If you’re ever debating whether you should buy a product marketed as a detox, don’t.
The only things your body needs to detoxify itself are a liver and a kidney.
Although, two kidneys are preferable.
But the kidneys themselves are mysterious little bean shaped black boxes.
“Dirty blood” goes in, clean blood and urine come out.
But how?
In today’s video, we’ll go over the anatomy of the kidneys and the physiology involved
in cleaning your blood.
Hello and welcome, if you’re new here, my name is Patrick and this channel is all about
anatomy and how the body works.
The kidneys have two a few jobs in our bodies like helping us regulate fluid levels in our
bodies, keeping different ions in check, and clearing the waste from our blood and excreting
it through pee.
Each kidney is about the size of your closed fist.
You can find them stradling the sides of the spinal column, just above the small of your
back.
We usually visualize these little beans from the back, because they’re actually behind
the abdominal cavity where you’d find the other big organs.
Each kidney has an adrenal gland on top that pumps out the hormone adrenaline.
And it’s actually kind of intuitive naming since the Latin root for kidney is renal.
The adrenal glands make adrenaline and sit on top of the kidneys.
Coming down from the kidneys are the two ureters, tubes that transport urine made by the kidneys
into the bladder.
From there, a tube called the urethra transports urine outside the body.
Urethras are quite a bit shorter in people with vaginas, making it easier for bacteria
to sneak into their bladders, which is one of the reasons they can experience urinary
tract infections more often.
But we get to see some fun stuff once we crack open the kidney itself.
If you’re learning this in class, you’ll see this classic cross section a lot.
In general, we can break it into three regions: the cortex, medulla, and renal pelvis.
I’m about to blow your mind with how easy these are to remember.
The cortex is the outermost layer of kidney tissue, just like the cortex in the brain
is the outermost layer.
I like to imagine the kidney like a big O and the C, for the cortex, wraps around it.
The medulla is the middle layer, which I remember because of the word overlap between medulla,
medium, medius, and middle.
Finally, the renal pelvis is the flattened, uppermost end of the ureter.
And while it might not look like a pelvis, it does go directly to the pelvis.
Plus, it’s usually illustrated as yellow on most color diagrams, implying that it’s
carrying pee.
This mess of blood vessels and tubules here is the renal pedicle, or hilum — it’s
where the renal artery brings blood in, the renal vein brings blood out, and the renal
pelvis ships waste out to the bladder.
Those are the main three layers, but we need to get more granular to understand how the
kidneys filter blood.
The cortex is a thin layer, but it’s where a lot of the action happens.
It’s filled with nephrons, the “functional units” of the kidney.
That phrase just means that each of these nephrons are the things that are actually
filtering the blood.
Multiply this times the 1 million nephrons you have in each kidney and voila, you’ve
got a functioning organ.
And since filtration happens mostly in the cortex, we need a way for blood to get in
and out, and for waste products to head to the bladder.
That’s where the medulla comes in.
Blood vessels run through these areas in the medulla, otherwise known as the renal columns
— this lets them transport blood to and from those filtration units in the cortex.
From there, these triangle-shaped tissues, which are appropriately called renal pyramids,
funnel those waste products to the renal pelvis.
Each pyramid has a bunch of collecting ducts that merge into bigger ducts, that merge into
even bigger ducts before merging with the renal pelvis.
We usually have around 8 of these renal pyramid, cortex combos, or lobes, per kidney.
I like to imagine each lobe like a pizza slice with urine colored cheese, and blood red crust.
Look, I know it’s gross, but you’ll remember it.
So all that anatomy taken together, we can start piecing together how the kidney filters
blood.
A LOT of blood comes in through the renal arteries, like 20% of cardiac output.
Those arteries then branch off into smaller and smaller blood vessels called arterioles.
And when those branches get to a nephron, they bundle up into clumps of special capillaries
called a glomerulus.
I remember this one because it looks like a bunch of blood vessels kinda /glommed/ together.
And they’re really really tiny.
Most people have 3 glomeruli per a square millimeter, or about 2,000 per square inch,
although everyone’s different.
And these capillaries actually have tiny holes in them, only about 50-100 nanometers wide.
You may hear them referred to as fenestrated capillaries — from the latin root for /windows/.
And these windows are big enough to let some small particles like glucose, amino acids,
and water through, but keep big red blood cells, immune cells, and other big proteins
from leaving.
It also has a few barriers in place to keep certain substances from passing through into
the tubules but that’s more complex.
That bundle of blood vessels is surrounded by a few layers of epithelial cells called
Bowman’s capsule, kind of like a wrap-around funnel that makes sure waste products end
up in the right tube.
Together this unit is called a renal corpuscle, which means little body.
From here, we come across a jumble of blood vessels and tubules that, I know, looks overwhelming.
But you can do this, I believe in you.
You got this.
Branching off Bowman’s capsule is a long tubule that starts in the renal cortex, dips
into the medulla, then comes back to the cortex where it empties waste products into a collection
duct.
The closest part to the corpuscle is the proximal convoluted tubule while the farthest is the
distal convoluted tubule, named with those directional terms you’re familiar with.
The middle section is called the Loop of Henle and it gets a different name because it actually
crosses from the cortex down into the medulla.
Then after filtering through the tubules, the liquid, or what’s called filtrate, empties
back into a big collecting duct where it heads towards the renal pelvis.
But you notice that there are a bunch of blood vessels twisting around these tubules too.
Do you remember how that arteriole came into Bowman’s capsule and jumbled around?
Well, that same arteriole exits the capsule and starts winding around those tubules, and
eventually becomes a renal vein.
They’re called peritubular capillaries when they’re scattered around the convoluted
tubules, hence peritubular.
Then they’re called the vasa recta, literally, straight vessel, when they dance around the
Loop of Henle in the medulla.
But why does it take such a weird and twisty route?
Well, there’s a point to this mess.
The glomerulus is really porous and it dumps A LOT of stuff into the capsule, including
things you actually WANT in your blood like water and glucose.
So after receiving this generous gift from the blood, the tubules actually move some
of those materials back into the blood vessel.
Then, get this, some of those materials flow back into the tubule for excretion.
If you’re ever feeling overwhelmed, come back to this idea: the glomerulus takes more
stuff than it needs from the blood, and the tubules give some of it back and only keeps
the waste.
The first step is glomerular filtration — which, as the name implies, only happens at the glomerulus.
Your kidneys filter about 125 milliliters of blood per minute this way.
Other variables like blood pressure or hormones can push more or less molecules into the capsule
too.
But after all is said and done, ions and water made it through, as well as glucose, lipids,
amino acids, and urea.
Now, these next two processes, tubular reabsorption and tubular secretion, happen at different
points in the tubules.
These processes are constantly happening with different substances at different rates in
your convoluted tubules and loop of Henle.
The exact specifics of how and why different substances cross the tubules is complex, and
I want to keep this simple.
So I’ll give you some of the highlights and I’m going to use a simplified diagram
to keep things focused on the substances, not the anatomy.
Hope that’s okay.
After coming out of the glomerulus, the Proximal convoluted tubules transport nearly everything
back into the blood including water, glucose, amino acids, and multiple ions.
One of the biggest deals is sodium — about 65% of the sodium that was filtered in the
glomerulus is reabsorbed back into the blood.
And where sodium goes, water follows.
So about 65% of the water that came out of the blood goes back into the blood from the
PCT since it’s following the sodium concentration.
At the same time hopefully, 100% of glucose gets reabsorbed at this point too, which makes
sense.
You don’t want to pee out valuable calories!
Now, about half of your urea, a byproduct of protein metabolism, is reabsorbed here
too.
We’ll come back to this since there’s urea in your urine.
The filtrate then heads to the Loop of Henle.
At this point, water in the filtrate flows OUT of the descending limb of the Loop.
But, in the ascending limb, NO water flows out.
Instead, a bunch of ions flow out like sodium, potassium, chloride, calcium, and magnesium.
And here’s what’s so cool.
Because that ascending limb dumped out all that sodium, it made the space outside of
the descending limb super salty, causing water to exit the descending limb but not the ascending
limb.
That’s because of something called osmolality — how much of one substance is dissolved
in a liquid.
A super sugary soda has higher osmolality of sugar than a lightly sweetened soda.
Water will flow from areas of low osmolality to high osmolality.
The Loop of Henle works because of that passive concentration gradient.
It’s mostly water following the small molecules across barriers.
So then we get to the distal convoluted tubule, which actually uses energy to pump even more
ions like potassium, sodium, and chloride out of the tubules.
The final collecting tubule has some similar mechanisms to the Distal Convoluted Tubule,
so it’s not just a static drain.
From there, it’s all off to the bladder.
Okay, quick summary: the glomerulus filtered a bunch of particles out of the blood, and
tubular reabsorption put many of those particles back into the blood.
But another process, tubular secretion, transports other materials from the blood vessels into
the tubules.
I know that calling it secretion makes it seem like the tubules are secreting something
out, but that’s not the case.
Common misconception.
So what gets secreted?
Basically any big molecules that we don’t want in our bodies — it’s a last sweep
of bad stuff that didn’t come out during glomerular filtration.
So drug molecules, big proteins like ammonia, and certain byproducts of metabolism.
This process is also responsible for keep blood at a consistent 7.4pH by taking hydrogen
ions from the blood, which increases pH, making it more alkaline.
Now this is important to note.
Yes, some urea went out from the proximal convoluted tubule into the capillaries, but
later on in the PCT, some urea comes back into the tubules.
The same thing happens again at the Loop of Henle.
This is why you have urea in your urine.
Okay, so now we’ve got urine.
It’s ready to flow through those renal pyramids, renal pelvis, then down the ureters to the
bladder.
Then you pee.
All in all, our kidneys make about 180 liters of filtrate every day and urinate about a
liter and half.
That’s a really big difference, and a quantitative example of how effective reabsorption is.
Before you go, I made this video to be an introduction.
So I had to leave out some of the more complicated details.
But I want to make a nerdier, more advanced version of this video, so let me know in the
comments what concepts you want me to explain in the next one.
Upvote other people’s comments too so I know which ones are more popular.
Have fun, be good.Thank you for watching.