The Nephron - Online Tutor, Practice Problems & Exam Prep

Nephrons are the kidney's functional units, comprising a renal corpuscle and renal tubule. The renal corpuscle filters blood through the glomerulus, a network of fenestrated capillaries, and the glomerular capsule, which contains podocytes that form filtration slits. The renal tubule, divided into proximal tubule, nephron loop, and distal tubule, modifies filtrate by reabsorbing or secreting substances. Collecting ducts further refine filtrate before it becomes urine. There are two nephron types: cortical nephrons, primarily in the cortex, and juxtamedullary nephrons, extending deep into the medulla, specializing in concentrated urine production.

Introduction to the Nephron

In this video, we're just going to do a quick introduction to the nephron. Now, everyone get comfortable because we're going to be spending the bulk of the chapter talking about the anatomy and physiology of these little guys. So nephrons are microscopic structures that are basically the functional units of the kidney. And when I say functional unit of the kidney, what I mean by that is that these are the structures that are actually going to be filtering blood and producing urine. And each kidney will usually contain about 1 to 2 million nephrons. So they are very, very abundant. If you look over at our image here, this is a nephron. And you know what? I'll say it, they're kind of weird-looking. So, if you're looking at that and you're like, what the heck is that thing? Fear not, friend, by the end of this chapter, you will understand every single microscopic inch of these little guys. I promise.

Now, nephrons are located in the renal cortex and renal medulla. You can see we have this lighter-colored region right there, that is our renal cortex. And then they can dip a bit deeper into the kidney, into this darker-colored region that is the renal medulla. And this makes sense. Right? Remember how in our last video we talked about how the cortex and the medulla are the urine-forming structures of our kidney. So it makes sense that the functional units of our kidney would be located within those regions. Now, nephrons have two main components: they have a renal corpuscle and a renal tubule. And I will see you guys in our next video to start diving into those in a bit more detail. See you there.

The Nephron Example 1

Okay. This one asks us which of the following statements is false? So let's run through them. So a reads, "Nephrons are the functional units of the kidney." This is true, so "a" is out. "B" reads that "Nephrons are located in the renal pelvis," and that is false. The renal pelvis is just the structure where urine basically drains into and collects until it goes down into the ureter. Remember, the urine-forming structures of the kidney are the renal cortex and renal medulla, and those regions are where our nephrons will be found. Keep in mind that nephrons do indeed filter blood and produce urine, which is their main function. And between both kidneys, we do indeed have 3 to 4 million nephrons. Each kidney individually has about 1 to 2 million nephrons. So our answer here is "B."

The Renal Corpuscle

Okay. So let's dive into that renal corpuscle. Now, the renal corpuscle filters blood. Okay. So that is its main purpose. Now we're going to talk about the physiology of how this filtration works in some upcoming videos. So for now, we're just going to stick to kind of the anatomy of what makes up this actual structure and the main filtration features of it. So each renal corpuscle has 2 parts. So first up, we have a glomerulus. So a glomerulus is a group of fenestrated capillaries, which are also sometimes called glomerular capillaries. Now, fenestrations are literally just pores within the membrane or within the plasma membrane of those capillaries. And so, the fenestrations or those pores make the capillaries highly permeable. Another way to think about these capillaries is that they're very leaky. So they're leaky and permeable and certain fluids can basically pass through them, which is an important part of this filtration unit. So if you look down at our image here, you can see we have our glomerulus labeled. So we basically have a blood supply coming into our renal corpuscle, and it just turns into this big messy looking ball of capillaries, and that is the glomerulus. And honestly, I think it kind of looks how it sounds, just like the word glomerulus just sounds like a big messy ball of something to me. So that's how it looks. Looks kind of weird, but that's okay. Still a great part of our body.

And then next up, we have our glomerular capsule. So our glomerular capsule is a sheath of tissue that surrounds our glomerulus. And the glomerular capsule consists of an outer parietal layer, which basically just forms the outer portion of the capsule. It kind of gives it, like, its shape and, like, structural integrity, gives it the actual capsule shape. And then it has an inner visceral layer. And that inner visceral layer is composed of podocytes, which are modified epithelial cells. And these podocytes actually surround the glomerulus and they literally wrap around the capillaries themselves. So, if you look at our image here, we have our glomerular capsule. And you may also hear this called Bowman's capsule. It means the exact same thing. So we have our glomerular capsule here and you can see how we have this outer layer that's kind of giving it its shape. So that is that outer parietal layer. And then we have these podocytes, which you can see are these little epithelial cells that are literally like wrapped around the capillary. And you can see them better on our little zoomed-in image. So, this right here would be a podocyte. Now, each podocyte has something called foot processes or pedicels. And those foot processes weave together to form filtration slits. So the foot processes literally look kind of like this and then they weave together and you can see how there are tiny gaps between my fingers there. And those gaps would be our filtration slits. So, on our image here, you can see how there are these little gaps where you can see capillary in between those foot processes. And those are the filtration slits. And again, that is acting as a really important part of this major filtration unit happening.

So the whole point of this is to get some fluid to come out of the blood. Right? And so the fluid that actually makes it through those fenestrated capillaries and podocytes is called filtrate. So filtrate is a plasma-derived fluid and it is going to contain water, electrolytes, nutrients, and all types of other small molecules. And the filtrate will be collecting in the capsular space, which is a hollow region basically between the parietal and visceral layers. So if you look at our image here, we have these kind of just empty hollow parts. So that is the capsular space and that is where filtrate is going to be building up until it eventually comes out of the renal corpuscle and enters the renal tubule.

Now I know that this is a lot of information. It's kind of some odd-looking anatomy to get used to. So take your time with it. I do recommend the most important terms to know here. We should know glomerulus, you know, know what a glomerular capsule is, understand that the glomerulus is permeable or leaky, understand that podocytes are going to act as one of the major filtration unit here, and then understand that the fluid that comes out of it is going to be called filtrate. Those are kind of the most important takeaways from this lesson. Alright. So I will see you guys in our next video. Bye bye.

The Nephron Example 2

Okay. So this one asks us true or false. If false, we're going to be choosing an answer that would correct the statement. The statement being the fluid that passes through the phosphorylated capillaries is urine, and that is false. This fluid is not yet considered urine. This is something that we call filtrate. So our answer is going to be B here. This fluid is called filtrate, which is easy to remember because it's basically just filtered blood, filtered filtrate. Right? This is no longer considered blood. It does not have any blood cells in it or platelets or things like that. This fluid is made mainly of water, electrolytes, nutrients, and other small molecules. So our answer here is B, this is false. The fluid that passes through those leaky capillaries is called filtrate.

Which component of the renal corpuscle forms filtration slits?

• A. Glomerulus.
• B. Glomerular capsule.
• C. Glomerular capillaries.
• D. Podocytes.

The Renal Tubule

Alright. Let's dive into that renal tubule. So you can see in this image on kind of the upper corner of your screen, we've kind of faded out that renal corpuscle. We already know all about that. We've also faded out this limb over here on the right. We're going to learn about that very soon. And what we are focusing on here is this winding tube that makes up the majority of our nephron, and that is our renal tubule. What we are seeing here is basically a very zoomed-in image, looking at the renal tubule that is all color-coded. And we're going to go through what each of those color-coded segments is in this video. So the renal tubule is a winding tube that basically modifies filtrate. And it does that by either conserving or eliminating certain substances. And we're going to talk about the physiology of this tube a ton in upcoming videos. So for now, we're just going to be talking about the anatomy of it. So our renal tubule has 3 main regions, and they each have a unique structure and function. So, first up is our proximal tubule. And proximal just refers to this being the initial or the first segment of our renal tubule. You can see that segment over here in purple. I'm going to label that as one for us, and the proximal tubule is located in the renal cortex.

Now, the proximal tubule is a bit unique because it contains prominent microvilli. And those microvilli are really important because they basically increase the surface area of the proximal tubule, which allows for a very high volume of filtrate to be modified very quickly. And that is important because about 65% of filtrate modification happens in this little tiny part of our tubule, this little purple part. So it has to be very efficient at what it's doing. And those microvilli really help with that. So that is our proximal tubule.

Next up, we have our nephron loop, and the nephron loop is this entire U-shaped loop in our renal tubule. And so our nephron loop is located in the renal cortex. It's always going to start in the renal cortex, but it can dip down into the renal medulla as well. And the nephron loop is divided into 2 limbs, and each limb does have a distinct function. So first, we have our descending limb, which you can see here in green. I'm going to label that as 2a for us. And this is the part of the limb that is basically traveling toward the renal medulla. And then we have our ascending limb, which you can see here in orange. I'm going to label that as 2b. And this is the part of the nephron loop that is traveling toward the renal cortex. And we'll talk about the unique function of those in upcoming videos as well.

And then finally, we have our distal tubule. So this is the final segment of our renal tubule, and you can see it here in blue. We're going to label that as 3 for us. And this is also located in the renal cortex. So typically, our distal tubule and our proximal tubule are going to be about level with each other, approximately. Now, one last structure of note in our renal tubule is the macula densa cells. So the macula densa cells are a group of densely packed cells. So densa literally just means dense here. And they are basically going to be located at the transition point between the ascending limb and the distal tubule. So what we are looking at in this image is basically this location over here. So right where that blue distal tubule is meeting that orange ascending limb of the nephron loop. And you can see, we have our kind of, slightly prominent densely packed, macula densa cells here that we've done in green. So those are the macula densa. And what these cells do is they're going to monitor the sodium chloride content of the filtrate. And that is going to actually play a role in regulating filtration rate as well as blood pressure. And we're going to have a whole video talking about how that happens as well. Alright. So that is our renal tubule, and I will see you guys in our next video. Bye bye.

The Nephron Example 3

Okay. So this one asks us, nephrons modify a high volume of filtrate very quickly. Which portion of the renal tubule contains prominent microvilli to aid in this process? And that is going to be our proximal tubules. It looks like our answer is going to be A. Remember, 65% of filtrate modification happens right in that initial segment, happens very quickly. And so, we have to have all those microvilli right front loaded in the first part of our tubule to increase the surface area to allow for more filtration to happen. So our answer is A, the proximal tubule.

Macula densa cells monitor the ________ concentration of the filtrate.

• A. $H_2O$
• B. $Na^+$
• C. $NaCl$
• D. $Ca^2$$^+$

The Collecting Duct

In this video, we're going to be talking about the collecting duct. So the collecting duct is a tubule that further modifies the filtrate. If you look at the image here on the top right, basically at this point, the filtrate has gone through the entire renal tubule and it is exiting from the end of that distal tubule. And it is exiting into this long straight duct right there, and that is our collecting duct. So this is considered the final portion of the nephron. Now what's interesting is that each collecting duct actually collects filtrate from multiple nephrons. So if you look at our image here in the center, you can see that there is that collecting duct in the middle with that blue highlight behind it, and we have 2 nephrons here, 1 on the left and one on the right. And both of their distal tubules are emptying into that one collecting duct in the center. So each collecting duct is attached to multiple nephrons. Now each collecting duct consists of 2 structures. Each one has a cortical collecting duct which is simply the portion of the duct located in the renal cortex. And, then they each have a medullary collecting duct which is simply the portion of the duct located in the renal medulla. So those are nice and easy to remember. So, if we're looking at our image here, this portion of our collecting duct in this lightly colored region would be considered our cortical collecting duct. And then this portion of our collecting duct located in that renal pyramid would be considered our medullary collecting duct. Now, as these collecting ducts are coming down the renal pyramid and approaching the renal pelvis, they actually fuse into something called a papillary duct. Okay, so this right here is our papillary duct, And it's called the papillary duct because it's located at the papilla or the tip of that renal pyramid. And this is kind of hard to picture from our little cartoon here and most textbook images don't really show this either, but is kind of imagine each renal pyramid has 100 of collecting duct and about, you know, as they're coming down toward the tip of that renal pyramid, about 5 to 10 of them will fuse into one of these papillary ducts. And when filtrate reaches the end of that papillary duct, it is finally officially considered urine. We did it, guys. We finally made urine. So just to kind of put all of this anatomy together for you a little bit, what's happening here is we have our nephron located in the renal cortex. Filtrate is moving through the renal tubule, getting modified the entire time. It's moving through the collecting duct. It's still getting modified that entire time. The collecting duct is going down the renal pyramid toward the tip toward the papilla, and that filtrate is finally going to be coming out of that papillary duct. At that point, it is considered urine. So the urine is coming out of the tip, the papilla of that pyramid, it is emptying into a minor calyce which will then empty, empty into a major calyce, which will then empty into the renal pelvis, which we know is continuous with the ureter. And so urine will travel down the ureter and then enter the urinary bladder. But just to kind of put all that together for you. Alright. So that is our collecting duct and I will see you guys in our next one. Bye-bye.

The Nephron Example 4

Okay. So, this one asks us at which location would filtrate officially be considered urine? Let's run through these options. So option A reads at the end of the proximal tubule, and remember that's the very first part of the renal tubule. So at that point, we are definitely still filtrate. So option A is out. Option B reads the ascending limb of the nephron loop. At that point, we're still in our renal tubule. This is still definitely filtrate. So option B is out. Option C reads the beginning of the collecting duct. Remember the collecting duct continues to modify filtrate. This is not the final product yet when it hits that collecting duct. So option C is out, and we are left with option D, which is the correct answer when it reaches the papillary duct. It is a long journey to get there, but by the time filtrate has gone through the entire renal tubule and the entire collecting duct, and it's coming out to the papilla, the very tip of that pyramid in the papillary duct, it is officially urine at that point. So our answer here is D.

Types of Nephrons

So up until this point, we've been talking about nephrons pretty generally, but there are actually 2 types of nephrons. So, we're going to go over these in this video. First up, we have cortical nephrons. Now, cortical nephrons, as their name implies, are located in the renal cortex. The majority of the nephron will be in that cortex, but the tip of their nephron loop can dip into the renal medulla. In terms of distinguishing features aside from just their location, these tend to have shorter nephron loops. If you look at our image here, you can see how the majority of this nephron is located in the renal cortex. It has this little tiny nephron loop that's just barely dipping into our renal medulla. Cortical nephrons are quite prevalent, making up about 80% of all nephrons. In terms of their function, they filter blood, modify filtrate, and also specialize in helping regulate blood pressure. Regarding their blood supply, these are fed by peritubular capillaries, which we will talk about in an upcoming video.

Then, we have our juxtamedullary nephrons. Now, this name is also very intuitive. These are located at the boundary of the renal cortex and the renal medulla. Juxtamedullary means 'next to the medulla.' For these, their nephron loop is going to go deep into the renal medulla. Because of that, these need to have long nephron loops. You can see that they look quite distinctive, located right much closer to the boundary of that cortex and the medulla. They have nice long nephron loops that go deep into that renal pyramid there. So, they look very distinctive when you see them side by side. These make up approximately 20% of all nephrons. Once again, in terms of their function, they are filtering blood, modifying filtrate, but juxtamedullary nephrons also specialize in creating concentrated urine, which we will talk about in a future video. In terms of their blood supply, these are also supplied by peritubular capillaries as well as a specialized structure called the vasa recta. So we will see that again in an upcoming video.

So, those are our 2 types of nephrons. Again, luckily, they were named very intuitively based on their location, so they're pretty easy to remember. Alright. I'll see you guys in our next one. Bye bye.

The Nephron Example 5

Okay. So this one asks us which of the following statements is correct? Now I'm not going to read all of these verbatim because they can be a little bit long, but just looking at these answers, I can tell that our correct answer is going to be d, which says that cortical nephrons have shorter nephron loops and are located mainly in the renal cortex, whereas juxtamedullary nephrons have those longer loops and can extend deeper into the renal medulla. So our answer is d. Now just to take a quick look to make sure that we all understand why a through c are incorrect.

A reads that cortical nephrons have longer loops and that juxtamedullary nephrons have shorter loops. It should be the reverse. Right? So a is incorrect.

B reads that juxtamedullary nephrons are more abundant. And then, for cortical nephrons, the opposite is true. Right? About 80% of nephrons are cortical nephrons whereas about 20% are juxtamedullary. So that's why B is incorrect.

And then for C, we do have the correct locations there, but we have incorrect nephron loop lengths on both of these. So that is why C is incorrect and our answer here is D.

Which of the following statements best describes the role of the juxtamedullary nephrons?

• A. Juxtamedullary nephrons filter large volumes of blood to increase urine volume.
• B. Juxtamedullary nephrons play a role in forming concentrated urine.
• C. Juxtamedullary nephrons play a key role in regulating blood pressure.
• D. Juxtamedullary nephrons control $K^+$ and $Ca^2$$^+$ levels.

Here’s what students ask on this topic:

What are the main components of a nephron and their functions?

A nephron consists of two main components: the renal corpuscle and the renal tubule. The renal corpuscle is responsible for filtering blood and includes the glomerulus, a network of fenestrated capillaries, and the glomerular capsule, which contains podocytes that form filtration slits. The renal tubule, divided into the proximal tubule, nephron loop, and distal tubule, modifies filtrate by reabsorbing or secreting substances. The proximal tubule reabsorbs a significant portion of filtrate, the nephron loop concentrates urine, and the distal tubule fine-tunes the filtrate composition. Collecting ducts further refine filtrate before it becomes urine.

How does the renal corpuscle filter blood?

The renal corpuscle filters blood through the glomerulus, a network of fenestrated capillaries that are highly permeable. Blood enters the glomerulus, and the fenestrations allow water, electrolytes, and small molecules to pass through while retaining larger molecules like proteins and blood cells. The filtrate then passes through the glomerular capsule, which contains podocytes with foot processes that form filtration slits. These slits further filter the fluid, allowing only certain substances to pass into the capsular space, where the filtrate collects before entering the renal tubule.

What is the function of the proximal tubule in the nephron?

The proximal tubule is the initial segment of the renal tubule and is located in the renal cortex. Its primary function is to reabsorb a significant portion of the filtrate, including water, electrolytes, and nutrients. The proximal tubule contains prominent microvilli that increase its surface area, allowing for efficient reabsorption. Approximately 65% of filtrate modification occurs in the proximal tubule, making it a crucial part of the nephron for maintaining the body's fluid and electrolyte balance.

What are the differences between cortical and juxtamedullary nephrons?

Cortical nephrons are primarily located in the renal cortex and have shorter nephron loops that barely dip into the renal medulla. They make up about 80% of all nephrons and are involved in filtering blood and regulating blood pressure. Juxtamedullary nephrons, on the other hand, are located at the boundary of the renal cortex and medulla, with long nephron loops that extend deep into the medulla. These nephrons make up about 20% of all nephrons and specialize in creating concentrated urine. Both types of nephrons are supplied by peritubular capillaries, but juxtamedullary nephrons also have a specialized blood supply called the vasa recta.

What role do collecting ducts play in the nephron?

Collecting ducts are the final portion of the nephron and play a crucial role in further modifying the filtrate before it becomes urine. Each collecting duct collects filtrate from multiple nephrons and consists of two parts: the cortical collecting duct, located in the renal cortex, and the medullary collecting duct, located in the renal medulla. As filtrate moves through the collecting duct, it continues to be modified by reabsorption and secretion processes. The collecting ducts eventually fuse into papillary ducts at the tips of the renal pyramids, where the filtrate is finally considered urine and empties into the minor calyces, then the major calyces, and finally the renal pelvis before being transported to the urinary bladder via the ureter.