Author: Dr Peter de Souza
Last modified: 17 December 2020


This is a tutorial on the internal structure of the kidneys.   We’re looking here at the two kidneys and you can see the abdominal aorta and the inferior vena cava giving off the renal artery and renal veins to the kidney. You can see the ureters coming out of the kidney.


The place where these structures enter the kidney is called the hilum. This is a vertical slit on the medial aspect of the kidneys where the various structures enter the kidneys.   You’ve got blood vessels, the ureter, lymphatics and the nerves which enter the kidney at the hilum.

For the rest of this tutorial, I'm going to switch over to a cross-section and talk you through some of the internal features of the kidney.  , let's just imagine take a slice through the kidney in this plane.


We're looking here at a section of the left kidney. The area where these structures enter is the hilum as I mentioned. The hilum continues as the renal sinus inside the kidney. The renal sinus contains all these structures that enter into and out of the kidney.   It’s the continuation of the hilum inside the kidney.


Also inside the renal sinus, you've got perinephric fat.   If you've watched my tutorial on the capsules of the kidney, you'll know that the kidney is contained in a sort of fatty capsule. This perinephric fat also passes into the sinus via the hilum.


The outer part of the kidney is called the cortex and the inner part is called the medulla. You can see these red pyramidal-shaped structures. You've got the outer cortex, renal cortex. 'Cortex' is Latin for bark or shell, sort of an outer covering.   If you remember the cerebral cortex in the brain, there's a thin outer layer of gray matter and you've got the adrenal cortex.   Cortex is this outer part.


The cortex in this model is in this orangey-browny color on the outside. You can see it projects in towards the center of the kidney.   You can see these little projections here between the medulla. These are called renal columns.


The medulla is this inner tissue, which is surrounded by cortex. It's shaped like a pyramid, so they're called medullary pyramids.


The base of these pyramids faces outwards into the cortex, but the tip of the pyramid, the apex points in towards the renal sinus.   The apex, this little tip of the renal medulla is called the renal papilla.


The word 'papilla' is Latin for nipple and it crops up quite often in anatomy. You've got the major duodenal papilla in the duodenum and it's kind of like nipple-shaped. This little tip is called the renal papilla.


Surrounding the renal papilla, you've got the minor calyx.   It’s this structure here surrounding the renal papilla. And urine passes into the minor calyx from the medullary pyramid via the collecting ducts of the nephron, which I'll show you in a moment.


You can see these minor calyces surrounding the tip of the medullary pyramids, the renal papilla.   The plural of calyx is calyces. These minor calyces come together to form major calyces (or singular is major calyx).   You’ve got the renal papilla draining urine into the minor calyx and the minor calyx combines to form the major calyx.


And then you've got the major calyces combining to form the renal pelvis. This bit here is the renal pelvis. And then the renal pelvis forms the ureter, which leaves the kidney.


I mentioned that the collecting duct of a nephron drains from the renal medulla into the minor calyx. What I'm showing you here is a diagram of a nephron. The nephron is the basic functional and structural unit of the kidney. Essentially, it filters the blood and it does this to regulate the amount of water, salt, sugar and urea in the body.


The filtrate that is produced is urine and this flows out like I showed you through the collecting duct into the minor calyx, into the major calyx, into the renal pelvis and then out through the ureters, into the bladder and then urethra and so on – out of the body.


I'll do a separate tutorial which covers the nephron in a lot more detail, but I just want to show you a few things here.


We've got three parts to the nephron essentially. We've got the renal corpuscle, which consists of the glomerulus and Bowman's capsule. The glomerulus is this network of capillaries which sits inside this capsule known as Bowman's capsule. This structure is the first point at which the blood is filtered. That's the renal corpuscle and that lies in the cortex of the kidney.


And then after the renal corpuscle, you've got the next part of the nephron, which is the renal tubule. You can see this wavy structure and then this loop and then more wavy structure. That's the renal tubule.


And then the final part is the collecting duct, which I mentioned before. This is where the urine is collected and it drains all the way down until it gets to the renal papilla where it passes into the minor calyx.


There are three basic parts of the nephron. You've got the renal corpuscle consisting of the glomerulus and Bowman's capsule. And then you've got the renal tubule and then the collecting duct in the end.


How does the nephron fit into our picture of the kidney? Well, I'll show you that now. We're going to zoom into this area. We're going to zoom into one of the renal pyramids.


the reason, I  switched over to the diagram is because I want to show you what kind of structures lie in the cortex and what structures lie in the medulla.


If I just draw your attention over to this side, you can see these blood vessels extending into the cortex.   Surrounding the renal medulla, medullary pyramids, you've got these blood vessels at the base of this pyramid.   This is the apex where the papilla is at the base. You've got an artery called the arcuate artery. This gives off some branches laterally at a 90° angle. These vessels are called interlobular vessels.   You’ve got an interlobular artery and an interlobular vein and you've got an arcuate artery and an arcuate vein.


In the renal cortex, you've got the arcuate vessels and the interlobular vessels. And then if we look at the nephron, you can see that the renal corpuscle lies in the renal cortex and you've got parts of the renal tubule as well.


It's worth pointing out at this point that this is obviously not to scale of any sort. It's just diagrammatic to show you what parts of the nephron are in the cortex and what parts are in the medulla.


In the medulla, we've got this part of the renal tubule. It kind of has this u-shaped hairpin bend. This is called the loop of Henle. This sits in the medullary pyramid. But the renal corpuscle is always in the cortex and the loop of Henle is always in the medullary pyramid.


And then as you know, we've got this tube here.   This is the collecting duct where the urine is collected. This lies in the renal medulla. It drains down. This is the papilla at the apex of the pyramid. The urine passes out to the collecting ducts into the minor calyx.


Just to put things into perspective a little, there are over one million nephrons in each kidney.   You could see that it probably doesn't look like this, but I hope it's given you some idea of what structures lie in the cortex, what lies in the medulla.


You can see roughly what's happening with the filtering of the blood here. Blood comes from the renal arteries into the kidneys. And then it comes up through these arteries around the side of the medullary pyramids. It passes up through the arcuate arteries into these interlobular arteries.


And the interlobular arteries give off branches called the afferent arteriole, which go in towards the renal corpuscle.   They go into this glomerulus and they're filtered, the blood is filtered and it's returned back into this interlobular vein, which returns ultimately to the renal vein.


But the stuff that's filtered out of the blood passes through this tubular system. There's a lot of complex physiology that I won't go into, but the stuff that's filtered out and excess water goes into the collecting duct and then passes out into the minor calyx, major calyx, renal pelvis, ureters and it's excreted out of the body.


That’s the internal structure of the kidney.