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Biology: Nephron: Filter Blood, Produce Urine

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About this Lesson

  • Type: Video Tutorial
  • Length: 12:33
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 135 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Animal Systems and Homeostasis (63 lessons, $84.15)
Biology: Human Excretion (3 lessons, $5.94)

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/biology. The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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Recent Reviews

Nopic_blu
great
08/07/2012
~ karina6

but i wish it was longer!!!!

Nopic_blu
great
08/07/2012
~ karina6

but i wish it was longer!!!!

So how do nephrons work their magic to somehow, someway get blood, take the bad stuff out, keep the good stuff in, and get the bad stuff into your bladder? Now that's a darn good question. And when we take a look at a nephron, we see that, well, you know, it really just seems to be a tube. But have I mentioned to you that structure follows function before? Because it does, and if we take a look at the structure of some of the parts of this nephron, you're going to see that, boy, that's where the function comes in.
Here's the scoop: When you bring in the fluid right here, well, let's just start talking about what's going to happen. First of all, you know that you're going to be bringing in materials out of your blood. What materials? Well, here's immediately one problem we're going to run into, and the problem is this: When you filter into Bowman's capsule, the material that enters the proximal tubule is very interesting - it's blood plasma - except certain things can't get through. So it's everything in the blood except no cells, because the cells can't filter through; large proteins can't filter through, no large proteins. But guess what else? Urea, water, salts, lots of small molecules, including glucose, and amino acids, and vitamins, and wait a minute.
Are you getting nervous? Is that a bad thing? Yes, it is. That's a bad thing, because you don't want glucose amino acids in urine going into your - and urea - going into your urine. You don't want that - problem number one. Problem number two: Therefore, everything that's in here, besides having useful materials, also has stuff that you want to get rid of and somehow want to get it concentrated down here. So the point is, it has this same concentration which plasma does. Now let me show you just - we're going to come back to this chart, but just to throw some numbers in here. The numbers represent a relative molar concentration of materials; it's called "milliosmoles." Just look at the relativity of the numbers, and not worry too much about the quantitative things here. But as we enter, the filtrate has a concentration of 300 milliosmoles. When it leaves, it's four times greater - 1200. So somehow we have to go from 300 to 1200; we have to concentrate that waste by a function of four. That's the magic, my friends, of the nephron. Let's talk about how it works.
A lot of the magic in a nephron happens right here, in the loop of Henle, because the loop of Henle is going to work as something called a "countercurrent multiplier." A countercurrent multiplier - what does that mean? Well, you guys know about countercurrent exchange systems, and if you don't, you may want to link back to our discussion of how heat might be transferred by a countercurrent multiplier, or a countercurrent exchange system. This is very much the same thing. We're going to have two different directions of flow - one going this way, one going that way - and what that is going to do, as we see, is that we are going to cause literally a multiplication of the osmolarity. So we're going to go from 300 to 1200 because of the idea of this countercurrent exchange. And, once again, there's a countercurrent this way, a countercurrent this way, and this is going down in this direction. So in essence we have three countercurrents set up here - it's pretty cool, very tricky, and relatively complex, so let's talk.
So, did I say structure follows function? I think I did, because we need to talk about the cells of the descending loop and the ascending loop. These descending loop cells, and the bottom part of the ascending loop, those cells - so the descending loop, and the bottom of the ascending loop - have very nondescript cells. The cells are flat, there are few mitochondria, there's very small surface area. In other words, these are not built for transport; they're basically built structurally. So this particular loop, the descending loop, will be permeable to water, but that's about it. So osmosis can happen through these cells, but nothing else.
But when you get to this thick part - you see how this thickens? - off the ascending loop, in here we have a very different kind of cell. Whereas these are just common epithelial cells, over on the top part of the ascending loop, we have what are called cuboidal epithelium - a different kind of epithelial cell. And these have lots of surface area, many mitochondria; these cells are built, they're built for active transport, for pumping. And, indeed, they have a major function, because you're going to see that they are going to pump salt - sodium chloride. There's that salt and that blood pressure story again; more on that later.
So let's go through the steps, let's see what happens. You kind of get a picture of what's there. I hope you've got the vocabulary down, because I'm going to talk to you in kidney-talk now. So let's move through.
Step number one: First thing that happens is we are going to - the material enters the proximal tubule. One, filtrate into the proximal tubule, and now a very important thing happens. In the proximal tubule, the good stuff is actively transported back, so I just took away that first problem. So amino acids, glucose, some ions, etc. - they are released into the interstitial fluid, and, in the interstitial fluid, they eventually end up back in the blood. So indirectly they're going to get back to the blood via the interstitial fluid and those capillaries I showed you that branched off and wrapped around the proximal tubule. So it's going to get back in the blood, I promise - that's good news.
Number two: Now the filtrate enters the descending tubule. So, two, the descending loop - filtrate to the descending loop. And what did I say that's permeable to? I said that that's permeable to water. So let's see what's going to happen. Well, you guessed it. What's going to happen is water's going to leak out of there. So look, it's getting darker and darker and darker; we're showing you more and more concentrated. So we're going to start to lose water out of this thing. So water is going to osmose out of here.
Now you're probably saying, "Well, how can that be? There has to be a higher concentration of water in here than out here for that to happen." I'll show you how that's established as soon as we get around and go around the loop. But, at this point, you can see what's happening - that water is out of the descending limb, and it's getting more and more concentrated. And that - you can see the numbers - is verified - 300, 400, 600, 900, 1200 - very concentrated.
Why? Here's why, guys. What's happening in the ascending loop is - now I want you to notice something. It looks like this is getting less and less concentrated here. It is, and I'll show you why that later. You can only do one thing at a time. This is pumping sodium chloride into this layer, the outer medulla, of the kidney. Remember the kidney has the cortex and the medulla? On the outer medulla, we start to get a buildup of sodium chloride, so we're going to get a lot of sodium chloride, which is going to change. So, in essence, what's going to happen is that's going to set up a hyperosmolar situation where there's - since it's salty out here, since it's all salty out here - water is going to go out, because there's a higher concentration of water here than here. So water is going to go out, water is going to go out. Is sodium chloride going to go in? No, that's important. Sodium chloride can't get in here, because it's not permeable to sodium chloride. So you've got a salty environment - water's out. Okay, we're almost done.
Now another thing happens. As this starts to go up, and the sodium chloride is leaving here, you'll notice this is getting more and more water. Why? It's losing sodium chloride. So what's happening is the osmolarity is going down, you see, because we're losing sodium chloride.
Well, then we get to the distal tubule. You guys with me so far? So far so good? So as we reach the top of the ascending loop, what has happened? We've lost a lot of sodium chloride, and look, we're right back to the same concentration of plasma we were before. But we've set up a concentration gradient, we've set up a hyperosmolar situation, and we still have the urea in here; we've put back our good stuff, and now we're ready to make some urine.
So let's get down to it and make some urine. So now we're going to get down to this area right here, which is the collecting tubule. And as we get there, we start to move our materials down. And now what's going to happen is this tubule, as the water starts - as the plasma - starts to move down through the cortex, into the outer medulla, and into the inner medulla, what's going to happen? Well, remember there's sodium chloride here. So water is going to start going out because of the sodium chloride, and you're going to get more and more concentrated. Then when you get down here - now, none of you guys have been good, none of you have said, "Well, wait a minute. Why is it so darn hyperosmolar down there, if all the sodium chloride is coming out down here?" Well, I was glad you didn't ask me that too soon, because I wanted to show you the last thing, and the most important thing that happens, because at this point, you start to also lose - besides water - you start to lose some - not all - of your urea.
Now think about that. That is going to set up another concentration gradient here, which is going to still more cause water to leave this thing. And that's how this got so darn concentrated down here, because of the urea gradient down in the bottom. So you've got a urea gradient down here, but then you're saying, "But wait, I'm pouring urea into my kidney; I'm not getting it out." Of course you are, because number one, you're only pouring some of it out; and number two, guess what that urea can do? It goes right back in there. So it literally becomes a urea cycle, if you will.
This is so clever, because look what we've done. We've gone from 300 to 300 - pretty much back to the same - and then, bam, we concentrated that baby by pouring out the urea, by pouring out the sodium chloride, by pouring out the sodium chloride, by pouring out the water, and setting up an osmotic gradient using a two-solute system - urea and sodium chloride. Countercurrent exchange - very important. Osmosis - very important. And most importantly, you're getting rid of those toxic wastes, that urea; it's a good thing.
Animal Systems and Homeostasis
Human Excretion
The Nephron: Blood Filtration and Urine Production Page [2 of 2]

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