If you're seeing this message, it means we're having trouble loading external resources on our website.

如果你被网页过滤器挡住,请确保域名*.kastatic.org*.kasandbox.org 没有被阻止.



肾元的次级主动运输. Sal Khan 创建




in the last video on the nephron we talked about the different parts of the nephron and how and and what I guess molecules are reabsorbed by the body in the different parts if you remember in the proximal convoluted tubules talked about maybe glucose and amino acids and sodium being reabsorbed when we talked about the ascending part of the loop of Henle we talked about salt so that sodium potassium chloride chlorine being reabsorbed in the distal convoluted tubule it was calcium other things but it at least in my mind when I first learned it says you know 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 or the inside of these tubes are to be reabsorbed out of the filtrate so let's start in the and the mechanism is actually reasonably similar in the different 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 so let's start with the proximal tubular I'd here so let's say if we were to zoom in 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 look something like this so this is 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 gonna draw it like this I'm gonna draw it like this because the proximal tubular these little things that stick out sometimes referred to as a brush border let me write that so this is our this inside right here this is our lumen that is the lumen that is where the filtrate is so the filtrate the glomerular filtrate is coming in in this direction this is our filtrate our filtrate is this is you can imagine the inside of the nephron and then the border of the two Buhl is made up by a bunch of cells so maybe this is one cell right here this is another cell right here it makes them cell that's another cell obviously this is a cross-section it would actually more of a cylinder it would go around like that but 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 you can imagine that's kind of the base of the cell so let me actually 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 tube you'll this is called the apical side apical apical and then this side is normally referred to as the basal lateral side or the basal lateral or this membrane if you view this as a membrane this would be the basolateral membrane this is true what regardless of what part of the nephron were in there's a proximal where there in the loop of Henle or there in the distal part what we have here and on the other sides of these cells will have our peritubular capillaries and that's another fancy word so our peritubular capillaries will look something like this they're actually cells as well 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 there forests 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 loom and how they're selectively reabsorbed and so and just to get that so this is the peritubular peri tubular tubular 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 to overlay it on this picture we have these capillaries that are going all around the tubes so that when things get secreted or reabsorbed out of the nephrons they're going into those capillaries so this is our peritubular this is our proximal convoluted membrane right here we said let's think about what happens with the glucose so what happens is we actually have sodium potassium pumps sodium potassium pumps on the basolateral side of these cells so this is sodium potassium pump so I'll just draw one right here and 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 sodium maybe I'll draw AZ 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 send 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 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 when it's in this open configuration I have a whole video on this it's very it at that point potassium likes to bond to it so potassium likes to bond to it it bonds to it maybe well maybe advance 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 it anymore and then it gets released because it's that 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 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 the potassium kind of comes in you can almost imagine passively it doesn't require ATP and that's why this is often called a sodium potassium ATPase which is means it's an 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 and we call this the very general term for this as a co transporter or a symporter co trans Porter or a symporter and symporter means that translate it transfers two types of molecules in the same direction code transporter 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 gonna have a low sodium concentration here low we have we're gonna have a low sodium concentration there the more we pump out the lower this is and eventually is gonna be lower than the sodium concentration in the lumen so the sodium concentration gradient if sodium if there was no membrane here sodium would want to go across this to kind of make up for all of the lost sodium's over here sodium would want to cross that if there was no barrier and what the the cells here is 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 to actually also transport maybe some glucose so this if you have to visualize it you could imagine um you know a protein that's on this apical membrane right here so let's say we have a big protein well let me draw it something like this so maybe it looks something like this this is to get some type of visualization maybe you know you have more sodium on this side then 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 it's going to change its shape to look something more like this when the two when they 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 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 the cycle over again but this is all stipulated on the idea that there's more there's more sodium over here to bump into this point to make this reaction happen so sodium is going to go down its concentration gradient it's taking glucose for the ride and then 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 the same exact process is happening not 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 same idea let me draw it so let's say that that right there is the lumen this is a cell that makes up the wall of the lumen so this is a this 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 that's why what's happening to potassium isn't that important but so sodium concentration becomes low here so what we have are symporters over here it's a symporter 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 grade we're going to have we're going to have we're gonna have chlorine ions and actually this symporter right here it's called the sodium potassium chlorine code transporter and it's actually it's the second variation that you actually get in the ascending loop of Henle and so eventually you're gonna 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 as along with the sodium same thing in the distal convoluted tubules err calcium it's a little bit different so if we're in the distal convoluted tubules let's say we're in the distal convoluted tubule these kind of villi these things that stick out this is only in the proximal convoluted tubule this brush borders but over there and just so you know this 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 secondary active transport it's nice to know and then just fine I'm finishing up at the distal convoluted tubules so this was the lumen let's say that this is the lumen right here let's say so we have cells on either side of that I think you get the general idea the distal is a little bit different so let's say that 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 we're pumping sodium out we're pumping sodium out sodium potassium pumps has a whole video on that and that pumps potassium in so you end up with a lot of sodium's over here the membrane the apical membrane that that faces the lumen it's porous to calcium so calcium has whatever the concentration of calcium here it's gonna be here so maybe you have calcium these are calcium ions just like that floating around and we write here what you have is an empty Porter so our concentration in the blood of sodium is going to be higher because we keep pumping it out and so sodium if if you let it go down its concentration gradient would go back in and so maybe right here you have some sodium going down its concentration gradient going back 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 yourself how protein would actually do that I kind of imagine a revolving door the sodium makes the door revolve the calcium is in the other part of the door and it gets spit out so this is called an anti porter because they're going in different directions but once again it's secondary secondary active transport because the only real way that this could work is if we have active transport using ATP of the sodium out of the basolateral membrane and every one of these cases anyway hopefully you found that useful it's a lot 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 all things a lot more concrete it helps me really kind of internalize what the nephron is up to