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  • What I want to do in this video is talk a little bit

  • about the kidney-- and this is a big picture of a kidney--

  • and to talk about how it operates at its-- I guess you

  • could call it its smallest functional level and that's

  • the nephron.

  • So we're going to talk about the kidney and the nephron.

  • And I think you might already know the kidney.

  • We have two of them.

  • They're the organ that, I guess, is most famous for

  • producing or allowing us to excrete waste.

  • But part of that process, it also helps us maintain our

  • water, the correct level, and actually the amount of salts

  • or electrolytes we have and our blood pressure, but I'll

  • just say maintain water.

  • And it also produces hormones and things, and I'm not going

  • to go into a lot of detail on that right now.

  • I really just want to focus on these first two to kind of

  • just understand the overview function of the kidney.

  • And most of us have two of these.

  • They're kind of closer to our back on either sides of our

  • spine behind our liver.

  • And this is a zoomed-in version of it.

  • If you're watching this in full screen, it's not going to

  • be as big as this picture is, but we've sliced it so we can

  • see what's going on inside the kidney.

  • Just to understand the different parts here, just

  • because it will actually be significant when we start

  • talking about the functional units or the nephron within

  • the kidney, this area right here from here to here, this

  • is called the renal cortex.

  • Whenever we talk about something with the kidney, if

  • you see a renal anything, that's actually referring to

  • the kidney.

  • So this right here is a renal cortex, that

  • outer part right there.

  • And then this area right here, this is the renal medulla.

  • And medulla comes from middle.

  • So you can almost view it as the middle of the kidney.

  • Besides just understanding these words, we're going to

  • see that they actually play a very important role in this

  • actual filtration or this excretion of waste and this

  • ability to not dump too much water or excrete too much

  • water when we're trying to filter out our blood.

  • So I've said before, and you might have heard it already

  • from other lectures or from other teachers, that the

  • functional unit of the kidney is the nephron.

  • And the reason why it's called a functional unit-- I'll put

  • it in quotes-- is because that's the level at which

  • these two things are happening.

  • The two major functions of the kidney: the waste excretion

  • and the maintenance of the water level

  • in our blood system.

  • So just to get an idea of how a nephron fits in within this

  • picture of a kidney-- I got this picture from Wikipedia.

  • The artist tried to draw a couple of nephrons over here.

  • So a nephron will look something like this, and it

  • dips down into the medulla, and then it goes back into the

  • cortex, and then it dumps into collecting ducts, and

  • essentially the fluid will end up in the ureters right here

  • and end up in our urinary bladder that we can later

  • excrete when we find a suitable time.

  • But that's about-- I guess you can imagine

  • the length of a nephron.

  • This is where it starts and then it dips down again.

  • So multiple nephrons are going to keep doing that, but

  • they're super thin.

  • These tubes or these tubules, maybe I should

  • say, are super thin.

  • Your average kidney will contain on the order of one

  • million nephrons.

  • You can't really say, my nephrons are microscopic.

  • They kind of have a-- at least their length when they dip

  • down, you can say, I can see that distance.

  • You can still jam a lot of them inside of one kidney.

  • With that said, let's actually figure out how a nephron

  • filters the blood and actually makes sure that not too much

  • water or not too much of the good stuff in our blood ends

  • up the urine.

  • So let me draw here a nephron.

  • So I'm going to start like this.

  • We'll start with the blood flow.

  • So the blood's going to come in an arterial-- that's an

  • arterial capillary, you could say.

  • So it's going to come in like that.

  • This is actually called the afferent arterial.

  • You don't have to know the names, but you

  • might see that sometime.

  • Blood is coming in.

  • Then it goes into this big windy place.

  • It really winds around like that.

  • This is called the glomerulus.

  • And then it leaves via the efferent arterial.

  • Efferent just means away from the center.

  • Afferent towards, efferent away from the center.

  • And I'll talk about it more in the future, but it's

  • interesting that we're still dealing with an

  • artery at this point.

  • It's still oxygenated blood.

  • Normally, when we leave a capillary system like the

  • glomerulus right there, we're normally dealing with the

  • venous system, but here we're still an arterial system.

  • And it's probably because arterial systems have higher

  • blood pressure, and what we need to do is we need to

  • squeeze fluid and stuff that's dissolved in the fluid out of

  • the blood and in the glomerulus right here.

  • So this glomerulus is very porous and it's surrounded by

  • other cells.

  • This is kind of a cross-section.

  • It's surrounded like that by this structure, and these are

  • cells here so you can imagine these are all cells over here.

  • And, of course, the actual capillaries have cells that

  • line them so there are cells here.

  • So when I draw these lines, these lines are actually made

  • up of little cells.

  • What happens is the blood comes in

  • at really high pressure.

  • This is very porous.

  • These cells out here, they're called podocytes.

  • They're a little bit more selective in what gets

  • filtered out, and essentially about a fifth of the fluid

  • that's coming in ends up going into this space right here

  • that's called the Bowman's space.

  • Well, actually, this whole thing is called

  • the Bowman's capsule.

  • It's a sphere with an opening in here that the capillary can

  • kind of wind around in, and the space right here, this is

  • the Bowman's space.

  • It's the space inside the Bowman's capsule, and the

  • whole thing has cells.

  • All these structures are obviously made-- or maybe not

  • so obviously-- they're made up of cells.

  • And so we end up having filtrate in it.

  • Filtrate is just the stuff that gets squeezed out.

  • We can't call it urine just yet because there's a lot of

  • steps that have to occur for it to earn the name urine.

  • So it's only filtrate right now, and essentially what get

  • squeezed out, I said it's about a fifth of the fluid,

  • and things that are easily dissolved in fluid, so small

  • ions, sodium, maybe some small molecules like glucose, maybe

  • some amino acids.

  • There are tons of stuff in here, but this is

  • just to give an idea.

  • The things that do not get filtered are things like red

  • blood cells or larger molecules, larger proteins.

  • They will not get filtered.

  • It's mainly the micromolecules that'll get filtered, that'll

  • be part of this filtrate that shows up here

  • in the Bowman space.

  • Now, the rest of what the nephron does, the Bowman's

  • capsule is kind of the beginning of the nephron, and

  • just to get an idea of our big picture of our kidney, let's

  • say we're near an arterial.

  • This is a Bowman's capsule right here.

  • It looks something like that, and the whole nephron is going

  • to be convoluted like this.

  • It's going to dip down into the medulla, and then come

  • back, and then it's going to eventually dump into a

  • collecting duct, and I'll talk more about that.

  • So what I've drawn just here, this is a zoomed-in version of

  • that part right there.

  • Now what I want to do is zoom out a little bit because I'm

  • going to run out of space.

  • So let me zoom out.

  • So we had our arterial go in.

  • It gets all bunched in the glomerulus, and then most of

  • the blood leaves, but one-fifth of it gets

  • essentially filtered in to the Bowman's capsule.

  • That's the Bowman's capsule right there.

  • I've just zoomed out a little bit.

  • So we have our filtrate here.

  • Maybe I'll make it a little bit yellow.

  • The filtrate that just comes out at this point,