Chapter 13: Respiratory Physiology and Gas Exchange

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Welcome to the Deep Dive.

Today we're looking at something pretty fundamental how animals, all animals really, keep their insides, well, stable.

Think about it, your body is this incredibly complex machine, right?

Absolutely, constantly working to keep everything just so, especially the fluids inside.

Exactly, maintaining that internal sweet spot for every single cell, no matter what's happening outside.

So our mission today is to explore this whole fascinating world of animal fluid and acid -base balance.

We want to see how they pull it off.

Yeah, from the tiniest molecular pumps all the way up to how whole organisms adapt to, say, salty oceans or super dry deserts.

And we're drawing our insights from animal physiology, from genes to organisms, second edition.

It's a great resource for understanding how animals manage their internal environment.

That famous milieu interior that Claude Bernard talked about, that stable internal world.

Right, so we'll unpack how animals deal with these challenges like salt water or lack of water and also how they keep their internal chemistry, especially pH, perfectly balanced.

It sounds simple, but it's incredibly complex.

So the big question is,

how do these cells, which are basically little bags of water, stop themselves from, you know, swelling up like balloons or shrinking like raisins, depending on where they live?

And how do they handle all the acids produced just by being alive by their own metabolism?

Okay, let's start with that core concept, homeostasis.

You mentioned Claude Bernard and Walter Cannon too, right?

They really drove this idea.

They did.

The key insight was that if you keep the fluid outside the cells, the intracellular fluid or ICF have a much easier job.

Ah, so the ECF acts like a buffer zone.

Exactly.

It shields the individual cells from constant disturbances, saves them a lot of energy.

And this ECF, that includes things like blood plasma and the fluid between tissues.

Precisely.

Plasma in your blood, interstitial fluid bathing the tissue cells, they're all part of the ECF.

And then you have the ICF inside the cells.

Separate, but definitely connected.

And this idea of balance input equals output.

That's the core principle.

For any substance, like salt, which your body doesn't make, what comes in must equal what goes out to keep levels stable.

Simple accounting, really, but vital.

Okay, so how is all this water distributed?

In vertebrates, roughly two thirds is inside the cells, ICF.

The other third is ECF.

And that ECF is mostly interstitial fluid, then plasma, and smaller bits like lymph and cerebrospinal fluid.

But here's the kicker, isn't it?

Even though plasma and interstitial fluid are pretty similar because stuff moves easily across capillaries,

the ECF and the ICF are totally different chemically.

Markedly different, yes.

And the gatekeeper is that highly selective plasma membrane around every cell.

It's incredibly picky about what gets in and out.

So what are the main differences?

Well, big ones are proteins inside the cell, certain organic molecules, but, crucially, the ions.

Sodium Na plus Lin is the main player outside in the ECF.

Potassium K plus A dominates inside the ICF.

And that difference is maintained by?

The Na plus K plus ATPase pump.

It's almost universal in animal cells.

Constantly pumping three sodium ions out for every two potassium ions it brings in.

Uses a ton of energy, actually.

And it's not just for fluid balance, right?

That gradient is key for - Oh, absolutely.

It's fundamental for nerve impulses, muscle contraction, transporting other solutes.

Everything, really.

It sets up the cell's electrical potential.

Okay, so if ions are kept so different and water just follows solute, how do cells avoid bursting or shrinking?

That's the osmotic problem.

Yeah, and the threats are everywhere.

The salinity of the environment is a big one.

Ocean versus freshwater.

On land, it's evaporation.

Even things like freezing.

Freezing locks up water, yeah.

And ingestion, excretion, even diseases like diabetes that change the ECF concentration.

It's a constant challenge.

So animals evolve basically two main strategies.

Broadly speaking, yes.

You have the Osmo conformers who just let their internal fluids match the environment's osmotic pressure.

They just go with the flow, osmotically speaking.

Pretty much.

And then you have the Osmo regulators who fight to maintain a stable internal osmotic pressure.

Often very different from the outside world.

Let's tackle

marine invertebrates.

Jellyfish, sea anemones, also hagfish.

Their ECF is very similar to sea water.

Maybe a thousand milliosmoles per liter, mostly sodium and chloride ions.

Okay, that makes sense.

But what about inside their cells?

Ah, that's where it gets clever.

The intracellular fluid, the ICF, doesn't just load up on sodium and chloride to match the ECF.

Instead, a huge chunk of its osmotic concentration, maybe 600 millios out of that 1 ,000, comes from organic osmolytes.

Organic molecules, like amino acids and stuff.

Exactly.

Free amino acids like glycine and taurine, methylamines, things the cell makes or accumulates.

But hang on, making those organic molecules costs energy, right?

Why not just let in the free sodium and chloride ions from the sea water?

Seems cheaper.

That's a really important question.

There are two main reasons.

First, cells rely on carefully controlled sodium gradients for all sorts of things, transport, nerve signals.

Letting loads of sodium flood in would mess that up.

Same for chloride, it's often kept low inside.

Okay, so maintaining those ion gradients is critical.

What's the second reason?

The second reason is even more fundamental.

High concentrations of inorganic ions, Na plus AAA, even K plus to some extent, can actually disrupt the structure and function of vital molecules like proteins and DNA.

They can make enzymes work poorly or even damage genetic material.

So salt is kind of harsh on the cellular machinery.

In high concentrations, yes.

But these organic osmolytes are what we call compatible solutes.

They contribute to osmotic pressure without interfering with how proteins and other macromolecules work.

They're much gentler.

That's clever.

Like internal cushions.

Sort of.

And some are even counteracting solutes.

A great example is

TMAO trimethylamine oxide.

It actually helps stabilize proteins against things that might damage them, like high concentrations of urea in sharks or high pressure in the deep sea, or even temperature changes.

TMAO.

Is that the fishy smell?

That's the one.

Or rather, its breakdown product is.

Another one is DMSP, which breaks down into DMS, that classic smell of the sea.

Okay, so most basic marine invertebrates, like jellyfish and starfish, they're osmo -conformers, loading up their cells with these compatible organic solutes.

Right.

They're often stenohaline, meaning they can only tolerate a narrow range of salinity.

If the water gets too fresh or too salty, they're in trouble.

But not all conformers are like that.

You mentioned estuaries.

Ah, yes.

You get urehaline organisms, like the eastern oyster.

Living in estuaries means dealing with big salinity swings with the tides.

These oysters are still osmo -conformers overall, but they actively regulate the levels of amino acids inside their cells to prevent swelling when the water gets fresher or shrinking when it gets saltier.

They adjust.

That's pretty adaptable.

Very.

Atlantic coast oysters use slightly different osmolytes, like taurine, but the principle is the same.

Adaptable osmo -conformity.

And then there are the real masters of survival.

Yeah.

The ones that dry out.

Oh, and hydrobiosis.

Yeah, some bacteria, yeasts, nematodes, brine shrimp, and the famous tardigrades, or water bears.

They can survive almost complete dehydration.

They accumulate massive amounts of sugars, especially a desaccharide called trehalose.

As the water leaves, trehalose essentially replaces it, hydrogen bonding to proteins and membranes, forming a kind of glassy protective matrix.

It puts the cell in suspended animation until water return.

Wow.

Like biological antifreeze, but for dryness.

Kind of, yeah.

It's so effective.

People are trying to use trehalose to preserve human blood cells for longer storage.

Fascinating.

And even less extreme examples like the nematode C.

elegans.

Right.

Research shows when C.

elegans is put in salty conditions, it ramps up production of glycerol, another osmolyte, to balance things out.

And importantly, it also boosts genes involved in repairing protein damage, suggesting even compatible solutes aren't perfectly harmless.

Okay.

So that covers typical osmo -conformers.

What about those sharks you mentioned?

They conform, but differently.

Exactly.

These are the hypoionic osmo -conformers.

Sharks, skates, raise the cartilaginous fish, and also co -allicants.

Their ECF salt concentration is actually lower than seawater.

So they're not quite conforming with salt.

Not with salt, no.

But they do match the overall osmotic pressure of seawater.

They make up the difference by accumulating huge amounts of two organic solutes throughout their bodies.

Urea and TMAO.

Urea.

But isn't urea like waste?

Isn't it toxic at high levels?

It normally is, yes.

That's what's so remarkable about sharks.

They live with urea concentrations that would kill most other vertebrates.

So how do they manage that?

It's swimming around in their blood.

Three key adaptations.

First, some of their proteins have evolved to simply be resistant to urea's potentially damaging effects.

They just aren't bothered by it as much.

Okay.

Resistant proteins.

Second, some of their proteins actually seem to require urea to function optimally.

They've become dependent on it.

Wow.

Okay.

Dependent.

What's the third?

The third, and probably most crucial, is TMAO.

Remember TMAO, the counteracting solute.

Sharks maintain a very specific ratio, usually about two parts urea to one part TMAO.

The TMAO directly counteracts the destabilizing effects of urea on proteins.

So the TMAO protects the proteins from the urea that's keeping the water balance right.

Exactly.

It's this incredible physiological balancing act.

They use a potentially toxic waste product, balanced by another organic molecule, to solve their osmotic problem without high internal salt.

That is really elegant.

And it's not just sharks.

The crab -eating frog that lives in mangrove swamps does something similar, and some arthropods too.

It's a successful strategy.

And you mentioned buoyancy earlier.

Does this relate?

A little bit, yeah.

Some jellies actually achieve buoyancy by pumping out heavier ions like sulfate and replacing them with lighter chloride.

Squids use ammonium.

And even in sharks, the relatively low density of urea and TMAO compared to salt contributes slightly to their neutral buoyancy.

Every little bit helps.

Alright, let's switch gears to the other major strategy.

The osmoregulators.

The fighters.

The ones maintaining internal stability regardless.

Yes.

These animals actively control their ECF osmotic pressure, keeping it constant even if the outside environment changes dramatically.

And most marine vertebrates fall into this category, right?

Bony fish, whales,

sea turtles.

Correct.

They are typically hypoosmotic regulators.

Their internal body fluids are much less concentrated, maybe 250 to 400 millisim compared to seawater at around 1 ,000 millisim.

Hypoosmotic.

So they're less salty than the sea, which means they're constantly losing water to the sea.

Exactly.

Water constantly tries to leave their bodies via osmosis, moving from their higher water concentration to the lower water concentration of the salty sea.

And at the same time, salt ions are constantly trying to diffuse in.

So double whammy.

Losing precious water, gaining excess salt.

How do they cope?

How do they replace the water and ditch the salt?

Well, first they have to drink.

They drink the seawater around them.

Drink salt water.

Doesn't that make it worse?

You'd think so, but their digestive tracts are specialized.

They can absorb the water while restricting some of the salt influx, but they still take in a lot of salt.

The real key is actively pumping that excess salt out.

They have specialized tissues packed with ion pumps.

In fish, it's mainly the gills, using structures called chloride cells.

These cells use transporters like the NKCC symporter and CFTR channels, the same channel, defective in cystic fibrosis, interestingly, to actively transport salt ions out into the water.

So the gills are pumping salt outwards.

Vigorously.

Marine reptiles and birds have evolved special salt glands, often near their eyes or nose, that do the same thing, excreting a very concentrated salt solution.

And marine mammals rely on incredibly efficient kidneys to produce concentrated urine, getting rid of salt that way.

Amazing how different groups solve the same problem.

But nature always has exceptions, doesn't it?

Always.

Some osmoregulators blur the lines.

Think about fish living in very cold polar waters or the deep sea.

Where they do.

Many of them accumulate high levels of TMAO, just like we saw in sharks, but maybe not as much urea.

And sometimes glycerol, too.

These act as antifreezes, preventing ice crystals from forming in their tissues.

Ah, so it's for cold protection.

Primarily, yes, but it has an osmotic side effect.

By increasing their internal solute concentration with TMAO and glycerol, they reduce the osmotic difference between themselves and the seawater.

This lessens the constant water loss and salt gain, making osmoregulation less energetically expensive.

High TMAO also helps counteract the effects of hydrostatic pressure in the deep sea.

Deep sea shrimp do something similar.

Clever.

What about animals that face drying out, but on land or in freshwater habitats?

Ah, estivation.

Some animals enter a dormant state during dry periods.

Lungfish are a classic example.

They burrow into the mud as their palm dries up.

And what happens then?

They dramatically slow their metabolism and they start accumulating urea.

Sometimes reaching incredibly high levels, like 400 millimolar, just like the sharks, but for a different reason.

Not for osmotic balance with seawater, but to hold on to water.

By making their body fluids very concentrated with urea, they reduce water loss by evaporation, and they also retain water produced metabolically.

It helps them survive drought.

The American spadefoot toad does this too, burrowing underground.

Some snails as well.

So urea can be a tool for water retention too.

And what about animals that move between salt and freshwater, like salmon?

Migratory fish are physiological marvels.

A salmon in the ocean is a hypoosmotic regulator, drinking seawater and pumping out salt via its gills.

But when it migrates into a freshwater river to spawn?

It's suddenly in an environment much less salty than its body fluids.

Exactly.

It becomes hyperosmotic relative to the river water.

Now the problem flips.

Water tries to flood in and precious salts try to diffuse out.

So it has to completely reverse its strategy.

It does.

And this switch is triggered by hormones, growth hormone, cortisol, prolactin.

These hormones cause dramatic changes in the gill cells.

The chloride cells that were pumping salt out basically change function, or new cells take over, and they start actively pumping salt in from the dilute river water.

The activity of the NAPlusK plus ATPase pump changes direction, functionally speaking.

That transformation is incredible.

It really is.

And you see similar flexibility elsewhere.

The green shore crab, for instance, is an invasive species known for its adaptability.

What does it do?

In high salinity water, it tends to osmo -conform, letting its internal fluids track the environment.

But if it moves into lower salinity water, like in an estuary, it switches to osmo -regulating.

It ramps up the activity of NAPlusK plus ATPase pumps in its gills to take up salt, and maintains a higher internal concentration than the surrounding water.

It also increases ATP production to fuel those pumps.

It's a flexible conformer regulator.

Okay, so that covers aquatic life.

What about us land dwellers?

We're regulators by necessity, right?

Absolutely.

On land, the big challenge is usually water loss to the dry air, and sometimes salt scarcity, too, depending on diet.

So terrestrial animals are osmo -regulators, mainly using their excretory systems, kidneys, and digestive systems to manage water and salt.

And some have really extreme adaptations, don't they?

I read about insects that can pull water out of the air.

It sounds like science fiction, but it's true.

Certain insects and arachnids, like some desert beetles, house dust mites, even the rat flea, have specialized structures, often in their rectum, that can actively absorb water vapor directly from unsaturated air as long as the humidity is high enough.

It's an amazing feat.

Oh, okay, but for most mammals, like us, how do we manage?

For mammals, fluid balance regulation really tackles two related but distinct things.

Regulating the volume of the ECF, which is critical for maintaining blood pressure, and regulating the osmolarity, the concentration, of the ECF, which is essential to prevent cells from swelling or shrinking.

Right, so volume for pressure, concentration for cell integrity.

How does water balance work?

Water input comes from what you drink, the water in your food, and also metabolic water, water produced chemically during cellular respiration.

Ah, making water from scratch, essentially.

A small but significant amount, yes.

Water output happens via urine produced by the kidneys in feces, and through evaporation, both insensible loss from skin and breathing, and sensible loss through sweating.

And the main controls are?

The kidneys are the primary regulators of output, adjusting urine volume and concentration,

and thirst is the primary regulator of input.

Both are heavily influenced by hormones like vasopressin, also called antidiuretic hormone, or ADH, which acts on the kidneys, and signals detected by osmoreceptors in the brain's hypothalamus.

Got it, kidneys and thirst.

What about salt balance?

Salt balance is a bit different.

Unlike water, we don't really produce salt metabolically.

Input comes solely from what we ingest in our diet.

And output?

Output occurs in sweat and feces, but the main regulated pathway is through the kidneys.

The kidneys don't just control water volume, they precisely control how much sodium is excreted or reabsorbed.

How do they control the sodium?

It involves regulating how much fluid is filtered in the first place, the glomerular filtration rate, or GFR, and then fine -tuning how much sodium is reabsorbed back into the body from the kidney tubules.

This reabsorption is heavily influenced by hormones,

especially the renin -angiotensin -aldosterone system, or RAAS, which promotes sodium retention, and atrial natriuretic peptide, or ANP, which promotes sodium excretion.

RAAS holds on to salt, ANP gets rid of it.

Basically, yes.

And this control is vital.

Think about herbivores.

Plants are often very low in sodium.

They often experience intense salt hunger and will actively seek out salt licks to meet their needs.

Their RAAS system is usually working hard to conserve every bit of sodium.

It all ties together.

Can we see all these systems working in concert?

A dramatic example is the response to hemorrhagic shock severe blood loss.

It triggers a massive coordinated response.

What happens?

Your cardiovascular system tries to maintain pressure.

Your nervous system kicks in.

Hormonal systems go into overdrive.

RAAS is activated strongly to retain salt and water.

Vasopressin is released.

Thirst becomes intense.

There's even a shift of fluid called autotransfusion from the interstitial space into the capillaries to try and boost blood volume quickly.

And over the longer term, the liver ramps up production of plasma proteins like albumin to help hold fluid within the blood vessels.

Wow.

It's like an emergency response team for the entire body, all focused on restoring volume and pressure.

Exactly.

It showcases just how integrated fluid and salt balance regulation is with cardiovascular and endocrine systems.

Incredibly complex.

Let's shift now to the other side of the internal balance coin,

acid -base regulation, keeping the pH right.

Just as critical, if not more so in some ways, because pH, the concentration of hydrogen ions, H +, affects virtually everything.

Why is it so important?

A little bit acidic, a little bit alkaline.

What's the big deal?

Oh, it's a huge deal.

Even tiny shifts in pH can drastically change the shape, and therefore the function of proteins think enzymes, hemoglobin carrying oxygen.

It messes with potassium levels, which is critical for nerves and muscles, and it directly affects the excitability of nerve and muscle cells.

Too much acid, acidosis, tends to depress the central nervous system.

You can become confused, disoriented, even comatose.

Too little acid, alkalosis, makes nerves and muscles hyper excitable, leading to pins and needles, muscle spasms, even convulsions.

So the body needs to keep pH in a very narrow range.

What is that range typically?

In mammalian arterial blood, it's tightly regulated around pH 7 .4, which is slightly alkaline.

Remember, the pH scale is logarithmic, so a change from 7 .4 to 7 .1 represents a doubling of hydrogen ion concentration.

It's a big change physiologically.

Different animals might have different set points.

Factors like body temperature matter, fish living in cold water, for example, often have a higher, more alkaline blood pH than mammals.

Okay, so where do these hydrogen ions, these acids come from?

Are we just constantly generating acid?

Pretty much, yes.

The biggest source is carbonic acid, formed when the CO2 produced by metabolism dissolves in water.

Then you have inorganic acids produced from breaking down proteins containing sulfur or phosphorus forming sulfuric and phosphoric acid.

Okay, CO2 and protein breakdown.

And also organic acids from metabolism like fatty acids and importantly, lactic acid during strenuous anaerobic exercise.

Plus, there can be external inputs, acid rain from pollution, and the big one globally, the increasing CO2 dissolving in the oceans from human activity.

So constant acid production.

How does the body defend against pH changes?

There are three main lines of defense operating on different time scales.

The first line is chemical buffers.

Buffers, like shock absorbers for pH.

Exactly.

They're right there in the body fluids, ready instantly within fractions of a second to minimize pH changes.

They work by binding H plus when it's in excess or releasing H plus when it's too scarce.

What are the main buffer systems?

The most important one in the ECF for dealing with acids other than carbonic acid is the CO2 bicarbonate system.

It uses bicarbonate ions, HCO3, to buffer H plus white.

The beauty is its components, CO2 and HCO3, are abundant and independently regulated by the lungs and kidneys.

We maintain about a 20 to 1 ratio of bicarbonate to dissolve CO2 in mammals to keep that pH at 7 .4.

The Henderson -Hasselbalch equation describes this relationship.

Okay, bicarbonate in the ECF.

What about inside cells?

Inside cells and also in plasma proteins are the primary buffers.

They're abundant and amino acids have acidic and basic groups that can accept or release H plus life.

The amino acid histidine is particularly important.

Diving mammals, for instance, have high concentrations of histidine containing dipeptides in their muscles to buffer the lactic acid built up during long dives.

Rubber.

Any others?

Hemoglobin inside red blood cells is a crucial buffer.

Especially for carbonic acid generated from CO2 picked up in the tissues.

Reduced hemoglobin is better at binding H plus than oxygenated hemoglobin is.

And finally, the phosphate buffer system.

It's important inside cells, but its unique role is as the main buffer in urine.

It allows the kidneys to excrete large amounts of H plus without making the urine incredibly acidic.

Phosphate in the urine.

And I heard about turtles using their shells.

That's an amazing adaptation.

The painted turtle can survive months underwater without oxygen, building up huge amounts of lactic acid.

Its shell and skeleton actually act as enormous buffers.

They demineralize, releasing calcium carbonate and phosphate into the ECF, which buffers the lactate, effectively taking the acid load out of circulation.

Incredible.

But buffers don't actually get rid of the acid, do they?

No, that's the crucial point.

Buffers are like sponges.

They soak up H plus temporarily, but they don't eliminate it from the body.

They just buy time.

So what's the second line of defense?

That's the respiratory system, the lungs or gills.

This kicks in within minutes.

It controls pH by adjusting the level of CO2, the source of carbonic acid.

How does breathing faster or slower help?

If your body fluids become too acidic, too much H plus, your respiratory centers are stimulated, and you start breathing faster and deeper hyperventilating.

This blows off more CO2.

Removing CO2 shifts the carbonic acid equilibrium, effectively reducing H plus concentration and raising pH back towards normal.

And if you become too alkaline?

If you're too alkaline, too little H plus, breathing slows down hypoventilation.

This retains CO2, increasing carbonic acid and H plus A, lowering pH back towards normal.

Even insects adjust how much they open their spiracles based on their hemolyph pH.

So lungs fine tune CO2, but they can't get rid of those other acids like sulfuric or lactic acid?

No, they can only compensate for them by adjusting CO2.

For actually eliminating those fixed acids, or for major adjustments in bicarbonate, you need the third line of defense, the excretory systems.

Kid using gills again?

Primarily, yes.

This is the most powerful system for correcting pH imbalances, but it's also the slowest, taking hours to days to fully respond.

How do they work?

Fish and crustacean gills have specialized cells with transporters like proton pumps, VAT paces, sodium hydrogen exchangers, chloride bicarbonate exchangers that can actively pump H plus out into the water or take up bicarbonate from it.

In mammals, the kidneys are the stars.

What do the kidneys do?

They can excrete varying amounts of H plus into the urine, and they can conserve virtually all the filtered bicarbonate and generate new bicarbonate to replenish the buffer system.

This happens mainly in the tubules.

Specialized cells, like the intercalated cells in the collecting ducts, have powerful proton pumps to secrete H plus into the urine.

And making new bicarbonate helps buffer more acid.

Exactly.

Often, the secretion of an H plus ion into the urine is coupled with the addition of a new bicarbonate ion to the blood.

The kidneys can also adjust how much bicarbonate they simply reabsorb versus excrete.

And there's another trick,

ammonia secretion.

When the body has a large acid load to excrete, the kidneys produce ammonia, NH3.

Ammonia diffuses into the tubules, where it combines with secreted H plus to form ammonium ions, NH4 plus.

Ammonium gets trapped in the urine and excreted.

This essentially allows the kidneys to shuttle out H plus without making the urine dangerously acidic, acting as a urinary buffer alongside phosphate.

Okay, three lines of defense.

Buffers, breathing, and kidney skills.

What happens when these systems are overwhelmed or something goes wrong?

Acid -based disorders.

Right.

Doctors often think in terms of six main categories.

Respiratory acidosis or alkalosis, metabolic acidosis or alkalosis, and sometimes environmental causes too.

They look at blood pH, CO2 levels, and bicarbonate levels to figure out the primary problem and whether the body is compensating.

So respiratory acidosis, that's from holding onto too much CO2.

Yes, caused by hypoventilation due perhaps to lung disease, injury to the respiratory center, or even just breathing air with high CO2 levels, like in a poorly ventilated space.

The kidneys will try to compensate slowly by retaining more bicarbonate.

And respiratory alkalosis.

That's from blowing off too much CO2 due to hyperventilation, maybe from anxiety, high altitude, or fever.

Compensation involves the kidneys excreting more bicarbonate.

Birds flying at high altitude have remarkable adaptations to manage this.

Okay, what about metabolic causes?

Metabolic acidosis?

This means too much acid from sources other than CO2 or excessive loss of bicarbonate.

Classic causes include severe diarrhea, losing bicarbonate -rich fluids,

diabetic ketoacidosis, producing acidic ketone bodies, strenuous exercise, lactic acid buildup, or kidney failure, uremic acidosis.

Why is uremic acidosis so bad?

Because when the kidneys fail, the body loses its most powerful tool for excreting fixed acids and regenerating bicarbonate.

It's a very serious situation.

And metabolic alkalosis.

Too little acid.

Usually caused by losing acid from the body, most commonly through vomiting, which removes acidic stomach contents, or sometimes from excessive intake of alkaline substances.

Compensation involves slower breathing to retain CO2, and the kidneys excreting excess bicarbonate.

You mentioned environmental causes.

What about those?

Well, environmental acidosis can happen from things like acid rain, which introduces sulfuric acid into aquatic environments.

But the really big global issue now is ocean acidification.

Right, from CO2 dissolving in the ocean.

Exactly.

All that anthropogenic CO2 we pump into the atmosphere, a lot of it dissolves in the oceans, forming carbonic acid, which lowers the pH.

Since the industrial revolution, the average ocean surface pH has dropped by about 0 .1 units, from 8 .2 to 8 .1.

That doesn't sound like much.

But remember the logarithmic scale.

That 0 .1 unit drop represents about a 25 -30 % increase in hydrogen ion concentration.

And it's having measurable impacts.

We're seeing reduced growth rates in corals documented in places like the Great Barrier Reef.

It's harder for organisms that build shells or skeletons out of calcium carbonate, like corals, snails, clams, and oysters.

The lower pH makes carbonate ions less available.

Like those oyster die -offs in Oregon.

That was a major wake -up call.

Larval oysters were dying in huge numbers in hatcheries because the acidified seawater corroded their developing shells.

Even fish aren't immune.

Studies on Pollock show they have to expend more metabolic energy just to regulate their internal pH in more acidic water, leaving less energy for growth or reproduction.

It's affecting the whole food web.

Potentially, yes.

And controversially, some have even proposed dumping CO2 into the deep sea, but the potential impacts there are largely unknown and concerning.

Interestingly, while many organisms suffer, some evidence suggests jellyfish might actually thrive in warmer, more acidic oceans, possibly shifting ecosystem dynamics.

A worrying trend.

Is there environmental alkalosis?

It's much rarer.

But you can find naturally alkaline lakes, like Lake Magadi in Africa or Pyramid Lake in Nevada, that have high concentrations of basic minerals like sodium carbonate.

Fish living there, like certain tilapia or trout, have adaptations to excrete excess base, often through their gills.

Wow.

What an incredible intricate dance it is maintaining this internal balance.

From tiny molecules to whole ecosystems adapting to change.

It truly is.

We've covered how vital fluid and acid -base balance are for every single cell.

Enabling life to exist in such diverse environments.

From the bottom of the sea to the driest desert, it's all about those constant tiny adjustments.

Yeah, keeping everything in that sweet spot.

And it really underscores the interconnectedness of physiology,

molecular pumps,

cellular function, organ systems, and how organisms interact with their environment.

It all ties together.

It leaves you thinking, doesn't it?

If our own bodies go to such extraordinary lengths to maintain their internal balance, their milieu interior,

what's our responsibility for maintaining the balance of our planet's external environment, especially with challenges like ocean acidification staring us in the face?

That's a profound question to ponder.

Indeed.

Well, that's all the time we have for today.

Thank you so much for joining us on this deep dive into the world of animal physiology.