 Okay everybody, Dr. O'Hare, let's take a deeper dive into the nephrons. We covered an introductory one where you got the big picture of what's happening in each location. Let's start here with the blood supply. The nephrons are considered the functional units of your kidney. Each kidney should have about 1.25 million of them and the action starts up there in the upper left hand corner with the afferent arterioles. The afferent arterioles are going to feed these knots of capillaries, these high pressure capillaries, which are going to squeeze a lot of fluid out called the glomerulus. And then this fluid as it's being filtered out of these capillaries is going to be captured by what's called the glomerular capsule or the Bowman's capsule. When you put them both together, the knot of capillaries and the glomerulus and the glomerular capsule, you're going to see that called the renal corpuscle sometimes just so you know. So then the fluid that isn't lost in this knot of capillaries will leave through the efferent arterioles, but then notice we have the peritubular capillary network. And then the time you have these straight arterioles here, they're called vasorecta. Not a huge deal. But these blood vessels are important because we just lost 50 gallons of fluid and a whole bunch of good stuff like vitamins, minerals, glucose, amino acids. We have to actively reabsorb that and get that back in our blood. So that's why you see that massive blood supply surrounding each individual nephron. Okay. So that's the blood supply. Let's go ahead and look at each part. So here we do have the renal corpuscle. So we have the glomeruli, which is going to be that knot of capillary beds you see there. And we'll talk about the special cells covering them in just a moment. On the outside, you're going to have that glomerular capsule or Bowman capsule. So this is a filter. In a word, that's all that the renal corpuscle here can do. All the glomerulus can do is filter. So like all filters, they're based on size. So we'll talk about the size and where the filtration occurs in just a second. Good news is we can filter out a lot of small metabolic waste products. The bad news is we have metabolic waste products that are too small to filter out. And also we have lots and lots of good things that we need that we need that are tiny. So they're going to be filtered out. Silly analogy. But have you ever heard the don't throw the baby out with the bath water? Well, it's almost like we throw the baby out with the bath water, but then we immediately grab the baby and ribs and reabsorb the baby. So you'll see that. That's weird. But the reabsorption at the proximal convoluted tubule is going to be a very, very big deal. So we're filtering out good stuff and bad stuff and way too much fluid for us to survive. So you'll see that the rest of the system is designed to reabsorb 99 to 100% of most of those things. So these special cells here is called podocytes. Let's take another look at those. So what actually creates what's called this filtration membrane is number one, we have fenestrated capillaries. So remember fenestrated capillaries have windows or larger openings in them. Then surrounding these capillaries is a thicker than usual basement membrane. So we're trying to make sure that only tiny things get through here. So we have fenestrated capillaries, but then we have a thick basement membrane, but the key is these cells. So you saw they're called podocytes and these podocyte cells have these feet called pedicles. And I'll point to them here, but these pedicles are these finger-like extensions that are surrounding these podocyte cells. So if you take the fenestrated capillary, the thick basement membrane, and then the slits created by these pedicles, these feet, then we now have the filtration membrane. And if you're small enough to travel through the filtration membrane, then you will make it into filtrate. And that's good if you're small and bad and we want you gone, but it's bad if you're small and good. And let's move on to the next step. So now we're at the proximal convoluted tubule. Its primary function is reabsorption. It's going to reabsorb 70% of water, the water that we lost, those 50 gallons, and it's going to reabsorb 99% to 100% of vitamins, minerals, glucose, amino acids, all the good stuff we can't afford to lose. It's going to absorb it 100% unless there's too much of something. So if you're taking high doses of vitamins, don't be surprised to see some vitamins in your urine. That's totally fine, totally normal. Some people look at that and think, isn't that evidence that multivitamins don't work if I'm just peeing it out? Well, here's the thing. It actually is the opposite. It's evidence they do work. If you were to take a multivitamin and you find vitamins in your urine, that means that that multivitamin pill or whatever it was, was digested, was absorbed, got into your bloodstream, and the excess was lost as urine. So just kind of keep that in mind. Glucose is the key example, though, of where we might reach what's called a transport maximum or a renal threshold. So you've only got so many transporters that can reabsorb glucose. If you overwhelm them because there's too much glucose in your blood, then some will spill into the urine. The number is going to be, I used to say 180, but the renal threshold is somewhere between 180 and 200 milligrams per deciliter, meaning that if you find glucose in your urine, that means your blood glucose levels are way higher than normal because a blood sugar above 125 would be diabetic. 100 to 125 would be pre-diabetic. So that's fasting. So now you're looking at double that. So if your blood sugar is 200, then you're going to see glucose spilling into your urine. So that's going to be, just imagine you've got these transporters grabbing every glucose they can find once each of them is saturated and working as hard as it can, as it can, any excess will be found in your urine. So there's lots of things that have a renal threshold. The renal threshold for amino acids is usually about 65 milligrams per deciliter. So a diet really high in protein and amino acids could lead to some of them being in your urine as well. All right. One of the ways, just quickly, one of the ways the PCT or Proximal Confidatubule does reabsorb so much water is it actively reabsorbs sodium and then water follows it. OK, now we're down to the loop of Henley here. Big picture, the primary role of the loop of Henley depends on which side you're looking at, but its primary function is to reabsorb 15 more percent of that 50 gallons of filtrate. But you also see it does reabsorb sodium as well. Before we get there, though, there are two types of nephrons based on how long the loop of Henley is. So the 85% of your nephrons are cortical nephrons with short loop of Henley's like this one. 15% would be juxtamedullary nephrons. Imagine that loop of Henley going to the floor. So the cortical nephrons, they do most the reabsorption and secretion. But juxtamedullary nephrons, because their loops are so long, they reabsorb more water. So the loop of Henley's primary job is to further concentrate urine by reabsorbing water. You see here the descending loop has permanent aquaporn channels. Aquaporn channels are any channels that allow only water to come through. The more aquaporn channels you have, the more water is going to be reabsorbed. So in this case, they're always here. So they're always going to be reabsorbing water. Whereas the collecting duct is going to have aquaporn channels that can be put there if you need them based on how much ADH, antidiuretic hormone you have. So the descending loop of Henley thinks reabsorb water. A sending loop on the way back up, it reabsorbs sodium primary function there. All right, so here you see what's actually happening here. As you travel down the left-hand side, you'll see the osmolarity is increasing to 300 to 1200 because water is freely being reabsorbed here. So as you reabsorb water into your body, you're going to be changing the concentration. And then on the way back up, you're going to see that sodium and chloride are being actively pumped back in. And that's why the descending limb of the loop of Henley is for reabsorption of water. A sending limb, reabsorption of sodium. I don't think anything else needs to be said there. Now we're at the distal convoluted tubule. In a word, secretion is the key thing to remember. But it does have three functions. It actively secretes things like ions, acids, drugs, and toxins. So this would be how drugs and toxins might end up in a year analysis. That would be, again, if your pH is dropping, then you're going to want to be getting rid of more acids. This would be why if you're exercising, because you're producing more metabolic acids, the pH of your urine is going to drop. These acids are going to be dumped out here at the distal convoluted tubule, so the pH of your urine might drop two points. But that's good because you're keeping your blood pH normal even though you're exercising. So secretion is the key, but there is reabsorption here. Sodium, the big one here is the distal convoluted tubule is calcium. So under the influence of parathyroid hormone, you will reabsorb more calcium here, which is why if someone has calcium-based kidney stones, the advice used to always be low-calcium diets, stop this from happening, but your body wouldn't be actively reabsorbing as much calcium if you had enough of it. So if you eat a diet higher in calcium, your parathyroid hormone levels will drop. So I think, and this is not medical advice, but personally, I think, and research shows this too, higher calcium intake will lower your kidney stone risk, not lower calcium intake. Just kind of an interesting tidbit there. So distal convoluted tubule, there is reabsorption, but secretion is the key. And that also will reabsorb water if it's needed. It's called the selective reabsorption of water because this would only be happening if there was an increase in antidiuretic hormone, that type of thing. All right, then we get into the collecting ducts. Remember, they're not actually part of a nephron, but nephrons will connect to these collecting ducts. Their job is to determine the final concentration of your urine. If you have perfectly functioning kidneys, at this point, when you reach that end of the distal convoluted tubule, you've turned 50 gallons of filtrate into 27 liters, but we can't afford to lose that last 15%, that 27 liters of urine a day. So we have to start to actively reabsorb water here. The reabsorption before was just a passive process, primarily caused by osmosis, but the hormone ADH will determine how much of that 27 liters you reabsorb. This is why I always tell people how often are you urinating? How much urine are you producing? That will tell you whether or not you're hydrated. If you're hydrated, your kidneys will allow excess water to pass through as urine, if you're dehydrated, it won't. So if you're only urinating every five or six hours, I would say you're probably dehydrated and need to be consuming more fluid. All right, so how does it actually work? I've already mentioned this, but ADH opens extra aquaporin water channels. So the more ADH you have, the more aquaporin channels there are, the more water will be reabsorbed, and now we have our urine. So where does it go? You'll see here, we have the entire nephron we just talked about, the collecting system there. The tuft of collecting systems will fuse here at the bottom and form the renal papilla. So let me go ahead, so about 30 of them, the collecting ducts are gonna fuse and about 30 of these ducts will form this renal papilla. If you look at the kidneys here, you see the renal papilla at the end of the renal pyramid and they're going to dribble or drain urine into a minor calyx, which will fuse to become a major calyx, which will drain that urine to the renal pelvis where it will then drain to the ureter, the bladder, the urethra and the toilet. So now we've gone from the fluid being filtered out of your blood, everything being reabsorbed we need, hopefully, and then what we don't need and the excess gets lost in your urine. So that's the nephron and how it works. I hope this helps, have a wonderful day, be blessed.