Tubes of the Nephron

00:01 Let's have a look at some details now of the tubule system, diagrams to remind you of the tubules, the types of tubules.

00:10 And on the right, you can see a glomerulus sitting in Bowman's space, Bowman's capsule. And you can just see the beginnings of the urinary pole, the beginnings of the proximal convoluted tubule. There's the urinary pole labelled and proximal convoluted tubules. You know the glomerulus or at least the kidney overall filters about 180 liters perhaps, of blood a day. And of that 180 liters of blood, or should I say 180 liters of filtrate that passes through these glomeruli into Bowman's space, about 140 is reabsorbed in the proximal convoluted tubules. The proximal convoluted tubules are responsible for absorbing back into the blood 65% to 70% of everything that was passed out in the filtrate. So they're extremely busy cells. And for that reason, they stain very eosinophilic in this section, in normal H&E sections, because they've got large factories, mitochondria, to provide energy for all the active transport processes for absorbing all that material back into the system. They also have various basal foldings and mitochondria there to again have channels and transport mechanisms to take material back from the lumen of the proximal convoluted tubule that's been filtered back into the blood, back into the body. So they're extremely active cells.

01:57 And they have a microvillus, brush border, which you don't appreciate here because it's very often hard to see them and to even preserve the tissue well enough to see them, particularly in H&E sections. But you see a rather undulating luminal border, representing microvilli, and also the very large cells of these proximal convoluted tubules. They are very, very busy cells. And they're easy to distinguish from the distal convoluted tubules, which are rather pale staining, more cuboidal, and they are doing less of the workload I guess. They are doing very important functions that the physiology lecturers will explain to you. But at least here, now you should be able to distinguish, the glomerulus, Bowman's space, and now, the proximal convoluted and distal convoluted tubules. Up in the cortical region, you'll also have collecting tubules. These are very small stained lightly structures you see, very thin. And if you look very very carefully along the epithelial cells, you often see a hint of the little lines between the cells representing where the lateral borders of these cells are joined together to stop material leaking back into the blood from the urinal space or the space within the collecting tubule. They're often hard to see. But certainly, at least in this section, you can make out the very eosinophilic stained proximal convoluted tubules in this region. Here, a section through the straight tubules.

03:54 You're looking now down, perhaps, in the medullary region, or at some part of the medullary rays in the cortex. Again, you can make out proximal straight tubules from distal straight tubules, because of the staining characteristics between the two that I've described before because of the high activity of these proximal tubules. And on the right-hand section, you can see profiles of the thin segments, the descending thin limb, the ascending thin limb, and even the loop region. They're also often hard to see because they are very very thin, as their name suggests, and they're lined by just a simple squamous epithelium. And they're seen best here when they're cut transversely.

04:43 When we want to identify collecting ducts, it's quite an easy exercise, particularly if we're looking in the papillary area or the pyramid area of the medulla. You can identify them because again, like the collecting tubule, these cells have very fine lateral borders that you can just see in sections, in H&E sections.

05:11 Have a look carefully along the epithelium lining this collecting duct, and you can certainly see the nice round nuclei, but you can also see little fine pink lines that represent the junction or borders between these cells. Again, it's to stop contents leaking back into the system, into the vascular system, and then back into the body, because these tubules now contain the urine that's now destined to be passed down through the ureter to the bladder and then eliminated. And then finally, when the urine passes down through the collecting ducts to the papilla of the pyramid, the apex of the pyramid that then passes into the minor calyx and then down the ureter towards the bladder for storage.

06:07 Note the epithelium on the minor calyx. It becomes rather stratified.

06:16 We're going to call that transitional epithelium. We're going to refer it also to urothelium. And just finally, well, as we're talking about the nephron, sometimes the term uriniferous tubule is named. That refers to the nephron and the collecting duct to which that nephron passes urine too. Let me now explain the vascular supply to the nephron. I've already explained that blood enters the glomerulus from the afferent arteriole arriving. It then breaks into the capillary bed, the glomerulus that I've described, where the podocyte wrap around. And then the blood leaves via the efferent arteriole, the exiting arteriole. That blood then forms a capillary network, another capillary network.

07:18 And in the cortex, in the cortical nephrons, this capillary network is called the peritubular network. It wraps around all the tubules and then drains into corresponding veins and then out of the kidney. And remember, they're the important capillaries that send oxygen levels and then can secrete erythropoietin. In contrast, the efferent arteriole in the juxtamedullary nephrons, they do a different thing. They form a network that then follows parallelly all the straight tubules going down into the medulla, in the pyramids. And this is a very important relationship. Running parallel to these tubules, these capillaries called the vasa recta are able to participate in a countercurrent mechanism that concentrates our urine. And again, the physiologist will explain about the role that these vasa recta have in doing that.

08:28 On the right-hand side, you can see a section through the medullary portion of the kidney, the pyramids. And you can see the vasa recta stained here, the red blood cells. They form an enormous network around these straight tubules.

08:48 And then finally, that blood passes out again through the venous system into the renal vein.

08:54 I want to now go back to what I was talking about earlier when I mentioned