Chapter 26: Oxygen, Carbon Dioxide, and Internal Transport: Diving by Marine Mammals

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Alright, let's unpack this.

Today we're diving deep into one of nature's, well, most incredible mysteries.

How do these massive, warm -blooded, air -breathing predators like seals and whales spend so much of their lives hundreds, even thousands of feet underwater

without oxygen?

I mean, you take the Weddell Seal, it's nearly a thousand pounds living in the brutal Antarctic cold, a place where, frankly, it feels like nothing should survive, let alone, you know, flourish.

Absolutely.

And yet they do.

Populations near a million.

It's a huge ecological success story.

Right.

Homeothermic, warm -blooded predators hunting in these icy depths.

But the question, the real head -scratchers, how do they do it without air?

And that's our mission today.

We're going to try and crack open the physiological playbook of these marine marvels.

We'll explore the mind -blowing adaptations, allowing them to manage oxygen, withstand crushing pressure, fine -tune their metabolism, and we'll even peek into how scientists have, you know, wrestled with these questions over time.

Yeah, it's a real deep dive into animal physiology.

Looking at those comparative strategies, what they mean for survival.

And the clever, sometimes really basic, experimental methods that revealed all these secrets.

Okay, let's unpack this then, starting with just how incredible these animals really are.

Humans have, well, we've always been fascinated by the ocean depths, haven't we?

Yeah.

And the creatures down there, you think about classic tales like Moby Dick, right?

Yeah.

Descriptions of sperm whales sounding.

Yeah, just disappearing straight down into the abyss.

For centuries, these were just observations, these grand mysteries.

Modern tech.

Wow.

It's really shown us that fact is sometimes stranger than fiction.

And what's fascinating here is how we even started to understand this stuff.

I mean, back in the early days, that scientist, Kuhnman.

Gerald Kuhnman, yeah, he did some pioneering work.

He literally capitalized on the behavior of Waddell seals.

Oh, so.

Well, they tend to return to the same breathing holes in the Antarctic ice.

So he developed these primitive, but really innovative data recorders.

And attach them to the seals in the Antarctic.

That's not challenging.

Oh, absolutely ingenious.

And the results were just fantastic revelations.

We learned Waddell seals were voluntarily staying submerged for over an hour, sometimes up to 80 minutes.

80 minutes voluntarily.

Voluntarily and diving to nearly 600 meters.

That's almost half a mile deep.

Wow.

Think about that.

A warm -blooded, air -breathing mammal over an hour voluntarily, nearly half a mile deep.

It didn't just impress scientists.

It fundamentally challenged what they thought was physiologically possible for a big animal.

Right.

But you mentioned that's not their typical dive.

Exactly.

It's crucial to note that while they can do these extreme dives, the vast majority, maybe 90 percent, are much shorter, usually 20 to 25 minutes, and shallower too, maybe 200 meters or less.

OK.

But the records just keep getting crazy, right?

Like northern elephant seals.

Oh, yeah.

Those guys are incredible.

They migrate across the entire Pacific and they've been diving up to 1 ,600 meters.

1 ,600 meters?

And one individual was tracked going down to an astonishing 2 ,050 meters.

That's what?

1 .33 miles straight down.

Straight down.

And then you have the real titans, the sperm whales and beaked whales regularly hitting 1 ,900 to 2 ,000 meters.

It really puts our own abilities in perspective.

You think about human breath -hold divers.

Like the Amma divers of Korea and Japan.

Yeah.

Legendary.

But they typically manage, what, 60 to 80 seconds at 15 to 25 meters.

And even the elite competitive free divers, pushing the limits under super controlled, dangerous conditions.

Maybe 200 meters, four minutes plus breath -hold, but compared to an hour or over a mile deep.

It's really stark contrast.

An evolutionary masterpiece, really.

Yeah.

OK.

So here's where it gets really interesting for me.

Because finding all this out forced scientists to rethink how they were even studying these animals, right?

That's exactly right.

For a long time, diving physiology research was heavily based on forced diving studies in the lab.

So basically holding an animal underwater.

Pretty much.

And observing its physiological responses.

This led to this idea of a stereotype sort of all or nothing diving reflex.

But that wasn't the whole picture.

Not at all.

When we started getting data from animals freely diving in the wild voluntary diving, we call it, it became clear the responses weren't always so rigid.

They were more flexible, more graded.

Which means the method really matters.

Profoundly.

It forced this crucial realization.

How you conduct the experiment dramatically affects the results.

You have to

Was the dive forced or voluntary?

Short or long?

Was the animal resting or active?

The answers completely change what you observe and, well, what conclusions you can actually draw.

OK, so they're doing these incredible dives without breathing.

My first question, and I bet a lot of listeners are wondering this too, where are they storing the oxygen?

It can't just be the lungs, right?

What else is going on?

Fantastic question because this is where you see the first layer of their physiological genius.

Marine mammals have three main internal O2 stores.

Blood hemoglobin, muscle myoglobin, and yes, the air in their lungs.

And I'm guessing these aren't just regular stores.

Not even close.

They're absolutely supercharged compared to us.

Take their blood.

Some species like Waddell seals, sperm whales, have blood with dramatically more oxygen carrying capacity.

How much more?

Think nearly double the concentration compared to human blood.

Plus they just have far more blood overall.

More volume?

Yeah.

Sometimes two to four times the volume per kilogram of body weight compared to humans.

Waddell seals, elephant seals, sperm whales are way up there.

So it's like having a fuel tank that's not only bigger, but holds super concentrated fuel.

Exactly.

It gives them something like four to six times the total blood oxygen storage compared to us.

It's massive.

OK, blood makes sense.

Then you mentioned myoglobin.

Ah, myoglobin.

This is truly fascinating.

Myoglobin is the protein in muscles that binds oxygen, right?

In diving mammals, the concentrations are just astronomically high.

Their muscles are so incredibly dense with it, they literally look almost black.

Black, wow.

So how much more oxygen can they store there?

We're talking 10 to 15 times more oxygen stored directly in the muscle tissue compared to what you'd find in human muscle.

So it's like a private, dedicated oxygen tank just for the muscles.

Precisely.

Ready to fuel them directly during the dive.

OK, huge blood stores, super dense muscle stores.

What if the lungs you'd think that just take a massive gulp of air before diving?

You'd think so, wouldn't you?

But it's a bit more complex.

While the lungs are an oxygen store for the really deep divers, a large lung full of air actually causes problems at depth.

Problems like what?

Well, first, buoyancy.

Too much air makes it harder to dive down.

They have to fight to sink.

Second, a lot of that oxygen can get sequestered, basically trapped in the non -respiratory parts of the airways as the lungs compress under pressure.

So it's not even available.

OK.

And critically, there's the risk of nitrogen absorption at high pressure.

That leads to decompression sickness, the bends, which we definitely need to talk more about.

Right.

So because of all that, many deep diving seals actually exhale before they make a big dive to minimize the air in their lungs.

They exhale.

That seems counterintuitive.

It does, but it avoids those bigger problems.

OK, so summing up the stores,

massive oxygen in blood, massive oxygen in muscles far beyond us.

Yeah.

But here's the kicker, you said.

Even with all that?

Even with all that, it's still utterly inadequate to sustain their resting metabolism for their maximum dive duration.

How inadequate?

Well, take that Weddell Seal again.

Its total calculated oxygen store, if it was just resting underwater, should only last about 17 to 20 minutes.

But we know they dive for 60, even 80 minutes, and they're hunting, not just resting.

Exactly.

So what does this all mean?

If their fuel tank, even supercharged, isn't quite big enough for the whole trip,

how do they stretch that limited supply?

How do they survive?

Yeah, how do they stretch it?

This is where their incredible circulatory adjustments come in.

They become the real unsung heroes of the deep.

One of the very first discoveries made by Paul Burt and Ducks way back in 1870 was something called diving bradycardia.

Bradycardia, slow heart rate.

A profound slowing of the heart rate, yeah.

But that wasn't the whole story.

Irving and Schollander, brilliant minds, took the crucial next step.

What did they find?

They observed that during a dive, if you, say, stimulated a seal's paw to make it bleed slightly, the blood flow was severely reduced.

It clued them into peripheral vasoconstriction.

Peripheral vasoconstriction, meaning blood vessels clamping down.

Exactly.

It's not just a fancy term.

Blood vessels at the body's periphery, like in the limbs, the skin, even some organs like the gut and kidneys, severely constrict.

They basically shut off blood flow to those areas.

Wow.

So where does the blood go?

It gets rerouted.

Bronze x -ray studies on seals showed this vividly.

Blood flow is dramatically diverted away from those regions.

Their bodies essentially become this highly efficient heart, lung, brain machine.

Blood is preferentially sent only to the most critical oxygen -dependent organs, the brain and the heart.

Everything else is put on metabolic hold, more or less.

So the slow heart rate, the bradycardia, and this vasoconstriction, they work together.

Absolutely.

It's an integrated response.

Think of it like a sophisticated plumbing system.

Your heart's the main pump, maybe six hoses running from it.

Okay.

Instead of just slowing the pump down equally for all hoses, they effectively turn off the faucets to five of those hoses.

All the pressure and flow get directed to the one or two critical ones, brain and heart.

That's a genius way to match the heart's output to the much smaller active circulatory system.

Precisely.

And crucially, this isn't just an on -off switch, especially when they dive voluntarily.

Back to the voluntary versus forced dive distinction.

Right.

Forced dives often show that stereotyped rigid response.

But in voluntary dives, you see a much more graded response.

Heart rate might slow down a lot or just a little, depending on what the animal's doing.

Kidney function might be reduced but not totally shut off.

So give them flexibility.

Incredible flexibility.

A whole spectrum of physiological strategies adapting to whether they're just cruising calmly or in a high -speed chase after a fish.

And there's another trick too, right?

Something about the spleen.

Ah, yes.

Another

store a huge reserve of extra red blood cells packed away in their spleen.

Like a blood bank.

Kind of.

When they start a dive, the spleen contracts and squeezes these extra red cells out into the active circulation.

Giving them an oxygen boost.

An extra boost of oxygen carrying capacity, exactly.

But it also helps them manage blood viscosity, how thick the blood is during these big changes in flow.

Pretty neat.

It really is.

It connects back to evolution, doesn't it?

These aren't totally new tricks.

Not fundamentally new, no.

These responses, the bradycardia, the vasoconstriction, they're actually specializations of really ancient responses to asphyxic conditions, low oxygen situation.

You see similar, though much less perfected responses in fish, even in human fetuses.

Marine mammals have just taken these basic survival mechanisms and honed them, perfected them to an absolutely astonishing degree over millions of years.

Okay, so they store oxygen brilliantly.

They ration it incredibly carefully through circulatory changes.

But the big question I still have is about metabolism during those super long dives.

Are they just pushing into anaerobic limits, running on fumes?

Or is something more sophisticated going on?

Yeah, it's a critical distinction.

During those really protracted, super long dives, their body essentially performs a metabolic subdivision.

Metabolic subdivision.

The tissues that absolutely need oxygen, like the brain and the heart, they stay aerobic.

They use that carefully conserved, redirected oxygen supply.

Okay, the essentials stay online.

Right.

But other areas, less critical perhaps for that moment, like the hardworking skeletal muscles propelling them through the water, they can actually switch and go anaerobic.

Anaerobic, meaning without oxygen, like sprinting.

Exactly.

They switch to a kind of emergency power system that doesn't need oxygen immediately.

But like sprinting, it comes at a cost.

And that cost is lactic acid, right?

Yeah.

That burning feeling in your muscles.

Precisely.

Lactic acid starts to accumulate in those tissues that have gone anaerobic.

Now the clever part is that this lactic acid often stays sequestered, basically trapped, within those muscles during the dive.

So it doesn't immediately flood the system?

Not immediately.

But the moment the animal surfaces and starts breathing again and circulation opens back up, that lactic acid floods into the general bloodstream.

And that requires recovery time.

A significant recovery time, yeah.

It has to be metabolized, cleared out.

That Waddell seal example again.

After a 43 -minute dive, which is well beyond its aerobic capacity,

it might need 70 minutes resting at the surface to recover before it can dive effectively again.

70 minutes recovery for a 43 -minute dive.

That's inefficient.

Very inefficient if you need to hunt.

And this observation led Gerald Koeman to develop the concept of the aerobic dive limit, or ADL.

The ADL.

Okay, what's that exactly?

It's essentially the maximum amount of time a particular species can stay underwater using primarily aerobic metabolism before it starts accumulating a significant net amount of lactic acid before it incurs that oxygen debt.

And for the Waddell seal, that's?

Around 20 to 25 minutes.

So dives shorter than that, they typically surface with little to no net lactic acid buildup.

And this ADL varies hugely between species depending on their size, oxygen stores, metabolic rate, and so on.

And the adaptive significance, the benefit of staying within that ADL, must be huge.

Absolutely massive.

It's all about minimizing that surface recovery time.

Right.

If you keep your dives aerobic within the ADL, you pop up, maybe spend just one to four minutes recovering, getting your breath back.

And then you can dive again, compared to tens of minutes or even over an hour recovering from a deep anaerobic dive.

Which means you maximize your time underwater where the food is.

Exactly.

For a Waddell seal sticking to aerobic dives, it might spend 80 % of its total time underwater foraging.

If it constantly pushes past the ADL, that might drop to only 40 % because of all the surface recovery needed.

And it probably helps maintain overall body balance homeostasis too.

Definitely.

And the fascinating thing is this hypothesis is really strongly supported by field observations.

The vast majority, we're talking 90 to 95 % of all the voluntary dives recorded for various species are actually shorter than that species calculated ADL.

So they know their limits.

Their behavior is finely tuned to their physiology.

It seems so.

And beyond just managing oxygen stores and circulation, they actively work to reduce metabolic costs while diving too.

Several ways.

They can allow peripheral tissues to cool down, which saves energy and inhibits shivering.

Some, like gray seals, seem to delay food processing.

They postpone digestion until after a dive bout.

Putting digestion on hold.

Clever.

And they use incredibly energy -sparing ways of moving.

Lots of gliding or using a stroke and glide swimming style like Waddell seals do, making every single movement, every bit of storage, rationing, metabolism,

incredible stuff.

But there's still that other hidden danger lurking at those crushing depths.

The one human divers fear most.

The bends.

Decompression sickness.

How on earth do marine mammals avoid that?

Ah, the bends.

Yeah, it's a question that puzzled scientists for a very, very long time.

For human divers using compressed air, nitrogen gas gets forced into the blood and tissues under pressure.

And if you come up too fast?

The pressure drops, the dissolved nitrogen comes out of solution and forms bubbles in the blood and tissues.

It can be agonizing, cause tissue damage, neurological problems, even death.

And marine mammals seem immune, mostly.

Mostly, yeah.

Although, interestingly,

repeated deep breath -hold dives in humans can also lead to decompression sickness symptoms.

So it's a potential risk for any air -breathing creature diving deep and returning.

So what was the classic explanation for how whales and seals avoided it?

For about 70 years, the leading hypothesis was alveolar collapse.

The idea was that under the immense pressure at depth, the alveoli, those tiny air sacs in the lungs where gas exchange happens, would simply collapse.

This collapse would push the remaining lung air, including the nitrogen, into the upper airways, the bronchi and trachea, where gas exchange doesn't happen.

So the nitrogen would be sequestered, trapped away from the bloodstream, preventing it from dissolving in large amounts and causing bubbles on ascent.

Seemed like a neat solution.

Air sacs collapse, nitrogen gets parked safely out of the way.

It did seem neat, but here's where we run into a fascinating, kind of unresolved phenomenon in recent years.

Uh oh, science getting complicated again.

Always.

Recent evidence, especially from things like ultrasound studies on diving animals and some tragic incidents involving beaked whales.

Well, it suggests that alveolar collapse might not be as complete, or happen as early, or maybe as consistently as we thought.

Meaning, gas exchange might still be happening at greater depths than the classic theory predicted.

And in fact, bubbles have sometimes been observed in the tissues of deep diving marine mammals that have died accidentally.

Bubbles.

The bends.

Possibly, or at least nitrogen bubbles.

It raises a really important question.

And there's also this unsettling link that's emerged between deep diving whales, particularly beaked whales, exhibiting symptoms consistent with decompression sickness after exposure to mid -frequency military sonar.

A sonar causing the bends in whales.

How?

The idea is it might startle them, cause disorientation, or somehow disrupt their normal, carefully controlled ascent behaviors, perhaps forcing them up too quickly, or altering their physiology in a way that allows bubbles to form.

It's still being heavily researched, but what's truly going on with

So the simple lungs collapse story might not be the whole story.

It seems not.

What's fascinating here is that the lungs, which we often think of as just simple scuba tanks for holding air, they might actually be performing a far more complex role, especially concerning both nitrogen and oxygen at depth and during ascent.

This raises an important question.

Which is?

Well, the new thinking, or at least a developing hypothesis, is that maybe that partial or

collapse isn't just about preventing nitrogen problems.

Maybe it's also about actively retaining just enough oxygen within the lungs during that incredibly rapid ascent from crushing depths.

Retaining oxygen?

Why?

Think about it.

As the animal sheets towards the surface, the pressure drops dramatically and the air in its lungs expands massively, very quickly.

If there wasn't enough oxygen left in that expanding air, the partial pressure of oxygen could plummet, potentially leading to hypoxia, blackout, right near the surface.

Ah, so they need to manage the oxygen level in the expanding lung air on the way up, too.

Potentially, yes.

It might be a delicate balancing act, managing nitrogen risk while ensuring enough oxygen remains available in the lungs to safely complete the final, rapid phase of ascent.

It's about seeing these incredible physiological challenges from the animal's unique point of view, understanding how all these adaptations have to work together seamlessly for survival in one of the most extreme environments on Earth.

Wow.

That puts a whole new perspective on it.

Not just avoiding the bends, but managing the ascent itself.

Exactly.

It highlights how much we're still learning.

We truly hope this deep dive into the remarkable physiology of diving marine mammals has given you some incredible aha moments today, and maybe a shortcut to being well informed about one of nature's truly astounding feats.

We really encourage you to keep exploring the world of animal physiology.

The adaptations out there allowing life to thrive in the most extreme conditions are just, well, they're endlessly fascinating.

Thank you so much for being part of the Deep Dive family.