Human biology
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The Lungs and Pulmonary System
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Red blood cells
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Circulatory System and the Heart
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Hemoglobin
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Anatomy of a Neuron
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Sodium Potassium Pump
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Correction to Sodium and Potassium Pump Video
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Electrotonic and Action Potentials
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Saltatory Conduction in Neurons
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Neuronal Synapses (Chemical)
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Myosin and Actin
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Tropomyosin and troponin and their role in regulating muscle contraction
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Role of the Sarcoplasmic Reticulum in Muscle Cells
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Anatomy of a muscle cell
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The Kidney and Nephron
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Secondary Active Transport in the Nephron
Secondary Active Transport in the Nephron Secondary Active Transport in the Nephron
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- In the last video on the nephron, we talked about the
- different parts of the nephron and what, I guess, molecules
- are reabsorbed by the body and the different parts.
- If you remember, in the proximal convoluted tubule, we
- talked about maybe glucose and amino acids and sodium being
- reabsorbed.
- We talked about the ascending part of the loop of Henle.
- We talked about salts, so that sodium, potassium, chlorine
- being reabsorbed.
- In the distal convoluted tubule, it was
- calcium, other things.
- But at least in my mind, when I first learned it, I said how
- does that happen?
- How do we actively pump out these things, especially
- against their own concentration gradients?
- What I want to do in this video is get a little bit more
- depth on exactly what's happening on the borders of
- these tubules to actually allow these ions to be
- selectively transported out of the lumen or the inside of
- these tubes or to be
- reabsorbed out of the filtrate.
- The mechanism's actually reasonably similar in the
- different parts of the nephron, but let's look at
- each of the parts, because they're each reabsorbing
- different types of molecules.
- And I won't go through all of the molecules, but I'll just
- give you a sense of things.
- Let's start with the proximal tubule right here.
- So let's say if we were to zoom in right over on that
- part-- so let me draw the inside of the nephron.
- The inside of the nephron maybe looks
- something like this.
- So this the inside.
- This is where our filtrate is right here.
- Actually, let me draw it a little bit
- different than that.
- So the inside, I'm going to draw it like this because the
- proximal tubule has these little things that stick out,
- sometimes referred to as a brush border.
- So this inside right here, this is our lumen.
- That is where the filtrate is.
- The glomerular filtrate is coming in this direction.
- This is, you can imagine, the inside of the nephron.
- And then the border of the tubule is made up
- by a bunch of cells.
- So maybe this is one cell right here, this is another
- cell right here, that's another cell.
- Obviously, this is a cross-section.
- It would be actually more of a cylinder.
- It would go around like that.
- This is to give an idea.
- That's another cell right there.
- And maybe this is their basal side right there.
- And when we say basal, we can imagine that's kind of the
- base of the cell.
- Those are good words to know, fancy words.
- So the side of the cells that are facing the lumen, or kind
- of facing the inside of our tubule, this is called the
- apical side.
- And then this side is normally referred to the basal lateral
- side, or this membrane, if you view this as a membrane, this
- would be the basolateral membrane.
- This is true regardless of what part of the nephron we're
- in, whether the proximal, whether the loop of Henle, or
- whether we're in the distal part.
- What we have here, and on the other sides of these cells,
- we'll have our peritubular capillaries.
- That's another fancy word.
- So our peritubular capillaries will look something like this.
- They're actually cells as well.
- Actually, instead of drawing the cells, I'll just draw it
- as kind of the tube of-- I'll just draw it like this.
- They're porous.
- So this is actually blood flow right here.
- This is blood right here.
- This is blood right here.
- I'm not going to do too much detail on the actual cells of
- the capillary walls.
- I really want to give you the idea of how things are
- transported out of the lumen, how they're selectively
- reabsorbed.
- So this is the peritubular capillary.
- And once again, fancy word, but peri means
- around, like perimeter.
- So it's around the tubes.
- These capillaries go around the tubes.
- If I were overlay it on this picture, we have these
- capillaries that are going all around the tubes.
- So when things get secreted or reabsorbed out of the
- nephrons, they're going into those capillaries.
- So this is our proximal convoluted
- membrane right here.
- Let's think about what happens with the glucose.
- So what happens is we actually have sodium-potassium pumps on
- the basolateral side of these cells.
- So this is sodium-potassium pumps.
- I'll just draw one right here.
- You might want to watch the video on
- sodium-potassium pumps.
- I have a whole video on it.
- But the idea here is that sodium, maybe I'll draw as
- plus particles right there, they'll attach on the inside
- right here, ATP will come along.
- When ATP attaches to the right part of this protein, it'll
- change its shape, its conformation, and then the
- protein will essentially close on this side and open on that
- side, and then when it's in that conformation, the sodium
- doesn't want to bond as much to the protein and it will go
- outside or it'll cross the basolateral membrane and
- eventually make its way into the blood.
- And then on the other side, it's a sodium-potassium pump.
- When it's in this kind of open configuration-- I'll draw it
- over here; I have a whole video on this-- at that point,
- potassium likes to bond to it.
- So potassium likes to bond to it.
- Maybe it bonds to it over here.
- This is a gross oversimplification.
- That causes the protein to change its conformation.
- It doesn't require ATP at that point, and it goes back to
- this conformation, and then the potassium doesn't want to
- bond anymore, and then it gets released, because the protein
- is now a different shape.
- So the general idea: Sodium bonds.
- ATP bonds.
- The ATP gets its phosphate popped off of it.
- That changes the shape of the protein to this.
- Now the sodium wants to get released, and now potassium
- wants to join.
- When potassium joins, we get to our original one.
- The end product of this is we're having sodium being
- pumped out of the cell and we're having potassium being
- pumped into the cell, and this is active transport.
- Why is it active transport?
- Because we're using ATP to drive sodium against its
- concentration gradient to keep pumping the sodium out of the
- cell, and then potassium kind of comes in, you could almost
- imagine, passively.
- It doesn't require ATP.
- And that's why this is often called a sodium-potassium
- ATPase, which means it's a protein or an enzyme that
- breaks ATP.
- But it breaks ATP, it uses that energy to change its
- shape to pump sodium out and potassium in.
- Well, anyway, this is all a review of what we learned in
- those videos, but how does that help us, for example, get
- glucose out of our lumen?
- Well, what we have over here is we have other proteins.
- I'll just do the example of glucose.
- Let's say we have a protein here.
- There's a very general term for this.
- It's a cotransporter or a symporter.
- Symporter means it transfers two types of molecules in the
- same direction.
- Cotransporter means one molecule wants to go through
- because of its concentration gradient and the other
- molecule kind of goes along for the ride.
- So you can imagine, we're actively pumping out sodium.
- So if we're actively pumping out sodium over here on the
- basolateral side, then we're going to have a low sodium
- concentration here.
- The more we pump out, the lower this is, and eventually
- it's going to be lower than the sodium
- concentration in the lumen.
- So the sodium concentration gradient, if there was no
- membrane here, sodium would want to go across this to kind
- of make up for all of the lost sodiums over here.
- Sodium would want to cross that if there was no barrier.
- These cells here take advantage of sodium wanting to
- move down its concentration gradient, which is happening
- because of this active transport over here, but it
- uses that energy of sodium going down its concentration
- gradient to actually also transport, in this case, maybe
- some glucose.
- So if you had to visualize it, you could imagine a protein
- that's on this apical membrane right here.
- Maybe it looks something like this.
- This is to get some type of visualization.
- Maybe you have more sodium on this side than you have on
- this side, so sodium is more likely to bond here.
- Maybe glucose will bond here.
- This is just a simplification, but when they bond, this
- protein is going to change its shape to look something more
- like this when they bond, and now the sodium is going to be
- here and the glucose is going to be here.
- We're essentially on the inside of the cell now, and in
- this conformation, they don't want to bond as much to the
- amino acids or whatever else is in the protein, and then
- they get released.
- And when they get released, then the protein will change
- its shape back to this right here and we can do this cycle
- over again.
- But this is all stipulated on the idea that there's more
- sodium over here to bump into this point to make this
- reaction happen.
- So sodium's going to go down in its concentration gradient.
- It's taking glucose for the ride.
- And so essentially glucose concentration will go up high
- here, and then if we make this porous to glucose so glucose
- can go through, then glucose will eventually, if this gets
- high enough, it'll just go down its concentration
- gradient eventually into the blood.
- And this same exact process is happening, maybe not exactly
- with glucose, but throughout the entire nephron.
- If we go to the loop of Henle, if we go to the ascending part
- right here, where we're trying to get the salts out of the
- picture, same idea.
- So let's say that that right there is the lumen.
- This is a cell that makes up the wall of the lumen.
- We're in the loop of Henle now and you have a
- sodium-potassium pump out here.
- You have sodium being pumped out.
- You have potassium gets pumped in, but actually, potassium
- channels are leaky, so potassium can often make its
- way back out in either direction.
- So what's happening to
- potassium isn't that important.
- But so sodium concentration becomes low here.
- So what we have are symporters over here, just like we had
- with glucose, but in this case, sodium wants to enter
- just as the case with glucose, but here we're trying to
- transport chlorine and potassium ions.
- So that's what we're going to join.
- That's what's going to take advantage of sodium's
- concentration gradient.
- We're going to have potassium and we're going to have
- chlorine ions.
- And actually, this symporter right here, it's called the
- sodium-potassium-chlorine cotransporter, and it's
- actually the second variation that you actually get in the
- ascending loop of Henle.
- So eventually, you're going to end up with a lot of chlorine
- here-- actually, potassium from both directions-- but as
- long as this is porous to chlorine, if this
- concentration gets high enough, the chlorine is going
- to make its way out and help make the medulla that much
- saltier along with the sodium.
- Same thing in the distal convoluted tubule.
- There, calcium.
- It's a little bit different.
- So if we're in the distal convoluted tubule, these kind
- of villi, these things that stick out-- this is only in
- the proximal convoluted tubule, those brush borders.
- But over there-- and just so you know, this idea where
- we're using a concentration gradient that's driven by some
- type of active transport to transport other things, this
- is called secondary active transport.
- That's nice to know.
- And then just finishing up at the distal convoluted tubule.
- So this was the lumen.
- Let's say that this is the lumen right here, so we have
- cells on either side of that.
- I think you get the general idea.
- The distal's a little bit different so let's say this is
- a cell, and let's say that this is a peritubular
- capillary right here.
- This is our blood.
- What we have here is once again, we're
- pumping sodium out.
- Sodium-potassium pumps.
- I have a whole video on that, and that pumps potassium in,
- so you end up with a lot of sodiums over here.
- The apical membrane that faces the lumen,
- it's porous to calcium.
- Whatever the concentration of calcium here,
- it's going to be here.
- So maybe you have calcium.
- These are calcium ions just like that floating around.
- And right here, what you have is an antiporter.
- So our concentration in the blood of sodium is going to be
- higher because we keep pumping it out.
- And so sodium, if you let it go down its concentration
- gradient, it would go back in.
- And so maybe right here you have some sodium going down,
- its concentration gradient going back in, and then when
- that goes in, that you can almost imagine it's some type
- of a rotating door, it makes the calcium go out.
- You can try to visualize it yourself how a protein would
- actually do that.
- I kind of imagine a revolving door.
- The sodium makes the door revolve.
- The calcium is at the other part of the door and
- it gets spit out.
- So this is called an antiporter because they're
- going in different directions, but once again, it's secondary
- active transport, because the only way that this could work
- is if we have active transport using ATP of the sodium out of
- the basolateral membrane in every one of these cases.
- Anyway, hopefully, you found that useful.
- It's more detailed than you normally get on how the
- nephron is actually pumping things out of the lumen into
- the peritubular capillaries, but for me, it made things a
- lot more concrete.
- It helps me really kind of internalize what the
- nephron is up to.
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