Video: Kidney histology

Did you know that your kidneys will filter the entire volume of your blood up to 20 times each day? What’s more, each kidney will produce up to one-and-a-half liters of urine which will be sent to the bladder for storage. This means that whether we are running a marathon or watching our favorite shows on a Sunday afternoon, the one million nephron units present within our kidneys are working their socks off.

In this video, we’ll take a look behind the scenes at the micromachinery responsible for keeping all of these processes in check. We’ll go on a virtual exploration down the microscope to uncover anything and everything we can identify along the way. So, let’s get these neurons and nephrons fired up!

It’s time to take a look at the histology of the kidney.

Let’s start by checking out what we’ll cover in this video. We’ll begin by reminding ourselves about the gross anatomy of the kidneys including a reminder of what blood vessels supply the various parts of the kidney. Then we’ll delve into the internal structure of the kidney by taking a look at a coronal histological section of a kidney tissue, stained with a type of trichrome strain. If you aren’t familiar with this one, don’t worry. It’s a great chance for us to explore our histology in a slightly different way to what you may have seen before.

Then we’ll magnify our lens and take a closer look at the kidney ultrastructure with our good old friend, the H&E stain. And, finally, we’ll contrast the adult and fetal kidney to point out some characteristic features of kidney development. And once we’ve whisked through all this content, your brain – or perhaps your bladder – will be full to the brim with new and exciting kidney facts to impress your peers in anatomy class.

So, without further ado, let’s get cracking with the gross anatomy of the kidney.

So, we know that the kidneys are bilateral, or paired organs, found in the retroperitoneal space of the abdomen. They’re found in a really posterior position, as we can see here in this radiology image, posterior to the liver or spleen right next to the vertebra and deep muscles of the back.

They’re bean-shaped and can be found between the vertebral levels T12 and L3, with the right being positioned slightly lower than the left. Each kidney contains a major convexity on the lateral surface and a minor concavity on the medial surface, and it’s here at the medial concavity where we have the hilum of the kidney from which the ureter emerges on its final pathway to the urinary bladder. You’ll find the neurovasculature of the kidney entering and exiting the organ at this point.

Let’s turn our attention now to the internal anatomy of the kidney. There’s only one way to uncover this information and that’s to take a look inside the kidney itself to uncover what lies beneath the surface, which is where we’re going to be starting our journey.

The surface of the kidney is called the renal capsule and it surrounds the kidney tissue itself which is called the renal parenchyma. This tissue can be further divided into an outer layer known as the renal cortex and an inner layer known as the renal medulla.

So, let’s start on the outside with the renal cortex.

The cortex has a somewhat striated appearance due to the presence of medullary rays which extend from the underlying medulla. I’ll explain a little bit more about these a little bit later on in the tutorial.

Next stop is the renal medulla, which can be divided into 10 to 18 masses of tissue known as renal pyramids and between each of these pyramids are extensions of the renal cortex, which are known as renal columns. Taken together, one renal pyramid and its overlying section of renal cortex is known as a renal lobe.

As I mentioned, on the medial border of each kidney, there’s a central depression called the renal hilum through which blood vessels and the renal pelvis enter and exit the kidney. This hilum opens up into a central space called the renal sinus.

Okay, so in the introduction to this tutorial, I mentioned that the basic functional unit of the kidney is called the nephron, and it’s these structures that operate tirelessly to ensure that hundreds of liters of blood are filtered each day to produce urine which are sent to the bladder for storage. But what are they exactly and what makes them tick? Let’s take a look.

Essentially, each nephron is divided into a renal corpuscle and renal tubule. The renal corpuscle is the first dilated portion of the nephron containing a massive capillary surrounded by an epithelial capsule. The second part of the nephron is the renal tubule which is divided into many parts including the proximal tubule, the nephron loop which you may know as the loop of Henle, the distal tubule, and the collecting tubule, which forms the last part of the nephron. Along its length, the renal tubule passes through the cortex and medulla of the kidney parenchyma carrying the urinary filtrate to be finally drained into the pelvicalyceal system which we’ll get into later.

But it’s a little bit more complex than that. You see, we also have blood vessels passing through the cortex and the medulla, all branching away from a stem vessel called the renal artery. This artery enters the kidney through the renal hilum before multiple smaller branches start to emerge, which include the segmental arteries, the interlobar arteries, and the arcuate arteries, just to name a few. And that’s just the arteries. In reality, these vessels travel with their associated venous counterparts, a.k.a., the veins, which, fortunately for us, carry the same names as the arteries. Examples of which include the arcuate vein, the interlobar vein, and the renal vein.

Now that the nephrons are fired up and ready to go, let’s put this knowledge to the test as we go down the microscopic lens to uncover some of the structures we just mentioned. Let’s waste no time and get to it.

Okay, so we’re now all familiar with some basic anatomical structures on schematics such as these, but in reality, it isn’t this simple, and it looks a little more like this. This slide is actually a low magnification trichrome stained section of kidney tissue, and unlike other staining methods such as H&E, trichrome staining allows extracellular components to be visualized whilst also maintaining a really clear image of the nuclei and the cytoplasm.

But what can we see exactly? Well, you should be able to make out some basics without your magnifying glass. Check out the renal capsule on the outermost surface of the kidney along with the renal cortex, the renal medulla, and the renal sinus at its core. In order to visualize the finer details, let’s magnify our lens further.

Now that we’ve zoomed in a little, we can see the outermost layer of the kidney a little clearer, which is known as the renal capsule. As we touched on earlier, the capsule is a tough and fibrous layer which wraps around its outer surface. It can be divided into two layers – a thinner outer layer of the renal capsule, and a thicker inner layer of the renal capsule directly underneath.

The outer layer is made up of both type I and type III collagen fibers, which appear purplish-blue with trichrome staining. We may also see the occasional spindle-shaped fibroblasts scattered among these collagen fibers and it’s these fibroblast cells found within all connective tissues throughout the body that are responsible for the synthesis of extracellular matrix and collagen.

Much the same as the outer layer, the inner layer is also made up of both type I and type III collagen fibers, but instead of fibroblasts, we see another type of cell known as the myofibroblast, which can be recognized by their irregular-shaped membrane. These cells play a role in wound healing in many organs including the kidney and the liver.

So, we’ve taken a closer look at the capsule, what will we find next? It’s the renal cortex. The renal cortex surrounds the entire kidney except at the renal hilum as this is the area where blood vessels and the renal pelvis enter and exit the kidney. It is essentially a thin band of pale-staining tissue approximately six to nine millimeters in thickness which surrounds the outer part of the kidney. The cortex is known as having a granular appearance because it contains many ovoid-shaped filtration units called renal corpuscles, which we’ll see later in this video.

To see it in a little greater detail, let’s magnify our lens further.

So, what are we looking at here exactly? Well, in the center of this slide are highlighted examples of the medullary ray. These structures are a series of straight tubules in the center of a so-called renal lobule, which you should think of as a group of nephrons surrounding and draining into a single collecting duct. There are approximately five hundred of these in the adult kidney and are orientated in a perpendicular direction to the renal capsule.

The secondary of the lobule is the renal cortical labyrinth and is found between the medullary rays centrally and the cortical vessels laterally. This area contains the renal corpuscle, the proximal convoluted tubule, and the distal convoluted tubule. Unlike the straight tubes of the medullary ray, the cortical labyrinth has a far less regular arrangement with concentric-looking tubules instead. Later in this video, we’ll go through the whole nephron in depth. So don’t worry, we’ll be coming back to this.

Now that we’re familiar on the cortex of the kidney, let’s move down another layer and check out the renal medulla. As we mentioned earlier, the medulla can also be split into defined regions at a gross level involving structures called renal pyramids. The renal medulla is divided into an outer or more superficial part known as the outer stripe and an inner or deeper part known as the inner stripe. These are each characterized by what parts of the nephron they contain. But we’ll see more on this later.

The pyramid has a flat base located adjacent to the next structure here highlighted in green – the corticomedullary border – also known as the corticomedullary junction of the kidney. The corticomedullary junction is the point at which the renal cortex and the renal medulla meet and is the place that divides the short-looped nephrons of the cortex from the long-looped nephrons of the medulla.

The apex of the renal pyramid is known as the renal papilla. It is composed of the terminal portions of the collecting ducts which unite to form larger renal papillary ducts which empty into the renal pelvis. The papillary ducts have large and wide lumens and aligned by simple cuboidal or columnar epithelial cells. Openings of the numerous papillary ducts form a sieve-like appearance at the tip of the renal papilla known as the cribriform area. It's here where the contents of each renal papillary duct is emptied into the minor renal calyx. This in turn empty into major calices which come together to form the renal pelvis, and taken together, this is what we mean by the pelvicalyceal system.

The renal pelvis passes through the hilum of the kidney where it becomes known as the ureter, and interestingly, the renal pelvis along with the ureter and the urinary bladder are lined by a specialized type of transitional epithelium known as urothelium. Note how the urothelium has large cuboidal cells where binucleate cells can often be found, which is quite different from the simple columnar epithelium of the renal papilla which you can see here on this image highlighted in green.

So now we’ve spotted some major anatomical features of the kidney, let’s take a closer look at the vessels across these regions.

So as we’ve encountered before, the renal artery is the major stem vessel entering the kidney. Once through the renal hilum, the artery splits into two major branches, notably, the anterior branch and the posterior branch. It’s these branches which give rise to a series of segmental arteries which course through the renal sinus. Examples we can see here include the anterior inferior segmental artery and the superior and posterior segmental artery, to name a few.

From the segmental branches, interlobar arteries emerge which course through the medulla to the level of the corticomedullary junction, and once the interlobar arteries have reached this level, they divide again to form the arcuate arteries known as arcuate because they form a large arching network around the base of each renal pyramid.

And, finally, can you guess what artery radiates from this arc? Yep, you got it. The interlobular arteries which travel through the cortex and give off microvessels surrounding the functional unit of the kidney – the nephron. Important to take note of is that these interlobular arteries are also commonly referred to as cortical radiate arteries.

But what does all of this look like histologically? Well, zooming in again, we can see multiple examples. So check this one out. It’s an interlobular artery involved with dividing the cortex into lobule compartments we saw earlier. Like with all arteries, the interlobular artery has a thickened tunica media or muscular layer and a smaller lumen than its counterpart. What’s more, we can also see an interlobular vein in longitudinal section in the same region which is recognizable for its thinner wall. And if we move our lens slightly deeper into the cortex, we should be able to see the larger arcuate arteries and arcuate veins bundled together and passing along the corticomedullary junction.

So now that we’ve covered pretty much everything on gross kidney anatomy, let’s check out some microanatomy of this region using a different stain which is shown here. Yup, you guessed it. In this section, we’ll be looking at some H&E sections. Using H&E, the nuclei stained blue and are basophilic because they’re heavily abundant with ribosomes and the cytoplasm stains pink and is acidophilic due to the presence of cytoplasmic proteins.

Let’s kickstart this section with a quick recap of where we are at so far.

At the outermost edge of the section is the renal capsule which can be divided into an outer layer and an inner layer. Moving deeper into the kidney parenchyma, the next layer we’ll encounter is the renal cortex. And the next layer? You got it! This is the renal medulla, which we’ve already mentioned, is divided into different parts itself.

So now that you’re equipped with all the necessary tools to voyage further into the microanatomy of the kidney, let’s begin by looking at the nephron.

The nephron is the basic functional unit of the kidney and can be classified as one of three types – cortical nephrons which have their glomeruli in the outer parts of the renal cortex and they have shorter nephron loops which extend only into the outer region of the medulla; juxtamedullary nephrons whose proximal tubules lie at the level of the corticomedullary junction and have relatively long nephron loops reaching deep into the medulla; or midcortical nephrons, which are an intermediate between the two.

Okay, so I know what you’re thinking. All of this stuff sounds terribly complex and convoluted. But, fear not. It’s all pretty logical. Let me take you on yet another adventure down the lens to take a look at each portion in turn.

So here we’re looking at the first part of the nephron which is the renal corpuscle. It’s formed from a core of capillary tufts known as the glomerulus at the center and is surrounded by a double-layered epithelial shell known as the Bowman’s capsule, also known as the glomerular capsule, around the outside. Each corpuscle contains a urinary or tubular pole where the proximal tubule begins, and a vascular pole where the blood vessels enter and exit the corpuscle. The blood entering the renal corpuscle will be filtered through the filtration barrier before forming what’s known as the urinary filtrate, containing both waste products and useful ions which will later be reabsorbed along the nephron.

But where does this blood come from? Well, this would be the afferent glomerular arteriole which delivers blood to the renal corpuscle. The afferent arteriole stems from the interlobular vessels – those guys which stemmed from the arcuate arteries at the corticomedullary junction. From here, each arteriole travels toward a renal corpuscle where it provides a blood supply.

Moving our lens along, we can see that as soon as the arteriole is at the vascular pole, it delivers blood into a network of capillary tufts known as the glomerulus at the center. It is here where filtration occurs and molecules small enough to pass through the barrier end up in the urinary space. But what is the filtration barrier and what is it made of?

As I mentioned already, glomerulus is surrounded by a double-layered capsule – an outer parietal layer and an inner visceral layer. The parietal layer consists of a simple squamous epithelium surrounded on the outside by a basal lamina. The visceral layer adheres to the glomerular capillaries and is formed by irregularly-shaped epithelial cells known as podocytes and taken with the endothelium forming the capillary itself forms the barrier responsible for ultrafiltration.

Let’s take a closer look at these podocytes, which are one of the major components of this filtration barrier. They are specialized stellate, or star-shaped cells, which secrete factors such as angiopoietin-1 and vascular endothelial growth factor, which maintain the capillary endothelium of the glomerulus. The space between the parietal and visceral layers of the glomerular capsule is known as the glomerular capsular space, or urinary space, which receives the urinary filtrate from the glomerular arterioles. The glomerular capsular space is continuous with the proximal tubule, which we’ll look at in a moment.

And when that’s all over and the filtrate has been formed, the blood leaves the renal corpuscle via the efferent glomerular arteriole, again, via the vascular pole which we can see here. On a side note, it’s pretty important to mention that the lumen of the efferent arteriole is smaller than that of the lumen of the afferent arteriole. Do you know why this is?

This difference in diameter of the lumen generates pressures high enough for glomerular filtration to occur. It involves specialized cells known as juxtaglomerular cells, which we can see now that we’ve zoomed in to the afferent arteriole. These cells are found in the wall of the arteriole and the distal tubule acting to maintain an appropriate blood pressure in the glomerulus for filtration to occur.

So now that we know about the renal corpuscle, the glomerulus, the glomerular capsule, and the juxtaglomerular cells, let’s take a look at another group of cells found within and adjacent to the corpuscle known as mesangial cells. These cells can be found in the spaces between the glomerular capillaries. They serve a wide range of functions related to the general function of the glomerulus.

One of these functions is to phagocytose material accumulated in the glomerular space preventing any clogs of debris from forming which might interrupt filtrate flow. They also help to maintain an optimal filtration rate in the glomerulus. They do this by their ability to contract and relax which regulates capillary flow. In addition, they also provide physical support to the capillaries within the glomerulus. And finally, these cells can release cytokines and prostaglandins which act to repair and maintain the filtration barrier. Unfortunately, for us, these cells are pretty hard to spot in histological section and are often mistaken for a podocyte, but in reality, they often stain a little darker than their podocytic neighbors.

To sum this all up, I kind of think of the mesangial cells as the handyman of the corpuscle, keeping the glomerulus in check and fixing any problems which might creep up.

Let’s turn our attention now to the renal tubule, which is divided into several parts, the first of which is the proximal tubule. It is specialized for ion reabsorption and make sure that we reclaim the nutrients, water, and electrolytes which have passed through the filtration barrier. In fact, the proximal tubule reabsorbs around 65 percent of the filtrate drained in the renal corpuscle. Once reabsorbed, these molecules pass through the cell walls and into the network of the peritubular capillaries in the cortex. These peritubular capillaries are branches of the efferent arteriole, which surrounds the renal corpuscle.

The first part of the proximal tubule is a region known as the proximal convoluted tubule. Convoluted refers to the fact that it looks like a little bit like spaghetti in appearance compared with the straight tubules which look more linear. The tubules are formed from a single layer of cuboidal epithelium with a granular cytoplasm. It has many cool features which make it highly specialized for absorption. Check out the apical surface. It contains many microvilli which gives a kind of fuzzy appearance to the luminal border of the cell. This increases surface area for absorption in the tubule.

For much the same reason, the basal surface is highly folded and contains many transporters, pumps, and channels which move ions from the lumen into the peritubular capillaries. Inside the cell, the cytoplasm contains many vacuoles which assist in storage and transport. For example, protein stored here may be broken down by lysosomes before being released into the capillaries. The cytoplasm is also highly acidophilic as it is packed out with many mitochondria. The mitochondria facilitate the production of ATP which supports the active movement of ions by pumps and transporters at the basal surface.

The proximal convoluted tubule continues as the proximal straight tubule which passes into the outer medulla of the kidney. The cells here are not as specialized for absorption and have a less developed brush border of microvilli and fewer mitochondria for sustaining gross transport of the molecules.

The proximal straight tubule is also the first segment of what’s referred to as the nephron loop or the loop of Henle. This explains why the proximal straight tubule also is sometimes referred to as the thick descending limb of the nephron loop.

So, we’re going to move on now to what’s know as the thin segment of the nephron loop, and as we noted before, the length of the thin segment varies with the location of the nephron within the cortex. Juxtamedullary nephrons have the longest limbs while cortical nephrons have the shortest.

The first part of the thin segment of the nephron loop is called the thin descending limb. It’s continuous with the proximal straight tubule and has a luminal diameter of just 30 micrometers, which explains the whole thing about being thin, of course. It’s lined by simple squamous epithelium which allows it to be highly permeable to water, but not specialized for the absorption of ions and solutes remaining in the urinary filtrate. Because the interstitial fluid of the surrounding medulla is hyperosmotic, water diffuses out of this nephron segment.

Following the band of the nephron loop, next is the thin ascending limb of the nephron loop. Like its neighbor, the thin descending limb, it too is lined by a simple squamous epithelium. Unfortunately, for us, the thin ascending loop looks pretty similar to the vasa recta down at microscope. But all is not lost, as the vasa recta have a thinner epithelial lining than the thin ascending limb, and of course, have lots of red blood cells inside their lumens.

The thin ascending limb of the nephron loop does not actively transport ions from the filtrate to the blood nor is it permeable to water. However, it does allow for passive diffusion of sodium chloride or salt, meaning that the filtrate inside becomes hypoosmotic and the interstitium becomes hyperosmotic.

Now we’ve moved our lens, we can see that in the distal straight tubule, also known as the thick ascending limb, our lining changes to simple cuboidal epithelium. It also contains many mitochondria to help reduce enough ATP to sustain active support of sodium and chloride ions. Once absorbed, these ions pass into the interstitial space surrounding the tubule. What’s more, this increasing concentration of ions in the interstitial space draws in water molecules from the thin descending limb, making the filtrate a bit more concentrated.

The distal straight tubule has large cuboidal cells which have a lightly stained cytoplasm and nuclei located towards the apical end of the cell which causes the cells to appear to bulge into the lumen of the tubule. They do not have a well-developed brush border of microvilli which helps us identify them from the proximal straight tubules, and you can expect to find these distal straight tubules in the outer stripe of the medulla or within the overlying cortex.

Let me quickly remind you about the stripes of the medulla. You remember earlier we mentioned that the renal medulla has what we called outer and inner stripes. We can now define exactly what is meant by these terms.

The outer stripe is the more peripheral portion of the medulla and is only traversed by thick segments of nephron tubules. The inner stripe, however, is structurally distinct from the outer stripe due to the fact that it contains both thick and thin segments of nephron tubules.

The next structures which I want to draw your attention to is this thickening along the distal straight tubule as it contracts the renal corpuscle. This is known as the macula densa of the distal straight tubule. These cells are tall and narrow columnar epithelial cells with large nuclei which can usually be found at the apical surface close to the lumen. The cells of the macula densa are also components of the juxtaglomerular apparatus – that whole feedback system responding to changes in glomerular blood flow.

When blood flow decreases in the glomerulus, downstream changes in the pressure mean that more salt is reabsorbed from the proximal tubule. As this occurs, less salt reaches the lumen of the distal tubule which is detected by the cells of the macula densa and leads to the secretion of prostaglandins. This stimulate renin release from the juxtaglomerular cells which bring the flow rate up again by vasodilation.

I know all of this sounds super complex right now, but it’s essentially just a feedback system keeping flow constant.

Similar to the proximal tubule, the distal tubule also contains a distal convoluted tubule which also functions in further ion reabsorption and secretion. It’s lined by a simple cuboidal or square-shaped epithelium with no distinguishable brush border. As a result, the lumen has no fussy appearance like in the proximal tubule, but rather is wide and smooth.

The cuboidal cells are smaller and slightly lighter staining than their proximal counterparts and the cell borders are very noticeable. Distal convoluted tubules are shorter and less convoluted than proximal tubules, so you’ll notice that they’re less frequently seen in sections of renal cortex.

And lastly, we’re at the collecting tubules. They’re the shortest in terminal part of the nephron where excess water is reabsorbed from the filtrate. In the cortex, multiple tubules converge to form collecting ducts in the region of the medullary ray. These are somewhat more flattened or squamous compared to the surrounding convoluted tubules.

As the collecting ducts continue into the medulla, they are lined by a single layer of cuboidal epithelium which later joins up with other ducts to form much greater ducts of Bellini in the outer stripe of the medulla. The ducts of Bellini travel towards the apex of the medullary pyramid where multiple ducts join to form a single papillary duct.

The collecting duct contains many pale-staining cells called principal cells, also known as light cells or simply collecting duct cells. These can be identified by their relatively empty cytoplasm with hardly any organelles. The boundaries of each cell stain very darkly and appear very pronounced at an ultrastructural level and the basal membrane is highly folded which increases the surface area for transporters and channels.

Also noticeable are the small vesicles in the cytoplasm, which contain a membranous protein known as aquaporins. Aquaporins regulate the amount of water present in the filtrate, and therefore, the concentration of our urine. In between the principal cells are masses of darker cells known as intercalated cells which secrete hydrogen ions or bicarbonate ions which regulate the acidity of the filtrate. Ultrastructurally, these guys are recognizable for having many mitochondria and folds which project into the lumen.

So, we know all about the anatomy and the microanatomy of the kidney. Now for something a little different – looking at a section of a fetal kidney.

Due to its small size, a fetal kidney allows a larger tissue section to be produced. The trichrome stain highlights the major characteristic features of embryological development in the kidneys. Can you identify the renal capsule? Just like with the adult, it is located around the outside of the kidney parenchyma and contains many capsular progenitor cells identifiable by their irregularly-shaped nuclei, but these tend to vary greatly in number from one kidney to another.

Next, take a look at the convex border. It is highly irregular and bumpy in appearance due to the development of separate renal lobes. Remember, there are approximately 10 to 18 of these lobes in the adult kidney. You may be able to recognize a darker staining renal cortex around the outer surface of the organ. Remember that when using a trichrome stain, areas containing large quantities of collagen stain dark purple. This make sense as the cortical tissue is collagen-rich, and within the cortex itself, you may have noticed a series of minute white holes which represent the developing ovoid filtration units called the renal corpuscles.

The next layer inwards is the pale-staining renal medulla on the inner surface. You may recognize multiple developing medullary papillae, the apex of which is directed inwards towards the pelvicalyceal space which will eventually become the renal sinus. The renal sinus is the central space through which structures known as major calices pass before forming the renal pelvis at the hilum of the kidney.

Okay, almost there. So, let’s look at an example of what happens when filtration becomes disrupted and what can be done to alleviate the symptoms.

So, although it sure is different to what we’ve already seen, this is a renal corpuscle of a patient suffering from glomerulonephritis, which is a disease resulting in damage to the capillaries in the glomerulus. In this disease, the filtration barrier becomes blocked with immune complexes disrupting filtration in the kidney. The basement membrane, which is the layer below the surface endothelium becomes greatly thickened. In addition, the lumen of the capillaries becomes radically reduced due to the proliferation and invasion of both activated endothelium and mesangial cell debris. Over time, more protein is lost in the urine and the excess water and salt accumulate in the body. This is called nephrosis, and although treatment is limited, doctors may prescribe diuretic drugs to control water levels in the body and help with edema while also suggesting a diet low in salts.

Okay, guys, now that we’re coming to the end of our microscopic kidney quest, let’s wrap this up and recap what we did today.

To begin, we jump straight in with some basic kidney anatomy explaining that the kidneys are bilateral and retroperitoneal organs situated at approximately levels of T12 to L3. We then took a closer look at the kidney in coronal section and identified some major features that we should look out for in trichrome-stained images. These are the renal capsule surrounding the outer part of the kidney divided into an outer layer and inner layer. We then looked at the renal cortex and discussed that this was divided into lobules by the presence of interlobular vessels. In the center of each lobule was the medullary ray and either side of the ray are the convoluted tubules of the renal cortical labyrinth.

Before moving into the medulla, we noted that the arcuate vessels could be found in the corticomedullary junction and give rise to the interlobular vessels which divide up the cortex. Beyond the corticomedullary junction, we mentioned that the cortex extends multiple fibrous bands known as renal columns into the medulla which divided up into many pyramidal-shaped sections called the renal pyramids. We also mentioned that the medulla can be divided into an inner medulla and an outer medulla, respectively.

Then we looked into the apex of the renal pyramid to the papillary ducts which conveys the filtrate to the renal calyx. The renal calyx, we explained, conveys urine from the renal medulla and into the renal pelvis which forms the ureter. Following this, we mapped out the nephron of the kidney. Starting from the renal corpuscle which is formed by the glomerular or Bowman’s capsule on the outside with both a visceral and parietal layer and the glomerular capillary tuft on the inside formed from a fenestrated epithelium.

Between these two structures was the intervening urinary space which led into the other structures of the renal tubule. These are the proximal tubule with both straight and convoluted parts, the nephron loop with descending and ascending portions, and the distal tubule again with both straight and convoluted parts. And, finally, the collecting tubules which lead to the collecting ducts and into the calyx which drains into the ureter.

And that’s a wrap! Remember that you can watch this video as many times as you like and don’t forget to head over to the atlas where you’ll find all the images we used in this tutorial.

Have a wonderful day and remember to come back soon to learn some awesome new Kenhub knowledge for your anatomy class. See you soon!