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  • Voiceover: So, we've already talked about

  • this part of the nephron, right?

  • Remember, it's called the glomerulus, the glomerulus,

  • and it's the structure that receives an arteriole

  • that's called the afferent, meaning going towards,

  • arteriole, that's the arteriole that branches off of

  • the renal artery, and it gives off this vessel right there.

  • This is called the efferent arteriole,

  • efferent meaning going away from,

  • and these are all talking about the glomerulus.

  • So, the afferent arteriole goes in, becomes really

  • really squiggly, and then it comes off as a

  • single vessel, the efferent arteriole.

  • In this process, a ton of fluid is filtered out of the blood

  • and goes into this yellow capsule right here.

  • This is Bowman's capsule, Bowman's capsule,

  • which is the first part of the nephron to actually

  • collect fluid that's going to be filtered to become urine,

  • and in this sense, the kidneys are pretty amazing.

  • Do you know in desert rats, these are animals

  • that may never see free water in their entire life,

  • because they live in the desert, their kidneys

  • are so good at concentrating urine and absorbing water

  • that sometimes their pee can actually be solid crystal.

  • That sounds pretty painful,

  • but it works for the desert rodents.

  • Let's figure out how our nephrons work for us.

  • After the glomerulus, the next part of the nephron is

  • this guy right here, which is pretty convoluted,

  • wouldn't you agree, but it's close to the glomerulus,

  • so, we name it according to those features.

  • It's proximal to the glomerulus, so we call this a proximal,

  • and because it's so windy, we call it convoluted.

  • It's the proximal convoluted tubule,

  • proximal convoluted tubule.

  • Kind of a mouthful, but these words

  • perfectly describe what it is.

  • The proximal convoluted tubule is very important

  • for reabsorbing ions, like sodium and chloride,

  • but also some of our other builders of macromolecules,

  • like amino acids and even glucose,

  • and remember when we're absorbing things like this,

  • and especially with sodium,

  • we're going to take water with it as well.

  • So, water is reabsorbed at the

  • proximal convoluted tubule too.

  • In fact, I've read somewhere that the

  • proximal convoluted tubule reabsorbs about

  • 65 percent of all of these important nutrients.

  • That's the most of anywhere else in the nephron

  • that we're going to talk about.

  • So, it's pretty important.

  • Where does the nephron go next?

  • Well, it actually becomes this loop right here.

  • It descends deep into the kidney,

  • and then it ascends back up again.

  • This entire structure is affectionately called

  • the loop of Henle, the loop of Henle,

  • and it's got two limbs to it, as I've shown here,

  • and they're going in opposite directions.

  • One is called the descending limb,

  • descending because it's diving deep into the kidney,

  • so this is the descending limb, and the other part of it,

  • this guy right here, is called the ascending limb,

  • ascending because it rises up, ascending limb,

  • and the reason why this is significant is because

  • remember the kidney is kind of broken up

  • into two main overall parts, right?

  • There's a part that we call

  • the renal cortex, it's above here,

  • and then there's a part we call the renal medulla,

  • which is down here, and the thing to remember is

  • that the renal medulla is very salty,

  • very salty because we have a lot of

  • ion reabsorption happening here.

  • Now, I should further specify that

  • the descending limb and the ascending limb

  • of the loop of Henle reabsorb very separate things.

  • The descending limb reabsorbs water,

  • so we have mainly water coming here, and, in fact,

  • there are no ions that are reabsorbed at this point.

  • It is impermeable to ions.

  • On the other hand, the ascending limb

  • does the exact opposite.

  • Here, we reabsorb things like sodium, chloride, potassium,

  • and, in fact, this part is impermeable to water.

  • No water will be reabsorbed here in the ascending limb,

  • and because of this, we have

  • a very beautiful system that occurs as a result.

  • This is called countercurrent multiplication,

  • countercurrent multiplication, which is also, I'll admit,

  • a mouthful, but it completely makes sense, I promise.

  • Countercurrent multiplication.

  • We say countercurrent because the descending limb

  • and the ascending limb go in opposite directions.

  • That's why it's countercurrent.

  • Multiplication means that when we reabsorb ions

  • in the ascending limb here and make the medulla salty

  • by not reabsorbing water, that drives water to be

  • reabsorbed passively in the descending limb,

  • and we have a video that goes into detail

  • about transport processes in the nephron,

  • but here just remember that water is reabsorbed passively,

  • so no energy is expended to reabsorb water,

  • and this is because we have used energy

  • here in the ascending limb to reabsorb these ions.

  • So, active transport is used here,

  • and by actively pumping ions into the medulla,

  • and no water in the ascending limb to make it salty,

  • we can multiply the amount of water that is

  • reabsorbed passively, because it's driven into this

  • space around the tubule or the nephron.

  • The space around the tubule is just called interstitium.

  • I'll write that off right here.

  • So, this is the interstitium.

  • This is anything that is not the tubule or vessels,

  • and that's just space around here.

  • This is all just interstitium,

  • just hanging out right here, and so all this ions

  • that are reabsorbed into the medullary interstitium

  • down here, the interstitium of the medulla,

  • drive the passive reabsorption of water.

  • All right, so I think we have a pretty good understanding

  • of the loop of Henle and the countercurrent multiplication

  • process that happens here.

  • The next part of the nephron is this guy,

  • that kind of loops back and just kisses

  • the glomerulus right there, and I'm sure you have

  • astutely noted that just like the proximal convoluted tubule

  • this tubule is also certainly very convoluted.

  • So, it's a tubule that is convoluted, let's say,

  • but it's not as close to the glomerulus

  • as the proximal convoluted tubule was.

  • So, instead, we shall call this guy

  • the distal convoluted tubule, and this dude

  • is responsible for the reabsorption of other ions,

  • like sodium and chloride, and it helps to just pick up

  • more of these important nutrients that we'd like to have

  • in our bloodstream, that we don't want to pee away.

  • The other thing I should mention now,

  • that I promise we go into more depth in

  • in a separate video, is this very scientific kiss

  • that happens here, when the distal convoluted tubule

  • comes by the glomerulus again.

  • This produces something that's called,

  • and this is a mouthful,

  • the juxtaglomerular, juxtaglomerular apparatus,

  • juxtaglomerular apparatus, and all this is responsible for

  • is to control blood pressure.

  • This is part of the kidney that's used

  • to control blood pressure, and we'll talk about this

  • in detail in a separate video.

  • So, now that the distal convoluted tubule's

  • come up here and kissed the glomerulus

  • and kind of come out here, it's time to collect whatever

  • leftover fluid we have, and we do so in this guy right here.

  • This is called the collecting tubule or the collecting duct.

  • So, it collects things that we have left over

  • in the lumen, or inside of this nephron,

  • and one thing to note is that there are many

  • DCT's, or distal convoluted tubules, that feed into this

  • single collecting duct.

  • So, there is a DCT, there is a DCT, and there is

  • another one down here, right?

  • And we actually reabsorb a couple of things

  • in the collecting tubule as well.

  • The main thing that we reabsorb into our interstitium

  • is water, and another thing we reabsorb,

  • that I'll show deep in the medulla right here, is urea.

  • Urea is one of the main waste components

  • that we actually pee away, but sometimes

  • the kidneys like to hold onto urea to increase

  • the osmolarity in the medulla, to help drive

  • water reabsorption in the loop of Henle.

  • This goes into a process that's called urea recycling,

  • if you've heard of that term before,

  • but we're not going to go into detail for right now on that.

  • Instead, we'll just mention here that

  • urea is reabsorbed to maintain osmolarity,

  • maintain the osmolarity in our medulla that will help

  • drive water reabsorption in the loop of Henle.

  • And, finally, I want to close the loop on what happens

  • to this poor efferent arteriole right here,

  • because we came off the afferent arteriole,

  • and this I promised you would turn into a capillary

  • and then a venule, and here's where I'm going to

  • talk about that, because we've reabsorbed

  • all of these awesome things here in blue,

  • but we don't have a way to put them

  • into the bloodstream yet.

  • Well, the efferent arteriole gives us a way to do that,

  • and it does so by kind of coming off this way,

  • and just like all good arterioles, it branches off

  • into even smaller branches, so much so that we

  • branch off into smaller capillaries,

  • and these capillaries will dance across our nephron

  • and collect all this good stuff that we've talked about,

  • here in blue, that gets reabsorbed

  • into our interstitium, and I should mention that because

  • these capillaries kind of hang out all over the place,

  • where our tubules are, we say that they are peritubular,

  • peritubular meaning just around the tubule,

  • so, sure enough, their official name is

  • peritubular capillary, or we call them all

  • peritubular capillaries, and so after we've

  • collected our nutrients in these peritubular capillaries,

  • we come back here, where we then start doing

  • the exact opposite, because now we've lost our oxygen

  • but we've reabsorbed these nutrients

  • into our bloodstream, and then this will

  • kind of come back together and head off into,

  • I think you guessed it, the renal vein,

  • and the renal vein will then take this back to the

  • rest of the body, and that leaves the rest

  • of what we've collected here in our tubules,

  • in our lumen right here, that goes away,

  • and this is going to become our urine.

  • Our collecting tubule is where we first have urine

  • that's going to be sent off into our renal calyces

  • and then further on to be peed away.

Voiceover: So, we've already talked about

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B2 中高級

腎臟的逆流增殖|腎臟系統生理學|NCLEX-RN|可汗學院 (Countercurrent multiplication in the kidney | Renal system physiology | NCLEX-RN | Khan Academy)

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    柳夙芯 發佈於 2021 年 01 月 14 日
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