Chapter 23: External Respiration: The Physiology of Breathing

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Have you ever just stopped and, you know, really thought about breathing?

Just that simple in and out.

It feels so automatic, doesn't it?

Exactly.

But for literally every animal out there, getting oxygen in and CO2 out is this incredibly complex, vital process.

I mean, without it, our cells just stop.

No energy.

No life, basically.

Okay, so let's unpack this.

Today we're doing a deep dive into external respiration in animals.

We're drawing from Chapter 23 of Animal Physiology by Hill, Weisen, Anderson, a classic text.

Right.

And our goal here, really, is to get under the hood.

Understand the physiology, the mechanics,

the incredibly diverse ways animals pull this off, how they get O2 ditched CO2, and why these adaptations are so crucial for how they live.

Yeah, we'll be looking at different strategies, why they evolve that way, and you're going to hear about some truly mind -bending adaptations.

Seriously amazing stuff.

Absolutely.

From fish that are just hyper efficient to insects breeding with bubbles, it's quite the story.

So let's start right at the beginning.

External respiration.

What exactly is happening at that boundary between the animal and the world?

Well, think of it as the crucial exchange point.

It's all about moving oxygen towards a specialized, really thin tissue layer, that's the gas exchange membrane, and then moving carbon dioxide away from it.

It's the interface.

And that movement, that exchange across the membrane.

Yeah.

How does that work?

Ah, it's always diffusion.

Always.

Which means oxygen only moves in if the partial pressure outside is higher than inside.

Simple physics, but it dictates everything.

Okay, so diffusion.

That means the membrane itself must be really important, like its size and thickness.

Exactly.

The rate of diffusion depends massively on the surface area.

Bigger is faster, and the thickness thinner is faster.

So the more oxygen an animal needs, the more extreme these features tend to become.

And this is where tunas come in, right?

They're like the ultimate example.

Oh, absolutely.

Tunas are phenomenal.

Think of them as the ocean's marathon runners.

Constantly swimming, migrations over huge distances.

Like a hundred kilometers a day, you mentioned?

That's incredible.

It is.

And to fuel that, their breathing system is just off the charts.

Their gills, for instance, have something like eight times the surface area of a similar sized rainbow trout's gills.

Eight times.

How big is that, practically?

Get this.

Take a one kilogram tuna.

If you could flatten out its gill membranes, you'd have a square over a meter on each side.

Huge.

And it's not just area.

The membranes are incredibly thin.

We're talking 0 .6 micrometers.

Compare that to a trout's, which is maybe five micrometers.

So much, much thinner.

That speeds up diffusion dramatically, I guess.

Precisely.

It's a key adaptation for their high oxygen demand, faster uptake.

And this connects to how they swim, doesn't it?

Ram ventilation.

Yes, exactly.

Most fish actively pump water over their gills using mouth and gill cover muscles that buckle with percular pumping.

But tunas, they just swim with their mouths open.

Letting the water ram through.

That's it.

And the critical thing is they have to do this.

They're obligate ram ventilators.

If they stop swimming, they can't breathe, they suffocate.

So their swimming muscles are literally powering their breathing.

What a trade off.

It's a fascinating link, and it leads nicely into the basic types of breathing organs we see.

Generally, you have gills or lungs.

Right.

Gills sticking out, lungs tucked inside.

Pretty much.

Gills are evaginated.

They project into the environment.

We use the term branchile for them.

Lungs are invaginated, folded inwards, containing a bit of the environment.

And that's pulmonary.

And that structure makes sense for where they're used, right?

Lungs inside give support on land.

Exactly.

While gills, being delicate, get support from the water's buoyancy, perfect for aquatic life.

OK.

So organs.

What about how animals actually move the air or water?

Ventilation.

Good point.

It can be active.

The animal uses energy, usually muscles, to create flow.

Or passive relying on environmental currents.

Active gives control, but costs energy.

And the flow itself can differ, too.

Yes.

Three main patterns.

Unidirectional flows one way across the membrane.

Think most fish gills or even bird lungs, surprisingly.

Then tidal flow.

In and out through the same tubes.

That's us mammals.

And finally, non -directional flow.

You see this sometimes with external gills just waving around in the water.

Less common for internal systems.

Interesting.

But it's not always one or the other, is it?

Some animals can use both air and water.

Ah yes.

Dual breathers or bimodal breathers.

Air breathing fish, amphibians.

They often have structures for both and switch depending on, say, if the water's low in oxygen.

Remarkable flexibility.

It really is.

Now, you mentioned efficiency before with tunas.

How do we measure that?

We use something called the oxygen utilization coefficient.

It's basically the percentage of O2 removed from the inhaled air or water.

So like how much oxygen they actually grab.

Exactly.

A rainbow trout might grab, say, 33 % from the water.

Tunas, being tunas, can pull out 50%, even 60%.

Highly efficient.

And you said that efficiency isn't just about flow rate, but how the blood flows relative to the water or air.

Yes.

This is absolutely key.

The spatial relationship, the direction of flow makes a huge difference.

This is where we get into different exchange types.

Okay, lay it out for us.

All right.

First, tidal exchange, like our lungs.

Fresh air mixes with stale air that's already there.

So the O2 level inside is always lower than outside air.

Which means the blood leaving can't get fully saturated at the outside level.

Precisely.

The blood O2 pressure ends up lower than even the air we exhale.

It works, obviously, but there's a limit.

Then there's co -current exchange, medium air or water and blood flow in the same direction.

Slide by side.

As they flow together, they exchange oxygen, but they gradually approach an equilibrium, a midpoint.

The blood can never get more R2 than that equilibrium point allows.

So less efficient.

Right.

That makes sense.

But then, the star player, counter -current exchange, like in fish gills.

Medium and blood flow in opposite directions.

Okay.

Head -on collision, basically.

Metaphorically, yes.

And what this does is maintain an O2 partial pressure difference along the entire length of the exchange surface.

The blood is always meeting fresher water with more O2.

So it just keeps grabbing oxygen the whole way.

Exactly.

It allows the blood O2 pressure to get really, really close to the level in the water coming in.

It's intrinsically the most efficient design for grabbing oxygen.

Wow.

Okay, counter -current is top tier.

Any others?

One more main type.

Cross -current exchange.

This is what birds use.

The blood flow sort of splits up and crosses the path of the airflow multiple times.

So not directly opposite, not the same way, but across.

Kind of like a grid.

Its efficiency is intermediate, better than tidal or co -current, but not quite as good as pure counter -current.

So ranking them.

Counter -current wins, then cross -current, then tidal and co -current are sort of less efficient.

That's the hierarchy, generally speaking, for O2 uptake.

It's a beautiful example of evolutionary optimization.

Okay, fascinating.

We focus a lot on oxygen because it's often the challenge.

What about getting rid of CO2?

Is that different?

Hugely different, especially between water and air -breathers.

Water -breathers, like fish, have much lower CO2 levels in their blood compared to air -breathers like us.

Really?

Why is that?

It's mostly about the physics of the medium.

Water is actually really good at holding CO2, much better relatively than it holds O2.

And CO2 diffuses rapidly.

So for a fish, shutting CO2 into the water across the gills is, well, easy.

So easy it doesn't really drive their breathing rate.

Exactly.

For most fish, low O2 is the big stimulus to breathe more.

High CO2, much less of a trigger.

For us mammals, it's the opposite CO2 buildup is the primary driver telling us to breathe.

That's a fundamental difference.

How do animals even know if oxygen is low?

How do they sense it?

They have systems operating at different levels.

At the whole body level, many monitor the oxygen partial pressure in their blood.

If it drops hypoxia, they trigger responses.

Like breathing faster or deeper?

Yep.

Fish increase water flow over gills.

We increase lung ventilation.

It's a rapid adjustment.

And inside the cells, is there a response there too?

Oh, absolutely.

When individual cells experience hypoxia, a molecular pathway kicks in centered around proteins called hypoxia -inducible factors, HIV.

HIV -1.

It's HIV -1 and HIV -2.

HIV -1, what do they do?

They're ancient transcription factors.

When oxygen is low, they become more active and turn on a whole suite of genes, genes for making more red blood cells, for boosting anaerobic energy production, making mitochondria use R2 more efficiently.

And even growing new blood vessels.

Angiogenesis.

Bringing the oxygen supply closer to the cells by shortening diffusion distances.

It's a multi -pronged defense system.

That's incredible.

Can we see this in specific animals?

Definitely.

Think about epaulette sharks.

They often hang out in tide pools where oxygen can get really low.

Studies show they actually ramp up their HIV system activity to cope.

It's a fantastic example of acclimation.

Acclimation, right.

Adjusting to the environment.

And this whole HIV system isn't just for sharks, it's crucial for us too.

People living at high altitudes, for example, rely on HIV pathways to adapt to the thin air, managing red blood cell production and other things.

It's a deeply conserved, fundamental survival toolkit.

Wow.

Okay, let's zoom out again and look across the vertebrates.

How has breathing evolved?

Can we see trends?

We can see trends, definitely, though we have to be careful not to draw straight lines between modern groups.

But comparing them gives insights.

Look at surface area, for instance.

Amphibians and reptiles, their lung surface area is kind of in the same ballpark as fish gill area for similar sized animals.

But then mammals and birds, there's a huge jump, much, much larger lung surface areas.

Linked to being warm -blooded, higher metabolism.

That's the general thinking, yes.

Needs more oxygen, so needs more area.

Our own lungs, if spread out, could cover a big room.

And again, tunas are the exception among fish.

Their gill area is up there with mammals and birds.

And thickness, does that trend, too?

Yes.

Generally, vertebrate lungs have thinner barriers than fish gills.

Birds have the absolute thinnest, just 0 .2 micrometers on average.

Then mammals, then reptiles.

Tunas, again, buck the fish trend, with gill thickness similar to mammals.

It's amazing how tuna keep popping up as exceptional.

They really are optimized for that high -performance lifestyle.

What about breathing through skin?

We see that in some animals, right?

We do.

Mammals, birds.

Most reptiles, our skin isn't very permeable.

Good for keeping water in, bad for breathing.

But amphibians, and some fish and reptiles.

Highly permeable skin.

Like those lungless salamanders.

Exactly.

They rely entirely on skin breathing.

It's a trade -off.

You gain breathing surface, but you're much more vulnerable to drying out.

Makes sense.

And what controls the rhythm of breathing in vertebrates?

The muscles contracting.

That rhythm comes from the brainstem.

Specifically, groups of neurons in the medulla, called central pattern generators, or CPGs.

They generate the basic in -out rhythm automatically.

Even if you isolate that part of the brainstem?

Remarkably, yes.

Experiments show the rhythm persists.

It's the pacemaker.

And the pattern isn't always constant, is it?

Some animals pause their breathing.

Right.

We tend towards continuous breathing, breath after breath.

But many reptiles and amphibians show intermittent or periodic breathing.

They'll take a few breaths, then hold their breath for a while, apnea with lungs inflated, then breathe again.

Probably saves energy.

Fascinating.

OK, let's dive into some specifics for each group.

Fish first.

We've touched on gills.

Right, recap.

Gills with those lamellae, that amazing countercurrent flow.

Most use that two -pump system buckle and opercular pumps working together for continuous one -way water flow.

Except tunas doing ram ventilation.

Yep.

And their gills even have structural reinforcements to handle that high -speed flow without collapsing.

We also mentioned O2 is the main stimulus, CO2 less so.

And they can adjust how much of their gill surface they use lamella recruitment based on oxygen needs.

And those air -breathing fish?

Oh yeah, about 400 species.

Especially common in freshwater that might get stagnant and low in O2.

They've evolved all sorts of organs to breathe air, modified parts of the gut, the swim bladder.

It's like evolutionary tinkering.

A real grab bag of solutions.

It is.

A key challenge for them, though, is losing the oxygen they gain from air back out to the water through their gills.

So many have adaptations to reduce gill function or shunt blood away from them when air -breathing.

Clever.

What about amphibians?

Frogs and salamanders.

Big transition there.

Larvae, like tadpoles, usually have gills and use skin.

Adults lose the gills, develop lungs, but the skin often remains really important, especially for getting rid of CO2.

And how do they fill their lungs?

You mentioned it was different.

They use a buccopharyngeal pressure pump.

Basically, they gulped air into their mouth cavity, closed nostrils and mouth, then squeezed the floor of the mouth to push the air into the lungs.

It's a hand -me -down mechanism from their fish ancestors.

Oh, interesting.

Reptiles next.

Lizards, snakes, turtles.

Reptiles made a major leap.

They primarily use suction or aspiration ventilation.

They expand their chest cavity, usually with rib muscles, creating negative pressure that sucks air into the lungs.

Like us.

More like us, yes.

And this was huge because it freed up the mouth and throat from having to push air in.

The mouth could then specialize for feeding, sound production, other things.

Their lungs vary, too.

Some are simple sacks.

Others, like an active monitor lizard, are more complex, multi -cameral, with more surface area.

OK, mammals.

Us.

Elaborate lungs, you said.

Incredibly elaborate.

Our airways branch maybe 23 times, leading down to millions of tiny sacks, the alveoli.

That's where the gas exchange happens across incredibly thin walls.

The total surface area is massive.

And we use tidal breathing in and out the same way.

Right, which means the air in the alveoli is always a mix of fresh and used air, keeping the gas pressures relatively stable.

Ventilation is driven mainly by the diaphragm muscle below the lungs and the intercostal muscles between the ribs.

Inhale is active.

Exhale is passive, usually.

At rest, yes.

Inhalation requires muscle contraction.

Exhalation is mostly the elastic recoil of the lungs and chest wall.

During exercise, though, exhalation becomes active, too, using abdominal muscles.

And control.

You mentioned CO2 is key for us.

Very key.

The basic rhythm comes from the pre -Butzinger complex in the brainstem.

But the rate and depth are strongly modulated by CR2 levels and associated acidity, detected both in the brainstem and by peripheral sensors.

Oxygen sensors, the carotid and aortic bodies, kick in mainly if O2 drops significantly, like at altitude.

Right, triggering hyperventilation.

Exactly.

And breathing rate scales with size small mammals breathe much faster than large ones.

One more thing for mammal surfactant.

Ah, yes, pulmonary surfactant.

Absolutely critical.

It's a substance lining the alveoli that reduces surface tension, especially as the alveoli get smaller during exhalation.

Without it, they'd collapse.

Which is why it's so important for premature babies.

Precisely.

Their lungs might not produce enough yet.

Surfactant therapy was a huge medical breakthrough.

Okay, now, birds.

Very different system you mentioned.

Radically different.

Bird lungs are relatively small and rigid.

They don't expand and contract much.

The exchange happens in tiny tubes called parabranchi, which allow air to flow through them.

So how does the air move?

They have a system of air sacs connected to the lungs, but mostly not involved in gas exchange themselves.

These sacs act like bellows.

When the bird inhales, sacs expand, drawing air in.

When it exhales, sacs compress, pushing air through the parabranchi.

This creates unidirectional flow.

Yes.

Air flows continuously in one direction through the parabranchi during both inhalation and exhalation.

It's incredibly efficient, combined with that cross -current exchange pattern.

Allows them to fly at insane altitudes.

It certainly contributes.

Birds flying over Mount Everest.

Their respiratory system is up to the challenge.

It's arguably a very, very effective design, maybe even superior to ours in some ways.

Wow.

Okay.

Shifting gears dramatically.

Invertebrates.

What about things without backbones?

Insects.

Insects are a whole different world again.

They use a tracheal system.

It's a network of air -filled tubes, the trachea, that open to the outside through little pores called spiracles.

Tubes going into the body.

Right.

These tubes branch finer and finer, ending in tiny tracheals that reach nearly every cell in the body.

Gas exchange happens directly between the tracheal air and the cells.

So the blood doesn't really carry oxygen.

Largely no.

The tracheal system delivers oxygen directly, bypassing the circulatory system for O2 transport.

It's a completely different strategy.

How does air move in these tiny tubes?

Just diffusion.

Diffusion is key, especially over short distances.

But we now know there's more to it.

Many insects actively ventilate, pumping their abdomens.

There's even evidence of rhythmic compression and expansion of the trachea themselves, microscopic ventilation.

And that discontinuous breathing.

Yeah, that's fascinating.

Some insects keep their spiracles closed much of the time, letting CO2 build up, then release it in bursts.

Meanwhile, O2 uptake can be steadier, sometimes creating a slight vacuum that helps suck air in when spiracles open.

It likely helps conserve water.

Clever.

What about insects living in water?

Oh, amazing adaptations there too.

Some have gills, basically modified parts of the tracheal system.

Others trap an air bubble against their body and use it like a temporary gill.

Oxygen diffuses in from the water as the insect uses it up.

A scuba tank they carry.

Kind of.

And some have evolved a permanent version called a plastron, a layer of specialized hairs or structures that hold a thin film of air indefinitely.

The air film acts as a permanent gill.

They can stay submerged for ages.

That's incredible.

From tunas to plastrons, what a range.

It truly is.

So pulling it all together, what's the big takeaway?

We've seen just this incredible diversity, all aimed at the same fundamental problem.

Getting oxygen in, CO2 out.

Life has innovated in countless ways.

Whether it's maximizing surface area, minimizing thickness, optimizing flow patterns like countercurrent exchange, or even bypassing circulation entirely like insects.

It's all about adapting to the environment and the specific needs of the animal.

The elegance of these solutions is just staggering.

It really underscores the power of comparative physiology seeing how different groups solve similar problems.

And the adaptive significance, how each system fits the animal's lifestyle and environment.

Evolution is an amazing engineer.

So next time you take a breath of that seemingly simple act,

maybe think about that tuna ramming water through its super gills or that tiny insect with its internal network of air tubes.

What might the future hold for breathing systems?

Maybe even ours facing new challenges.

What can we learn about efficiency from the rest of the animal kingdom?

Great questions to ponder.

It just shows there's always more to discover and more to understand about how life works.

Thank you so much for joining us on this deep dive.

It's been fascinating.

My pleasure.

Hope everyone listening found it insightful.

Keep asking questions.

Warmly thanking you for being part of the Last Minute Lecture family.