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肾元对肾脏的血液过滤以及水和其它分子的再吸收的作用. Sal Khan 创建




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 edits I guess you could call it it's smallest functional level and that's the nephron so we're gonna talk about the kidney 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 excrete waste but part of that process let me write excrete waste 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 gonna 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 me 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 a kidney isn't as big as if you're watching this in full-screen it's not gonna 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 renal cortex whenever we talk about something with the kidney when if you hear 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 area right here this is the renal medulla and medulla comes from middle so it's the you can almost view it as the middle of the kidney renal medulla and besides just its understanding these words we're gonna 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 have said before and you might have heard it already from other lectures from other teachers that the functional unit of the kidney is the nephron the functional functional unit 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 you know the the two major function of the kidney the waste excretion and the maintenance of the water level in our in our blood system so just to get an idea of how a nephron fits in within this picture of a kidney the artist actually I got this picture from Wikipedia the artist tried to do it draw a couple of nephrons over here so a nephron will look something like this and it goes and 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 into your readers 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 it goes you know this is where it starts and it dips down again so multiple nephrons are gonna keep doing that but they're super thin these tubes or these tubules maybe I should say are super thin and actually an actual kidney your average kidney will contain on the order on the order of 1 million nephrons 1 million nephrons so even though you can't really say mike nephrons are microscopic they kind of have a you know you there they're at least their length when they dip down you say oh that's it you know I can see that distance you can still Jam a lot of them inside of one kidney well with that said let's actually figure out how a nephron does how it filters the blood and actually make sure that not enough not too much water not too much of the good stuff in our blood ends up in the urine so let me draw let me draw here a nephron so I'm gonna start like this so let me start with the blood flow so the Bloods going to come in an arteriole that's an arterial capillary you could say so it's gonna come in like that this is actually called the afferent afferent arteriole you don't have to know the names but you might see that sometime arterial blood is coming in then it goes into this big windy place this big windy place and it really winds around like that this is called the glomerulus this is the glomerulus glomerulus glomerulus and then it leaves via the efferent arteriole efferent efferent let me switch colors this red is getting monotonous this is the efferent arteriole afferent and 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 that 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 in the in a in 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 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 so let me just draw this kind of a cross-section it's surrounded let me do it like this it's surrounded like that by this structure and these are cells here so you could imagine these are 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 filter it they're a little bit more selective in what's gets filter out and essentially you have about a fifth of the fluid that's coming in about a fifth of the fluid that comes 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 you can imagine it as a some kind of a 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 the Bowman's capsule and the whole thing has sales all of these structures are obviously made or maybe not so obviously they're made up of cells and so we end up having filtrate and the filtrate is just the stuff that gets squeezed out so I'll call it filtrate we can't call it urine just yet because there's a lot of steps that have to occur for it to be kind of to earn the name urine so it's only filtrate right now and essentially what gets squeezed out I said it's about a fifth of the fluid and things that are easily dissolved in fluid so small ions so you know sodium may be some small molecules like glucose maybe some amino acids amino acids there are tons of stuff in here but this should give an idea the things that do not get filtered are things like red blood cells red blood cells or larger molecules larger proteins they will not get filtered it's mainly the the micro micro molecules that will get filtered and that'll be part of this filtrate that shows up that shows up here in the Bowman's 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 on our big picture of our kidney if let's say that this is a this is a Bowman's let's say we're near an arteriole this is a Bowman's capsule right here it'll look something like that and the whole nephron is gonna get B convoluted like this and it's gonna 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 gonna run out of space so let me zoom out so we had our we had our arteriole go in it gets all bunched in and the glomerulus and then most of the blood leaves but one fifth of it gets 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 I'll make it a little bit maybe I'll make it a little bit yellow so that we have our filtrate right here that's all you know the filtrate that just comes out at this point sometimes it's called the glomerular filtrate because it's been filtered by the glomerulus but it's also been filtered by the Poteau site cells on the inside of the Bowman's capsule but now it's ready to go to the proximal tubules so the proximal tubules sounds like a very so let me draw something like this and obviously this is not exactly what it looks like but it gives you the sense this right here this right here this is the proximal tubules and it sounds like a very fancy word but proximal just means near and tubular you can imagine it's it's a small tube so it's the small tube that's near the beginning that's why it's called a proximal tubules and it has two parts it's often the whole thing is often called a proximal convoluted tubules eluted tubulin that's because it's all convoluted I the way I've drawn it it's all curvy and I just drew it curvy in two dimensions it's actually curvy in three dimensions but that the reality is there's a curvy part of them there's a straight part near the end of the proximal tubules so they'll call this whole thing the proximal tubule this is the convoluted part that's the straight part but we don't have to get too picky but the whole point of this part of the nephron and just to give just to remember where we are we're now at this point of the nephron right there the whole point is to start reabsorbing some of the stuff that is in the filtrate that we don't want to lose we don't want to lose glucose that's that's harder and stuff that we ate that was good for energy we don't want to lose necessarily as much sodium that's we've seen in multiple videos that that's a useful uh that's a useful ion to have around we don't want to lose lose amino acids those are useful for building up proteins and other things so these are things we don't want to lose so we start absorbing them back and I'll do a whole video on exactly how that happens but it's done actively it's since we're we're using ATP and just as a bit of a summary you're using ATP to actually pump out the sodium and then that actually helps bring in the other things that's just a kind of a tidbit on what's happening so we're reabsorbing so just imagine what's happening you have cells you have cells lining the proximal tubular right now and actually they have little things that jut out I'll do a whole video on that because it's actually interesting so you have cells out here on the other sides of the cells you have in our arterial system or a capillary system I should say actually so let's say you have a capillary system here that is very close to the cells lining the proximal tubules and so this this stuff actually gets actively pumped especially the sodium but all of it using energy gets pumped back into the blood selectively and maybe a little bit of our water so we're gonna have so we're pumping back some sodium some glucose glucose and we'll start pumping a little bit of the water back in because we don't want to lose all of that water if all of the water that was originally the filtrate we were just leaving our urine weed weed weed weed B excreting gallons and gallons of water every day which we do not want to do so that's the whole point we're starting the absorption process and then we'll enter the loop of Henle and actually this is in my mind the most interesting part of the nephron so entering the loop of Henle and it dips down it dips down and then comes back up so the loop of Henle and so most of the length of the nephron is the loop of Henle and if I go back to this diagram right here if I'm talking about the loop of Henle I'm talking about this whole thing right there and you can see something interesting here it crosses the border between the cortex this light brown part and the renal medulla this kind of reddish or orange part right there and it does that for a very good reason and I'm going to draw it here so let's say this is the dividing line this is the dividing line right here this right here was the cortex this right here is the medulla is the medulla so the whole point well there's two points of the loop of Henle loop of Henle one point is to make the medulla salty make the medulla the renal medulla salty and it does this by actively pumping out salts so it actively pumped out salts and it does that in the ascending part of the loop of Henle so this is sending part so it actively pumps out salts sodium potassium chloride or chlorine I should say chlorine ions it actually actively pumps out it actively pumps out these salts right here too make the entire medulla to make the entire medulla salty or if we think about it in terms of kind of osmosis make it hypertonic you have more solute out here than you have in the filtrate that's going through the tubules so what it does is it makes it and it uses ATP to do this all this stuff requires ATP to actively pump against a concentration gradient so this is salty it's salty and it's salty for a reason it's not just to take back the the you know these salts from the filtrate although that's part of the reason but by making this salty so this part the ascending part is only permeable to these salts in these ions so only on it's not permeable to water so we could say not permeable to water not water the descending part of the loop of Henle is only permeable to water only water so what's gonna happen if this is all salty because the ascending part is actively pumping out salt what's gonna happen to water as it goes down the descending loop well it's hypertonic out here water will naturally want to go and kind of try to make the the concentrations balanced out I've done a whole video on that it doesn't happen by magic and so the water will because this is hypertonic it's more salty and this is only permeable to water the water will leave the membrane on the descending part of the loop of Henle right now and this is a major part of water reabsorption and if you know I've thought a lot about you know why why don't we just actively pump why don't we use ATP somehow to actively pump water and the answer there is there's no easy way to do that biological systems are good at using ATP to pump out ions but it can't actively pump out water water is kind of a hard thing to up for proteins to operate on so the solution is to make it salty out here by pumping out ions and then water if you make this pores only to water water will naturally flow out so this is a major mechanism of gaining back a lot of the water that gets filtered out up here and the reason why this is so long is to give time for this water to secrete out and that's why I dips nice pretty nice and pretty far down into this salty portion so then we're leave the loop of Henle and then we're almost done with the nephron then we're in another convoluted tubules and you might even guess the name of this convoluted tubules if this was the proximal one this is the distal one and actually just to make my drawing correct it actually passes very close to the the Bowman's capsule so let me do it in a different color the distal convoluted tubules it I wanted to do that in a different color the distal convoluted tubules actually goes pretty close to the Bowman's capsule and once again I made it all convoluted in two dimensions but it's actually convoluted in three and it's not that long but I just have to get over here and I wanted to get over that point right there it's called distal distal is further away it's convoluted and it's a tube you'll so this right here is the distal distal convoluted tubules and here we have more reabsorption calcium more sodium reabsorption we're just reabsorbing more things that we didn't want to lose in the first place and there's a lot of things we could talk about what get reabsorbed but this is just the overview and we're also reabsorbing a little bit of more water but then at the end right here our filtrate has been processed a lot of the waters been taken out it's a lot more concentrated we've reabsorbed a lot of the salts electrolytes that we want we've reabsorbed the glucose and a lot of the amino acids everything that we want we've taken back we've reabsorbed and so this is mainly mainly waste products and water that we don't need anymore and then this gets dumped into collecting ducts this gets dumped into collecting ducts and you can kind of view this as the trash chute of the kidney where multiple multiple multiple nephrons are going to dump into this so that might be another that might be the distal tubules of another nephron right here this is a collecting duct collecting duct which is just a two of that's collecting all the byproducts of the of the nephrons and the interesting that is collecting ducts further goes into the medulla again it goes into the medulla again to the salty part again so if we're talking about the collecting duct maybe the collecting ducts coming back into the medulla collecting all of the filtrate from the different nephrons and because it goes back through that super salty spot in the medulla we actually have four hormones called antidiuretic hormone that can dictate how porous these this collecting this collecting tube is and if it makes it very porous if it makes it very porous it allows more water to leave as we go through the medulla because this is very salty so the water will leave if this is porous and when we do that what that does is it makes the filtrate and we can maybe start calling it your now even more concentrated so we don't lose so we lose even less water and it keeps collecting collecting collecting until we end up here and it leaves the kidney and and goes via our your readers to the urinary bladder so hopefully you found that helpful and I you know I think the neatest thing here is just how we actively reabsorb the water and how we actually fur in my mind that is the the neatest part in the loop of Henle