hi everyone and welcome to Miss Estre Biology in this video it's an updated version of my nephron video going through all the information you need to know about how the nephron filters the blood so let's get into it so what we'll cover in this video is the structure of the nephron function of the renal capsule proximal convoluted tubule loop of henley distal convoluted tubule and the collecting ducts we'll go through how ultra filtration occurs and where it happens and what selective reabsorption is and again where it happens so starting off then just with an overview of the kidney you don't actually need to know the structures in detail all you need to know is that the filterine and osma regulation which will be in a later video occur in the nephrons the nephrons are found within the medulla which is just here so the nephrons then what they actually are is these long tubules which are surrounded by capillaries and you have about 1 million nephrons in each of your kidneys so the structure of the nephron just zooming in on one here you have as we said the capillaries surrounding them and leading into the nephron you have an apherent arterial which then branches into lots and lots of smaller capillaries and that is what we call the glaris so that there is lots of capillaries the glarerus now those lie inside of this capsule which is called the renal capsule sometimes called the Bowman's capsule or on this picture here the glomela capsule but the away spec calls it the renal capsule so that's what I'll be using after that it then leads into the proximal convoluted tubule which we can see winding around here into the loop of henley then up to the distal convolute tubule and the collecting duct so we'll go through what happens at each of these positions in the nephron to create urine so that's the overall function of the nephron is to create urine um and that is because you're filtering the blood to remove waste so any excess water ura is going to be removed useful substances will be selectively reabsorbed back into the blood so the urine will only contain water excess water dissolved salts or mineral ions ura and any other small substances that um can be filtered out so it could be hormones or excess vitamins in a healthy person you should never find proteins blood cells or glucose and this is actually from GCSE as well the knowledge of knowing why you'd never find a protein or a blood cell in urine and why you'd never find glucose so just as a reminder the proteins and blood cells are both too big to be filtered out so they'll always remain in the blood glucose does get filtered out but all of the glucose is reabsorbed by active transport in the selective reabsorption stage which occurs in the proximal convoluted tubule or PCT for short so an overview view first of all of all of this filtering and reabsorption step one is in the glareris you have this ultrailtration of water and small molecules due to high pressure and that will force out the small molecules and water into the renal capsule then you'll have your filtrate which is called the glarerous filtrate passing into the proximal convoluted tubule and at this stage 85% of that filtrate gets reabsorbed back into the blood the loop of Henley is the next stage and at this point the sodium ion gradient is maintained and that is to enable water to be reabsorbed by osmosis into the blood and then the final step we've got our distal convoluted tubial and collecting duct where further water is removed barsis or diffuses out barsis and is reabsorbed back into the blood and any of the remaining liquid in the collecting duct goes on to form urine so let's go through each of these stages then in detail starting with the ultrailtration so at this point we have blood entering through the apherrant arterial and the arterial splits into lots and lots of smaller capillaries and as we said at the start that is what the glarerus is because you've gone from a wider lumen or a larger space into lots of smaller narrower capillaries you end up creating this hydrostatic pressure so that high pressure forces out small molecules and water and it has to be small because it has to be small enough to be able to fit through the tiny gaps in the cells in the epithelium of the capillaries that forms the glarus filtrate so we call it glarus filtrate because it's formed from the glarerus staying in the blood you'll have large proteins and blood cells because they're too big to fit out of the gaps so they will then pass out of the efferent arterial and continue to circulate around the blood in the body so just to show you where that is happening we've got zoomed in here we can see our apherrant arterial splitting to make the glarus and then leaving we have the epherent arterial so the ultra filtration will be occurring here and the glomelaris filtrate will be going into this renal capsule so just here so we're going to go through in a bit more detail here how that filtration happens so we've talked about how the high hydrostatic pressure is generated and now we're going to zoom in on the capillaries so this is a capillary within that glareris and the capillaries have just a single layer of cells making up their endothelium so that's what these structures here are representing really really zoomed in looking at those cells making up the endothelium and there are tiny gaps between those cells that is your first place where the filtration happens so a bit like a civ anything that's small enough to fit through those gaps will pass through then you have a basement membrane which again acts as a filter and then finally on the outside you have ptoytes which are these cells that wrap around the capillary and there are tiny gaps between those as well which adds another filtration layer so here are the pocytes we can see on the outside of the capillaries wrapping around those and looking at it from a different angle those are the pocytes and there's tiny gaps again so you've essentially got three filtrations we've got the gaps between the endothelium of the capillary basement membrane and then the gaps between the ptoytes so that's the ultra filtration so that glarous filtrate is now going to flow and pass into the proximal convoluted tubule and this is where selective reabsorption happens so 85% of that filtrate that's just been created gets reabsorbed back into the blood um at this stage in the proximal convoluted tubule so we're going to go through how that happens before we do that just going to point out some adaptations of the cells lining the proximal convoluted tubule and that's what we're looking at here so this bit is the lumen so that is the space in the middle that the filtrate passes through this bit proximal convoluted tubial cells so those are the epithelial cells we then got a slight gap which we call the interstitial space which is the gap between the proximal convoluted tubule and the capillaries that surround it and then here we have our capillary so the two key adaptations are the proximal convoluted tubial cells have all of these microvilli and that creates a really large surface area to maximize the reabsorption of glucose there's also lots of mitochondria within these cells and that's because energy is needed for active transport at this stage so those are our adaptations what we need to look at next then is how selective reabsorption happens so step one the concentration of sodium ions in the proximal convoluted tubule which I've just abbreviated PCT um is low within that cell that is low because sodium ions are actively transported out of the PCT into the bloodstream so that is why we have all these mitochondria to provide energy for the active transport of sodium ions out of the proximal convoluted tubal cells into the blood the impact that has is the cell here has um a very very low concentration of sodium ions compared to the glomelarus filtrate which is going to be flowing through the lumen so that then means that the sodium ions can move into the proximal convoluted tubule by diffusion going down their concentration gradient now the protein that the sodium ions diffuse through is a coransporter protein and that particular protein both sodium ions and glucose attach to so when that sodium ion attaches so does glucose and therefore that is how glucose gets from the glarous filtrate into the proximal convoluted tubule so the final step is you'll now have a large concentration of glucose within your proximal convoluted tubial cell and because you've got that high concentration compared in the blood you have a concentration gradient so the glucose can diffuse from the PCT cell into the bloodstream and that is how all of the glucose is reabsorbed that was initially um filtered out now one thing I just want to emphasize is reabsorbed if you just say absorb that is incorrect because absorbed means it's the first time it was taken in this is reabsorption because it was already in the blood then it was filtered out but then we take it back into the blood so whenever you are talking about the kidneys you will only ever be using the term reabsorb so next then the uh filtrate would have passed through the PCT and now it's leading into the loop of Henley and at this stage the function of the loop of Henley is to maintain a sodium ion gradient so that's what we're going to have a look at but first of all just to see the structure in more detail so the loop of henley we describe as being made up of two limbs so we have an ascending limb and a descending limb and this is named after the direction that the filtrate is moving in so so to go through the stages then of what's happening at the loop of Henley step one there are mitochondria within the walls of the ascending limb and that's to provide energy for the active transport of sodium ions so sodium ions are actively transported out of the filtrate into what we call the interstitial space which is the space between the nephron and the pillaries in doing this there's an accumulation of sodium ions in this interstitial space in the medulla and it creates a really low water potential now the numbers that you can see here in the loop of Henley and in the interstitial space that is representing water concentration so we're having um a lower water concentration here we've got more sodium ions moving in so we have a more concentrated solution or a lower water potential because there's a lower water potential that means the water in the descending limb of the loop of Henley will move out by osmosis into the interstitial space and then it'll be reabsorbed into the blood so that's how the water is reabsorbed into the blood final thing to point out on the loop of Henley is step four down here and it's at the very very base of the ascending limb because there's now very dilute um solution or very low concentration of sodium ions at the base some of the sodium ions will move out by diffusion so the next step in our nephron we've gone through the renal capsule proximal convoluted tubule the loop of henley next is the distal convoluted tubule and the collecting ducts next then is the distal convoluted tubule and the collecting duct and one thing just to point out although I've used PCT and DCT at different points in this video in the exam you do have to write out those full terms so proximal convoluted tubule and now the distal convoluted tubule so due to all the sodium ions that have been actively transported out of the ascending loop of Henley by the time the filtrate gets to the top so at the distal convoluted tubule you actually have a very dilute filtrate that is remaining inside the tubule especially in comparison to the water potential of the medulla so as that filtrate moves into the distal convoluted tubule and then the collecting ducts you have that concentrated solution or very negative water potential and that is what causes even more water to move out by osmosis from the distal convoluted tubule and the collecting duct and whatever filtrate remains in the collecting duct goes on to form the urine so that is the whole process of ultrailtration and reabsorption and how urine is created an example of an application question that comes up to do with a loop of henley is the one we've got here so in the past students have been asked to suggest how the length of the loop of henley will differ for a desert animal compared to a human and then you'd be asked to explain why so he said the loop of Henley the function is to maintain the sodium concentration gradients so that more water can be reabsorbed so if they're in the desert they're going to need more water to be reabsorbed and therefore they'll have a longer loop of henley so that more sodium ions can be um actively transported out and therefore more water will be reabsorbed so if you've got this longer loop of henley there's a larger surface area for sodium ions to be actively transported out so you'll then have even more sodium ions lowering the water potential more water will move out by osmosis and that gets reabsorbed into the blood and as a result they get more water going back into the blood and very very concentrated urine which for a desert animal that is essential for survival because their environment there's very little water so whatever water they do get from their food or from their water it's essential that that gets reabsorbed back into the blood rather than lost and wasted in the urine so just to summarize then the nephron is made up of the renal capsule the PCT loop of henley DCT and collecting ducts and they're surrounded by capillaries the glar filtrate is created in the renal capsule glucose and water are reabsorbed back into the blood by the PCT the sodium ion gradient is maintained in the loop of henley and that's to enable water reabsorption and then further reabsorption happens in the DCT and the collecting ducts so that is it for filtering re and reabsorption in the nephron make sure to watch the next video on osmo regulation so you can see the negative feedback and homeostasis of how the permeability of the distal convoluted tubule and the collecting duct changes um depending on how much water you have in your blood if you want to have a go at some practice questions to test your knowledge head over to Miss Estrich and if you aren't already subscribed make sure you click the button to subscribe and give it a thumbs up if you have found this video helpful