Chapter 61: Environmental Physiology

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Have you ever really stopped to think about the environments we live in?

I mean, really think.

It's kind of mind -blowing when you do?

Yeah.

From, you know, the air in this room, hopefully nice and conditioned, to the crushing deep sea or way up high on a mountain or even space.

Our bodies are constantly dealing with it all.

Exactly.

And that's our deep dive today.

We're jumping into Chapter 61, Environmental Physiology, from Boron and Bullpapes Medical Physiology, the updated edition.

A classic.

It is.

And our mission, for you listening, whether you're studying for exams or already heading into clinical work,

is to sort of untangle these complex ideas.

Make them stick.

Right.

Give you those aha moments, make it clear, accurate, but, you know, without just drowning you in facts.

We'll build it up.

Big picture first, then the details.

And crucially, how it connects to real patients, real medicine.

Yep.

And no diagrams needed.

We'll paint the picture for you right here.

You know, what's really cool is that studying these extremes,

deep sea, high altitude,

it actually teaches us fundamental stuff about how we adapt, even in everyday life.

How we cope.

Yeah, our limits,

our resilience.

It's all about that constant push and pull, our body trying to keep its internal balance against, you know, pressure, temperature, gravity, all these outside forces.

Okay, so let's unpack that balancing act.

It really starts with Claude Bernard, right?

His idea of the milieu and terrier.

The internal environment.

Yeah, that constant internal state, the extracellular fluid that our cells absolutely need.

It's what allows for free, independent life, as he put it.

Such a foundational concept.

But then there's the outside world, the milieu, exterior.

And it's not just one boundary, is it?

It's like layers.

Oh yeah, definitely layers.

Starts right at your skin, then the air touching your skin, your clothes, the air outside them, the building you're in.

All the way out to the natural environment.

It's like nested shells.

It's like a physiological bubble wrap around us.

And the amazing part is how our bodies interact with all those layers.

We've got these incredibly sophisticated feedback controls.

Right, the automatic stuff and the conscious stuff.

Exactly.

There's the involuntary physiological feedback.

That's the subconscious adjustments, blood pressure, breathing, keeping your temperature steady.

You don't think about it, it just happens.

But then there's voluntary feedback.

That's when you feel something consciously, like it's hot.

You decide to move into the shade.

Or turn on the fan.

Or turn on the fan.

Or if you're cold, you put on a jumper.

It's this whole negative feedback system.

Sensors detect a change, the brain figures out what to do, and then you act.

Or your body acts automatically.

Makes sense.

And think about the clinical side.

What happens when medication or disease or just getting older messes with these systems?

Yeah, that's crucial for anyone going into healthcare.

Yeah, absolutely.

I mean, if someone's unconscious, they can't make that voluntary choice to cool down.

So you, the caregiver, have to understand their physiological needs and anticipate them.

That's a really important point.

Okay, so these constant adjustments.

Let's zoom in on specifics.

Temperature first.

Good place to start.

A big one.

Hard core temp stays pretty tight, right?

Like 36 to 38 degrees Celsius.

Yep.

Remarkably stable.

And that neutral zone, where you're naked, comfy, 50 % humidity, it's around 26, 27 degrees C.

Makes you think twice about complaining about the often thermostat set to 26.

Huh, maybe.

But step outside that narrow band, and things change fast.

Go up to 28, 29 C, you feel warm, start sweating a bit.

Hit 30, 32 C, it's getting uncomfortable.

Then 35, 37 C, definitely hot, uncomfortable, and heat stroke becomes a real possibility.

And beyond that?

Well, 39, 43 C is really hot.

Your body might start failing to regulate.

Get up towards 46 C, and it's unbearable.

Heat stroke is pretty much imminent.

You're losing fluids rapidly.

Circulatory collapse.

It can be fatal.

Wow.

And the cold side is just as structured.

Oh yeah.

24, 25 C feels cool.

21, 32 C, slightly uncomfortable.

Down at 19, 20 C, you feel properly cold, blood vessels in your hands and feet constrict.

Get that chilly feeling, yeah.

Right.

And your muscles might even start to ache.

So beyond temperature, what about the air itself, making sure it's breathable and safe?

Critically important.

Room ventilation isn't just about smells or feeling stuffy.

It's about maintaining the right oxygen, PO2, and carbon dioxide, PCO2 levels.

Okay.

What are we aiming for?

Well, think about dry air at sea level.

PO2 is about 159 millimeters of mercury.

PCO2 is tiny, like 0 .2.

In a work environment, you want PO2 above 148, and PCO2 kept below about 3 .8.

Got it.

You mentioned submarines.

Yeah, interesting example.

Sometimes they keep the PO2 a bit lower there, helps reduce fire risk, but it's still carefully managed so the crew stays alert.

How do they measure if a room is well ventilated?

Couple ways.

One is the steady state method, looking at CO2 production versus removal.

Another is the exponential decay method.

You release a harmless test gas and see how quickly it clears out.

Like timing how fast the CO2 disappears.

Exactly.

Imagine a small room, maybe three by three meters.

If the CO2 level halves in, say, 10 minutes, that tells you the air exchange rate.

Pretty neat.

It is.

Now, carbon monoxide.

You mentioned insidious threats.

Ah, yes, CO.

It's so dangerous because it's invisible, odorless, and it basically hijacks your hemoglobin.

Doesn't it bind way stronger than oxygen?

Hugely stronger.

So it effectively suffocates you from the inside by preventing oxygen transport.

The scary part is that symptomless encroachment.

You don't realize it's happening.

Until it's too late.

Potentially, yes.

It can be lethal when it saturates about half your hemoglobin.

That can happen at a PCO of just 0 .13 millimeter Hg, or about 170 parts per million.

And it sticks around.

The half time in your body is roughly four hours.

Four hours.

So even after you're out of the environment.

It takes time to clear.

And the early symptoms are so vague, headache, nausea, maybe feeling drowsy, things you might easily ignore.

By the time you hit 25 % saturation, you could have impaired mental function, maybe lose consciousness.

Which is why CO detectors are just essential.

Absolutely non -negotiable.

And in workplaces, you have things like threshold limit values, TLVs for safe air levels, and biological exposure indices, BEIs, looking at the actual effect on workers' bodies.

Okay, shifting gears now.

Let's talk about a force we always feel but barely notice.

Gravity and acceleration.

G -force.

Right.

The force pulling us down right now.

We call it plus one G, just standing here.

But it's amazing how our bodies are built to resist it, isn't it?

Think about jumping.

When you land, you might hit 3G briefly.

Your spine has to handle that.

I read that a small part of a vertebra, like 10 square centimeters, can take maybe 200 kilos if your back's straight.

Something like that, yeah.

Maybe 5 .7 G worth of force.

But if your back is curved.

Much less.

Drops to about 3G.

Which is exactly why pilots ejecting need to keep their spine absolutely straight.

Any curve, and you risk a vertebral crush fracture.

Serious stuff.

And the flip side is also important, right?

What happens without that G force?

Oh, absolutely.

Bone demineralization is a big one.

Bones get weaker with age, sure.

But also with being immobilized.

Like bed rest.

Exactly.

And very relevantly, during spaceflight, there was this famous bed rest study.

Tell me about it.

Subjects stayed in bed for six or seven weeks.

The results were striking.

They lost significant calcium, about 14 grams.

Lost muscle mass, about 1 .7 kilos.

Their calf muscle strength dropped 21 percent.

Blood volume went down 6 percent.

Wow.

Just from lying down.

Just from removing that gravitational loading.

They even got faint when they were tilted upright afterwards.

Could they recover?

Yes.

It was reversible.

But regaining the muscle strength alone took about four weeks.

It really shows how vital that constant G force is.

Not just for astronauts, but for patients recovering from illness or surgery, too.

Okay, so gravity shapes us constantly.

Now, let's take a plunge.

Deep sea diving.

Pressure becomes the big player down there.

Huge player.

Barometric pressure PB.

And unlike air, water doesn't compress much, right?

So the pressure increase is steady.

Exactly.

It goes up by one atmosphere for every 10 meters you descend in seawater.

Nice and linear.

Are body tissues, being mostly water, handle that okay?

Generally, yes.

Fluids and solids are pretty incompressible.

Yeah.

But the air spaces, that's different.

Lungs, ears.

Right.

They follow the, boils a lot, pressure goes up, volume goes down.

Think about your ears.

If your Eustachian tube is blocked on descent.

Ouch.

Yeah, the pressure difference builds up.

Big time.

You get pain.

The middle ear might fill with blood or, worst case, the eardrum ruptures because it can't equalize.

And just holding your breath and diving down, that has its own issues.

Definitely.

Your lungs compress significantly.

Alveolar PCO2 can actually double, maybe hit 80 millimeter Hg.

Double?

Wow.

Yeah.

And initially, that high PCO2 can even drive CO2 back into your blood briefly before your metabolic CO2 starts building up.

But the real sneaky danger is often on the way up.

Shallow water blackout.

That's the one.

As you ascend, the pressure drops, the partial pressure of oxygen in your lungs falls rapidly, and your arterial PO2 can plummet, potentially causing you to lose consciousness just below the surface.

Very dangerous.

So technology steps in.

SUBA gear.

Right.

From early diving bells to modern SQA, it lets you breathe air that's compressed to match the water pressure around you.

Keeps your lungs normally inflated.

But breathing compressed air?

That brings new problems.

It certainly does.

First big one.

Nitrogen Narcosis.

Rapture of the Dia.

That's the poetic term.

It follows Henry's law.

Higher pressure means more gas dissolves in liquids.

So at depth, more nitrogen dissolves in your blood and tissues.

Especially fatty tissues like the brain.

Exactly.

And that dissolved N2 seems to interfere with nerve cell function.

Kind of like an anesthetic.

Reduces ion conductance.

Makes you feel, well, intoxicated.

The martini's law thing.

Yeah.

The rough rule of thumb is that every 15 meters or 50 feet of depth feels like drinking one extra martini.

Lethargy, drowsiness,

impaired judgment.

It creeps up on you.

Potentially fatal if you make a bad decision down there.

Okay.

So nitrogen's one issue.

What else?

Oxygen toxicity.

This is counterintuitive, maybe?

Because we need oxygen.

We do.

But too much under pressure is bad.

While your hemoglobin gets saturated pretty quickly, the amount of physically dissolved O2 in your blood keeps increasing linearly with pressure.

So at, say, five atmospheres.

Your arterial PO2 could be around 700 millimeter Hg.

Breathing that for too long can damage your lungs inflammation, pulmonary edema.

Fluid in the lungs?

Right.

And eventually the lung tissue can consolidate.

Yeah.

But the really acute danger is to the central nervous system.

Yes.

Breathe air at maybe 10 atmospheres, giving you a PO2 around 1500 millimeter Hg.

And you could have seizures and fall into a coma within minutes.

You might get warnings like muscle twitching or disorientation.

What's causing that damage?

Oxygen -free radicals.

Highly reactive molecules that damage cell membranes, enzymes.

Basically wreak havoc at the cellular level.

So diving deep for long periods like saturation diving for undersea work,

how do they manage these risks?

Nitrogen marcosis, oxygen toxicity?

The key is often switching the gas mixture.

They use helium instead of nitrogen.

Helium?

By helium.

Several good reasons.

One, it has way less narcotic effect than nitrogen, so less martini effect.

Two, it dissolves less in tissues overall.

Three, it's less dense, so it's easier to breathe at high pressure, less airway resistance.

Though side note, it conducts heat better, so diverse need warmer environment.

Right, keeps them from getting chilled.

And fourth, crucially, helium diffuses out of tissues faster than nitrogen during decompression.

That's a big safety advantage.

And they adjust the oxygen level too.

Absolutely.

To avoid oxygen toxicity, they reduce the percentage.

For instance, at 10 atmospheres, they might use a mix with only 2 % oxygen.

That gives you the same safe PO2 you'd get breathing normal air at sea level.

Clever.

But even with all that, there's still the risk on the way up, right?

Decompression sickness?

Yes.

Decompression sickness, DCS, or the bends.

It's the classic problem.

If you ascend too quickly after being saturated with a gas at depth.

It's like opening a soda bottle.

Exactly like that.

The dissolved gas, mostly nitrogen, if breathing air, or helium, if using that comes out of solution too fast and forms bubbles in your tissues and blood.

And that can happen to pilots too.

Or flying after diving.

Yep.

Rapid decrease in ambient pressure is the trigger.

The bubbles cause problems in a few ways.

They can form locally in tissues, causing pain.

They can become emboli, blocking blood vessels.

Especially if they get into the arterial side.

Yes.

Particularly dangerous if there's a patent form in OVIL.

A small hole between the heart's chambers.

Or you can get arterial gas embolization directly if expanding air in the lungs tears tissue and enters the circulator, potentially lodging in the brain.

Catastrophic.

How is DCS classified?

Is it always severe?

No.

There's a range.

Type I DCS is milder.

Maybe some skin itching or rash, short -lived pains, niggles.

Or that classic deep throbbing pain in muscles or joints, the actual bends.

Type II DCS is serious.

It involves the central nervous system dizziness, paralysis, sometimes from bubbles affecting the spinal cord's myelin or the lungs that chokes.

With burning pain, cough, difficulty breathing from emboli.

It can also cause circulatory shock.

And the third type.

That's the arterial gas embolization we mentioned.

Very severe.

Often with immediate neurological consequences.

So prevention is key following dive tables.

Absolutely critical.

Decompression schedules based on depth and time spent underwater.

And if DCS does happen.

Recompression chamber.

Immediately.

Get the person into a hyperbaric chamber.

Increase the pressure to force the gas bubbles back into solution.

Then decompress very, very slowly and carefully according to specific treatment protocols.

Right.

Let's completely switch gears from the crushing depths to the thin air high above altitude.

A whole different set of challenges.

Unlike water, air gets thin fast, right?

Exponentially.

That's right.

Barometric pressure roughly halves for every 5 ,500 meters or about 18 ,000 feet you go up.

So Everest Base Camp, around that height, pressure and oxygen are half of sea level.

Pretty much.

And at the summit, 848 meters.

Pressure's down to about a third.

The inspired PO2, once humidified in your airways, is incredibly low.

Maybe 44 millimeter Hg.

Hypoxia, lack of oxygen is the overwhelming issue.

Even on a plane, the cabin isn't sea level pressure, is it?

No, usually pressurized to the equivalent of about 1800 to 2400 meters.

6 to 8 ,000 feet.

Fine for most, but...

If you have lung issues, like COPD?

You might need supplemental oxygen, yeah.

Because even that moderate altitude reduces your arterial oxygen levels.

But our bodies have some built -in protection, don't they, with hemoglobin?

They do, thankfully.

Up to around 3 ,000 meters, about 10 ,000 feet, your arterial O2 content doesn't drop as dramatically as the pressure does.

Why's that?

It's because of the shape of the oxygen hemoglobin dissociation curve.

That top part is relatively flat.

Hemoglobin holds onto oxygen quite well, even when the PO2 starts to drop.

But go higher, and it gets tougher.

Much tougher.

The curve steepens, meaning small drops in PO2 cause bigger drops in saturation.

Plus, the diffusion gradient for oxygen getting from your lungs into your blood shrinks.

At very high altitudes, blood might pass through your lungs too quickly to even fully equilibrate with the alveolar air.

So the body has to compensate.

How does it do that acutely, like in the first few days?

First thing is, you breathe faster and deeper.

Ventilation increases immediately, stimulated by peripheral chemoreceptors sensing the low O2.

Makes sense.

Get more air in.

Right.

But the side effect is you blow off more CO2.

That causes respiratory alkalosis, which actually puts a break on those chemoreceptors, dampening the breathing response a bit.

A balancing act again.

Always.

You also get an increased heart rate, driven by the sympathetic nervous system, trying to deliver the limited oxygen more quickly.

Okay, that's the immediate reaction.

What about acclimatization over days or weeks?

Ventilation actually increases further, progressively.

How?

If the alkalosis was breaking it?

Well, the pH in your cerebrospinal fluid gradually decreases, counteracting the alkalosis' effect on the central chemoreceptors.

And your kidneys adjust too, excreting more bicarbonate, which helps normalize blood pH.

Plus, long -term hypoxia makes the peripheral chemoreceptors even more sensitive.

So you end up breathing much, much harder.

Dramatically harder.

Think about those Everest climbers at the summit.

Their alveolar PCO2 was down to 7 or 8 mmHg.

That means their ventilation was maybe five times higher than normal, just to get enough oxygen for basic resting metabolism.

Insane effort.

And there are longer -term changes too.

Adaptations.

Oh yes, profound ones.

Often driven by this protein called HIF1, hypoxia -inducible factor 1.

These changes make the whole process more efficient, less energetically costly.

Like making more red blood cells.

That's a big one.

Hematocrit increases slowly.

Your kidneys release EPO, erythropoietin, stimulating red blood cell production.

Hemoglobin might go up to over 18 GDL.

What else?

Your lungs get better at transferring oxygen.

Pulmonary diffusing capacity can increase two or three times, partly due to more blood volume in the lung capillaries.

More pipes for the oxygen to travel through?

Sort of, yeah.

And you actually grow new capillaries in your tissues, angiogenesis driven by factors like VEGF.

That happens pretty quickly, within days.

Better delivery network.

Exactly.

Yeah.

And finally, your mitochondria get better at using the oxygen they receive through enhanced expression of oxidative enzymes.

Amazing adaptations.

But things can still go wrong.

Altitude sickness.

Unfortunately, yes.

Even mild symptoms of hypoxia, drowsiness, headache, nausea, fuzzy thinking can hit some people as low as 2 ,100 meters, 7 ,000 feet.

And most feel it by 3 ,500 meters, about 11 ,500 feet.

Probably due to low oxygen to the brain and maybe some mild swelling.

And acute mountain sickness, AMS.

That's more serious.

Develops within about a day, usually above 3 ,000 meters.

Worse headache, fatigue, dizziness, shortness of breath, nausea, sometimes vomiting, maybe swollen hands or feet.

Last few days, usually.

What causes AMS?

It seems to be progressive edema, swelling, in both the brain and the lungs.

The lung part, high -altitude pulmonary edema, HAPE, is linked to hypoxic pulmonary veas constriction blood vessels in the lungs constricting due to low oxygen.

AMS, especially Haber high -altitude cerebral edema, HACE, can be fatal if you don't get down or get oxygen quickly.

And some people are just much more susceptible than others.

And chronic mountain sickness, that's different again.

Yeah, that develops after living at high altitude for a long time.

It's basically an over -the -top red blood cell production.

Humatocrit goes way up, over 60 % polycythemia.

Too much of a good thing.

Exactly.

The blood becomes incredibly thick, viscous, that increases vascular resistance, puts a huge strain on the right side of the heart, and can eventually lead to right heart failure.

Wow, okay.

From the highest peaks to leaving Earth entirely.

Spaceflight.

G -forces are back in the picture here, aren't they?

They are, especially during launch and reentry.

Astronauts in the shuttle might hit plus 3G or plus 4G.

Less than fighter pilots, though.

Oh yeah, military pilots doing high -speed maneuvers pull much higher Gs, sometimes for longer.

That's where you really see the effects.

Like blood being pulled away from the head.

Precisely.

G -forces shift blood volume, compress tissues, high positive Gs reduce blood flow to the brain.

First, you might get gray -out loss of peripheral vision, colors fading as the retina loses oxygen.

And then?

Then black -out total loss of consciousness.

That's why G -suits are so vital for fighter pilots.

They inflate around the legs and abdomen.

Squeeze the blood back upwards?

Essentially, yes.

Provide counter pressure to stop blood pooling in the lower body and keep enough blood flowing to the brain.

Okay, so G -forces are intense, but then in orbit, it's the opposite extreme, weightlessness or microgravity.

And technically, you're still feeling most of Earth's gravity up there.

Maybe 94 % at 200 kilometer altitude.

But you feel weightless because the spacecraft is constantly falling around the Earth.

Its orbital path balances out gravity.

So what does that lack of G -force do to the body acutely?

Well, our bodies are tuned for plus one G, fighting gravity's pull on our blood.

Take that away and blood immediately shifts upwards towards the head.

The cephalid shift.

Is that why astronauts look kind of puffy -faced?

Exactly.

Within 24 hours, you see that bloated look.

It's plasma water filtering out of the capillaries into the facial tissues because of the fluid shift.

How does the body react to that fluid shift?

Does it think there's too much blood volume?

It seems to perceive it that way.

Even if central venous pressure doesn't always measure a huge increase, the stretch on the heart's right atrium triggers release of ANP, Atrial Natriuretic Keptide.

Tells the kidneys to get rid of salt and water.

Yep, and stimulation of low pressure receptors inhibits ADH, antidiuretic hormone.

Same result, more salt and water excretion.

The body corrects this perceived overload.

Which is why astronauts actually tend to be a bit dehydrated.

That's right.

It's a reflex response to the fluid shift.

Okay, that's the initial reaction.

What about longer term in microgravity?

Well, motion sickness is common initially.

Over half get a conflicting signals between the eyes and the inner ear's vestibular system.

Usually clears up in a few days.

But the bigger physiological changes?

They're quite significant.

Reductions in total body water, plasma volume, red blood cell mass, muscle mass decreases.

And bone mass calcium and phosphate loss is a major issue.

Does the bone loss stop?

It seems to be continuous as long as you're weightless.

The other changes, like fluid volumes and muscle mass, happen mostly in the first few weeks.

And the consequence of all this?

Reduced maximal cardiac output, less muscle strength, weaker bones,

makes coming back to earth really challenging.

So countermeasures are crucial.

Exercise.

Absolutely essential.

Astronauts use bungee cords for resistance, cycle on ergometers.

But the best seems to be walking or running on a treadmill inside a special chamber.

Yeah, a negative pressure chamber around the lower body, often combined with positive pressure pants.

It creates a pressure difference that mimics the effects of gravity on the body.

Pulling fluids down, loading the bones and muscles more normally,

helps fight that deconditioning significantly.

So finally, the return trip.

Coming back to 1G.

That must be tough after weeks or months without it.

It really is.

The main issues are that lower blood volume we talked about, plus the leg bed vessels have lost some of their tone, their ability to constrict properly.

So when you stand up?

Blood pools in your legs, cardiac preload drops, and you can feel faint or actually pass out.

Or the static intolerance.

Exercise capacity is also reduced.

Explains why they often look a bit shaky right after landing.

Yes, they need time to readapt.

They're often kept out of the public eye initially.

Can they do anything just before coming back to help?

They try.

Intense exercise shortly before reentry can boost plasma volume a bit, maybe 10 % in 24 hours by increasing plasma albumin.

They also try to load up on salt and fluids.

Though that's hard if the body's still trying to excrete fluid.

Exactly, it's a challenge.

But the goal is to minimize those effects so they can regain normal standing colorants, usually within hours or maybe a day after returning.

The body's adaptability is pretty amazing, even in recovery.

It really is.

So we've gone from our comfy homes deep into the ocean, up to the highest mountains, out into space.

It's just incredible how tuned our physiology is to where we are.

And how adaptable we are.

Yeah, we hit the milior interior and exterior, feedback loops, the nitty gritty of pressure, temperature, G -force, those tricky gases like nitrogen and CO.

Nitrogen, narcosis, oxygen, toxicity, the bends.

Altitude sickness, space deconditioning, quite the tour.

It really puts our own bodies in perspective, doesn't it?

Understanding these extremes, it genuinely helps you appreciate the resilience we have, that constant dance between inside and outside.

And for you listening, knowing this stuff isn't just for exams, it helps you think like a clinician, anticipating how different environments might affect a patient.

Absolutely.

It makes you a better physiologist and a more informed future doctor or healthcare professional.

Remember, you just navigated a really dense chapter and hopefully it felt more like a conversation.

You're building a fantastic knowledge base, you're part of the Deep Dive family, and you've totally got this.

Keep digging into it.

Okay, final provocative thought for you to chew on.

Think about your everyday life.

What seemingly small things may be getting dehydrated on a hot day or standing up too quickly might be triggering some of these same extreme physiological responses just in a much subtler scale.

Something to ponder.

Good one.

That's it for this Deep Dive.

Until next time, keep exploring, keep asking questions, and stay well -informed.