Chapter 62: Cerebral Blood Flow, Cerebrospinal Fluid, and Brain Metabolism

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If I were to pinch off the primary blood supply to your brain right now, you wouldn't just feel a little groggy.

No.

Not at all.

You wouldn't get a headache.

In about, I'd say, five to ten seconds, you would be completely, entirely unconscious.

Yeah.

It's incredibly fast.

Right.

And as you sit here listening to this, your brain only accounts for roughly, what, two percent of your total body weight.

So for take, yeah.

Two percent.

But it is currently stealing this massive, like, fifteen percent of your resting cardiac output.

So welcome to the most demanding, greedy, and honestly fragile organ in your entire body.

It really is.

I mean, it's remarkably greedy.

And understanding that greed is kind of the perfect entry point into the medical physiology of the brain.

The whole reason it demands so much blood, so constantly, is its absolute inability to store energy or oxygen.

Like it has no reserves at all.

Basically none.

It lives entirely in the present tense, second by second.

The metabolic fires inside those neurons are just burning so hot and fast that without a continuous, uninterrupted stream of oxygen.

Just shuts down.

Exactly.

The system simply shuts off to protect itself.

Wow.

Five to ten seconds to unconsciousness.

That really sets the stakes.

Okay, let's unpack this.

Our mission in this deep dive is to trace this meticulous life support system from start to finish.

Sounds good.

We need to map out the anatomy of how the blood gets there without, you know, destroying the tissue.

Then explore the brilliant chemical mechanisms the brain uses to regulate its own supply.

Right.

The auto -regulation.

Yeah.

And then look at how the integrated fluid systems physically cushion it.

And finally, understand the severe clinical outcomes when, well, any part of this plumbing fails.

And to understand the function, we really have to start with the physical architecture.

Okay.

Lay it out for me.

So,

blood travels up the neck through four main arterial highways.

You've got the two carotid arteries in the front and the two vertebral arteries in the back.

But wait.

If you have these massive high -pressure arteries coming straight off the aorta and they just slam directly into the delicate, kind of jelly -like brain tissue,

wouldn't the sheer pressure just cause damage?

Oh, it absolutely would.

Which is exactly why they don't go straight in.

Oh, they don't.

No.

They actually merge at the base of the brain into this, like, specialized arterial roundabout called the circle of Willis.

Oh, right.

The circle of Willis.

Yeah.

By pooling the blood there first, it equalizes the pressure.

And from that roundabout, the blood travels along the outer surface of the brain through what are called pile arteries.

Pile arteries.

And then from the surface, smaller branches called penetrating arteries and arterioles literally dive down vertically into the brain tissue.

Almost like roots diving down into soil.

Yes.

Great analogy.

And as those penetrating vessels dive in, they actually bring a tiny microscopic extension of the space surrounding the brain down with them.

Wait, a space around the vessel?

Yeah, it's called the Virchow -Robbins space.

Virchow -Robbins.

Right.

And we will definitely come back to that space later.

But eventually, those penetrating vessels branch out into a massive, really dense network of capillaries.

And that's where the magic happens.

Exactly.

That is where the actual life or death exchange of oxygen, nutrients, and carbon dioxide happens.

Normal flow down in those capillaries is about 50 to 65 milliliters per 100 grams of brain tissue every minute.

OK, but wait.

If the brain is so greedy, how does it adjust that flow?

Like, I'm sitting here passively listening, right?

But then I suddenly start doing, I don't know, complex calculus.

Right.

Your metabolic demand goes up.

Yeah, my neurons are firing harder.

They need more energy.

How do those blood vessels deep in the tissue actually know to open up and deliver more blood?

Well, it relies on this brilliant chemical feedback loop.

And it's based entirely on the brain's own waste products.

Waste products, really?

Yeah.

The primary chemical regulators are carbon dioxide, hydrogen ions, and oxygen.

Let's trace the CO2 pathway first, because it's honestly the most powerful.

OK, let's trace it.

So when your neurons fire harder during that calculus problem, their metabolic rate spikes, right, and they produce more CO2 as a byproduct.

Makes sense.

That CO2 diffuses out of the cells and into the surrounding tissue fluids, where it immediately combines with water to form carbonic acid.

And acids are unstable, right?

So that carbonic acid is going to, like, break down.

It dissociates almost instantly.

And when it breaks apart, it releases hydrogen ions, or H+.

Right.

It is actually these hydrogen ions that trigger the smooth muscle in the blood vessels to relax and dilate wide open.

Ah.

So, to trace the cause and effect here, the brain works hard, produces CO2, which turns into acid.

Right.

And then that acid forces the local blood vessels to dilate.

More blood rushes in, which washes away the excess acid, bringing the local pH back to normal.

You nailed it.

That makes perfect sense, because, I mean, if the tissue stays too acidic, wouldn't that start to depress the neuronal activity entirely?

That is the exact mechanism.

Yeah, it regulates its own environment to prevent a shutdown, and the sensitivity is just incredible.

How sensitive are we talking?

Well, if we look at the data mapping arterial CO2 against cerebral blood flow, the relationship is just a steep cliff.

If your arterial CO2 increases by just 70%, the blood flow to your brain literally doubles.

Wait, it doubles just from a 70 % bump in CO2?

Literally doubles.

Wow.

What about the other regulator you mentioned, like oxygen?

Oxygen acts more as a critical safety net.

So normal oxygen pressure in the brain tissue is around 35 to 40 millimeters of mercury.

If that pressure drops below 30, the blood vessels immediately dilate.

The brain essentially senses it's suffocating, you know, and it opens the floodgates to prevent derangement of function, or a coma, which actually occurs if tissue oxygen drops below 20.

Man.

Okay, so that explains the chemistry, but there's this like mechanical disconnect here that I'm struggling with.

What do you mean?

Well, a blood vessel is just a tube of muscle and endothelial cells, right, it's not a neuron itself.

Right.

So when the neuron is firing, how is the message actually passed from the nerve synapse over to the blood vessel?

Oh, that is such a good question.

That brings us to the star players of the microcirculation.

The astrocytes.

Astrocytes.

Yeah, these are specialized star -shaped glial cells.

They aren't neurons, but they provide the structural and chemical scaffolding for the brain.

They act as the vital middlemen.

Okay.

An astrocyte has these tiny extensions called foot processes.

It wraps some of these feet around the active neuronal synapses, and it wraps other feet tightly around the nearby blood vessels.

Oh, so it's physically touching both systems at the exact same time.

Precisely.

So when an excitatory neuron fires, it releases a neurotransmitter called glutamate into the synapse.

The astrocyte foot process absorbs some of that glutamate.

Gotcha.

That triggers a sudden influx of calcium inside the astrocyte, and then that calcium wave travels through the astrocyte over to the feet wrapped around the blood vessel.

And that does what?

It causes the astrocyte to release vasoactive metabolites, primarily nitric oxide, and that nitric oxide diffuses directly into the vessel wall, forcing it to dilate.

That is wild.

The neuron fires, the astrocyte basically hears the chemical signal and immediately commands the blood vessel to bring more supplies.

Exactly.

But how do we actually observe this happening in a living brain?

Like how do we know this?

Well, researchers originally mapped this by injecting a radioactive substance, like xenon, into a patient's carotid artery.

Whoa, radioactive.

Yeah, and by placing hundreds of tiny radiation detectors against the skull, they could track the speed of the radioactive blood flowing through different regions.

That sounds intense.

It was.

But today, the gold standard is FMRI, or functional magnetic resonance imaging.

It measures the BOLD -D signal, the blood oxygen level dependence signal.

How does the BOLD signal actually work, though?

It relies on this fascinating quirk of physics.

Hemoglobin that is carrying oxygen, oxyhemoglobin, is diamagnetic.

That means it slightly repels a magnetic field.

But when it drops that oxygen off to the hungry neurons, it becomes deoxyhemoglobin.

And deoxyhemoglobin is paramagnetic, meaning it is attracted to a magnetic field.

Oh, so the MRI machine is essentially acting like a giant, highly sophisticated metal detector.

Basically, yeah.

It's looking for the iron in the blood, but it can only really see that iron when the firing neurons have stripped the oxygen away from it.

That is a brilliant way to picture it.

The machine detects that magnetic shift, allowing us to indirectly estimate regional blood flow based on where oxygen is being rapidly consumed.

That's so smart.

There is a classic experiment detailing this process.

Researchers took a cat and mapped the blood flow in its occipital cortex, you know, the visual processing center at the back of the brain.

And they simply shined intense light into the cat's eyes for half a minute.

Let me guess.

The blood flow in that specific area just spiked.

Immediately.

If you look at the charting from the experiment, the moment the light turns on, blood flow specifically in that visual area shoots up to almost 140 % of normal.

And the instant the light turns off, the line drops right back down to baseline.

It proves how incredibly localized and rapid this blood flow coupling really is.

OK, so that handles local demand when a specific brain area is working hard.

But what about whole body pressure changes?

Right.

Like if I'm lying down on the couch and then I suddenly jump up and sprint to catch a bus, my systemic blood pressure fluctuates wildly.

Doesn't that blast those fragile brain capillaries with a massive surge of pressure?

It would, but that is prevented by a system called autoregulation.

The brain has this remarkable ability to keep its internal flow steady despite the chaos happening in the rest of the body.

How steady?

Extremely.

If you chart mean arterial pressure on a horizontal axis from about 60 all the way up to 150 millimeters of mercury,

the corresponding line for cerebral blood flow is completely flat, horizontal.

So even if my body's overall blood pressure drops to 60 or spikes all the way up to 150,

the volume of blood entering my brain doesn't change at all.

Not at all.

It's like a water pressure regulator on a house.

The water main out of the street might be surging or dropping, but the regulator at the intake pipe ensures the shower inside always has the exact same steady flow.

Yes, exactly.

The brain's blood vessels actively constrict when systemic pressure is high to block the surge and they actively dilate when systemic pressure drops to pull more blood in.

That's incredible.

It is, but there is a dangerous clinical catch.

What is it?

If someone has chronic high blood pressure, chronic hypertension,

this entire auto -regulation window shifts to the right toward higher pressures.

Wait, why does it shift?

Does the brain just get used to it?

It physically changes.

It undergates vascular remodeling, the smooth muscle in the blood vessels, hypertrophies.

It thickens to withstand the constant pounding of the high pressure.

Oh, I see.

And this remodeling successfully protects the delicate brain capillaries from the high pressure, but it creates a severe vulnerability on the low end.

Because they can't open up.

If a chronically hypertensive patient's blood pressure drops rapidly, even to a level that you or I would consider completely normal, they can suffer severe brain ischemia.

Their thickened vessels just can't dilate enough at that lower pressure because their baseline has been permanently altered.

Wow.

So the body adapts to survive the hypertension, but that very adaptation creates a totally new deadly vulnerability.

Exactly.

What happens in extreme acute emergencies, though, like if a powerlifter is straining to lift a massive weight and their pressure spikes way past 150?

Does the auto -regulation just fail?

In those extreme moments, the sympathetic nervous system acts as the ultimate emergency break.

Oh, really?

Yeah.

Normally, sympathetic nerves don't play a huge role in day -to -day brain blood flow.

But during strenuous exercise or massive circulatory stress, they fire aggressively.

They powerfully constrict the large and intermediate arteries feeding the brain.

So they just clamp down.

Right.

This creates an intentional bottleneck, purposefully restricting flow to prevent those stroke -inducing high pressures from ever reaching the tiny capillaries deeper inside.

OK, so the big pipes are fiercely guarded.

But once we get past those bottlenecks and down to the microcirculation, is the plumbing just like uniform everywhere in the brain tissue?

Not at all.

Capillary density directly mirrors metabolic need.

So the gray matter is where the actual neuronal cell bodies and synapses live.

It has a metabolic rate four times higher than the white matter, which is mostly just the connecting axons.

Consequently, gray matter has four times the density of capillaries compared to white matter.

And the physical structure of these capillaries is really unique.

Unique how?

Like, they are built differently than the capillaries in my arm or my lungs.

Very differently.

In most peripheral tissues, capillaries have these tiny slits between the endothelial cells that allow fluids and nutrients to leak out easily into the surrounding tissue.

But brain capillaries are famously non -leaky.

They are buttressed on all sides by glial feet, those tiny projections from the same astrocytes we discussed earlier.

Oh, the astrocytes again?

Yep.

The astrocytes physically wrap around the capillary wall, structurally supporting it against pressure and strictly preventing unwanted fluid from leaking into the brain tissue.

Okay, so it is a pristine, heavily guarded,

tightly sealed plumbing system.

But here's where it gets really interesting.

What happens when that meticulous plumbing gets clogged or bursts?

Well, when the plumbing fails, you get a stroke.

Right.

Up to 10 % of elderly people eventually suffer a serious disruption of brain function due to a stroke.

And the most common type is an ischemic stroke.

There's a blockage, right?

Yes.

It occurs when an arteriosclerotic plaque triggers the formation of a blood clot that totally blocks an artery, cutting off the downstream flow.

So the symptoms a patient shows in the ER depend entirely on exactly which artery got blocked.

Precisely.

Let's trace a classic presentation.

Suppose the middle cerebral artery on the left side of the brain is blocked by a clot.

That specific artery supplies the mid -portion of the left hemisphere.

The patient will likely suffer damage to the Wernicke area, meaning they lose speech comprehension.

Wow.

They will also lose function in the Broca area, meaning they lose the motor control required for word formation.

They physically cannot speak the words they want to say.

That's terrifying.

And because the left hemisphere of the motor cortex controls the right side of the body, they will suffer spastic paralysis on their right side.

It's devastatingly precise.

Cut one pipe and you can map the exact loss of humanity.

What about the other mechanism of stroke?

Well, in about 15 to 20 percent of strokes, the vessel doesn't block, it ruptures.

This is a hemorrhagic stroke, and it is usually driven by high blood pressure.

So the blood just spills out?

Yeah, blood spills violently into the brain vault, compressing and destroying the local tissue through sheer physical pressure.

We also have to acknowledge silent strokes.

Silent strokes.

What are those?

These are— But physically, the brain is a soft, jelly -like organ trapped inside a hard -gone box.

It must have some kind of physical shock absorber, or every time we jogged, we'd give ourselves a concussion.

It does.

It has the cerebrospinal fluid or CSF system.

The entire cranial cavity has a volume of about 1 ,600 to 1 ,700 milliliters.

About 150 milliliters of that is Cure CSF, and the brain actually has a specific gravity that is almost identical to this fluid.

There's only about a 4 % difference.

Oh, wow.

Because they weigh almost exactly the same relative to their volume, the brain literally floats in this fluid bath.

Okay, I have to push back on the physics of this, though.

Okay, shoot.

If the fluid perfectly cushions the brain, and they have the same specific gravity so they move together, how do boxers get brain damage from a punch?

Why do concussions happen at all if the shock absorber is perfectly calibrated?

That is an excellent question, and it comes down to inertia and the mechanics of coup en contre coup injuries.

Coup en contre coup.

Let's say a boxer takes a massive right hook to the jaw.

The impact causes the skull to rapidly accelerate to the left.

The fluid on the side of the impact is incompressible, so it pushes the brain along with the skull.

Usually there is no damage on the side of the head that actually got hit.

Wait, really?

But the brain itself is a heavy mass.

It wants to stay still.

Yes.

The sudden acceleration of the skull causes the bone on the opposite side of the impact to pull away from the brain for a split second.

This literally creates a momentary vacuum space in the fluid on the far side.

But the skull quickly stops accelerating, and that vacuum violently collapses.

The brain is pulled by the collapsing vacuum and slams into the inner wall of the skull on the side opposite the original blow.

That is a conche coup injury.

Oh, wow.

So a hit to the right side of the head actually causes a massive bruise on the left side of the brain because of a collapsing vacuum.

Exactly.

And if the brain bounces back and bruises on the same side as the impact, that's called a coup injury.

The poles of the frontal and temporal lobes are incredibly vulnerable to this because they sit right next to sharp bony ridges at the base of the anterior skull.

Ouch.

So where does all this protective fluid actually come from?

It's not just stagnant water sitting in the skull.

Not at all.

It is a continuously flowing river, produced and absorbed every single day.

Picture a river system originating from deep underground springs.

OK, I'm picturing it.

Those springs are called the choroid plexuses.

They are cauliflower -like bundles of blood vessels located deep inside the brain's hollow spaces, primarily in the two lateral ventricles.

How do those vessels actually turn blood into this clear CSF fluid?

Through active transport.

The epithelial cells lining the choroid plexus actively pump positively charged sodium ions out of the blood and into the ventricle space.

Because positive electrical charges attract negative charges, chloride ions get pulled right along with the sodium.

Now you have a high concentration of sodium chloride salts sitting in the ventricle space.

Ah, and water always follows salt through osmosis.

Yes.

The high salt concentration creates an osmotic gradient that pulls water directly out of the blood plasma, creating the fresh cerebrospinal fluid.

And then where does it go?

From those lateral ventricle springs, the fluid flows into the third ventricle, then down a narrow, treacherous tube called the aqueduct of Sylvius and into the fourth ventricle.

Got it.

From there, it exits the deep brain through three small openings, the foremena of Lusca and Magendi, and flows into the cisterna magna, which is a large pooling area at the base of the brain.

Finally, it washes up over the entire exterior surface of the brain through the subarachnoid space.

Well, a river has to empty into an ocean eventually, otherwise it floods, right?

Where does the CSF go?

It empties back into the venous blood.

It gets absorbed through microscopic structures called arachnoidal villi.

Arachnoidal villi.

Yeah.

These are finger -like projections that push from the subarachnoid space into the large venous sinuses.

They function as one -way pressure valves.

When the CSF pressure is higher than the venous blood pressure, the valves blow open and the fluid empties into the veins.

And as it washes over the brain, it acts as the brain's unique trash disposal system, right?

The glymphatic system.

How does that physically work without traditional lymph nodes?

It's an ingenious mechanism.

Remember those Vircho Robin spaces we talked about earlier?

Right.

The tiny spaces surrounding the penetrating arteries as they dive down into the brain tissue.

Exactly.

The brain uses those paravascular spaces as a makeshift lymphatic system.

And here is how it drives the fluid.

It relies on the physical pulsing of the arteries themselves.

Wait.

Just the heartbeat?

Yep.

Every time your heart beats, the arteries expand slightly.

That physical pulsation acts like a mechanical pump, driving the CSF down into the tissue along the arteries, flushing the tissue of dead white blood cells and leaked proteins.

That is so elegant.

And then pushing that waste out along the veins into the subarachnoid space to be cleared.

Okay, so we have this intricate river system.

Production,

circulation, flushing, and absorption.

But what happens if that river gets dammed up?

That's bad news.

You said normal CSF pressure is about 10 millimeters of mercury.

If a blockage happens, where does the pressure go?

It causes a condition called hydrocephalus, which literally translates to excess water in the cranial vault.

Clinically, we divide it into two types.

Okay, what's the first?

If the blockage happens deep inside the plumbing, say a tumor blocking that narrow aqueduct of sylveus, it is called non -communicating hydrocephalus.

The fluid is trapped deep inside the ventricles.

And it just keeps building up?

Because it keeps being produced, yes.

It violently expands the ventricles, crushing the brain tissue outward against the rigid skull.

Oh, God.

And the second type?

Communicating hydrocephalus.

That occurs when the fluid makes it out of the deep ventricles just fine.

But the arachnodal villi, those one -way exit valves, are blocked.

Blocked by what?

Often caused by debris from a severe infection or red blood cells from a hemorrhage gumming up the valves.

The fluid collects on the outside of the brain.

In babies where the skull bones haven't completely fused together yet, either type causes the entire head to swell tremendously as the pressure builds.

How do you fix that?

The most common treatment is surgically placing a shunt, a long silicone tube from the ventricle all the way down to the abdominal cavity to give the fluid an artificial exit route.

So the physical pressure is managed.

But what about chemical invaders?

How does the brain stop toxins floating in the blood from slipping through those capillaries and poisoning the neurons?

It uses the famous blood -brain barrier.

We mentioned the tight junctions between the endothelial cells of the brain capillaries earlier.

Yeah, we had a monoleaky one.

They are tightly fused together.

They allow vital molecules like water, oxygen, CO2, and lipid -soluble substances like alcohol to pass through easily.

But they act as an impenetrable brick wall for large, non -lipid -soluble molecules and plasma proteins.

Which is great for keeping out toxins.

But I mean, that makes treating brain diseases incredibly frustrating for doctors, right?

Because many life -saving drugs just bounce right off the barrier.

It's a huge hurdle in pharmacology, yes.

But isn't there a physiological exception?

Some parts of the brain have to know what's in the blood.

You're absolutely right.

Certain sensory areas, like the hypothalamus, actually lack this strict barrier.

And they have to.

To monitor things.

Right.

The hypothalamus contains sensors for osmolality, glupose, and hormones like leptin and angiotensin II.

If it were blocked off, it couldn't do its job.

It has to be able to physically taste the blood directly in order to accurately regulate your thirst, appetite, and your sympathetic nervous system.

That makes perfect sense.

But what if the protective mechanisms fail and the actual brain tissue itself starts swelling?

Say, from a massive concussion or from hyponatremia where someone drinks so much excess water that it dilutes their blood sodium levels.

Brain edema is a nightmare scenario for an ER doctor.

Because the skull absolutely cannot expand.

Once the brain tissue begins to swell, it triggers two lethal positive feedback loops.

Two of them?

Okay, what's the first?

First, the swollen tissue mechanically crushes the local blood vessels.

Crushed vessels mean ischemia, a severe lack of blood flow.

The brain panics at the sudden lack of oxygen and triggers massive vasodilation to try and force more blood in.

But dilating the vessels just dramatically increases the local capillary pressure, doesn't it?

Exactly.

Which pushes more fluid into the tissue, causing even more edema.

Wow.

A completely vicious cycle.

The cure literally makes the disease worse.

And the second loop?

The ischemia deprives the cells of oxygen.

Without oxygen, the nones cannot synthesize ATP.

Without ATP, the sodium pumps on the neuron cell membranes shut down.

So sodium builds up?

Massively inside the cells.

And just like we saw in the choroid plexus, water always follows sodium.

Water rushes into the cells through osmosis, causing the neurons themselves to swell until they literally burst.

Oh my god.

The vessels are leaking, the cells are bursting, and the skull is completely unyielding.

How do you even begin to treat that before the patient dies?

You need heroic measures to rapidly reverse the physics.

You might inject a highly concentrated osmotic substance, like intravenous mannitol.

What does that do?

It makes the blood plasma so hypertonic that it forcefully sucks the water out of the brain tissue back into the blood by brute force osmosis.

Or you surgically puncture the ventricles with a needle to drain the fluid directly and rapidly relieve the mechanical pressure.

You mentioned the ATP pumps failing.

That brings us to the core of everything we've talked about.

Yeah.

Energy.

Let's finish by looking at how the brain actually fuels all this activity.

It all comes down to pumping ions.

Every single time a neuron fires an action potential, positively charged sodium and calcium rush into the cell and potassium rushes out.

The vast majority of the brain's massive energy demand is spent simply running the pumps that force those ions back to their resting states so the neuron is ready to fire again.

And to run those pumps, it needs continuous oxygen and glucose.

But unlike a muscle cell, it has zero backup plan.

Like if I sprint as hard as I can, my leg muscles will run out of oxygen, but they can switch to anaerobic metabolism.

Exactly.

They produce lactic acid, they cramp up, but the cells survive.

The brain can't do that at all, can it?

No.

It has virtually zero capacity for anaerobic metabolism.

It relies, well, in a type 1 diabetic who secretes absolutely zero insulin, their brain function is perfectly fine because the glucose in the blood still freely enters the neurons without needing the insulin key.

Oh, wow.

But if a diabetic patient accidentally injects too much insulin over treatment, what happens to the rest of the body?

Oh, I see.

The massive dose of insulin forces all the muscle and liver cells in the body to throw their doors open and frantically suck up all the available glucose out of the bloodstream.

Yes.

The muscles and liver vacuum the blood completely clean of glucose.

Now, the blood reaching the brain has almost no glucose left in it.

And the brain has no backup stores.

Right.

It relies on that second -by -second delivery and suddenly it's starving.

The patient will suffer acute mental derangement, hallucinations, and very quickly a hypoglycemic coma.

All of this is caused by an overdose of insulin, not a lack of it.

Because the brain is so greedy, so absolutely demanding, that it will just shut down the entire human experience if its metabolic demands aren't met perfectly.

That is the fragile reality of our biology.

And here's a final thought for you to ponder as you digest all of this.

Given how fiercely the brain regulates its oxygen, its glucose, and its pressure, and knowing how the glymphatic system relies on the physical pulsing of blood vessels to wash away daily metabolic debris, consider the long -term impact.

Think about how subtle chronic disruptions to our daily blood pressure or even our daily hydration might quietly impact our cognitive longevity over decades.

It is not always the sudden, violent stroke that harms us.

Sometimes it's the slow, quiet disruption of the plumbing.

That is definitely something to think about the next time you feel a bit dehydrated during a late -night study session.

Alright.

Welcome to the dynamic, intricate, and high -stakes world of medical physiology.

Thank you for listening from the Last Minute Lecture Team.