Chapter 11: Digestion and Absorption of Nutrients

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You know, you build a house, you kind of expect the walls to, well, stay put.

Right.

Yeah.

I mean, it's a structure.

You lay the foundation, you put up the drywall, and you just assume it's permanent.

Exactly.

Yeah.

It would be terrifying if your drywall just spontaneously dismantled itself while you were trying to live in the house.

Right.

Like just dissolving right in front of you.

But imagine a house that completely tears down and rebuilds its own inner walls.

Yeah.

Literally from the ground up every three to six days.

And doing all that while you're still inside.

Yeah.

While you're actively cooking meals and running the plumbing.

Yeah.

That incredibly dynamic,

just almost violent cycle of destruction and renewal is the exact reality of human gastrointestinal physiology.

It really is.

It's an extreme environment.

And that is exactly what we are focusing on today.

We are welcoming you, the college students staring down GI physiology for the very first time to a custom tailored deep dive.

So our mission today is to conquer Chapter 11 of your textbook, Gastrointestinal Physiology.

Chapter 11.

We are going to focus entirely on how the food you eat is broken down into small absorbable molecules and then how it's transported into the blood.

And we should mention we're doing this using zero outside information.

Exactly.

Just exactly what is mapped out in your text.

No extra confusion.

Okay.

Let's unpack this.

The textbook spends a lot of time talking about the microscopic architecture of the small intestine.

Uh -huh.

The physical structure.

Yeah.

And it keeps mentioning this thing called the brush border as if it's some sort of, I don't know, magical barrier.

What exactly are we looking at structurally?

So to understand the brush border, you really have to think about surface area.

I mean, the small intestine's whole job is absorption.

Right.

And to do that efficiently, it needs maximum contact with your food.

So it creates folds upon folds.

First you have Kirk rings folds, which are these macroscopic ridges you can actually see.

Okay.

Like wrinkles in a rug.

Yeah, exactly.

And projecting off of those ridges are tiny finger -like structures called villi.

And it gets even smaller from there, right?

It does.

On the very surface of the columnar epithelial cells, which by the way we call enterocytes, you have even tinier projections called microvilli.

So it's basically like a towel covered in loops, and then those loops are covered in microscopic fuzz.

That microscopic fuzz is the brush border.

It massively multiplies the absorptive surface area.

But as we hinted at with the house metaphor, this environment is intensely dynamic.

Right, the rebuilding process.

Yeah.

Down at the base of these villi are valleys called intestinal crypts.

That's where the stem cells live.

Oh, so that's the construction crew.

Exactly.

They constantly divide, creating new enterocytes for absorption and goblet cells, which secrete protective mucus.

These new cells basically act like they're on a conveyor belt, constantly migrating up the side of the villus.

So what happens when they actually reach the top?

Do they just fall off?

Basically, yeah.

They are extruded and shed right into the open space or the lumen of the intestine.

Wow.

But interestingly, not all the daughter cells move up.

Some migrate down to the very base of the crypt and become paneth cells.

Paneth cells.

What do they do?

They stay put.

They act as the mucosal defense force, secreting bacterial destroyers like lysozymes and defensins.

Which brings up a massive clinical reality, actually.

Because this entire population of epithelial cells turns over in just three to six days, the intestinal mucosa is incredibly vulnerable.

Oh, absolutely.

Like if a patient is undergoing radiation or chemotherapy,

those treatments are literally designed to target rapidly dividing cells, you know, to kill cancer.

Right.

They attack fast -growing tissue.

But the gut takes a massive hit because it's practically the fastest rebuilding house in the body.

It's a profound vulnerability, and I think it really highlights why the environment must remain perfectly calibrated.

Definitely.

So let's talk about the actual breakdown process.

The text divides digestion into two distinct phases, luminal digestion and membrane digestion.

I always just assumed digestion was just stomach acid and enzymes floating around like a big soup.

Well, you're not wrong, but that soup is only luminal digestion.

The text also calls it cavital digestion.

Okay, cavital, like in a cavity.

Exactly.

That involves the enzymes from your saliva, your stomach, and your pancreas just floating freely in the open space of the gut, hacking away at the complex molecules in your food.

But that soup only gets you partway there.

Right.

The second phase, membrane digestion, is where the fine -tuning happens.

The textbook calls it contact digestion.

Does that mean the enzymes are actually like physically attached to the intestinal wall?

Yes.

And honestly, membrane digestion is really the more accurate term.

These are hydrolytic enzymes that are synthesized right inside the enterocytes and inserted directly into their own apical membranes.

Wow.

So they are built into the wall itself.

Literally built in.

So a nutrient trying to get from the center of the intestine into your blood isn't just floating through an open door, it's facing a microscopic obstacle course.

Oh, it's a huge obstacle course.

If you look at figure 11 .2 in your text, it maps out this barrier beautifully.

It's kind of helpful to think of it like a medieval castle's defenses.

I love a good castle metaphor.

Let's hear it.

So a nutrient has to survive eight distinct layers of a siege.

First, it hits the moat, which is an unstirred layer of fluid.

Second, it faces spiked vines, that's the glycocalyx, which is a fuzzy coat over the microvilli.

Okay, two layers down.

Third, it hits the outer wall, which is the apical cell membrane itself.

Okay, so it breaches the wall, now it's inside the cell.

Right.

Then fourth, it has to cross the castle courtyard, which is the cytoplasm of the enterocyte.

Fifth, it hits the back wall to exit the basolateral cell membrane.

So it's out of the cell, but still not in the blood.

Exactly.

Sixth, it enters the intracellular space outside the cell.

Seventh, it has to pierce the basement membrane holding the tissue together.

And finally, layer eight, it crosses the actual membrane of the capillary or lymph vessel to actually get into the bloodstream.

Layer eight.

That is a serious microscopic gauntlet.

It really is.

With that obstacle course in mind, let's explore how our bodies actually push specific macronutrients across it.

I want to start with the most abundant fuel source carbohydrates.

Good place to start.

Carbohydrates account for about 50 % of the calories we ingest, mostly as starch, sucrose, and lactose.

And starch is the big complex one, right?

It is.

It's made of two polysaccharides.

You have amylose, which is a straight chain of glucose molecules linked together by alpha -1 filler for glycosidic bonds.

Okay, straight chains.

And then you have amylopectin, which is similar but features branched alpha -1 -theral -6 linkages every 20 to 30 units.

So we have straight chains and we have branches.

How does the soup phase, the luminal digestion, handle those?

Salivary and pancreatic alpha amylase are our first line of attack.

They float in the lumen and randomly snip the interior alpha -1 -theral -4 bonds of the starch.

But they're limited, aren't they?

Very limited.

Figures 11 .3 and 11 .4 in the text illustrate this perfectly.

Amylase cannot break those branch points, the 1 -theral -6 linkages, and it can't even break the bonds right next to them.

So it's like having a pair of scissors that can cut a straight string, but it just completely jams whenever it hits a knot.

That is exactly it.

So after amylase does its job, it leaves behind a mess of fragments, maltose, maltotrios, and these branched pieces called alpha -limit dextrins.

And this is where those enzymes built into the wall of the membrane digestion phase take over.

Right, the finishing team.

Yeah, the text describes a whole finishing team on the brush border.

You have glucoamylase handling the remaining straight chains.

Isomaltase specifically snips those stubborn branch knots.

Because the amylase couldn't do it.

Exactly.

And then you have enzymes for the other sugars you ate.

Sucrase breaks down sucrose, lactase splits lactose, and trihalase breaks down tricolus.

So by the end of this assembly line on the membrane wall, all those complex, bulky carbohydrates are reduced to just three simple sugars, glucose, galactose, and fructose.

But chopping them up is only half the battle.

They still have to get across that eight -layer castle defense.

How do these simple sugars actually force their way into the blood?

So the primary entry mechanism for glucose and galactose is a fascinating piece of cellular machinery called SGLT -1.

SGLT -1.

Yeah, figure 11 .6 diagrams this.

SGLT -1 is a secondary active transport system.

The carrier protein binds to the sugar, but it also binds to sodium.

For every sugar molecule, it drags two sodium ions into the cell.

Wait, why use sodium?

Why not just have a tunnel that lets glucose passively slide through on its own?

Because passive diffusion is just too slow and inefficient to capture all the calories you need.

The cell uses sodium to create a literal chemical vacuum.

A vacuum, how?

On the back wall of the cell, the basolateral membrane, there's a pump that uses ATT energy to constantly shove sodium out of the cell.

This makes the inside of the cell highly depleted of sodium.

Oh, I see.

So the SGLT -1 transporter on the front wall essentially opens a valve, and the sodium from the gut comes rushing in to fill the vacuum, forcibly dragging the glucose along with it.

You nailed it.

It's brilliant.

And the text proves how powerful this vacuum effect is with an experiment in figure 11 .5.

The guinea pig intestine one.

Yes.

Researchers perfused a guinea pig intestine with glucose.

When they provided oxygen, meaning the cells could make ATP and run those sodium pumps on the back wall, the glucose transport curve skyrocketed and eventually hit a maximum saturation point.

Which proves it's an active, mechanically driven system.

Exactly.

So what happened when they cut off the oxygen?

Well, in an anaerobic environment, the cells couldn't make ATP, so the sodium pumps failed.

The vacuum disappeared.

Oh, wow.

Suddenly, the active transport flatlined, and the graph just showed a slow, linear crawl of passive diffusion.

It perfectly isolates the mechanism.

That's super elegant.

It is.

Now, I should mention fructose is the odd one out here.

It ignores the sodium vacuum entirely and just uses a transporter called GLUT5 for simple, facilitated diffusion.

But once they are all safely inside the courtyard of the cell,

glucose, galactose, and fructose all use the same exit door in the back wall, right?

Yes.

They all exit the basolateral membrane via a transporter called GLUT2, finally entering the blood.

So what happens when this perfectly tuned sugar factory breaks down?

The most common mechanical failure is lactase deficiency.

In many populations, the activity of that lactase enzyme on the brush border naturally drops off after childhood.

Right.

Lactose intolerance.

Exactly.

Without it, lactose isn't broken down into absorbable glucose and galactose.

Which means it just sits there in the intestinal lumen.

Yeah.

And because lactose is a large, osmotically active molecule, it's basically going to act like a sponge and draw water straight into the gut.

Oh, yeah.

Plus, the resident bacteria are going to absolutely feast on it.

Leading to the classic symptoms of bloating, cramps, and osmotic diarrhea.

Unfortunately, yes.

And we can actually trace this failure in real time using a lactose tolerance test.

The patient drinks 50 grams of lactose, and we monitor their blood glucose.

So normally, as the lactose is digested and the glucose is absorbed by SGLT1, you'd expect a spike, right?

Right.

You'll see blood glucose rise by at least 25 milligrams per deciliter.

But if you see a flat curve, meaning blood glucose doesn't rise at all, it confirms the lactose is just trapped in the gut, pointing directly to a lactase enzyme failure.

Exactly.

Now, moving from carbs to proteins, the body shifts from a relatively safe disassembly process to playing with a biochemical fire.

I'm glad you brought that up, because breaking down sugars seems pretty gentle.

But breaking down proteins means we need enzymes powerful enough to dissolve meat.

It's terrifying if you think about it.

How does the body do that without accidentally digesting its own internal organs?

It requires a highly dangerous, perfectly timed cascade.

Protein digestion begins in the stomach with an enzyme called pepsin, which only activates in low pH acid.

But the really heavy lifting happens in the small intestine thanks to the pancreas, right?

Table 11 .1 details the pancreatic arsenal.

You have endopeptidases like trypsin, chemitrypsin, and elastase that attack interior peptide bonds.

And then exopeptidases like carboxypeptidases A and B that chew away at the external ends.

But the pancreas can't secrete them in their active meat -dissolving form, or it would literally dissolve itself.

It secretes them as inactive precursors, or zymogens.

Right.

They need a spark.

So what provides the spark that lights this whole explosive cascade?

Figure 11 .8 maps the ignition sequence.

The spark is an enzyme sitting safely on the brush border of the intestine called enterocinase.

When the inactive precursor trypsinogen floats out of the pancreas and bumps into the intestinal wall, enterocinase cleaves a specific hexapeptide off its end.

Suddenly, it becomes active trypsin.

And trypsin is the master switch.

The text says it acts autocatalytically.

It does.

So once that first domino falls, trypsin acts as its own trigger, creating a runaway chain reaction to activate more trypsin plus all the other dangerous pancreatic enzymes.

It's a massive explosive amplification.

Wait, hold on.

If trypsin is this powerful and it activates itself, what happens if a little bit of it accidentally activates inside the pancreas before it even reaches the intestine?

Why doesn't the organ just liquefy?

What's fascinating here is the layered security system the body evolved for exactly that scenario.

First, the pancreas synthesizes specific trypsin inhibitors and packages them right alongside the precursors.

Okay, so a chemical buffer.

Second, it isolates the enzymes away from lysosomes within the cells.

But the ultimate failsafe is a literal self -destruct button built into the trypsin molecule itself.

A self -destruct button.

How does a molecule self -destruct?

There is a specific amino acid, an arginine residue, located at position 117 on the trypsin molecule.

If trypsin prematurely activates while still inside the pancreas, other trypsin molecules recognize that exact arginine spot, digest it, and irreversibly destroy the rogue enzyme before the chain reaction can start.

That is brilliant.

It uses its own destructive power against itself to stop a leak.

But let me guess, the textbook gives an example of what happens when that failsafe breaks.

Of course it does.

There is a genetic mutation known as R117H where that critical arginine at position 117 is swapped out for a histidine.

Oh no.

So the self -destruct button is physically gone.

The failsafe is completely disabled.

Trypsin can no longer inactivate itself.

The result is that premature activation cascades violently inside the organ, causing a devastating and painful condition called hereditary pancreatitis.

Wow.

Okay, so assuming the failsafe's work and the dietary proteins are successfully chopped into tiny pieces, we reach an incredible plot twist regarding how they actually cross into the cell.

The transport twist.

Figure 11 .9 shows a graph comparing the absorption rate of an intact dipeptide glycyllucine against a mixture of completely free glycine and free leucine.

You would totally assume the completely broken down free amino acids would cross the barrier faster because they're smaller, right?

That is the logical assumption, yeah.

But the experimental data in the graph proves the exact opposite.

The intact dipeptide is absorbed significantly faster.

Which is so weird.

It is.

But this data proved the existence of an entirely separate parallel transport system dedicated just to small peptides completely distinct from the free amino acid transporters.

And it doesn't even use the sodium vacuum we talked about earlier.

The free amino acids use sodium gradients, just like the sugars.

But this specialized small peptide transporter, called PEP2, uses a hydrogen ion gradient.

Driven by an acidic microclimate hovering right at the surface of the brush border.

The clinical evidence for this parallel system is iron -clad, too, as shown in Figure 11 .11.

The text discusses two hereditary conditions, cystinuria, where a patient genetically lacks the membrane transporter for cationic amino acids,

and Hartnup's disease, where they lack the transporter for neutral amino acids.

Right, so their primary transporter is just broken.

So if you feed them those free amino acids, the main door is locked.

They can't absorb them at all.

But if you feed those exact same patient's dipeptides containing those exact same amino acids, they absorb them flawlessly.

Because the dipeptides use the secondary PEPT1 doorway, bypassing the broken single -ano acid doors entirely.

Exactly.

And once inside the enterocyte, cytoplasmic peptidases snip the dipeptides into single amino acids, which then exit into the blood.

I love that.

It's an elegant built -in backup system.

Okay, we've successfully digested carbs and proteins, but both of those dissolve in water.

They can just float right up to the castle walls.

The body faces a massive physics problem with our next macronutrient.

When you eat a cheeseburger, you are asking a water -based gastrointestinal tract to process And as we all know, oil and water do not mix.

Lipid assimilation is arguably the most complex physiological challenge in the gut.

The text outlines four required steps to pull this off.

The secretion of bile and lipases,

the emulsification of the fats, the enzymatic hydrolysis of the ester linkages, and finally, solubilization into micelles.

Step one actually starts with mechanical force, right?

The physical churning of the stomach literally smashes the giant fat globs into tiny one -micrometer emulsion droplets, vastly increasing their surface area.

Right.

And then the pancreas drops its heavy -hitter pancreatic lipase.

Figure 11 .2 shows the strict positional specificity of this lipase.

Specificity.

How so?

Well, the triglyceride looks kind of like a capital letter E.

The lipase only cleaves the top and bottom rungs, the one and three ester linkages.

It leaves behind two free fatty acids and a two monoglyceride.

But there is a huge mechanical flaw in this system.

Bile salts are required to coat the fat droplets so they don't just merge back into a giant oil slick.

Right.

They act as emulsifiers.

But those same bile salts physically block the lipase from touching the fat.

The enzyme is basically locked out of its own workspace.

It's a huge problem, but the solution is a specialized polypeptide called colipase.

The pancreas secretes it right alongside the lipase.

And what does colipase do?

It acts as an anchor.

It binds firmly to the fat droplet, piercing right through the protective bile salts.

Then the lipase binds directly to the colipase.

It literally tethers the enzyme to the fat so it can do its job.

Here's where it gets really interesting.

So the fats are cut up, but they still hate water.

They're facing that first layer of the castle defense, the unstirred water layer, and they refuse to dissolve into it.

How did they cross the moat?

They use a microscopic fairy service called a micelle.

Figure 11 .13 diagrams this structure beautifully.

A micelle.

What does that look like?

It's a tiny sphere formed by bile salts.

The bile salts arrange themselves with their hydrophilic, water -loving sides facing outward into the intestinal fluid and their hydrophobic, fat -loving sides facing inward.

Oh, clever.

Yeah.

They scoop up all the insoluble fatty acids and monoglycerides into their dry center, creating a true water -clear solution.

Which is mind -blowing chemistry.

The text notes that these micelles diffuse across the unstirred water layer and increase the concentration of lipids right at the cell membrane by 100 to 1000 times.

That's a massive concentration gradient.

They ferry the cargo across the moat and unload the fats directly into the cell membrane.

But the danger isn't over once the fats breach the cell wall.

Free fatty acids act exactly like soap detergents.

Wait, really?

Yeah.

If left alone to float in the cytoplasm, they will literally dissolve the enterocytes' own lipid membranes from the inside out.

They are highly toxic to the cell.

So the cell uses bounders.

As soon as a free fatty acid crosses the apical membrane, a fatty acid binding protein, or

FABP, grabs it.

The bouncer steps in.

Exactly.

The FABP acts like a biochemical straitjacket, shielding the cell's internal machinery from the toxic fat and escorts it directly to the smooth endoplasmic reticulum.

And this is where the cell does something seemingly counterintuitive.

Figure 11 .14 shows that once inside the smooth ER, the cell rebuilds the fat back into triglycerides.

Rebuilds them?

Yep.

There are two routes for this.

The major route is the monoglyceride acylation pathway, where CoA -activated fatty acids are reattached to the two monoglycerides.

Okay, and the other route?

The minor route is the phosphatidic acid pathway, which builds them from scratch, starting from alpha -glyceraphosphate.

Wait.

Let me get this straight.

We spent all this energy churning, anchoring, and micell -faring just to tear the fat apart so it could fit through the door.

And the second it gets inside, we glue it all back together.

Why would the body waste energy doing that instead of just bringing it in whole?

Because of toxicity and logistics.

Bringing it in whole is physically impossible due to the water barrier, but leaving it broken inside the cell is toxic.

Rebuilding it solves the toxicity problem and prepares it for shipping.

Shipping.

Yeah, you can't just dump raw fat droplets into the blood.

They would cause massive embolisms.

Ah.

So rebuilding them is about packaging them safely.

Exactly.

The rebuilt triglycerides are packaged into massive lipoprotein transport vehicles called chylomicrons.

The cell coats the fat with apolipoproteins, phospholipids, and cholesterol.

And getting them into those vehicles requires a specific enzyme, doesn't it?

It does.

The critical assembly step requires an enzyme called MTP -microsomal triglyceride transfer protein.

MTP's job is to physically load the lipids onto apolipoprotein B.

Okay, so the chylomicron is loaded up.

Once it's fully assembled, it is pushed out the back wall of the cell.

But because it's so massive, it can't fit into the tiny blood capillaries.

Where does it go?

It has to enter the wider, more porous gaps of the lacteals, traveling through the lymphatic system before eventually dumping into the bloodstream near the heart.

And when any single part of this massive lipid assembly line breaks, you get statoria excess fat in the stool.

The text outlines several ways this can fail.

It's a fragile system.

Like if a patient has Zollinger -Ellison syndrome,

they secrete so much stomach acid that it overwhelms the duodenum's buffering capacity.

The pH drops so low that it literally denatures the pancreatic lipase.

The scissors break.

Or consider bacterial overgrowth syndrome.

Excess bacteria in the gut prematurely deconjugate the bile acids.

Meaning the micelles can't form.

Exactly.

Without properly conjugated bile acids, those micelle ferryboats can't form.

And the fats can never cross the moat to reach the membrane.

And then there's beta -lipoproteinemia, which is a completely misleading name, right?

Very misleading.

The name implies the patient genetically cannot make apolipoprotein B.

But their ApoB is actually perfectly fine.

So what's the real defect?

The actual defect is that they are missing MTP, that essential loading enzyme.

Without MTP, the triglycerides can't be merged with the ApoB.

Chylomicrons can't form.

So the fats just pile up.

The assembly line halts.

And massive amounts of fat just accumulate, trapped inside the enterocytes.

Which is a disaster, not just because you lose the calories from the fat, but because fat absorption is a required vehicle for our final category of vitamins.

Right.

Table 11 .3 divides vitamins into water -soluble and fat -soluble.

The fat -soluble ones, A, D, E, and K, are relatively simple conceptually.

You just ride along with the fat, right?

Yeah.

They simply hitch a ride inside the lipid micelles and get packaged into the chylomicrons.

If a patient has fat malabsorption, they will inevitably develop fat -soluble vitamin deficiencies.

And the water -soluble vitamins mostly use simple passive diffusion, though folic acid is highly electronegative and requires its own active transport pump.

True.

But the absolute king of complex, high -maintenance vitamin absorption has to be vitamin B12.

Oh, without a doubt.

If we connect the B12 journey to the bigger picture of the GI tract, it is a spectacular multi -organ escort mission.

First, food preparation and stomach acid release B12 from dietary proteins.

And once it's free, it needs protection.

Yes.

Once free, it binds to a glycoprotein in your saliva called haptocorin.

So the saliva provides a bodyguard to protect the B12 from being destroyed by the brutal acid of the stomach.

Precisely.

The bodyguard escorts it safely to the duodenum.

There, the pancreatic enzymes digest the haptocorin away.

So the B12 is exposed again.

It is free again, but it immediately binds to a second escort intrinsic factor, which was secreted all the way back in the stomach by the gastric parietal cells.

And this new B12 intrinsic factor complex has to travel hand -in -hand all the way down to the very end of the small intestine, the terminal ilium.

That is the only place with specific receptors that recognize intrinsic factor and pull the B12 into the body.

This multi -step process raises a crucial clinical vulnerability.

If a patient has an autoimmune condition that destroys their gastric parietal cells, they can't produce intrinsic factor.

Or if a surgeon removes their terminal ilium due to disease, they lose the receptors.

In either case, the chain is completely broken.

The B12 cannot be absorbed, leading to a severe condition called pernicious anemia, where the body fails to produce enough red blood cells.

Though interestingly, the text points out that the liver stores a massive three to five year supply of B12.

So if that absorption chain breaks today, the symptoms of pernicious anemia won't actually show up until years later.

It's an incredible buffer built into a highly sensitive system.

So what does this all mean?

If we step back and look at the entire scope of Chapter 11, there is this beautiful, unrelenting logical chain to gastrointestinal physiology.

It all connects.

It really does.

The macroscopic anatomy, the folds, villi and microvilli, provides the massive surface area.

The membrane proteins like the STLT -1 vacuum and the PBT -1 doorway dictate the incredibly specific physics of transport.

And the complex micellar chemistry overcomes the physical barriers of water and fat.

Exactly.

It all relies on structure supporting function, resulting in the seamless systemic nourishment of the human body.

And it manages to orchestrate all of this under incredibly harsh, acidic and enzymatic conditions, constantly adapting to whatever combination of macronutrients we throw down the chute.

Which brings us to the end of today's Deep Dive.

From the team here at the Deep Dive and our special last minute lecture series, thank you for listening.

We wish you the absolute best of luck in your GI physiology studies.

You've got this.

But before we go, I want to leave you with a final thought to mull over.

We learned today that our enterocytes replace themselves entirely every three to six days.

The house rebuilds itself.

Right.

Think about the sheer scale of that.

Every time you eat a meal next week, you'll be feeding it to a completely new generation of intestinal cells that didn't even exist today.

That is wild.

How does the body maintain such flawless continuous memory of its enzymatic duties across millions of dying and birthing cells every single week?

Think about that as you review your notes.

The house that never stops rebuilding itself.

Until next time.