Chapter 39: Transport of Acids and Bases

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You know that feeling, right?

Staring at a chapter in a dense textbook, maybe something like Medical Physiology by Boron and Bull Pape, and just wishing someone could walk you through the really tough parts, make them clearer, maybe even a bit exciting.

Oh, absolutely.

Especially with physiology, where everything is so interconnected, it can feel overwhelming.

Exactly.

Like, how does your body actually manage something as complex as its pH balance second by second?

Well, that's what we're diving into today.

We're focusing specifically on the kidney, often thought of as just a filter, but its role in acid -base balance is, well, heroic.

Totally heroic.

And that's our mission for you today.

We're pulling from that cornerstone text, Boron and Bull Pape, to really unpack the kidney's role.

Not just what it does, but the nitty -gritty how at the cellular level.

And crucially, why it matters clinically.

How understanding this helps you make sense of diagnostics, disease, treatment, the whole picture.

Think of it as building the understanding from the ground up.

No textbook needed right now.

Just follow along as we make this complex stuff hopefully really clear and engaging.

So where do we start?

The big picture.

Yeah, let's start big.

The body's acid -base dream team.

The lungs and the kidneys.

Right.

The lungs are workhorses for CO2, carbon dioxide.

That's a potential acid, technically a volatile acid.

And they manage, get this, about 15 ,000 millimoles of it every single day.

It's staggering.

15 ,000.

Okay.

But they don't handle everything, do they?

No.

And that's the critical point.

The lungs deal with CO2, the volatile stuff.

But the kidneys,

they are the only effective way your body can neutralize and get rid of the non -volatile acids.

Non -volatile acids.

Okay, refresh my memory.

What falls into that category?

We're talking things like sulfuric acid, phosphoric acid, various organic acids.

They come from just, you know, normal metabolism, breaking down proteins and other molecules.

Plus some from your diet.

And how much are we talking about daily?

For an average person, it's around 70 millimoles a day.

Doesn't sound like much compared to the CO2 load maybe.

70.

Hmm.

Okay, maybe not compared to 15 ,000.

But still, if that just builds up day after day, what happens then?

Well, that's the danger.

If the kidneys don't handle that 70 millimole load, your blood pH starts to drop steadily.

Your bicarbonate reserves get used up buffering it.

And you end up in severe metabolic acidosis.

Which is life -threatening.

Absolutely.

It really highlights just how essential this kidney function is.

Think about kidney failure.

Acidosis is a major, major problem.

So the kidney has this like dual mission then.

What's job number one?

Job number one is actually a massive reclamation project.

Your kidneys filter an incredible amount of bicarbonate, about 4 ,320 millimoles every day.

Whoa.

Okay, 4 ,320 millimoles filtered.

Right.

And if you just peed all that out, catastrophic metabolic acidosis.

So the first absolutely non -negotiable task is grabbing almost all of that filtered bicarbonate back, reabsorbing it.

Preventing a massive loss of base.

Got it.

So once that huge reclamation job is underway, what's the second mission?

That's tackling the 70 millimoles of non -volatile acid we generate each day.

But here's the challenge.

Your urine can only get so acidic.

The absolute limit is a pH of about 4 .4.

And at pH 4 .4, you simply cannot dissolve and excrete 70 millimoles of free hydrogen ions.

There isn't enough room at that acidity level.

It's like trying to empty a bathtub with an eidropper.

So the kidney needs another strategy.

It can't just dump raw acid out.

Exactly.

It needs help.

And that help comes in the form of urinary buffers.

Things that can soak up those hydrogen ions.

Ah, the buffers.

Okay.

Yeah.

What are the main players here?

There are two main types.

First, you've got filtered buffers.

These are things already in the blood plasma that get filtered, like phosphate.

Phosphate is a star player because its pK is 6 .8, which is really close to pH, making it great at buffering H plus as the urine gets more acidic.

Creatinine and urate also chip in a bit.

Okay.

So filtered phosphate is key.

What's the second type?

The second type is really clever.

It's a synthesized buffer.

The kidney actually makes its own ammonia, NH3.

It makes ammonia.

Yep.

Primarily from the amino acid glutamine.

Yeah.

This ammonia diffuses into the tubule, finds a hydrogen ion, binds to it, and forms ammonium, NH4 plus C -O.

And that ammonium gets excreted.

Wow.

Okay.

So filtered buffers and kidney -made ammonia.

Let's zoom out again slightly.

How does the body handle an acid load overall?

What are the steps?

There's a sort of three -step strategy for dealing with those non -volatile acids.

Step one is super fast.

Extracellular buffering.

The immediate response.

Right.

The H plus hits the bloodstream and boom, most of it reacts immediately with bicarbonate, forming carbonic acid, which quickly becomes CO2 and water.

Other buffers in the blood, like proteins, help out too.

The result is just a tiny temporary dip in pH.

Okay.

Immediate buffering, then what?

Step two.

Step two involves the lungs again.

That CO2 produced from the bicarbonate buffering, your lungs breathe it right out, quick and efficient.

It gets rid of the immediate byproduct.

Breathing away the acid, essentially.

In a way, yes.

Yeah.

The volatile part of it.

But you still used up bicarbonate in that first step.

So step three is the long game.

And that's all kidney.

This is where they fix the deficit.

Exactly.

The kidneys have to regenerate the bicarbonate that was consumed.

They effectively make new bicarbonate, roughly 70 millimoles a day, to match the acid load that was buffered.

This keeps your blood bicarbonate levels stable long -term.

How do we actually measure that, that new bicarbonate creation?

We look at the urine and measure something called net acid excretion.

It's basically the sum of a few things.

One, the H plus excreted bound to those filtered buffers, phosphate that's called titratable acid.

Two,

the H plus excreted bound to ammonia as ammonium and H4 plus C.

Right.

And three, you have to subtract any bicarbonate that might have escaped reabsorption and ended up in the urine, though normally that's almost zero.

So titratable acid plus ammonium minus any lost bicarb equals net acid excretion.

That net acid excretion value essentially tells you how much new bicarbonate the kidney generated and added back to the body Okay, this is starting to connect.

Now let's get into the real nitty gritty.

How does the kidney actually move these hydrogen ions around?

What's the fundamental process?

The absolute core process underlying everything we've discussed is the secretion of hydrogen ions, H plus, from the tubule cell across its apical membrane into the tubular fluid, the lumen.

Pumping H plus into the forming urine.

Precisely.

Once that H plus is secreted into the lumen, it can meet one of several fates.

Okay, fate number one.

You mentioned the massive bicarbonate reclamation earlier.

How does secreted H plus help with that?

This is where most secreted H plus goes, especially early in the nephron.

The secreted H plus meets a filtered bicarbonate ion, HCO3, in the lumen.

They combine.

H plus plus HCO3 gives you H2CO3 carbonic acid.

Carbonic acid.

Right.

And carbonic acid is unstable.

It wants to become CO2 and water.

Now this reaction happens on its own, but it's kind of slow.

Too slow for the amount the kidney needs to handle.

Way too slow.

This is where the enzyme carbonic anhydrase comes in.

It's like a catalyst dramatically speeding up that conversion of H2CO3 to CO2 and water right there in the lumen.

There's also carbonic anhydrase inside the cell doing the reverse reaction.

So CO2 and water are formed in the lumen.

Then what?

The CO2 is lipid soluble, so it easily diffuses across the cell membrane into the tubule cell.

Inside the cell, carbonic anhydrase quickly combines it with water again to reform H2CO3, which then splits into H plus and HCO3.

So the H plus is recycled to be secreted again.

Exactly.

And the bicarbonate HCO3 that's formed inside the cell, that gets transported out the other side of the cell, the basolateral side, back into your blood.

So for every H plus created that combines with filtered bicarb, one new bicarb ends up back in the blood.

It's like an exchange.

That's precisely it.

It achieves the net reabsorption of filtered bicarbonate.

And remember, the scale, about 80 % of this happens in the proximal tubule.

High volume work.

The distal parts fine tune it and can create much steeper H plus gradients.

Okay, that covers reclaiming bicarbonate.

What's fate number two for secreted H plus -style forming titratable acid?

Right.

So some secreted H plus doesn't meet bicarbonate, instead it bumps into those other silted buffers, mainly phosphate HPO4 -2.

The dibasic phosphate.

Yep.

The secreted H plus binds to it, converting it into the monobasic form H2PO4.

This H2PO4 is then excreted in the urine.

We call this titratable acid because, well, historically that's how it was measured in the lab.

And phosphate is perfect for this because of its pK.

Exactly.

Its pK of 6 .8 means it's most effective at binding H plus in the typical pH range found in the tubules as urine gets acidified.

Creatinine and urate help too, but phosphate does the bulk of the titratable acid work.

And the amount depends on how much phosphate is filtered, right?

Which has clinical implications.

Absolutely.

If your GFR drops, like in kidney disease, you filter less phosphate.

That means less buffer available in the tubule, reducing your ability to excrete acid as titratable acid.

It's a direct contributor to the acidosis seen in renal failure.

Makes sense.

Okay, fate number three for secreted H plus -low.

This is the ammonia -ammonium story.

The kitty making its own buffer.

This is arguably the most important mechanism for excreting the daily acid load, especially during chronic acidosis.

The proximal tubule cells take up glutamine, metabolize it, and in the process they generate two key things.

Ammonia, NH3, and bicarbonate H2O3.

Two for one deal.

Kind of, yeah.

The bicarbonate gets transported into the blood.

This is genuinely new bicarbonate being added to your system.

The ammonia, NH3, being uncharged, diffuses easily into the tubule lumen.

And then it meets a secreted H plus -sat.

Exactly.

In the acidic lumen, NH3 immediately snaps up an H plus -sat, becoming ammonium, NH4 plus -sat.

Now NH4 plus is charged.

And charged molecules don't cross membranes easily.

Precisely.

It gets trapped in the lumen.

It can't easily diffuse back into the cell, so it stays in the urine to be excreted.

It's a very elegant way to export acid while simultaneously generating new base for the body.

And you mentioned recycling.

Yeah.

There's actually a fascinating loop where some NH4 plus gets reabsorbed later, particularly in the thick ascending limb, and concentrated in the kidney's inner medulla.

This allows for even more efficient secretion and trapping in the collecting ducts later on.

It maximizes the kidney's ability to get rid of acid this way.

And there's a liver connection here too, right?

Yes.

A crucial one.

The liver makes urea from amino groups, which actually consumes bicarbonate.

The kidney, by excreting ammonium, generates bicarbonate.

So they work together.

The liver handles most nitrogen waste via urea.

And the kidney handles the associated acid load via ammonium excretion, regenerating bicarb in the process.

Just a neat partnership.

Okay.

Amazing mechanisms.

Let's zoom right into the cell membrane now.

What are the specific pumps and transporters doing the H plus

secretion?

The machinery.

On that apical membrane facing the urine, there are three main players pushing H plus out.

The biggest workhorse is the NH exchanger, specifically NHE3.

It swaps one sodium ion coming into the cell for one hydrogen ion going out, driven by the sodium gradient.

Does a huge chunk of the work, especially in the proximal tubule.

Okay.

NHE3, the main exchanger.

What else?

Then you have the Electrogenic H plus pump, a V -type ATPase.

This one uses energy directly from ATP hydrolysis.

It's incredibly powerful and can pump H plus against a very deep gradient, allowing the urine pH to drop really low down to 4 .0 or 4 .5.

You find this mainly in specific cells called alpha intercalated cells, further down in the collecting ducts.

The heavy hitter for making urine really acidic?

What if that pump is broken?

Big problems.

Mutations affecting this pump cause a type of metabolic acidosis called Distal Renal Tubular Acidosis, or DRTA.

The kidney just can't acidify the urine properly.

Makes sense.

And the third mechanism.

The HK exchange pump, or HK ATPase, it pumps H plus out into the lumen while pulling potassium K plus back into the cell.

Its main job is often considered potassium conservation, but it definitely contributes to acid secretion too, especially in states of potassium depletion.

Ah, so it links acid base and potassium balance.

Very much so.

And it's implicated in the metabolic alkalosis you sometimes see with low potassium levels, hypokalemia.

We keep mentioning carbonic and hydrase.

Just how critical is it?

Indispensable.

It's needed both outside the cell in the apical membrane, CIV, to rapidly handle the filtered bicarbonate conversion, and inside the cell, CAI, to provide the H plus and HCO3 needed for transport.

Without it, the whole system grinds to a halt, or at least slows down dramatically.

Which is why CA inhibitor drugs have such a strong effect.

Exactly.

Drugs like acetazolamide blocks CA, massively impairing bicarbonate reabsorption.

You get very alkaline urine,

lose bicarbonate, and that can lead to metabolic acidosis.

Plus, the retained solute causes diuresis.

Okay, so H plus goes out the apical side.

Bicarbonate is generated inside the cell.

How does that bicarb get back into the blood on the other side, the basolateral membrane?

Two main routes there as well.

A major one, especially in the proximal tubule, is the electrogenic Ni -HCO3 co -transporter called NBC -We1.

It moves sodium and bicarbonate, usually three bicarbs per sodium, out of the cell together into the bloodstream.

And if that transporter has issues?

Mutations in NBC -1 cause another type of renal tubular acidosis, this time proximal RTA, or PRTA.

The proximal tubule just can't reclaim bicarb effectively, overwhelming the distal segments.

Severe acidosis results.

Wow, these transporters are critical.

What's the other way bicarb gets out?

Other mechanism is CLHCO3 exchange, swapping chloride for bicarbonate.

Different exchanger proteins handle this in different parts of the tubule, like AE1 and some collecting ducts.

Okay, that's a lot of machinery.

How does the kidney use all this to respond when our overall acid -base balance is disturbed, say respiratory acidosis, too much CO2?

Right, high CO2 levels.

Kidney senses this, and the response is to ramp up H plus secretion.

This means generating more new bicarbonate, primarily by boosting that ammonia production and ammonium excretion pathway we talked about.

It's trying to compensate by adding more base to the system.

And for metabolic acidosis?

Low bicarbonate.

It kicks into high gear.

Acutely, H plus secretion increases.

Chronically, though, it's really impressive.

There's a major upregulation of many of these transporters.

NHE3, the H plus pumps, the ammonia synthesis machinery.

Ammonium excretion can increase massively, tenfold or more, to maximize new bicarbonate generation.

But you mentioned a potential downside to that chronic response.

Yeah, interestingly, the changes in how the kidney handles citrate during chronic acidosis can actually increase the risk of forming certain types of kidney stones, particularly calcium phosphate stones.

It's a complex trade -off.

What about the opposite problem, metabolic alkalosis, too much bicarbonate?

The kidney does the opposite.

It dials down H plus secretion, tries to hang onto acid, effectively.

And if the alkalosis is severe,

specialized cells, the beta -intercalated cells in the collecting duct, can actually start secreting bicarbonate into the urine using transporters like pendrin on the apical membrane.

It actively tries to dump the excess base.

Fascinating flexibility.

What other factors influence how much acid the kidney secretes?

Does GFR play a role beyond filtering buffers?

It does.

There's something called glomerulotubular balance for bicarbonate, too.

Basically, if GSR increases, meaning more bicarbonate is filtered, the proximal tubule automatically increases its rate of bicarbonate reabsorption proportionally.

So it adjusts to keep things stable.

Exactly.

It prevents large swings in bicarbonate levels just because your GFR changed slightly.

And you mentioned volume status being a priority.

How does being dehydrated, for example, affect acid base?

This is key clinically.

When your body senses low volume or volume contraction, its top priority is to hold on to sodium and water to restore blood pressure.

Hormones like angiotensin II and aldosterone plus sympathetic nerves get activated.

And those signals also stimulate H plus secretion, particularly via NHE3 and the pumps in the collecting duct.

So the drive to conserve volume can actually lead to or worsen metabolic alkalosis because the kidney keeps pumping out H plus as it saves sodium.

Volume trumps pH balance in that scenario.

Volume wins.

What about potassium?

Big connection there, too.

Low potassium, hypokalemia tends to stimulate H plus secretion and also ammonia production.

So hypokalemia often goes hand in hand with metabolic alkalosis.

And high potassium, hyperkalemia.

That tends to suppress ammonia production and excretion.

So hyperkalemia can impair the kidney's ability to get rid of acid, contributing to metabolic acidosis.

So really important interplay.

Hormones must play a role, too.

Aldosterone, cortisol.

Definitely.

We mentioned aldosterone already.

It strongly stimulates H plus secretion in the collecting ducts.

Glucocorticoids like cortisol also stimulate the NaH exchanger in the proximal tubule.

The two in the system.

And finally, diuretics.

They make you pee more.

But how do they impact acid base?

It depends on how they work.

Carbonic anhydrase inhibitors like acetazolamide block bicarb reabsorption.

So they cause bicarbonate loss and tend towards acidosis, making urine alkaline.

Potassium -sparing diuretics also tend to alkalinize urine by reducing H plus secretion in the distal nephron.

But not all diuretic.

No.

Loop diuretics like furosemide and thiazide diuretics usually increase acid excretion and often lead to metabolic alkalosis.

This isn't a direct effect on H plus transport itself, but rather a consequence of causing volume contraction and potassium loss.

Both of which, as we just discussed, powerfully stimulate H plus secretion.

Wow.

Okay.

That really paints a picture of just incredible complexity and precision.

The kidney juggling bicarbonate reclamation, non -volatile acid excretion, buffer synthesis.

It's amazing.

It really is.

Understanding how all these transporters, enzymes, and regulatory factors work together is fundamental.

It's the basis for figuring out what's gone wrong in so many clinical situations involving acid -based disturbances.

We've just navigated a really complex piece of physiology.

Taking this deep dive means you've grasped concepts that are crucial for understanding health and disease.

Seriously.

Well done.

Absolutely.

Remember, you're part of the last -minute lecture family, and tackling challenging material like this is exactly what builds mastery.

You are definitely capable of getting this.

So, to leave you with a final thought.

We saw how the kidney sometimes prioritizes maintaining blood volume, even if it means letting pH drift slightly alkaline.

It makes you wonder, doesn't it?

What other hidden trade -offs, what other balancing acts are going on inside our bodies all the time that we aren't even aware of?

It's a great question.

Physiology is full of these intricate compromises and priorities, all aimed at keeping the whole system running.

There's always more to discover.