlling your blood pressure is less than two.

So to put that simply, if some extreme environment tries to push your internal body temperature off balance by 28 degrees,

a feedback gain of 27 means your body's regulatory system will aggressively fight back and correct 27 of those degrees, leaving you with only a tiny one degree error.

Exactly.

The temperature control system is mathematically one of the most powerful automated systems in human biology.

It is remarkably powerful.

But what's truly elegant is that the 37 .1 degree set point isn't entirely fixed.

It can anticipate trouble.

Oh right, the skin temperature.

Yes.

Researchers have found that skin temperature can actually alter the brain's set point.

If your skin is very cold, it shifts the hypothalamic set point to a slightly higher temperature.

This means your body will start shivering and producing heat at a higher internal temperature than it normally would.

Here's where it gets really interesting.

It behaves exactly like a smart home thermostat.

Okay, how so?

Well, if you have a high tech thermometer on the outside of your house that detects a blizzard rolling in, it talks to the main system and tells the furnace to kick on before the living room ever actually gets cold.

Your freezing skin anticipates the drop in your core temperature and preemptively turns on the internal furnace.

That's a perfect way to visualize it.

But there is one final override to all these autonomic loops sweating and shivering.

It's behavioral control, the sensation of psychic discomfort.

Meaning you just feel cold so your conscious brain decides to walk into another room or put on a sweater?

Yes.

And we know how vital this is because the text notes what happens if a person's spinal cord is severed in the neck.

Oh wow.

The hypothalamus is still working, but it loses its physical nerve connection to the sweat glands and the blood vessels in the trunk and limbs.

For those individuals, their autonomic regulation is gone.

Conscious, behavioral adjustments become the major critical defense they have against temperature changes.

We've built this incredibly complex, perfectly integrated physiological system.

Anatomy supports the function, the brain regulates it all, but Guyton Hall, don't stop there.

No, they don't.

We have to look at system failures.

What happens when this integrated system is hijacked by disease or entirely overwhelmed by extreme environments?

Let's start with a fever.

Fever is a literal hijacking of the set point.

It starts with pyrogens like endotoxins released by invading bacteria.

Your immune system spots these pyrogens.

White blood cells like macrophages consume them.

And then what?

In response, these immune cells release signaling molecules called cytokines, primarily one called interleukin -1.

An interleukin -1 acts as a chemical messenger.

It travels up to the hypothalamus in the brain and induces the formation of prostaglandin E2.

This prostaglandin acts directly on the thermostat neurons, and suddenly it cranks the set point way up.

Which, by the way, is exactly why aspirin works as a fever reducer.

It physically blocks the formation of prostaglandins.

So without prostaglandins, the set point stays normal.

But let's say you don't take aspirin.

This mechanism explains a very specific, miserable clinical phenomenon.

The chills versus the flush.

Oh, everyone has felt this.

Picture this.

You're lying in bed.

A bacterial pyrogen has caused your brain to suddenly crank your set point up to 103 degrees Fahrenheit.

But the actual physical tissues of your body are still sitting at a normal 98 .6.

So your brain perceives that you are dangerously cold.

Yes.

A fever isn't your body's heating system breaking.

It's a bacterial hacker breaking into your brain and cranking the thermostat up to 103.

Because your blood is at 98 .6, but the brain demands a 103, you feel freezing.

And the chills.

Exactly.

You get the chills.

You pull three heavy blankets over yourself, you shiver violently, and your blood vessels vasoconstrict to drive your actual temperature up to reach that new high set point.

And the absolute reverse happens when the fever breaks.

The bacteria are defeated, the pyrogen is cleared, and the brain's set point suddenly drops back down to normal 98 .6.

But now you're too hot.

Right.

Because of all that shivering under the blankets, your physical body is sweltering at 103 degrees.

The brain realizes you were way too hot, which results in the crisis or the flush.

You throw off the blankets, break into an intense sweat, and experience full body vasodilation to dump the excess heat.

Okay, so that's internal hijacking.

What about external environmental overwhelm?

Let's talk about heat stroke.

Heat stroke is incredibly dangerous.

It happens when the core gets pushed above 105 to 108 degrees Fahrenheit, especially in environments with high humidity where your sweat simply cannot evaporate.

The symptoms are severe dizziness, delirium, circulatory shock from massive fluid loss,

and most terrifyingly permanent neuronal damage.

The brain cells physically begin to break down, and once those neurons are destroyed, they do not regenerate.

The treatment for heat stroke is extremely aggressive cooling, usually plunging the person into a cold water or ice bath.

But if we connect this to the integrated physiology we've been discussing, there is a beautiful tragic irony here.

Right.

The body's defenses actually work against the cure.

Exactly.

If you plunge a heat stroke victim into an ice bath to save their brain,

their intact

hypothalamus senses the freezing cold water on the skin.

It thinks we are freezing to death and fights you.

Oh, man.

It triggers violent shivering, which produces massive amounts of internal heat.

The body literally tries to fight the very thing saving its life.

Yes.

That's why doctors often have to administer strong muscle relaxants to heat stroke patients.

They have to temporarily paralyze the body's thermogenic effectors, stop the shivering, so the cold water can actually bring the core temperature down without the muscles fighting back.

And on the complete other end of the spectrum, we have severe hypothermia and extreme cold.

Right.

If the core drops below 85 degrees Fahrenheit, the hypothalamic regulation essentially gives up and completely fails.

The read of chemical heat production becomes depressed and a profound sleepiness and coma set in.

And unfortunately, that coma actually prevents the body from shivering.

Which is when tissue damage starts.

Yeah.

If tissues freeze, ice crystals form inside the cells, destroying them and causing frostbite and gangrene.

But the body has one final desperate defense mechanism when it gets near freezing cold induced vasodilation.

Usually cold makes blood vessels clamp down, but near freezing, the smooth muscle in the blood vessels actually becomes paralyzed by the severe cold itself.

So they just open up.

When they paralyze, they relax and dilate, allowing a sudden flush of warm core blood to rush to the skin to try and prevent outright frostbite.

It's just wild how many fail safes are built in right up to the bitter end.

And modern medicine actually harnesses this failure point.

The text notes the use of artificial hypothermia for complex heart surgeries.

Oh, right.

The cooling protocol.

Yeah.

Doctors will administer strong sedatives to manually depress the hypothalamus, then pack the patient in ice to cool the body down below 90 degrees.

This drops the cellular metabolism so low that the body's cells can survive for over an hour with no blood flow while the surgeons operate on the heart.

It is a profound, lifesaving application of the exact physiology mapped out in this chapter.

And as we wrap up this exploration, I think there's a really fascinating final thought to take away.

Let's hear it.

We've spent this entire time detailing the immense microscopic complexity of continuous venous plexuses, TRPcation channels, uncoupled oxidative phosphorylation in brown fat, and high -gain hypothalamic feedback loops.

All these totally invisible, automated survival tools firing off inside you every second.

Yes.

But despite all of that elegant, deeply evolved biology happening under the hood, the absolute most powerful temperature control mechanism humans possess is the simple, conscious behavioral decision to put on a coat.

That really brings it all full circle.

That high -wire survival act happening inside you is brilliant, but sometimes it just needs you to step out of the hot sun or grab a thick blanket.

We hope this strictly translated tour of Chapter 74 helps you master the incredible mechanics of temperature regulation.

A warm, encouraging thank you for joining us today from the Last Minute Lecture Team.

Keep that balance steady.